Scientific Publications

On this page you will find a list of all peer reviewed KASC publications that have been published in a scientific journal.

Anomalies in the Kepler Asteroseismic Legacy Project Data A re-analysis of 16 Cyg A&B, KIC8379927 and 6 solar-like stars

Ian W Roxburgh.
Full text
NASA ADS
DOI

I compare values of the frequencies, separation ratios, errors and covariance matrices from a new analysis of 9 solar-like stars with the Legacy project values reported by Lund et al and, for 16Cyg A&B and KIC8379927, with values derived by Davies et al. There is good agreement between my results and Davies's for these 3 stars, but no such agreement with the Legacy project results. My frequencies differ from the Legacy values, there are inconsistencies in the Legacy frequency covariance matrices which are not positive definite, and the Legacy errors on separation ratios are up to 40 times larger than mine and the values and upper limits derived from the Legacy frequency covariances. There are similar anomalies for 6 other solar-like stars: frequencies and separation ratio errors disagree and 2 have non positive definite covariance matrices. There are inconsistencies in the covariance matrices of 27 the 66 stars in the full Legacy set and problems with the ratio errors for the vast majority of these stars

Metallicity effect on stellar granulation detected from oscillating red giants in Open Clusters

E. Corsaro, S. Mathur, R. A. García, P. Gaulme, M. Pinsonneault, K. Stassun, D. Stello, J. Tayar, R. Trampedach, C. Jiang, C. Nitschelm, D. Salabert.
Full text

Context. The effect of metallicity on the granulation activity in stars, and hence on the convective motions in general, is still poorly understood. Available spectroscopic parameters from the updated APOGEE-Kepler catalog, coupled with high-precision photometric observations from NASA's Kepler mission spanning more than four years of observation, make oscillating red giant stars in open clusters crucial testbeds.
Aims. We determine the role of metallicity on the stellar granulation activity by discriminating its effect from that of different stellar properties such as surface gravity, mass, and temperature. We analyze 60 known red giant stars belonging to the open clusters NGC 6791, NGC 6819, and NGC 6811, spanning a metallicity range from [Fe/H] $\simeq -0.09$ to $0.32$. The parameters describing the granulation activity of these stars and their frequency of maximum oscillation power, $\nu_\mathrm{max}$, are studied while taking into account different masses, metallicities, and stellar evolutionary stages. We derive new scaling relations for the granulation activity, re-calibrate existing ones, and identify the best scaling relations from the available set of observations.
Methods. We adopt the Bayesian code DIAMONDS for the analysis of the background signal in the Fourier spectra of the stars. We perform a Bayesian parameter estimation and model comparison to test the different model hypotheses proposed in this work and in the literature.
Results. Metallicity causes a statistically significant change in the amplitude of the granulation activity, with a dependency stronger than that induced by both stellar mass and surface gravity. We also find that the metallicity has a significant impact on the corresponding time scales of the phenomenon. The effect of metallicity on the time scale is stronger than that of mass.
Conclusions. A higher metallicity increases the amplitude of granulation and meso-granulation signals and slows down their characteristic time scales toward longer periods. The trend in amplitude is in qualitative agreement with predictions from existing 3D hydrodynamical simulations of stellar atmospheres from main sequence to red giant stars. We confirm that the granulation activity is not sensitive to changes in the stellar core and that it only depends on the atmospheric parameters of stars.

Asteroseismology and Gaia: Testing Scaling Relations Using 2200 Kepler Stars with TGAS Parallaxes

Daniel Huber, Joel Zinn, Mathias Bojsen-Hansen, Marc Pinsonneault, Aldo Serenelli, Victor Silva Aguirre, Christian Sahlholdt, Keivan Stassun, Dennis Stello, Jamie Tayar, Fabienne Bastien, Timothy R. Bedding, Lars A. Buchhave, William J. Chaplin, Guy R. Davies, Rafael A. Garcia, David W. Latham and 3 coauthors.
Full text

We present a comparison of parallaxes and radii from asteroseismology and Gaia DR1 (TGAS) for 2200 Kepler stars spanning from the main sequence to the red giant branch. We show that previously identified offsets between TGAS parallaxes and distances derived from asteroseismology and eclipsing binaries have been partially overestimated for stars beyond 100pc, and can be in part compensated by adopting a hotter Teff scale (such as the infrared flux method) instead of spectroscopic temperatures for dwarfs and subgiants. Residual systematic differences are at the  2% level in parallax across three orders of magnitude. We use TGAS parallaxes to empirically demonstrate that asteroseismic radii are accurate to  10% or better for stars between  0.8-8Rsun. We find no significant offset for main-sequence (< 1.5Rsun) and low-luminosity RGB stars ( 3–8Rsun), but seismic radii appear to be systematically underestimated by  5% for subgiants ( 1.5-3Rsun). We find no systematic errors as a function of metallicity between [Fe/H]   -0.8 to +0.4 dex, and show tentative evidence that corrections to the scaling relation for the large frequency separation (Dnu) improve the agreement with TGAS for RGB stars. Finally, we demonstrate that beyond  3kpc asteroseismology will provide more precise distances than end-of-mission Gaia data, highlighting the synergy and complementary nature of Gaia and asteroseismology for studying galactic stellar populations.

Near-degeneracy effects on the frequencies of rotationally-split mixed modes in red giants

S. Deheuvels, R. M. Ouazzani, S. Basu.
Full text

The Kepler space mission has made it possible to measure the rotational splittings of mixed modes in red giants, thereby providing an unprecedented opportunity to probe the internal rotation of these stars. Asymmetries have been detected in the rotational multiplets of several red giants. This is unexpected since all the red giants whose rotation have been measured thus far are found to rotate slowly, and low rotation, in principle, produces symmetrical multiplets. Our aim here is to explain these asymmetries and find a way of exploiting them to probe the internal rotation of red giants. We show that in the cases where asymmetrical multiplets were detected, near-degeneracy effects are expected to occur, because of the combined effects of rotation and mode mixing. Such effects have not been taken into account so far. By using both perturbative and non-perturbative approaches, we show that near-degeneracy effects produce multiplet asymmetries that are very similar to the observations. We then propose and validate a method based on the perturbative approach to probe the internal rotation of red giants using multiplet asymmetries. We successfully apply our method to the asymmetrical $l=2$ multiplets of the Kepler young red giant KIC7341231 and obtain precise estimates of its mean rotation in the core and the envelope. The observed asymmetries are reproduced with a good statistical agreement, which confirms that near-degeneracy effects are very likely the cause of the detected multiplet asymmetries. We expect near-degeneracy effects to be important for $l=2$ mixed modes all along the red giant branch (RGB). For $l=1$ modes, these effects can be neglected only at the base of the RGB. They must therefore be taken into account when interpreting rotational splittings and as shown here, they can bring valuable information about the internal rotation of red giants.

Beyond the Kepler/K2 bright limit with halo photometry: variability in the seven brightest members of the Pleiades

T. R. White, B. J. S. Pope, V. Antoci, P. I. Pápics, C. Aerts, D. R. Gies, K. Gordon, D. Huber, G. H. Schaefer, S. Aigrain, S. Albrecht, T. Barclay, G. Barentsen, P. G. Beck, T. R. Bedding, M. Fredslund Andersen, F. Grundahl and 6 coauthors.
Full text
NASA ADS
DOI

The most powerful tests of stellar models come from the brightest stars in the sky, for which complementary techniques, such as astrometry, asteroseismology, spectroscopy, and interferometry can be combined. The K2 Mission is providing a unique opportunity to obtain high-precision photometric time series for bright stars along the ecliptic. However, bright targets require a large number of pixels to capture the entirety of the stellar flux, and bandwidth restrictions limit the number and brightness of stars that can be observed. To overcome this, we have developed a new photometric technique, that we call halo photometry, to observe very bright stars using a limited number of pixels. Halo photometry is simple, fast and does not require extensive pixel allocation, and will allow us to use K2 and other photometric missions, such as TESS, to observe very bright stars for asteroseismology and to search for transiting exoplanets. We apply this method to the seven brightest stars in the Pleiades open cluster. Each star exhibits variability; six of the stars show what are most-likely slowly pulsating B-star (SPB) pulsations, with amplitudes ranging from 20 to 2000 ppm. For the star Maia, we demonstrate the utility of combining K2 photometry with spectroscopy and interferometry to show that it is not a `Maia variable', and to establish that its variability is caused by rotational modulation of a large chemical spot on a 10 d time scale.

Characterizing solar-type stars from full-length Kepler data sets using the Asteroseismic Modeling Portal

O. L. Creevey, T. S. Metcalfe, M. Schultheis, D. Salabert, M. Bazot, F. Thévenin, S. Mathur, H. Xu, R. A. García.
Full text
NASA ADS
DOI

The Kepler space telescope yielded unprecedented data for the study of solar-like oscillations in other stars. The large samples of multi-year observations posed an enormous data analysis challenge that has only recently been surmounted. Asteroseismic modeling has become more sophisticated over time, with better methods gradually developing alongside the extended observations and improved data analysis techniques. We apply the latest version of the Asteroseismic Modeling Portal (AMP) to the full-length Kepler data sets for 57 stars, comprising planetary hosts, binaries, solar-analogs, active stars, and for validation purposes, the Sun. From an analysis of the derived stellar properties for the full sample, we identify a variation of the mixing-length parameter with atmospheric properties. We also derive a linear relation between the stellar age and a characteristic frequency separation ratio. In addition, we find that the empirical correction for surface effects suggested by Kjeldsen and coworkers is adequate for solar-type stars that are not much hotter (T$_{\rm eff}~\lesssim 6200$ K) or significantly more evolved ($\log g~\gtrsim 4.2$, $\langle \Delta \nu \rangle ~\gtrsim 80\mu$Hz) than the Sun. Precise parallaxes from the Gaia mission and future observations from TESS and PLATO promise to improve the reliability of stellar properties derived from asteroseismology.

Kepler Observations of the Asteroseismic Binary HD 176465

T. R. White, O. Benomar, V. Silva Aguirre, W. H. Ball, T. R. Bedding, W. J. Chaplin, J. Christensen-Dalsgaard, R. A. Garcia, L. Gizon, D. Stello, S. Aigrain, H. M. Antia, T. Appourchaux, M. Bazot, T. L. Campante, O. L. Creevey, G. R. Davies and 17 coauthors.
Full text

Binary star systems are important for understanding stellar structure and evolution, and are especially useful when oscillations can be detected and analysed with asteroseismology. However, only four systems are known in which solar-like oscillations are detected in both components. Here, we analyse the fifth such system, HD 176465, which was observed by Kepler. We carefully analysed the system's power spectrum to measure individual mode frequencies, adapting our methods where necessary to accommodate the fact that both stars oscillate in a similar frequency range. We also modelled the two stars independently by fitting stellar models to the frequencies and complementary spectroscopic parameters. We are able to cleanly separate the oscillation modes in both systems. The stellar models produce compatible ages and initial compositions for the stars, as is expected from their common and contemporaneous origin. Combining the individual ages, the system is about $3.0\pm0.5\,\mathrm{Gyr}$ old. The two components of HD 176465 are young physically-similar oscillating solar analogues, the first such system to be found, and provide important constraints for stellar evolution and asteroseismology.

Convective-core overshoot and suppression of oscillations: Constraints from red giants in NGC 6811

T. Arentoft, K. Brogaard, J. Jessen-Hansen, V. Silva Aguirre, H. Kjeldsen, J. R. Mosumgaard, E. L. Sandquist.
Full text

Using data from the NASA spacecraft Kepler, we study solar-like oscillations in red-giant stars in the open cluster NGC 6811. We determine oscillation frequencies, frequency separations, period spacings of mixed modes and mode visibilities for eight cluster giants. The oscillation parameters show that these stars are helium-core-burning red giants. The eight stars form two groups with very different oscillation power spectra; the four stars with lowest $\Delta\nu$-values display rich sets of mixed $l=1$ modes, while this is not the case for the four stars with higher $\Delta\nu$. For the four stars with lowest $\Delta\nu$, we determine the asymptotic period spacing of the mixed modes, $\Delta$P, which together with the masses we derive for all eight stars suggest that they belong to the so-called secondary clump. Based on the global oscillation parameters, we present initial theoretical stellar modeling which indicate that we can constrain convective-core overshoot on the main sequence and in the helium-burning phase for these $\sim$2 M$_{\odot}$ stars. Finally, our results indicate less mode suppression than predicted by recent theories for magnetic suppression of certain oscillation modes in red giants.

Kepler sheds new and unprecedented light on the variability of a blue supergiant: gravity waves in the O9.5Iab star HD188209

Conny Aerts, Sergio Simon-Diaz, S. Bloemen, J. Debosscher, P. I. Pápics, S. Bryson, M. Still, E. Moravveji, M. H. Williamson, F. Grundahl, M. Fredslund Andersen, V. Antoci, P. L. Pallé, J. Christensen-Dalsgaard, T. M. Rogers.
Full text

Context. Stellar evolution models are most uncertain for evolved massive stars. Asteroseismology based on high-precision uninterrupted space photometry has become a new way to test the outcome of stellar evolution theory and was recently applied to a multitude of stars, but not yet to massive evolved supergiants.
Aims. Our aim is to detect, analyse and interpret the photospheric and wind variability of the O9.5 Iab star HD188209 from Kepler space photometry and long-term high-resolution spectroscopy.
Methods. We used Kepler scattered-light photometry obtained by the nominal mission during 1460 d to deduce the photometric variability of this O-type supergiant. In addition, we assembled and analysed high-resolution high signal-to-noise spectroscopy taken with four spectrographs during some 1800 d to interpret the temporal spectroscopic variability of the star.
Results. The variability of this blue supergiant derived from the scattered-light space photometry is fully in agreement with the one found in the ground-based spectroscopy.We find significant low-frequency variability that is consistently detected in all spectral lines of HD188209. The photospheric variability propagates into the wind, where it has similar frequencies but slightly higher amplitudes.
Conclusions. The morphology of the frequency spectra derived from the long-term photometry and spectroscopy points towards a spectrum of travelling waves with frequency values in the range expected for an evolved O-type star. Convectively-driven internal gravity waves excited in the stellar interior offer the most plausible explanation of the detected variability.

Evidence for compact binary systems around Kepler red giants

Isabel L. Colman, Daniel Huber, Timothy R. Bedding, James S. Kuszlewicz, Jie Yu, Paul G. Beck$, Yvonne Elsworth, Rafael A. García, Steven D. Kawaler, Savita Mathur, Dennis Stello, Timothy R. White.
Full text
NASA ADS
DOI

We present an analysis of 169 oscillating red giants from NASA's Kepler mission that exhibit anomalous peaks in their Fourier amplitude spectra. These peaks result from ellipsoidal variations which are indicative of binary star systems, at frequencies such that the orbit of any stellar companion would be within the convective envelope of the red giant. Alternatively, the observed phenomenon may be due to a close binary orbiting a red giant in a triple system, or chance alignments of foreground or background binary systems contaminating the target pixel aperture. We identify 88 stars in the sample as chance alignments using a combination of pixel Fourier analysis and difference imaging. We find that in the remaining 81 cases the anomalous peaks are indistinguishable from the target star to within 4$''$, suggesting a physical association. We examine a Galaxia model of the Kepler field of view to estimate background star counts and find that it is highly unlikely that all targets can be explained by chance alignments. From this, we conclude that these stars may comprise a population of physically associated systems.

KIC 8262223: A Post-Mass Transfer Eclipsing Binary Consisting of a Delta Scuti Pulsator and a Helium White Dwarf Precursor

Zhao Guo, Douglas R. Gies, Rachel A. Matson, Antonio García Hernández, Zhanwen Han, Xuefei Chen.
Full text

KIC 8262223 is an eclipsing binary with a short orbital period ($P=1.61$ d). The Kepler light curves are of Algol-type and display deep and partial eclipses, ellipsoidal variations, and pulsations of $\delta$ Scuti type. We analyzed the Kepler photometric data, complemented by phase-resolved spectra from the R-C Spectrograph on the 4-meter Mayall telescope at Kitt Peak National Observatory and determined the fundamental parameters of this system. The low mass and oversized secondary ($M_2=0.20M_{\odot}$, $R_2=1.31R_{\odot}$) is the remnant of the donor star that transferred most of its mass to the gainer, and now the primary star. The current primary star is thus not a normal $\delta$ Scuti star but the result of mass accretion from a lower mass progenitor. We discuss the possible evolutionary history and demonstrate with the MESA evolution code that the system can be understood as the result of non-conservative binary evolution similar to that for the formation of EL CVn type binaries. The pulsations of the primary star can be explained as radial and non-radial pressure modes. The equilibrium models from single star evolutionary tracks can match the observed mass and radius ($M_1=1.94M_{\odot}$, $R_1=1.67R_{\odot}$) but the predicted unstable modes associated with these models differ somewhat from those observed. This work presents a preliminary asteroseismic analysis of the abnormal $\delta$ Scuti pulsators, and we discuss the need for better theoretical understanding of such pulsating mass gaining stars.

The Kepler Cepheid V1154 Cyg revisited: light curve modulation and detection of granulation

A. Derekas, E. Plachy, L. Molnar, A. Sodor, J. M. Benko, L. Szabados, Zs. Bognar, B. Csak, Gy. M. Szabo, R. Szabo, A. Pal.
Full text
NASA ADS
DOI

We present a detailed analysis of the bright Cepheid-type variable star V1154 Cygni using 4 years of continuous observations by the Kepler space telescope. We detected 28 frequencies using standard Fourier transform method.We identified modulation of the main pulsation frequency and its harmonics with a period of  159 d. This modulation is also present in the Fourier parameters of the light curve and the O-C diagram. We detected another modulation with a period of about 1160 d. The star also shows significant power in the low-frequency region that we identified as granulation noise. The effective timescale of the granulation agrees with the extrapolated scalings of red giant stars. Non-detection of solar-like oscillations indicates that the pulsation inhibits other oscillations. We obtained new radial velocity observations which are in a perfect agreement with previous years data, suggesting that there is no high mass star companion of V1154 Cygni. Finally, we discuss the possible origin of the detected frequency modulations.

Asteroseismology of 1523 misclassified red giants using Kepler data

Jie Yu, Daniel Huber, Timothy R. Bedding, Dennis Stello, Simon J. Murphy, Maosheng Xiang, Shaolan Bi, Tanda Li.
Full text

We analyzed solar-like oscillations in 1523 Kepler red giants which have previously been misclassified as subgiants, with predicted νmax values [based on the Kepler Input Catalogue (KIC)] between 280 and 700 μHz. We report the discovery of 626 new oscillating red giants in our sample, in addition to 897 oscillators that were previously characterized by Hekker et al. from one-quarter of Kepler data. Our sample increases the known number of oscillating low-luminosity red giants by 26 per cent (up to ∼1900 stars). About three-quarters of our sample are classified as ascending red giant branch stars, while the remainder is red-clump stars. A novel scheme was applied to determine ∆ν for 108 stars with νmax close to the Nyquist frequency (240 μHz < νmax < 320 μHz). Additionally, we identified 47 stars oscillating in the super-Nyquist frequency regime, up to 387 μHz, using long-cadence light curves. We show that the misclassifications are most likely due to large uncertainties in KIC surface gravities, and do not result from the absence of broad-band colors or from different physical properties such as reddening, spatial distribution, mass or metallicity. The sample will be valuable to study oscillations in low-luminosity red giants and to characterize planet candidates around those stars.

Period spacings in red giants III. Coupling factors of mixed modes

B. Mosser, C. Pincon, K. Belkacem, M. Takata, M. Vrard.
Full text

The power of asteroseismology relies on the capability of global oscillations to infer the stellar structure. For evolved stars, we benefit from unique information directly carried out by mixed modes that probe their radiative cores. This third article of the series devoted to mixed modes in red giants focusses on their coupling factors that remained largely unexploited up to now. With the measurement of the coupling factors, we intend to give physical constraints on the regions surrounding the radiative core and the hydrogen-burning shell of subgiants and red giants. A new method for measuring the coupling factor of mixed modes is set up. It is derived from the method recently implemented for measuring period spacings. It runs in an automated way so that it can be applied to a large sample of stars. Coupling factors of mixed modes were measured for thousands of red giants. They show specific variation with mass and evolutionary stage. Weak coupling is observed for the most evolved stars on the red giant branch only; large coupling factors are measured at the transition between subgiants and red giants, as well as in the red clump. The measurement of coupling factors in dipole mixed modes provides a new insight into the inner interior structure of evolved stars. While the large frequency separation and the asymptotic period spacings probe the envelope and the core, respectively, the coupling factor is directly sensitive to the intermediate region in between and helps determining its extent. Observationally, the determination of the coupling factor is a prior to precise fits of the mixed-mode pattern, and can now be used to address further properties of the mixed-mode pattern, as the signature of the buoyancy glitches and the core rotation.

When dipole modes in red giants are simultaneously mixed and depressed

B. Mosser, K. Belkacem, C. Pincon, M. Takata, M. Vrard, C. Barban, MJ. Goupil, T. Kallinger, R. Samadi.
Full text

Seismic observations with Kepler have shown that dipole modes in evolved stars sometimes have lower amplitudes than expected from energy equipartition; they are called depressed modes. These observations were used to infer the presence of strong magnetic fields in the stellar core. This conclusion remains indirect, so that further studies are necessary to constrain the analysis and interpretation of depressed modes. The observation of mixed modes in red giants is used to characterize the seismic parameters of stars with low dipole visibility, in order to probe the damping of the oscillation. With the determination of global seismic parameters of mixed modes, we test the relevance of various hypotheses on the processes that construct mixed modes and explain the depressed modes. The observation of stars showing depressed dipole mixed modes is especially useful for deriving model-independent conclusions on the dipole mode damping. We explain with a simple model how mode visibilities provide a measure of the extra damping. Observations prove that dipole modes in red giants are mixed modes. Such a result disconfirms that depressed dipole modes are resulting from the suppression of the oscillation in the radiative core. Understanding the damping of mixed modes is necessary before extrapolating conclusions on the mechanism responsible for the mode depression. Measuring low visibilities is not enough for deriving the presence of a high internal magnetic field inside the red giant cores, so that the magnetic greenhouse effect remains speculative. Moreover, a high magnetic field cannot explain depressed modes in many stars.

Standing on the shoulders of dwarfs: the Kepler asteroseismic LEGACY sample II — radii, masses, and ages

Víctor Silva Aguirre, Mikkel N. Lund, H. M. Antia, Warrick H. Ball, Sarbani Basu, Joergen Christensen-Dalsgaard, Yveline Lebreton, Daniel R. Reese, Kuldeep Verma, Luca Casagrande, Anders B. Justesen, Jakob R. Mosumgaard, William J. Chaplin, Timothy R. Bedding, Guy R. Davies, Rasmus Handberg, Günter Houdek and 7 coauthors.
Full text

We use asteroseismic data from the Kepler satellite to determine fundamental stellar properties of the 66 main-sequence targets observed for at least one full year by the mission. We distributed tens of individual oscillation frequencies extracted from the time series of each star among seven modelling teams who applied different methods to determine radii, masses, and ages for all stars in the sample. Comparisons among the different results reveal a good level of agreement in all stellar properties, which is remarkable considering the variety of codes, input physics and analysis methods employed by the different teams. Average uncertainties are of the order of $\sim$2% in radius, $\sim$4% in mass, and $\sim$10% in age, making this the best-characterised sample of main-sequence stars available to date. Our predicted initial abundances and mixing-length parameters are checked against inferences from chemical enrichment laws $\Delta Y / \Delta Z$ and predictions from 3D atmospheric simulations. We test the accuracy of the determined stellar properties by comparing them to the Sun, angular diameter measurements, Gaia parallaxes, and binary evolution, finding excellent agreement in all cases and further confirming the robustness of asteroseismically-determined physical parameters of stars when individual frequencies of oscillation are available. Baptised as the Kepler dwarfs LEGACY sample, these stars are the solar-like oscillators with the best asteroseismic properties available for at least another decade. All data used in this analysis and the resulting stellar parameters are made publicly available for the community.

Standing on the shoulders of Dwarfs: the Kepler asteroseismic LEGACY sample I — oscillation mode parameters

Mikkel N. Lund, Víctor Silva Aguirre, Guy R. Davies, William J. Chaplin, Jørgen Christensen-Dalsgaard, Günter Houdek, Timothy R. White, Timothy R. Bedding, Warrick H. Ball, Daniel Huber, H. M. Antia, Yveline Lebreton, David W. Latham, Rasmus Handberg, Kuldeep Verma, Sarbani Basu, Luca Casagrande and 3 coauthors.
Full text
NASA ADS
DOI

The advent of space-based missions like Kepler has revolutionized the study of solar-type stars, particularly through the measurement and modeling of their resonant modes of oscillation. Here we analyze a sample of 66 Kepler main-sequence stars showing solar-like oscillations as part of the Kepler seismic LEGACY project. We use Kepler short-cadence data, of which each star has at least 12 months, to create frequency power spectra optimized for asteroseismology. For each star we identify its modes of oscillation and extract parameters such as frequency, amplitude, and line width using a Bayesian Markov chain Monte Carlo ‘peak-bagging’ approach. We report the extracted mode parameters for all 66 stars, as well as derived quantities such as frequency difference ratios, the large and small separations $\Delta\nu$ and $\delta\nu_{02}$; the behavior of line widths with frequency and line widths at $\nu_{\rm max}$ with $T_{\rm eff}$ , for which we derive parametrizations; and behavior of mode visibilities. These average properties can be applied in future peak-bagging exercises to better constrain the parameters of the stellar oscillation spectra. The frequencies and frequency ratios can tightly constrain the fundamental parameters of these solar-type stars, and mode line widths and amplitudes can test models of mode damping and excitation.

Tidally Induced Pulsations in Kepler Eclipsing Binary KIC 3230227

Zhao Guo, Douglas R. Gies, Jim Fuller.
Full text

KIC 3230227 is a short period ($P\approx 7.0$ days) eclipsing binary with a very eccentric orbit ($e=0.6$). From combined analysis of radial velocities and Kepler light curves, this system is found to be composed of two A-type stars, with masses of $M_1=1.84\pm 0.18M_{\odot}$, $M_2=1.73\pm 0.17M_{\odot}$ and radii of $R_1=2.01\pm 0.09R_{\odot}$, $R_2=1.68\pm 0.08 R_{\odot}$ for the primary and secondary, respectively. In addition to an eclipse, the binary light curve shows a brightening and dimming near periastron, making this a somewhat rare eclipsing heartbeat star system. After removing the binary light curve model, more than ten pulsational frequencies are present in the Fourier spectrum of the residuals, and most of them are integer multiples of the orbital frequency. These pulsations are tidally driven, and both the amplitudes and phases are in agreement with predictions from linear tidal theory for $l=2, m=-2$ prograde modes.

Seismic measurement of the locations of the base of convection zone and helium ionization zone for stars in the Kepler seismic LEGACY sample

Kuldeep Verma, Keyuri Raodeo, H. M. Antia, Anwesh Mazumdar, Sarbani Basu, Mikkel N. Lund, Víctor Silva Aguirre.
Full text
NASA ADS
DOI

Acoustic glitches are regions inside a star where the sound speed or its derivatives change abruptly. These leave a small characteristic oscillatory signature in the stellar oscillation frequencies. With the precision achieved by Kepler seismic data, it is now possible to extract these small amplitude oscillatory signatures, and infer the locations of the glitches. We perform glitch analysis for all the 66 stars in the Kepler seismic LEGACY sample to derive the locations of the base of the envelope convection zone and the helium ionization zone. The signature from helium ionization zone is found to be robust for all stars in the sample, whereas the convection zone signature is found to be weak and problematic, particularly for relatively massive stars with large errorbars on the oscillation frequencies. We demonstrate that the helium glitch signature can be used to constrain the properties of the helium ionization layers and the helium abundance.

Metal-Rich SX Phe stars in the Kepler Field

James M. Nemec, Luis A. Balona, Simon J. Murphy, Karen Kinemuchi, Young-Beom Jeon.
Full text

A spectroscopic and photometric analysis has been carried out for thirty-two candidate SX Phe variable blue straggler stars in the Kepler-field (Balona & Nemec 2012). Radial velocities (RVs), space motions ($U,V,W$), projected rotation velocities ($v$ sin $i$), spectral types, and atmospheric characteristics ($T_{\rm eff}$, $\log g$, [Fe/H], $\xi_t$, $\zeta_{\rm RT}$, etc.) are presented for 30 of the 32 stars. Although several stars are metal-weak with extreme halo orbits, the mean [Fe/H] of the sample is near solar, thus the stars are more metal-rich than expected for a typical sample of Pop. II stars, and more like halo metal-rich A-type stars (Perry 1969). Two thirds of the stars are fast rotators with $v$ sin $i$ $>$ 50 km/s, including four stars with $v$ sin $i$ $>$ 200 km/s. Three of the stars have (negative) RVs $>$ 250 km/s, five have retrograde space motions, and 21 have total speeds (relative to the LSR) $>$ 400 km/s. All but one of the 30 stars have positions in a Toomre diagram consistent with the kinematics of bona fide halo stars (the exception being a thick-disk star). Observed Rmer time delays, pulsation frequency modulations and light curves suggest that at least one third of the stars are in binary (or triple) systems with orbital periods ranging from 2.3 days to more than four years.

First observations of W Virginis stars with K2: detection of period doubling

E. Plachy, L. Molnár, M. I. Jurkovic, R. Smolec, P. A. Moskalik, A. Pál, L. Szabados, R. Szabó.
Full text
NASA ADS
DOI

We present the first analysis of W Vir stars observed by the Kepler space telescope in the K2 mission. We detected clear cycle-to-cycle variation in the light curves of KT Sco and the globular cluster member M80-V1. While the variations in the former star seems to be irregular on the short time scale of the K2 data, the latter appears to experience period doubling in its pulsation. Investigation of historical data revealed drastic period changes in both stars. For comparison we reexamine ground-based observations of W Vir, the prototype of the class, and conclude that the it shows period doubling instead of mode beating. These results support the notion that nonlinear dynamics plays an important role in the pulsation of W Virginis-type stars.

A new asteroseismic diagnostic for rotation in $\gamma$ Doradus stars

Rhita-Maria Ouazzani, S.J.A.J. Salmon, V. L. Antoci, T. R. Bedding, S. J. Murphy, and I. W. Roxburgh..
Full text

With four years of nearly-continuous photometry from Kepler, we are finally in a good position to apply asteroseismology to $\gamma$ Doradus stars. In particular several analyses have demonstrated the possibility to detect non-uniform period spacings, which have been predicted to be directly related to rotation. In the present work, we define a new seismic diagnostic for rotation in $\gamma$ Doradus stars that are too rapidly rotating to present rotational splittings. Based on the non uniformity of their period spacings, we define the observable $\Sigma$ as the slope of the period spacing when plotted as a function of period. We provide a univoque relation between this observable $\Sigma$ and the internal rotation, which applies widely in the instability strip of $\gamma$ Doradus stars. We apply the diagnostic to a handful of stars observed by Kepler. Thanks to g-modes in $\gamma$ Doradus stars, we are now able to determine the internal rotation of stars on the lower main sequence, which is still not possible for Sun-like stars.

Asteroseismic properties of solar-type stars observed with the NASA K2 mission: results from Campaigns 1-3 and prospects for future observations

Mikkel N. Lund, William J. Chaplin, Luca Casagrande, Víctor Silva Aguirre, Sarbani Basu, Allyson Bieryla, Jrgen Christensen-Dalsgaard, David W. Latham, Timothy R. White, Guy R. Davies, Daniel Huber, Lars A. Buchhave, Rasmus Handberg.
Full text
NASA ADS
DOI

We present an asteroseismic analysis of 33 solar-type stars observed in short cadence during Campaigns (C) 1-3 of the NASA K2 mission. We were able to extract both average seismic parameters and individual mode frequencies for stars with dominant frequencies up to ${\sim}3300\,\rm\mu Hz$, and we find that data for some targets are good enough to allow for a measurement of the rotational splitting. Modelling of the extracted parameters is performed by using grid-based methods using average parameters and individual frequencies together with spectroscopic parameters. For the target selection in C3, stars were chosen as in C1 and C2 to cover a wide range in parameter space to better understand the performance and noise characteristics. For C3 we still detected oscillations in $73\%$ of the observed stars that we proposed. Future K2 campaigns hold great promise for the study of nearby clusters and the chemical evolution and age-metallicity relation of nearby field stars in the solar neighbourhood. We expect oscillations to be detected in ${\sim}388$ short-cadence targets if the K2 mission continues until C18, which will greatly complement the ${\sim}500$ detections of solar-like oscillations made for short-cadence targets during the nominal Kepler mission. For ${\sim}30-40$ of these, including several members of the Hyades open cluster, we furthermore expect that inference from interferometry should be possible. le.

Asteroseismology of the Hyades with K2: first detection of main-sequence solar-like oscillations in an open cluster

Mikkel N. Lund, Sarbani Basu, Víctor Silva Aguirre, William J. Chaplin, Aldo M. Serenelli, Rafael A. García, David W. Latham, Luca Casagrande, Allyson Bieryla, Guy R. Davies, Lucas S. Viani, Lars A. Buchhave, Andrea Miglio, David R. Soderblom, Jeff A. Valenti, Robert P. Stefanik, Rasmus Handberg.
Full text
NASA ADS
DOI

The Hyades open cluster was targeted during Campaign 4 (C4) of the NASA K2 mission, and short-cadence data were collected on a number of cool main-sequence stars. Here, we report results on two F-type stars that show detectable oscillations of a quality that allows asteroseismic analyses to be performed. These are the first ever detections of solar-like oscillations in main-sequence stars in an open cluster.

Testing Asteroseismic Scalings for Red Giants with Eclipsing Binaries Observed by Kepler

P. Gaulme, J. McKeever, J. Jackiewicz, M. L. Rawls, E. Corsaro, B. Mosser, J. Southworth, S. Mahadevan, C. Bender, R. Deshpande.
Full text

Given the potential of ensemble asteroseismology for understanding fundamental properties of large numbers of stars, it is critical to determine the accuracy of the scaling relations on which these measurements are based. From several powerful validation techniques, all indications so far show that stellar radius estimates from the asteroseismic scaling relations are accurate to within a few percent. Eclipsing binary systems hosting at least one star with detectable solar-like oscillations constitute the ideal test objects for validating asteroseismic radius and mass inferences. By combining radial velocity (RV) measurements and photometric time series of eclipses, it is possible to determine the masses and radii of each component of a double-lined spectroscopic binary. We report the results of a four-year RV survey performed with the échelle spectrometer of the Astrophysical Research Consortium’s 3.5 m telescope and the APOGEE spectrometer at Apache Point Observatory. We compare the masses and radii of 10 red giants (RGs) obtained by combining radial velocities and eclipse photometry with the estimates from the asteroseismic scaling relations. We find that the asteroseismic scaling relations overestimate RG radii by about 5% on average and masses by about 15% for stars at various stages of RG evolution. Systematic overestimation of mass leads to underestimation of stellar age, which can have important implications for ensemble asteroseismology used for Galactic studies. As part of a second objective, where asteroseismology is used for understanding binary systems, we confirm that oscillations of RGs in

K2P2-reduced data from campaigns 0-4 of the K2 Mission

R. Handberg, M. L. Lund.
Full text
NASA ADS
DOI

Context: After the loss of a second reaction wheel the Kepler mission was redesigned as the K2 mission, pointing towards the ecliptic and delivering data for new fields approximately every 80 days. The steady flow of data obtained with a reduced pointing stability calls for dedicated pipelines for extracting light curves and correcting these for use in, e.g., asteroseismic analysis.
Aims: We provide corrected light curves for the K2 fields observed until now (campaigns 0–4), and provide a comparison with other pipelines for K2 data extraction/correction.
Methods: Raw light curves are extracted from K2 pixel data using the "K2-pixel-photometry" (K2P$^2$) pipeline, and corrected using the KASOC filter.
Results: The use of K2P$^2$ allows for the extraction of the order of 90.000 targets in addition to 70.000 targets proposed by the community — for these, other pipelines provide no data. We find that K2P$^2$ in general out-performs other pipelines in the metrics for photometric variability tested. In addition to stars, pixel masks are properly defined using K2P$^2$ for extended objects such as galaxies for which light curves are also extracted.

Detection of Solar-Like Oscillations, Observational Constraints, and Stellar Models for $\theta$ Cyg, the Brightest Star Observed by the Kepler Mission

J. A. Guzik, G. Houdek, W. J. Chaplin, B. Smalley, D. W. Kurtz, R.L. Gilliland, F. Mullally, J.F. Rowe, S. T. Bryson, M. D. Still, V. Antoci, T. Appourchaux, S. Basu, T. R. Bedding, O. Benomar, R. A. Garcia, D. Huber and 38 coauthors.
Full text

$\theta$ Cygni is an F3 spectral-type main-sequence star with visual magnitude V=4.48. This star was the brightest star observed by the original Kepler spacecraft mission. Short-cadence (58.8 s) photometric data using a custom aperture were obtained during Quarter 6 (June-September 2010) and subsequently in Quarters 8 and 12-17. We present analyses of the solar-like oscillations based on Q6 and Q8 data, identifying angular degree $l$ = 0, 1, and 2 oscillations in the range 1000-2700 $\mu$Hz, with a large frequency separation of 83.9 $\pm$ 0.4 $\mu$Hz, and frequency with maximum amplitude $\nu_{\rm max}$ = 1829 $\pm$ 54 $\mu$Hz. We also present analyses of new ground-based spectroscopic observations, which, when combined with angular diameter measurements from interferometry and Hipparcos parallax, give $T_\mathrm{eff}$ = 6697 $\pm$ 78 K, radius 1.49 $\pm$ 0.03 R$_{\odot}$, [Fe/H] = -0.02 $\pm$ 0.06 dex, log $g$ = 4.23 $\pm$ 0.03. We calculate stellar models matching the constraints using several methods, including using the Yale Rotating Evolution Code and the Asteroseismic Modeling Portal. The best-fit models have masses 1.35–1.39 M$_{\odot}$ and ages 1.0–1.6 Gyr. $\theta$ Cyg's $T_\mathrm{eff}$ and log $g$ place it cooler than the red edge of the $\gamma$ Doradus instability region established from pre-Kepler ground-based observations, but just at the red edge derived from pulsation modeling. The best-fitting models have envelope convection-zone base temperature of $\sim$320,000 to 395,000 K. The pulsation models show $\gamma$ Dor gravity-mode pulsations driven by the convective-blocking mechanism, with periods of 0.3 to 1 day (frequencies 11 to 33 $\mu$Hz). However, gravity modes were not detected in the Kepler data; one signal at 1.776 c d$^{-1}$ (20.56 $\mu$Hz) may be attributable to a faint, possibly background, binary. Asteroseismic studies of $\theta$ Cyg, in conjunction with those for other A-F stars observed by Kepler and CoRoT, will help to improve stellar model physics to sort out the confusing relationship between $\delta$ Sct and $\gamma$ Dor pulsations and their hybrids, and to test pulsation driving mechanisms.

The discovery of a planetary candidate around the evolved low-mass Kepler giant star HD 175370

M. Hrudková, A. Hatzes, R. Karjalainen, H. Lehmann, S. Hekker, M. Hartmann, A. Tkachenko, S. Prins, H. Van Winckel, R. De Nutte, L. Dumortier, Y. Frémat, H. Hensberge, A. Jorissen, P. Lampens, M. Laverick, R. Lombaert and 6 coauthors.
Full text
NASA ADS
DOI

We report on the discovery of a planetary companion candidate with a minimum mass Msini = 4.6 M_J orbiting the K2 III giant star HD 175370 (KIC 007940959). This star was a target in our program to search for planets around a sample of 95 giant stars observed with Kepler. This detection was made possible using precise stellar radial velocity measurements of HD 175370 taken over five years and four months using the coude echelle spectrograph of the 2-m Alfred Jensch Telescope and the fibre-fed echelle spectrograph HERMES of the 1.2-m Mercator Telescope. Our radial velocity measurements reveal a periodic (349.5 days) variation with a semi-amplitude K=133 m/s, superimposed on a long-term trend. A low-mass stellar companion with an orbital period of  88 years in a highly eccentric orbit and a planet in a Keplerian orbit with an ccentricity e=0.22 are the most plausible explanation of the radial velocity variations. However, we cannot exclude the existence of stellar envelope pulsations as a cause for the low-amplitude radial velocity variations and only future continued monitoring of this system may answer this uncertainty. From Kepler photometry we find that HD 175370 is most likely a low-mass red-giant branch or asymptotic-giant branch star.

Finding binary stars from phase modulation of pulsating stars with Kepler: IV Detection limits and radial velocity verification

Simon J. Murphy, Hiromoto Shibahashi, Timothy R. Bedding.
Full text

We explore the detection limits of the phase modulation (PM) method of finding binary systems among multi-periodic pulsating stars, finding that Jupiter-mass planets can be found around stars of $\sim$1.8 M$_{\odot}$ in the $\delta$ Sct instability strip. The difficulty of obtaining precise radial velocity measurements, due principally to rapid stellar rotation, makes PM an attractive method for finding non-transiting planets in the habitable zone of these stars. While such detections require oscillations with amplitudes of a few mmag, many $\delta$ Sct stars with these properties exist. In less favourable cases where the signal-to-noise of the oscillations is lower, objects with masses near the lower-limit for brown dwarfs ($\sim$13 M$_{\rm Jup}$) are detectable at periods longer than about 1 yr, and the lowest mass main-sequence stars ($\sim$75 M$_{\rm Jup}$) are detectable at all periods where the PM method can be applied. We use purpose-written Markov chain Monte Carlo (MCMC) software for the calculation of the orbital parameters of binary systems, which offers robust uncertainties that allow the PM solutions to be compared with radial velocity (RV) solutions. We verify that these two methods are in agreement, even at short orbital periods where the PM method undersamples the orbit. This is a direct consequence of our new theoretical developments detailing the corrections necessary to recover the orbital parameters of undersampled orbits. Such corrections are also necessary for the inclusion of RVs as observational data in the MCMC software. We show that combining RVs with the Rmer delays obtained with PM results substantially refines the orbital parameters because of the complementarity of the mathematical functions governing the spatial (PM) and velocity (RV) domains. We give examples using Kepler data and ground-based RV measurements, whereby 25-s precision on the orbital period and 40-ms precision on the projected light travel time across the orbit ($a_1 \sin i / c$) have been obtained, without transient events such as transits or eclipses for precise timing. Software outputs were tested through an extensive hare and hounds exercise, covering a wide range of orbital configurations including binaries containing two pulsators.

Asteroseismic determination of fundamental parameters of Sun-like stars using multilayered neural networks

Kuldeep Verma, Shravan Hanasoge, Jishnu Bhattacharya, H. M. Antia, Ganapathy Krishnamurthi.
Full text
NASA ADS
DOI

The advent of space-based observatories such as Convection, Rotation and Planetary Transits (CoRoT) and Kepler has enabled the testing of our understanding of stellar evolution on thousands of stars. Evolutionary models typically require five input parameters, the mass, initial Helium abundance, initial metallicity, mixing-length (assumed to be constant over time) and the age to which the star must be evolved. These parameters are also very useful in characterizing the associated planets and in studying Galactic archaeology. How to obtain the parameters from observations rapidly and accurately, specifically in the context of surveys of thousands of stars, is an outstanding question, one that has eluded straightforward resolution. For a given star, we typically measure effective temperature, surface metallicity spectroscopically and low-degree oscillation frequencies through space observatories. Here we demonstrate that statistical learning, using artificial neural networks, is successful in determining the evolutionary parameters based on spectroscopic and seismic measurements. Our trained networks show robustness over a broad range of parameter space, and critically, are entirely computationally inexpensive. This method is both computationally cheap and inferentially accurate, paving the way for analyzing the vast quantities of stellar observations from past, current and future missions.

Revisiting old friends with K2 reveals oscillating red giants in the open cluster M67

Dennis Stello, Andrew Vanderburg, Luca Casagrande, Ron Gilliland, Victor Silva Aguirre, Eric Sandquist, Emily Leiner, Robert Mathieu, David R. Soderblom.
Full text

Observations of stellar clusters have had a tremendous impact in forming our understanding of stellar evolution. The open cluster M67 has a particularly important role as a calibration anchor point for stellar evolution theory due to its near solar composition and age. As a result, it has been observed extensively, including attempts to detect solar-like oscillations in its main sequence and red giant stars. However, any asteroseismic inference has so far remained elusive due to the difficulty in measuring these extremely low amplitude oscillations. Here we report the first unambiguous detection of solar-like oscillations in the red giants of M67. We use data from the Kepler ecliptic mission, K2, to measure the global asteroseismic properties. We find a model-independent seismic-informed distance of 816+/-11pc, or (m-M)o=9.57+/-0.03mag, an average red-giant mass of 1.36+/-0.01Msun, in agreement with the dynamical mass from an eclipsing binary near the cluster turn-off, but we find a lower-than-usual age. We see no evidence of strong mass loss on the red giant branch. We also determine seismic log g of all the cluster giants with a typical precision of  0.01dex. Our results generally show good agreement with independent methods and support the use of seismic scaling relations to determine global properties of red giant stars with near solar metallicity. We further illustrate that the data are of such high quality, that future work on individual mode frequencies should be possible, which would extend the scope of seismic analysis of this cluster.

Fundamental Parameters of Main-Sequence Stars in an Instant with Machine Learning

Earl Patrick Bellinger, George C. Angelou, Saskia Hekker, Sarbani Basu, Warrick H. Ball, Elisabeth Guggenberger.
Full text

Owing to the remarkable photometric precision of space observatories like Kepler, stellar and planetary systems beyond our own are now being characterized en masse for the first time. These characterizations are pivotal for endeavors such as searching for Earth-like planets and solar twins, understanding the mechanisms that govern stellar evolution, and tracing the dynamics of our Galaxy. The volume of data that is becoming available, however, brings with it the need to process this information accurately and rapidly. While existing methods can constrain fundamental stellar parameters such as ages, masses, and radii from these observations, they require substantial computational efforts to do so.
We develop a method based on machine learning for rapidly estimating fundamental parameters of main-sequence solar-like stars from classical and asteroseismic observations. We first demonstrate this method on a hare-and-hound exercise and then apply it to the Sun, 16 Cyg A & B, and 34 planet-hosting candidates that have been observed by the Kepler spacecraft. We find that our estimates and their associated uncertainties are comparable to the results of other methods, but with the additional benefit of being able to explore many more stellar parameters while using much less computation time. We furthermore use this method to present evidence for an empirical diffusion-mass relation. Our method is open source and freely available for the community to use.
The source code for all analyses and for all figures appearing in this manuscript can be found electronically at https://github.com/earlbellinger/asteroseismology

Asteroseismic modelling of the two F-type hybrid pulsators KIC10080943A and KIC10080943B

Valentina S. Schmid, Conny Aerts.
Full text
NASA ADS
DOI

Pulsating binary stars are ideal targets for testing the theory of stellar structure and evolution. Fundamental parameters can be derived to high precision from binary modelling and provide crucial constraints for seismic modelling. High-order gravity modes are sensitive to the conditions near the convective core and therefore allow for a determination of parameters describing interior physics, especially the convective-core overshooting parameter. KIC10080943 is a binary system that contains two gravity- and pressure-mode hybrid pulsators. A detailed observational study has provided fundamental and seismic parameters for both components. We aim to find a model that is able to predict the observed g-mode period spacings and stellar parameters of both components of KIC10080943. By calculating model grids with the stellar evolution code MESA and the seismic code GYRE, we can compare theoretical properties to the observed mean period spacing and position in the Hertzsprung-Russell diagram. The masses of our best models are somewhat below the values estimated from binarity, which is a consequence of the low observed mean g-mode period spacing. We find that the amount of core overshooting and diffusive mixing can be well constrained by the equal-age requirement for the two stars, however, we find no significant difference for different shapes of core overshooting. The measured rotation rates are within the limit of validity for the first-order perturbation approximation. We can find a good fit by using the traditional approximation for the pulsations, when taking slightly younger models with a higher asymptotic period spacing. This is because the zonal modes experience a slight shift due to the Coriolis force, which the first-order perturbation approximation ignores.

Probing the deep end of the Milky Way with Kepler: Asteroseismic analysis of 851 faint Red Giants misclassified as Cool Dwarfs

S. Mathur, R.A. Garcia, D. Huber, C. Regulo, D. Stello, P.G. Beck, K. Houmani, D. Salabert.
Full text

Asteroseismology has proven to be an excellent tool to determine not only the global stellar properties with a good precision but also to infer stellar structure, dynamics, and evolution for a large sample of Kepler stars. Prior to the launch of the mission the properties of Kepler targets were inferred from broadband photometry, leading to the Input Catalog (KIC, Brown et al. 2011). The KIC was later revised in the Kepler Star Properties Catalog (Huber et al. 2014), based on literature values and an asteroseismic analysis of stars which were unclassified in the KIC. Here we present an asteroseismic analysis of 45,400 stars which were classified as dwarfs in the Kepler Star Properties Catalog. We found that around 2% of the sample shows acoustic modes in the typical frequency range that put them in the red-giant category rather than cool dwarfs. We analyse the asteroseismic properties of these stars, derive their surface gravities, masses, and radii and present updated effective temperatures and distances. We show that the sample is significantly fainter than the previously known oscillating giants in the Kepler field, with the faintest stars reaching down to a Kepler magnitude, Kp $\sim$ 16. We demonstrate that 404 stars are at distances beyond 5 kpc and that the stars are significantly less massive than for the original Kepler red-giant sample, consistent with a population of distant halo giants. The detections presented here will provide a valuable sample for galactic archeology studies.

SpaceInn hare-and-hounds exercise: Estimation of stellar properties using space-based asteroseismic data

D. R. Reese, W. J. Chaplin, G. R. Davies, A. Miglio, H. M. Antia, W. H. Ball, S. Basu, G. Buldgen, J. Christensen-Dalsgaard, H. R. Coelho, S. Hekker, G. Houdek, Y. Lebreton, A. Mazumdar, T. S. Metcalfe, V. Silva Aguirre, D. Stello and 1 coauthors.
Full text
NASA ADS
DOI

Context: Detailed oscillation spectra comprising individual frequencies for numerous solar-type stars and red giants are either currently available, e.g. courtesy of the CoRoT, Kepler, and K2 missions, or will become available with the upcoming NASA TESS and ESA PLATO 2.0 missions. These data can lead to a precise characterisation of these stars thereby improving our understanding of stellar evolution, exoplanetary systems, and the history of our galaxy.
Aims: Our goal is to test and compare different methods for obtaining stellar properties from oscillation frequencies and spectroscopic constraints. Specifically, we would like to evaluate the accuracy of the results and reliability of the associated error bars, and see where there is room for improvement.
Methods: In the context of the SpaceInn network, we carried out a hare-and-hounds exercise in which one group, the hares, produced "observed" oscillation spectra for a set of 10 artificial solar-type stars, and a number of hounds applied various methods for characterising these stars based on the data produced by the hares. Most of the hounds fell into two main groups. The first group used forward modelling (i.e. applied various search/optimisation algorithms in a stellar parameter space) whereas the second group relied on acoustic glitch signatures.
Results: Results based on the forward modelling approach were accurate to 1.5 % (radius), 3.9 % (mass), 23 % (age), 1.5 % (surface gravity), and 1.8 % (mean density), as based on the root-mean square difference. Individual hounds reached different degrees of accuracy, some of which were substantially better than the above average values. For the two 1 M(sun) stellar targets, the accuracy on the age is better than 10 % thereby satisfying the requirements for the PLATO 2.0 mission. High stellar masses and atomic diffusion (which in our models does not include the effects of radiative accelerations) proved to be sources of difficulty. The average accuracies for the acoustic radii of the base of the convection zone, the He II ionisation, and the $\Gamma_1$ peak located between the two He ionisation zones were 17 %, 2.4 %, and 1.9 %, respectively. The results from the forward modelling were on average more accurate than those from the glitch fitting analysis as the latter seemed to be affected by aliasing problems for some of the targets.
Conclusions: Our study indicates that forward modelling is the most accurate way of interpreting the pulsation spectra of solar-type stars. However, given its model-dependent nature, such methods need to be complemented by model-independent results from, e.g., glitch analysis. Furthermore, our results indicate that global rather than local optimisation algorithms should be used in order to obtain robust error bars.

Binary star detectability in Kepler data from phase modulation of different types of oscillations

D. L. Compton, T. R. Bedding, S. J. Murphy, D. Stello.
Full text

Detecting binary stars in photometric time series is traditionally done by measuring eclipses. This requires the orbital plane to be aligned with the observer. A new method without that requirement uses stellar oscillations to measure delays in the light arrival time and has been successfully applied to $\delta$ Scuti stars. However, application to other types of stars has not been explored. To investigate this we simulated light curves with a range of input parameters. We find a correlation between the signal-to-noise of the pulsation modes and the time delay required to detect binary motion. The detectability of the binarity in the simulations and in real Kepler data shows strong agreement, hence, we describe the factors that have prevented this method from discovering binary companions to stars belonging to various classes of pulsating stars.

Stellar Evidence that the Solar Dynamo may be in Transition

Travis S. Metcalfe, Ricky Egeland, Jennifer van Saders.
Full text
NASA ADS
DOI

Precise photometry from the Kepler space telescope allows not only the measurement of rotation in solar-type field stars, but also the determination of reliable masses and ages from asteroseismology. These critical data have recently provided the first opportunity to calibrate rotation-age relations for stars older than the Sun. The evolutionary picture that emerges is surprising: beyond middle-age the efficiency of magnetic braking is dramatically reduced, implying a fundamental change in angular momentum loss beyond a critical Rossby number (Ro 2). We compile published chromospheric activity measurements for the sample of Kepler asteroseismic targets that were used to establish the new rotation-age relations. We use these data along with a sample of well characterized solar analogs from the Mount Wilson HK survey to develop a qualitative scenario connecting the evolution of chromospheric activity to a fundamental shift in the character of differential rotation. We conclude that the Sun may be in a transitional evolutionary phase, and that its magnetic cycle might represent a special case of stellar dynamo theory.

Period spacing in red giant II. Automated measurement

Vrard M., Mosser B., & Samadi R..
Full text

The space missions CoRoT and Kepler have provided photometric data of unprecedented quality for asteroseismology. A very rich oscillation pattern was discovered for red giants, including mixed modes that are used to decipher the red giants interiors. They carry information on the radiative core of red giants stars and bring strong constraints on stellar evolution. Since more than 15000 red giant light curves were observed by Kepler, we aim at developing a simple and efficient methods for automatically characterizing the mixed-mode pattern and measuring the asymptotic period spacing. With the asymptotic expansion of the mixed modes, we have revealed the regularity of the gravity-mode pattern. The stretched periods were used to study the evenly-space periods with a Fourier analysis and to measure the gravity period spacing, even when rotation severely complicates the oscillation spectra. We automatically measured gravity period spacing for more than five thousand Kepler red giants. The results confirm and extend previous measurements made by semi-automated methods. We also unveil the mass and metallicity dependence of the relation between the frequency spacings and the period spacings for stars on the red giant branch. The delivery of thousands of period spacings combined with all other seismic and non-seismic information provides new bases for detailed ensemble asteroseismology.

Limits in the application of harmonic analysis to pulsating stars

J. Pascual-Granado, R. Garrido, J. C. Suárez.
Full text
NASA ADS
DOI

Using ultra-precise data from space instrumentation we found that the underlying functions of stellar light curves from some AF pulsating stars are non-analytic, and consequently their Fourier expansion is not guaranteed. This result demonstrates that periodograms do not provide a mathematically consistent estimator of the frequency content for this kind of variable stars. More importantly, this constitutes the first counterexample against the current paradigm which consid- ers that any physical process is described by a continuous (band-limited) function that is infinitely differentiable.

Sub-Inertial Gravity Modes in the B8V Star KIC 7760680 Reveal Moderate Core Overshooting and Low Vertical Diffusive Mixing

Ehsan Moravveji, Richard H. D. Townsend, Conny Aerts, Stephane Mathis.
Full text
NASA ADS
DOI

KIC 7760680 is so far the richest slowly pulsating B star, by exhibiting 36 consecutive dipole ($\ell=1$) gravity (g-) modes. The monotonically decreasing period spacing of the series, in addition to the local dips in the pattern confirm that KIC 7760680 is a moderate rotator, with clear mode trapping in chemically inhomogeneous layers. We employ the traditional approximation of rotation to incorporate rotational effects on g-mode frequencies. Our detailed forward asteroseismic modelling of this g-mode series reveals that KIC 7760680 is a moderately rotating B star with mass $\sim3.25$ M$_\odot$. By simultaneously matching the slope of the period spacing, and the number of modes in the observed frequency range, we deduce that the equatorial rotation frequency of KIC 7760680 is 0.4805 day$^{-1}$, which is 26% of its Roche break up frequency. The relative deviation of the model frequencies and those observed is less than one percent. We succeed to tightly constrain the exponentially-decaying convective core overshooting parameter to $f_{\rm ov}\approx0.024\pm0.001$. This means that convective core overshooting can coexist with moderate rotation. Moreover, models with exponentially-decaying overshoot from the core outperform those with the classical step-function overshoot. The best value for extra diffusive mixing in the radiatively stable envelope is confined to $\log D_{\rm ext}\approx0.75\pm0.25$ (with $D_{\rm ext}$ in cm$^2$ sec$^{-1}$), which is notably smaller than theoretical predictions.

Detection of solar-like oscillations in relics of the Milky Way: asteroseismology of K giants in M4 using data from the NASA K2 mission

A. Miglio, W. J. Chaplin, K. Brogaard, M. N. Lund, B. Mosser, G. R. Davies, R. Handberg, A. P. Milone, A. F. Marino, D. Bossini, Y. P. Elsworth, F. Grundahl, T. Arentoft, L. R. Bedin, T. L. Campante, J. Jessen-Hansen, C. Jones and 4 coauthors.
Full text
NASA ADS
DOI

Asteroseismic constraints on K giants make it possible to infer radii, masses and ages of tens of thousands of field stars. Tests against independent estimates of these properties are however scarce, especially in the metal-poor regime. Here, we report the detection of solar-like oscillations in eight stars belonging to the red-giant branch (RGB) and red-horizontal branch (RHB) of the globular cluster M4. The detections were made in photometric observations from the K2 Mission during its Campaign 2. Making use of independent constraints on the distance, we estimate masses of the eight stars by utilizing different combinations of seismic and non-seismic inputs. When introducing a correction to the Δν scaling relation as suggested by stellar models, for RGB stars we find excellent agreement with the expected masses from isochrone fitting, and with a distance modulus derived using independent methods. The offset with respect to independent masses is lower, or comparable with, the uncertainties on the average RGB mass (4–10 per cent, depending on the combination of constraints used). Our results lend confidence to asteroseismic masses in the metal-poor regime. We note that a larger sample will be needed to allow more stringent tests to be made of systematic uncertainties in all the observables (both seismic and non-seismic), and to explore the properties of RHB stars, and of different populations in the cluster.

Measuring the extent of convective cores in low-mass stars using Kepler data: toward a calibration of core overshooting

S. Deheuvels, I. Brandão, V. Silva Aguirre, J. Ballot, E. Michel, M. S. Cunha, Y. Lebreton, T. Appourchaux.
Full text

Context: Our poor understanding of the boundaries of convective cores generates large uncertainties on the extent of these cores and thus on stellar ages. The detection and precise characterization of solar-like oscillations in hundreds of main-sequence stars by CoRoT and Kepler has given the opportunity to revisit this problem.
Aims: Our aim is to use asteroseismology to consistently measure the extent of convective cores in a sample of main-sequence stars whose masses lie around the mass-limit for having a convective core.
Methods: We first test and validate a seismic diagnostic that was proposed to probe in a model-dependent way the extent of convective cores using the so-called $r_{010}$ ratios, which are built with $l=0$ and $l=1$ modes. We apply this procedure to 24 low-mass stars chosen among Kepler targets to optimize the efficiency of this diagnostic. For this purpose, we compute grids of stellar models with both the CESAM2k and MESA evolution codes, where the extensions of convective cores are modeled either by an instantaneous mixing or as a diffusion process.
Results: We find that 10 stars or our sample are in fact subgiants. Among the other targets, we are able to unambiguously detect convective cores in eight stars and we obtain seismic measurements of the extent of the mixed core in these targets with a good agreement between the CESAM2k and MESA codes. By performing optimizations using the Levenberg-Marquardt algorithm, we then obtain estimates of the the amount of extra-mixing beyond the core that is required in the CESAM2k code to reproduce seismic observations for these eight stars and we show that this can be used to propose a calibration of this quantity. This calibration depends on the prescription chosen for the extra-mixing, but we find that it should be valid also for the code MESA, provided the same prescription is used.
Conclusions: This study constitutes a first step towards the calibration of the extension of convective cores in low-mass stars, which will help reduce the uncertainties on the ages of these stars.

Near-uniform internal rotation of the main sequence gamma Doradus pulsator KIC 7661054

Simon J. Murphy, Luca Fossati, Timothy R. Bedding, Hideyuki Saio, Donald W. Kurtz, Luca Grassitelli, Edric S. Wang.
Full text
NASA ADS
DOI

We used Kepler photometry to determine the internal rotation rate of KIC 7661054, a chemically normal $\gamma$ Dor star on the main sequence at spectral type F2.5 V. The core rotation period of $27.25\pm0.06$ d is obtained from the rotational splittings of a series of dipole g modes. The surface rotation period is calculated from a spectroscopic projected rotation velocity and a stellar radius computed from models. Literature data, obtained without inclusion of macroturbulence as a line-broadening mechanism, imply that the surface rotates much more quickly than the core, while our detailed analysis suggests that the surface may rotate slightly more quickly than the core and that the rotation profile is uniform within the 1-$\sigma$ uncertainties. We discuss the pitfalls associated with the determination of surface rotation rates of slow rotators from spectroscopy in the absence of asteroseismic constraints. A broad signal is observed at low frequency, which we show cannot be attributed to rotation, contrary to previous suggestions concerning the origin of such signals.

What asteroseismology can do for exoplanets: Kepler-410A b is a Small Neptune in an eccentric orbit consistent with low obliquity

V. Van Eylen, M. N. Lund, V. Silva Aguirre, T. Arentoft, H. Kjeldsen, S. Albrecht, W. J. Chaplin, H. Isaacson, M. G. Pedersen, J. Jessen-Hansen, B. Tingley, J. Christensen-Dalsgaard, C. Aerts, T. Campante.
Full text

We confirm the Kepler planet candidate KOI-42b as a Neptune sized exoplanet on a $17.8$ day, eccentric, orbit around the bright ($K_\textrm{p} = 9.4$) star KOI-42A. KOI-42 consists of a blend between the fast rotating planet host star (KOI-42A) and a fainter star (KOI-42B), which has complicated the confirmation of the planetary candidate. Employing asteroseismology, using constraints from the transit light curve, adaptive optics and speckle images, and Spitzer transit observations, we demonstrate that the candidate can only be an exoplanet orbiting KOI-42A. Via asteroseismology we determine the following stellar and planetary parameters with high precision; M$_\star = 1.214 \pm 0.033$ M$_\odot$, R$_\star = 1.352 \pm 0.010$ R$_\odot$, Age $= 2.76 \pm 0.54$ Gyr, planetary radius ($2.838 \pm 0.054$ R$_\oplus$), and orbital eccentricity ($0.17^{+0.10}_{-0.05}$). In addition, rotational splitting of the pulsation modes allows for a measurement of KOI-42A's inclination and rotation rate. Our measurement of an inclination of $82.5^{+5.0}_{-5.2}$ [$^\circ$] indicates a low obliquity in this system.

Analysis of the acoustic cut-off frequency and the HIPs in 6 Kepler stars with stochastically excited pulsations

A. Jimenez, R. A. Garcia, F. Perez Hernandez, S. Mathur.
Full text
NASA ADS
DOI

Gravito-acoustic modes in the Sun and other stars propagate in resonant cavities with a frequency below a given limit known as the cut-off frequency. At higher frequencies, waves are no longer trapped in the stellar interior and they become traveller waves. Those waves produce different patterns of High Interference Peaks (HIPs) above the cut-off frequency depending on whether they are originated by outward waves interfering with inward waves that are either partially reflected at the back of the star or that have been directly refracted in the stellar interior. In this work we study six pulsating solar-like stars at different evolutionary stages observed by the NASA mission Kepler. These high signal-to-noise targets show a peak structure that extends at very high frequencies and are good candidates to study the transition region between the modes and the interference peaks or pseudo modes. Following the same methodology successfully applied on Sun-as-a-star measurements, we uncover the existence of pseudo modes in these stars with one or two dominant interference patterns depending on the evolutionary stage of the star. We also infer their cut-off frequency as the middle point between the last eigenmode and the first peak of the interference patterns. By using the ray theory we show that while the period of one of the interference pattern is very close to half the large separation the other one depends on the time phase of mixed waves thus carrying on additional information on the stellar structure and evolution.

Hot super-Earths stripped by their host stars

M. S. Lundkvist, H. Kjeldsen, S. Albrecht, G. R. Davies, S. Basu, D. Huber, A. B. Justesen, C. Karoff, V. Silva Aguirre, V. Van Eylen, C. Vang, T. Arentoft, T. Barclay, T. R. Bedding, T. L. Campante, W. J. Chaplin, J. Christensen-Dalsgaard and 12 coauthors.
Full text

Models predict that the envelopes of exoplanets orbiting close to their host star are stripped by evaporation, which should be evident as an absence of very hot super-Earth sized exoplanets. Recently, exoplanets in the midst of this process may have been observed. However, this absence in the distribution of exoplanets caused by evaporation has escaped a secure detection. Here we demonstrate, using asteroseismology on a sample of 162 exoplanets and exoplanet candidates observed during the Kepler mission that, while there is an abundance of super-Earth sized exoplanets with low incident fluxes, none are found with high incident fluxes. We do not find any exoplanets with radii between 2.2 and 3.8 Earth radii with incident flux above 650 times the incident flux on Earth. This gap in the population of exoplanets is explained by the evaporation of volatile elements and thus supports the theoretical predictions. The confirmation of a hot-super-Earth desert caused by evaporation will add an important constraint on simulations of the evolution of planetary systems, since they must be able to reproduce the dearth of close-in super-Earths.

Asteroseismic analysis of solar-like star KIC 6225718: constraints on stellar parameters and core overshooting

Zhijia Tian, Shaolan Bi, Wuming Yang, Yuqin Chen, Zhie Liu, Kang Liu, Tanda Li, Zhishuai Ge, Jie Yu.
Full text
NASA ADS
DOI

We analyze five seasons of short-cadence data of a solar-type star of spectral type F: KIC 6225718 observed by the Kepler . We obtain the power spectrum of this star by applying the Lomb-Scargle Periodogram to the smoothed time series. By applying the autocorrelation technique to the power spectrum, we derive the large frequency separation $\Delta\nu=105.78 \pm 0.65\ \mu$Hz and the frequency of the maximum power spectrum $\nu_{\rm{max}} = 2301 \pm 21\ \mu$Hz. We identify 33 p-modes with angular degrees of $l=0\sim2$ in the frequency range of 1600 - 2800 $\mu$Hz of the power spectrum with Bayesian Markov Chain Monte Carlo algorithms. In order to accurately determine the parameters of the star, we construct a grid of stellar models with core overshooting using the Yale stellar evolution code and then perform preliminary seismological analysis. With both asteroseismic and non-asteroseismic constraints, the range of stellar parameters are estimated: the mass $M=1.10^{+0.04}_{-0.03} M_{\odot}$, the radius $R = 1.22^{+0.01}_{-0.01}\ R_{\odot}$, and the age $t=3.35^{+0.36}_{-0.75}$ Gyr for this star. In addition, we analyze the effects of overshooting on stellar interiors, and find that the upper limit of the overshooting parameter $\alpha_{ov}$ is approximately 0.2 for this star.

Precise stellar surface gravities from the time scales of convectively driven brightness variations

T. Kallinger, S. Hekker, R. A. Garcia, D. Huber, J. M. Matthews.
Full text
NASA ADS
DOI

A significant part of the intrinsic brightness variations in cool stars of low and intermediate mass arises from surface convection (seen as granulation) and acoustic oscillations (p-mode pulsations). The characteristics of these phenome- na are largely determined by the stars’ surface gravity g. Detailed photometric measurements of either signal can yield an accurate value of g. However, even with ultraprecise photometry from NASA’s Kepler mission, many stars are too faint for current methods or only moderate accuracy can be achieved in a limited range of stellar evolutionary stages. This means that many of the stars in the Kepler sample, including exoplanet hosts, are not sufficiently characterized to fully describe the sample and exoplanet properties. We present a novel way to measure surface gravities with accura- cies of about 4%. Our technique exploits the tight relation between g and the characteristic time scale of the combined granulation and p-mode oscillation signal. It is applicable to all stars with a convective envelope, including active stars. It can measure g in stars for which no other analysis is now possible. Because it depends on the time scale (and no other properties) of the signal, our technique is largely independent of the type of measurement (for example, photometry or radial velocity measurements) and the calibration of the instrumentation used. However, the oscillation signal must be temporally resolved; thus, it cannot be applied to dwarf stars observed by Kepler in its long-cadence mode.

Tight asteroseismic constraints on core overshooting and diffusive mixing in the slowly rotating pulsating B8.3V star KIC 10526294

Ehsan Moravveji, Conny Aerts, Peter I. Pápics, Santiago A. Triana, Bram Vandoren.
Full text
NASA ADS
DOI

Context: KIC 10526294 was recently discovered to be a very slowly rotating and slowly pulsating late B-type star. Its 19 consecutive dipole gravity modes constitute a series with almost constant period spacing. This unique collection of identified modes probes the near-core environment of this star and holds the potential to reveal the size and structure of the overshooting zone above the convective core, as well as the mixing properties of the star. Aims: We revisit the asteroseismic modelling of this star with specific emphasis on the properties of the core overshooting, while considering additional diffusive mixing throughout the radiative envelope of the star. Methods: We pursued forward seismic modelling based on adiabatic eigenfrequencies of equilibrium models for eight extensive evolutionary grids tuned to KIC 10526294 by varying the initial mass, metallicity, chemical mixture, and the extent of the overshooting layer on top of the convective core. We examined models for both OP and OPAL opacities and tested the occurrence of extra diffusive mixing throughout the radiative interior. Results: We find a tight mass-metallicity relation within the ranges $M\in [3.13,3.25]$ M$_\odot$ and $Z\in [0.014,0.028]$. We deduce that an exponentially decaying diffusive core overshooting prescription describes the seismic data better than a step function formulation and derive a value of $f_{\rm ov}$ between 0.017 and 0.018. Moreover, the inclusion of extra diffusive mixing with a value of $\log D_{\rm mix}$ between 1.75 and 2.00 dex (with $D_{\rm mix}$ in cm$^2$ s$^{-1}$) improves the goodness-of-fit based on the observed and modelled frequencies by a factor $\sim\!11$ compared to the case where no extra mixing is considered, irrespective of the $(M,Z)$ combination within the allowed seismic range.
Conclusions: The inclusion of diffusive mixing in addition to core overshooting is essential to explain the structure in the observed period spacing pattern of this star. Moreover, for the input physics and chemical mixtures we investigated, we deduce that an exponentially decaying prescription for the core overshooting is to be preferred over a step function, regardless of the adopted mixture or choice of opacity tables. Our best models for KIC 10526294 approach the seismic data to a level that they can serve future inversion of its stellar structure.

MODE SUPPRESSION OF QUADRUPOLE AND OCTUPOLE MODES IN RED GIANTS OBSERVED BY KEPLER

DENNIS STELLO, MATTEO CANTIELLO, JIM FULLER, RAFAEL A. GARCIA, DANIEL HUBER.
Full text

The asteroseismology of red giant stars has continued to yield surprises since the onset of high-precision photometry from space-based observations. An exciting new theoretical result shows that the previously observed suppression of dipole oscillation modes in red giants can be used to detect strong magnetic fields in the stellar cores. A fundamental facet of the theory is that nearly all the mode energy leaking into the core is trapped by the magnetic greenhouse effect. This results in clear predictions for how the mode visibility changes with $\nu_{\mathrm{max}}$ as a star evolves up the red giant branch, and how that depends on stellar mass, spherical degree, and mode lifetime. Here we investigate the validity of these predictions with a particular focus on the visibility of different spherical degrees. We find that mode suppression weakens for higher degree modes with average reduced mode visibility of up to 49% for the least evolved stars in our sample, and no detectable suppression of octupole modes, in agreement with the theoretical predictions. We furthermore find evidence for the influence of increasing mode lifetimes on the measured visibilities along the red giant branch, in agreement with with previous independent observations. These results support the theory that strong internal magnetic fields are responsible for the observed suppression of non-radial modes in red giants. We also find preliminary evidence that stars with suppressed dipole modes on average have slightly lower metallicity than normal stars.

Internal rotation of the red-giant star KIC 4448777 by means of asteroseismic inversion

M. P. Di Mauro, R. Ventura, D. Cardini, D. Stello, J. Christensen-Dalsgaard, W. A. Dziembowski, L. Paternò, P. G. Beck, S. Bloemen, G. R. Davies, K. De Smedt, Y. Elsworth, R. A. García, S. Hekker, B. Mosser, A. Tkachenko.
Full text

We study the dynamics of the stellar interior of the early red-giant star KIC 4448777 by asteroseismic inversion of 14 splittings of the dipole mixed modes obtained from Kepler observations. In order to overcome the complexity of the oscillation pattern typical of red-giant stars, we present a procedure to extract the rotational splittings from the power spectrum.
We find not only that the core rotates from a minimum of 8 to a maximum of 17 times faster than the surface, confirming previous inversion results generated for other red giants \citepdeheuvels2012,deheuvels2014, but we also estimate the variation of the angular velocity within the helium core with a spatial resolution of $0.001R$ and verify the hypothesis of a sharp discontinuity in the inner stellar rotation. The results show that the entire core rotates rigidly and provide evidence for an angular velocity gradient around the base of the hydrogen burning shell; however we do not succeed to characterize the rotational slope, due to the intrinsic limits of the applied techniques. The angular velocity, from the edge of the core, appears to decrease with increasing distance from the center, reaching an average value in the convective envelope of $68\pm22$ nHz. We conclude that a set of data which includes only dipolar modes is sufficient to infer quite accurately the rotation of a red giant not only in the dense core but also, with a lower level of confidence, in part of the radiative region and in the convective envelope.

Oscillation frequencies for 35 Kepler solar-type planet-hosting stars using Bayesian techniques and machine learning.

G.R. Davies, V. Silva Aguirre, T.R. Bedding, R. Handberg, M.N. Lund, W.J. Chaplin, D. Huber, T.R. White, O. Benomar, S. Hekker, S. Basu, T.L. Campante, J. Christensen-Dalsgaard, Y. Elsworth, C. Karoff, H. Kjeldsen, M. Lundkvist and 2 coauthors.
Full text

Kepler has revolutionised our understanding of both exoplanets and their host stars. Asteroseismology is a valuable tool in the characterisation of stars and Kepler is an excellent observing facility to perform asteroseismology. Here we select a sample of 35 Kepler solar-type stars which host transiting exoplanets (or planet candidates) with detected solar-like oscillations. Using available Kepler short cadence data up to Quarter 16 we create power spectra optimised for asteroseismology of solar-type stars. We identify modes of oscillation and estimate mode frequencies by “peak bagging” using a Bayesian MCMC framework. In addition, we expand the methodology of quality assurance using a Bayesian unsupervised machine learning approach. We report the measured frequencies of the modes of oscillation for all 35 stars and frequency ratios commonly used in detailed asteroseismic modelling. Due to the high correlations associated with frequency ratios we report the covariance matrix of all frequencies measured and frequency ratios calculated. These frequencies, frequency ratios, and covariance matrices can be used to obtain tight constraint on the fundamental parameters of these planet-hosting stars.

KIC 9246715: The Double Red Giant Eclipsing Binary With Odd Oscillations

Meredith L. Rawls, Patrick Gaulme, Jean McKeever, Jason Jackiewicz, Jerome A. Orosz, Enrico Corsaro, Paul G. Beck, Benoît Mosser, David W. Latham, Christian A. Latham.
Full text
NASA ADS
DOI

We combine Kepler photometry with ground-based spectra to present a comprehensive dynamical model of the double red giant eclipsing binary KIC 9246715. While the two stars are very similar in mass ($M_1 = 2.171\substack{+0.006 \ -0.008} \ M_{\odot}$, $M_2 = 2.149\substack{+0.006 \ -0.008} \ M_{\odot}$) and radius ($R_1 = 8.37\substack{+0.03 \ -0.07} \ R_{\odot}$, $R_2 = 8.30\substack{+0.04 \ -0.03} \ R_{\odot}$), an asteroseismic analysis finds one main set of solar-like oscillations with unusually low-amplitude, wide modes. A second set of oscillations from the other star may exist, but this marginal detection is extremely faint. Because the two stars are nearly twins, KIC 9246715 is a difficult target for a precise test of the asteroseismic scaling relations, which yield $M = 2.17\pm0.14 \ M_{\odot}$ and $R = 8.26\pm0.18 \ R_{\odot}$. Both stars are consistent with the inferred asteroseismic properties, but we suspect the main oscillator is Star 2 because it is less active than Star 1. We find evidence for stellar activity and modest tidal forces acting over the 171-day eccentric orbit, which are likely responsible for the essential lack of solar-like oscillations in one star and weak oscillations in the other. Mixed modes indicate the main oscillating star is on the secondary red clump (a core-He-burning star), and stellar evolution modeling supports this with a coeval history for a pair of red clump stars. This system is a useful case study and paves the way for a detailed analysis of more red giants in eclipsing binaries, an important benchmark for asteroseismology.

An RR Lyrae family portrait: 33 stars observed in Pisces with K2-E2

L. Molnár, R. Szabó, P. A. Moskalik, J. M. Nemec, E. Guggenberger, R. Smolec, R. Poleski, E. Plachy, K. Kolenberg, Z. Kolláth.
Full text

A detailed analysis is presented of 33 RR Lyrae stars in Pisces observed with the Kepler space telescope over the 8.9-day long K2 Two-Wheel Concept Engineering Test. The sample includes not only fundamental-mode and first overtone (RRab and RRc) stars but the first two double-mode (RRd) stars that Kepler detected and the only modulated first-overtone star ever observed from space so far. The precision of the extracted K2 light curves made it possible to detect low-amplitude additional modes in all subtypes. All RRd and non-modulated RRc stars show the additional mode at $P_X/P_1\sim0.61$ that was detected in previous space-based photometric measurements. A periodicity longer than the fundamental mode was tentatively identified in one RRab star that might belong to a gravity mode. We determined the photometric [Fe/H] values for all fundamental-mode stars and provide the preliminary results of our efforts to fit the double-mode stars with non-linear hydrodynamic pulsation models. The results from this short test run indicate that the K2 mission will be, and has started to be, an ideal tool to expand our knowledge about RR Lyrae stars. As a by-product of the target search and analysis, we identified 165 bona-fide double-mode RR Lyrae stars from the Catalina Sky Survey observations throughout the sky, 130 of which are new discoveries.

The internal rotation profile of the B-type star KIC 10526294 from frequency inversion of its dipole gravity modes

Santiago A. Triana, Ehsan Moravveji, Peter I. Papics, Conny Aerts, Steven D. Kawaler, Joergen Christensen-Dalsgaard.
Full text
NASA ADS
DOI

The internal angular momentum distribution of a star is key to determine its evolution. Fortunately, the stellar internal rotation can be probed through studies of rotationally-split nonradial oscillation modes. In particular, detection of nonradial gravity modes (g modes) in massive young stars has become feasible recently thanks to the Kepler space mission. Our aim is to derive the internal rotation profile of the Kepler B-type star KIC 10526294 through asteroseismology. We interpret the observed rotational splittings of its dipole g modes using four different approaches based on the best seismic models of the star and their rotational kernels. We find that the dipole mode splittings hint towards internal counter-rotation within the radiative envelope if a smooth rotation profile is assumed. We discuss alternative solutions and their viability.

Constraints on the structure of 16 Cyg A and 16 Cyg B using inversion techniques

G. Buldgen, D. R. Reese, M. A. Dupret.
Full text
NASA ADS
DOI

Context Constraining additional mixing processes and chemical composition is a central problem in stellar physics as their impact on stellar age determinations leads to biases in our studies of stellar evolution, galactic history and exoplanetary systems. In two previous papers, we have shown how seismic inversion techniques could be used to offer strong constraints on such processes by pointing out weaknesses in current theoretical models. The theoretical approach having been tested, we now wish to apply our technique to observations. In that sense, the solar analogues 16CygA and 16CygB, being amongst the best targets in the Kepler field, are probably the current most well observed stars to test the diagnostic potential of seismic inversions. Aims We wish to use seismic indicators obtained with inversion techniques to constrain additional mixing processes in the structure of the components of the binary system 16Cyg. The combination of various seismic indicators will help to point out the weaknesses of stellar models and thus implies more constrained and accurate fundamendal parameters for these stars. Methods First, we will use the latest seismic, spectroscopic and interferometric observational constraints in the litterature for this system to determine suitable reference models independently for both stars. We will then carry out seismic inversions of the acoustic radius, the mean density and of a core conditions indicator. These additional constraints will be used to improve the reference models for both stars. Results The combination of seismic, interferometric and spectroscopic constraints allows us to obtain accurate reference models for both stars. However, we note that a degeneracy exists for these models. Namely, changing the diffusion coefficient or the chemical composition within the observational values could lead to $5 \%$ changes in mass, $3 \%$ changes in radius and up to $8 \%$ changes in age. We used acoustic radius and mean density inversions to further improve our reference models then carried out inversions for a core conditions indicator, denoted $t_{u}$. Thanks to the sensitivity of this indicator to microscopic diffusion and chemical composition mismatches, we were able to reduce the mass dispersion to $2 \%$, namely $\left[ 0.96M_{\odot},1.0M_{\odot} \right]$, the radius dispersion to $1 \%$, namely $\left[ 1.188R_{\odot},1.200R_{\odot} \right]$ and the age dispersion to $3 \%$, namely $\left[7.0,7.4\right]$, for $16$CygA. For $16$CygB, $t_{u}$ offered a consistency check for the models but could not be used to reduce independently the age dispersion. Nonetheless, assuming consistency with the age of $16$CygA could help to further constrain its mass and radius. We thus find that the mass of $16$CygB should be between $0.93$ $M_{\odot}$ and $0.96$ $M_{\odot}$ and its radius between $1.08$ $R_{\odot}$ and $1.10$ $R_{\odot}$

Dynamo-generated magnetic fields common in the convective cores of intermediate-mass stars

Dennis Stello, Matteo Cantiello, Jim Fuller, Daniel Huber, Rafael A. Garcia, Tim R. Bedding, Lars Bildsten, Victor Silva Aguirre.
Full text

Magnetic fields play a role in almost all stages of stellar evolution. Most low-mass stars, including the Sun, show surface fields that are generated by dynamo processes in their convective envelopes. Intermediate-mass stars do not have deep convective envelopes, yet 10% exhibit strong surface fields that are presumed to be residuals from the stellar formation process. These stars do have convective cores that might produce internal magnetic fields, and these might even survive into later stages of stellar evolution, but information has been limited by our inability to measure the fields below the stellar surface. Here we use asteroseismology to study the occurrence of strong magnetic fields in the cores of low- and intermediate-mass stars. We have measured the strength of dipolar oscillation modes, which can be suppressed by a strong magnetic field in the core, in over 3600 red giant stars observed by Kepler. About 20% of our sample have mode suppression but this fraction is a strong function of mass. Strong core fields only occur in red giants above 1.1 solar masses (1.1Msol), and the occurrence rate is at least 60% for intermediate-mass stars (1.6–2.0Msol), indicating that powerful dynamos are very common in the convective cores of these stars.

Period spacings in red giants I. Disentangling rotation and revealing core structure discontinuities

B. Mosser, M. Vrard, K. Belkacem, S. Deheuvels.
Full text

Asteroseismology offers an efficient probe for testing the physical conditions inside the core of red giant stars. This relies on the properties of the oscillations with a mixed character that are highly sensitive to the physical properties of the core. Unfortunately, the mixed-mode pattern is often described as the mixed-mode forest since overlapping rotational splittings and mixed-mode spacings result in complex structures. This motivates further work. % This work aims at making the mixed-mode oscillation pattern as clear as possible in order to disentangle the rotational splittings from the mixed-mode spacings, and then open the way to a fully automated analysis of large data sets. % An analytical development of the mixed-mode asymptotic expansion is used to derive the period spacing between two consecutive mixed modes. This expression allows us to correct for the distortion of the mixed-mode pattern induced by the coupling of gravity and pressure waves; it puts in evidence the contribution of the gravity waves, regardless of the coupling. Period-corrected échelle diagrams clearly exhibit the structure of the gravity modes and of the rotational splittings. % We propose a new view on the mixed-mode oscillation pattern based on corrected periods, called stretched periods, that mimic the evenly spaced gravity-mode pattern. We also exhibit the signature of the structural glitches on mixed modes and show that we can distinguish deep and shallow buoyancy glitches. % % This work opens the possibility to derive all seismic global parameters in an automated way, including the identification of the different rotational multiplets and the measurement of the rotational splitting, even when this splitting is significantly larger than the period spacing.

KIC 10080943: a binary star with two $\gamma$ Doradus/$\delta$ Scuti hybrid pulsators. Analysis of the g modes

M. A. Keen, T. R. Bedding, S. J. Murphy, V. S. Schmid, C. Aerts, A. Tkachenko, R.-M. Ouazzani, D. W. Kurtz.
Full text

We use four years of Kepler photometry to study the non-eclipsing spectroscopic binary KIC 10080943. We find both components to be $\gamma$ Doradus/$\delta$ Scuti hybrids, which pulsate in both p and g modes. We present an analysis of the g modes, which is complicated by the fact that the two sets of $\ell=1$ modes partially overlap in the frequency spectrum. Nevertheless, it is possible to disentangle them by identifying rotationally split doublets from one component and triplets from the other. The identification is helped by the presence of additive combination frequencies in the spectrum that involve the doublets but not the triplets. The rotational splittings of the multiplets imply core rotation periods of about 11 d and 7 d in the two stars. One of the stars also shows evidence of $\ell=2$ modes.

Evidence of amplitude modulation due to Resonant Mode Coupling in the $\delta$ Scuti star KIC 5892969

S. Barceló Forteza, E. Michel, T. Roca Cortés, R. A. García.
Full text

A study of the star KIC 5892969 observed by the Kepler satellite is presented. Its three highest amplitude modes present a strong amplitude modulation. The aim of this work is to investigate amplitude variations in this star and their possible cause. Using the 4 years-long observations available, we obtained the frequency content of the full light curve. Then, we studied the amplitude and phase variations with time using shorter time stamps. The results obtained are compared with the predicted ones for resonant mode coupling of an unstable mode with lower frequency stable modes. Our conclusion is that resonant mode coupling is consistent as an amplitude limitation mechanism in several modes of KIC 5892969 and we discuss to which extent it might play an important role for other $\delta$ Scuti stars.

KIC 10080943: An eccentric binary system containing two pressure and gravity mode hybrid pulsators

V. S. Schmid, A. Tkachenko, C. Aerts, P. Degroote, S. Bloemen, S. J. Murphy, T. Van Reeth, P. I. Papics, T. R. Bedding, M. A. Keen, A. Prsa, J. Menu, J. Debosscher, M. Hrudkova, K. De Smedt, R. Lombaert, P. Nemeth.
Full text
NASA ADS
DOI

$\gamma$ Doradus and $\delta$ Scuti pulsators cover the transition region between low mass and massive main-sequence stars, and as such, are critical for testing stellar models. When they reside in binary systems, we can combine two independent methods to derive critical information, such as precise fundamental parameters to aid asteroseismic modelling. In the Kepler light curve of KIC 10080943, clear signatures of gravity- and pressure-mode pulsations have been found. Ground-based spectroscopy revealed this target to be a double-lined binary system. We present the analysis of four years of Kepler photometry and high-resolution spectroscopy to derive observational constraints with which to evaluate theoretical predictions of the stellar structure and evolution for intermediate-mass stars. We used the method of spectral disentangling to determine atmospheric parameters for both components and derive the orbital elements. With phoebe, we modelled the ellipsoidal variation and reflection signal of the binary in the light curve and used classical Fourier techniques to analyse the pulsation modes. We show that the eccentric binary system KIC 10080943 contains two hybrid pulsators with masses $M_1=2.0\pm0.1~M_\odot$ and $M_2=1.9\pm0.1~M_\odot$, with radii $R_1=2.9\pm0.1~R_\odot$ and $R_2=2.1\pm0.2~R_\odot$. We detect rotational splitting in the g and p modes for both stars and use them to determine a first rough estimate of the core-to-surface rotation rates for the two components, which will be improved by future detailed seismic modelling.

A seismic and gravitationally-bound double star observed by Kepler: implication for the presence of a convective core

T. Appourchaux, H. M. Antia, W. Ball, O. Creevey, Y. Lebreton, J. Montalbá, K. Verma, S. Vorontsov, T. L. Campante, G. R. Davies, P. Gaulme, C. Régulo, E. Horch, S. Howell, M. Everett, D. Ciardi, L. Fossat and 3 coauthors.
Full text

Solar-like oscillations have been observed by Kepler and CoRoT in many solar-type stars, thereby providing a way to probe the stars using asteroseismology.
The derivation of stellar parameters has usually been done using single stars. The aim of the paper is to derive the stellar parameters of a double-star system (HIP93511), for which an interferometric orbit has been observed along with asteroseismic measurements.
We used a time series of nearly two years of data of the double star to detect the two oscillation mode envelopes that appear in the power spectrum. Using a new scaling relation based on the use of luminosity, we derive the radius and mass of each star. We derive the age of each star using two proxies: one based upon the large frequency separation and a new one based upon the small frequency separation. Using stellar modelling, the mode frequencies allowed us to derive the radius, the mass and age of each component. In addition, speckle interferometry performed since 2006 has enabled us to recover the orbit of the system, and the total mass of the system.
From the determination of the orbit, the total mass of the system is 2.34$_{-0.33}^{+0.45}$ $M_{\odot}$ The total seismic mass using scaling relations is 2.47 $\pm$ 0.07 $M_{\odot}$. The seismic age derived using the new proxy based upon the small frequency separation is 3.5 $\pm$ 0.3 Gyr. Based on stellar modelling, the common age of the system is 2.7-3.9 Gyr. The total seismic mass of the system is 2.34-2.53 $M_{\odot}$, consistent to what we determined with the orbit. The stellar models provided the radius, mass and age of the stars as: $R_{A}=1.82-1.87$ $R_{\odot}$, $M_{A}=1.25-1.39$ $M_{\odot}$, ${\rm Age}_{A}$=2.6-3.5 Gyr; $R_{B}=1.22-1.25 R_{\sun}$, $M_{B}=1.08-1.14$ $M_{\odot}$, ${\rm Age}_{B}$=3.35-4.21 Gyr. We detect a convective core in Star A while Star B does not have any. For the metallicity of the binary system of $Z \approx 0.02$, we set the limit between stars having a convective corer or not at about 1.2 $M_{\odot}$.

LAMOST observatons in the Kepler field. I. Database of low-resolution spectra

P. De Cat, J.N. Fu, A.B. Ren, X.H. Yang, J.R. Shi, A.L. Luo, M. Yang, J.L. Wang, H.T. Zhang, H.M. Shi, W. Zhang, S. Dong, G. Catanzaro, C.J. Corbally, A. Frasca, R.O. Gray, J. Molenda-akowicz and 20 coauthors.
Full text

The nearly continuous light curves with micromagnitude precision provided by the space mission Kepler are revolutionising our view of pulsating stars. They have revealed a vast sea of low-amplitude pulsation modes that were undetectable from Earth. The long time base of Kepler light curves allows an accurate determination of frequencies and amplitudes of pulsation modes needed for in-depth asteroseismic modeling. However, for an asteroseismic study to be successful, the first estimates of stellar parameters need to be known and they can not be derived from the Kepler photometry itself. The Kepler Input Catalog (KIC) provides values for the effective temperature, the surface gravity and the metallicity, but not always with a sufficient accuracy. Moreover, information on the chemical composition and rotation rate is lacking. We are collecting low-resolution spectra for objects in the Kepler field of view with the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST, Xinglong observatory, China). All of the requested fields have now been observed at least once. In this paper we describe those observations and provide a database of use to the whole astronomical community.
(This is an updated version of the paper with the same title with "status:unpublished" in the list of KASOC Publications)

Determination of fundamental asteroseismic parameters using the Hilbert transform

René Kiefer, Ariane Schad, Wiebke Herzberg, Markus Roth.
Full text
NASA ADS
DOI

Context. Solar-like oscillations exhibit a regular pattern of frequencies. This pattern is dominated by the small and large frequency separations between modes. The accurate determination of these parameters is of great interest, because they give information about e.g. the evolutionary state and the mass of a star.
Aims. We want to develop a robust method to determine the large and small frequency separations for time series with low signal-to-noise ratio. For this purpose, we analyse a time series of the Sun from the GOLF instrument aboard SOHO and a time series of the star KIC 5184732 from the NASA Kepler satellite by employing a combination of Fourier and Hilbert transform.
Methods. We use the analytic signal of filtered stellar oscillation time series to compute the signal envelope. Spectral analysis of the signal envelope then reveals frequency differences of dominant modes in the periodogram of the stellar time series.
Results. With the described method the large frequency separation $\Delta\nu$ can be extracted from the envelope spectrum even for data of poor signal-to-noise ratio. A modification of the method allows for an overview of the regularities in the periodogram of the time series.

KIC 3858884: a hybrid δ Sct pulsator in a highly eccentric eclipsing binary.

C.  Maceroni, H. Lehmann, R. da Silva, J. Montalbán, C.-U. Lee, H. Ak, R. Deshpande, K. Yakut, J. Debosscher, S.-L. Kim, J. W. Lee, J. Southworth.
Full text
NASA ADS
DOI

The analysis of eclipsing binaries containing non-radial pulsators allows: i) to combine two different and independent sources of information on the internal structure and evolutionary status of the components, and ii) to study the effects of tidal forces on pulsations. KIC 3858884 is a bright Kepler target whose light curve shows deep eclipses, complex pulsation patterns with pulsation frequencies typical of δ Sct, and a highly eccentric orbit. We present the result of the analysis of Kepler photometry and of high resolution phase resolved spectroscopy. Spectroscopy yielded both the radial velocity curves and, after spectral disentangling, the primary component effective temperature and metallicity, and line-of-sight projected rotational velocities. The Kepler light curve was analyzed with an iterative procedure devised to disentangle eclipses from pulsations which takes into account the visibility of the pulsating star during eclipses. The search for the best set of binary parameters was performed combining the synthetic light curve models with a genetic minimization algorithm, which yielded a robust and accurate determination of the system parameters. The binary components have very similar masses (1.88 and 1.86 M⊙) and effective temperatures (6800 and 6600 K), but different radii (3.45 and 3.05 R⊙). The comparison with the theoretical models evidenced a somewhat different evolutionary status of the components and the need of introducing overshooting in the models. The pulsation analysis indicates a hybrid nature of the pulsating (secondary) component, the corresponding high order g-modes might be excited by an intrinsic mechanism or by tidal forces.

Oscillating red giants observed during Campaign 1 of the Kepler K2 mission: New prospects for galactic archaeology

Dennis Stello, Daniel Huber, Sanjib Sharma, Jennifer Johnson, Mikkel N. Lund, Rasmus Handberg, Derek L. Buzasi, Victor Silva Aguirre, William J. Chaplin, Andrea Miglio, Marc Pinsonneault, Sarbani Basu, Tim R. Bedding, Joss Bland-Hawthorn, Luca Casagrande, Guy Davies, Yvonne Elsworth and 6 coauthors.
Full text

NASA's re-purposed Kepler mission – dubbed K2 – has brought new scientific opportunities that were not anticipated for the original Kepler mission. One science goal that makes optimal use of K2's capabilities, in particular its 360-degree ecliptic field of view, is galactic archaeology – the study of the evolution of the Galaxy from the fossil stellar record. The thrust of this research is to exploit high-precision, time-resolved photometry from K2 in order to detect oscillations in red giant stars. This asteroseismic information can provide estimates of stellar radius (hence distance), mass and age of vast numbers of stars across the Galaxy. Here we present the initial analysis of a subset of red giants, observed towards the North Galactic Gap, during the mission's first full science campaign. We investigate the feasibility of using K2 data for detecting oscillations in red giants that span a range in apparent magnitude and evolutionary state (hence intrinsic luminosity). We demonstrate that oscillations are detectable for essentially all cool giants within the $\log g$ range $\sim 1.9$–3.2. Our detection is complete down to $\mathit{Kp}\sim 14.5$, which results in a seismic sample with little or no detection bias. This sample is ideally suited to stellar population studies that seek to investigate potential shortcomings of contemporary Galaxy models.

Observational $\Delta\nu-\rho$ relation for $\delta$ Sct stars using eclipsing binaries and space photometry

A. García Hernández, S. Martín-Ruiz, Mário J. P. F. G. Monteiro, J. C. Suárez, D. R. Reese, J. Pascual-Granado, R. Garrido.
Full text
NASA ADS
DOI

Delta Scuti (δ Sct) stars are intermediate-mass pulsators, whose intrinsic oscillations have been studied for decades. However, modeling their pulsations remains a real theoretical challenge, thereby even hampering the precise determination of global stellar parameters. In this work, we used space photometry observations of eclipsing binaries with a δ Sct component to obtain reliable physical parameters and oscillation frequencies. Using that information, we derived an observational scaling relation between the stellar mean density and a frequency pattern in the oscillation spectrum. This pattern is analogous to the solar-like large separation but in the low order regime. We also show that this relation is independent of the rotation rate. These findings open the possibility of accurately characterizing this type of pulsator and validate the frequency pattern as a new observable for δ Sct stars.

The Blazhko effect and additional excited modes in RR Lyrae stars

J. M. Benk, R. Szabó.
Full text

Recent photometric space missions, such as CoRoT and Kepler revealed that many RR Lyrae stars pulsate – beyond their main radial pulsation mode – in low amplitude modes. Space data seem to indicate a clear trend, namely overtone (RRc) stars and modulated fundamental (RRab) RR Lyrae stars ubiquitously show additional modes, while non-Blazhko RRab stars never do. Two Kepler stars (V350 Lyr and KIC 7021124), however, apparently seemed to break this rule: they were classified as non-Blazhko RRab stars showing additional modes. We processed Kepler pixel photometric data of these stars. We detected small amplitude, but significant Blazhko effect for both stars by using the resulted light curves and O$-$C diagrams. This finding strengthens the apparent connection between the Blazhko effect and the excitation of additional modes. In addition, it yields a potential tool for detecting Blazhko stars through the additional frequency patterns even if we have only short but accurate time series observations. V350 Lyr shows the smallest amplitude multiperiodic Blazhko effect ever detected.

The potential for super-Nyquist asteroseismology with TESS

Simon J. Murphy.
Full text

The perfect 30-min cadence of the full-frame images from the Transiting Exoplanet Survey Satellite (TESS) will impose a hard Nyquist limit of 24 d$^{-1}$ ($\approx 278$ $\upmu$Hz). This will be problematic for asteroseismology of stars with oscillation frequencies at or around that Nyquist limit, which will have insurmountable Nyquist ambiguities. TESS does offer some observing slots at shorter cadences, but these will be limited in number and competitive, while the full frame images will be the main data product for many types of variable stars. We show that the Nyquist ambiguities can be alleviated if, when TESS resumes observations after a downlink, integrations are not resumed at perfect cadence with those before the downlink. The time spent idling before integrations are resumed need only be around five minutes for satisfactory results, and observing time can be recouped from the downlink event if the telescope does not wait for a return to perfect cadence before resuming integrations. The importance of imperfect cadence after downlink is discussed in light of phase coverage of transit events.

KIC 4768731: a bright long-period roAp star in the Kepler Field

B. Smalley, E. Niemczura, S. J. Murphy, H. Lehmann, D. W. Kurtz, D. L. Holdsworth, M. S. Cunha, L. A. Balona, M. Briquet, H. Bruntt, P. De Cat, P. Lampens, A. O. Thygesen, K. Uytterhoeven.
Full text
NASA ADS
DOI

We report the identification of 61.45 d$^{-1}$ (711.2 $\mu$Hz) oscillations, with amplitudes of 62.6-$\mu$mag, in KIC 4768731 (HD 225914) using Kepler photometry. This relatively bright ($V$=9.17) chemically peculiar star with spectral type A5 Vp SrCr(Eu) has previously been found to exhibit rotational modulation with a period of 5.21 d. Fourier analysis reveals a simple dipole pulsator with an amplitude that has remained stable over a 4-yr time span, but with a frequency that is variable. Analysis of high-resolution spectra yields stellar parameters of $T_{\rm eff} = 8100 \pm 200$ K, $\log g = 4.0 \pm 0.2$, [Fe/H] = $+0.31 \pm 0.24$ and $v \sin i = 14.8 \pm 1.6$ km s$^{-1}$. Line profile variations caused by rotation are also evident. Lines of Sr, Cr, Eu, Mg and Si are strongest when the star is brightest, while Y and Ba vary in anti-phase with the other elements. The abundances of rare earth elements are only modestly enhanced compared to other roAp stars of similar $T_{\rm eff}$ and $\log g$. Radial velocities in the literature suggest a significant change over the past 30 yr, but the radial velocities presented here show no significant change over a period of 4 yr.

Seismic evidence for a weak radial differential rotation in intermediate-mass core helium burning stars

S. Deheuvels, J. Ballot, P. G. Beck, B. Mosser, R. stensen, R. A. García, M. J. Goupil.
Full text

The detection of mixed modes that are split by rotation in Kepler red giants has made it possible to probe the internal rotation profiles of these stars, which brings new constraints on the transport of angular momentum in stars. Mosser et al. (2012) have measured the rotation rate in the central regions of intermediate-mass core helium burning stars (secondary clump stars).
Our aim is to exploit the rotational splittings of mixed modes to estimate the amount of radial differential rotation in the interior of secondary clump stars using Kepler data.
We select a subsample of Kepler secondary clump stars with mixed modes that are clearly rotationally split. By applying a thorough statistical analysis, we show that the splittings of both gravity-dominated modes (trapped in central regions) and p-dominated modes (trapped in the envelope) can be measured. We then use these splittings to estimate the amount of differential rotation by using inversion techniques and by applying a simplified approach based on asymptotic theory (Goupil et al. 2013).
We obtain evidence of a weak radial differential rotation for six of the seven targets that were selected, with the central regions rotating 1.8 +/- 0.3 to 3.2 +/- 1.0 times faster than the envelope. The last target is found to be consistent with a solid-body rotation.
This demonstrates that an efficient redistribution of angular momentum occurs after the end of the main sequence in the interior of intermediate-mass stars, either during the short-lived subgiant phase, or once He-burning has started in the core. In either case, this should bring constraints on the angular momentum mechanisms that are at work.

Automated determination of g-mode period spacing of red-giant stars

Abhisek Datta, Anwesh Mazumdar, Umang Gupta, Saskia Hekker.
Full text
NASA ADS
DOI

The Kepler satellite has provided photometric timeseries data of unprecedented length, duty cycle and precision. To fully analyse these data for the tens of thousands of stars observed by Kepler, automated methods are a prerequisite. Here, we present an automated procedure to determine the period spacing of gravity modes in red giant stars ascending the red giant branch. The gravity modes reside in a cavity in the deep interior of the stars and provide information on the conditions in the stellar core. However, for red giants the gravity modes are not directly observable on the surface, hence this method is based on the pressure-gravity mixed modes that present observable features in the Fourier power spectrum. The method presented here is based on the vertical alignment and symmetry of these mixed modes in a period echelle diagram. We find that we can obtain reliable results for both model frequencies and observed frequencies. Additionally, we carried out Monte Carlo tests to obtain realistic uncertainties on the period spacings with different set of oscillation modes (for the models) and uncertainties on the frequencies. Furthermore, this method has been used to improve mode detection and identification of the observed frequencies in an iterative manner.

Helium signature in red giant oscillation patterns observed by Kepler

Vrard M., Mosser B., Barban C., Belkacem K., Elsworth Y., Kallinger T., Hekker S., Samadi R., and Beck P..
Full text

The space-borne missions CoRoT and $\Kepler$ have provided a large amount of precise photometric data. Among the stars observed, red giants show a rich oscillation pattern that allows their precise characterization. The usual description of red giant solar-like oscillations can be understood at first order in terms of the universal asymptotic oscillation pattern, which corresponds to a second-order asymptotic developement. We aim to, first, investigate the deviations observed from this universal pattern and, second, characterize them and identify them as the signature of sound-speed variations due to the second helium ionisation region. We also intend to characterize its variations along stellar evolution. We first measure the frequencies of radial modes in each spectra using a Maximum Likelihood Estimator method, then we isolate the modulation corresponding to the glitch component by taking the departure to the universal pattern into account. We measure the modulation component of the radial mode frequency spacings in more than one hundred red giants. We attribute the modulation to glitches due to the second ionisation of helium. We find a correlation between the evolutionary status of the stars and the signature of the second helium ionisation zone. We also bring constraints on the position of the second helium ionisation zone for these stars.

Asteroseismic modeling of 16 Cyg A & B using the complete Kepler data set

Travis S. Metcalfe, Orlagh L. Creevey, Guy R. Davies.
Full text
NASA ADS
DOI

Asteroseismology of bright stars with well-determined properties from parallax measurements and interferometry can yield precise stellar ages and meaningful constraints on the composition. We substantiate this claim with an updated asteroseismic analysis of the solar-analog binary system 16 Cyg A & B using the complete 30-month data sets from the Kepler space telescope. An analysis with the Asteroseismic Modeling Portal (AMP), using all of the available constraints to model each star independently, yields the same age ($t=7.0\pm0.3$ Gyr) and composition ($Z=0.021\pm0.002$, $Y_{\rm i}=0.25\pm0.01$) for both stars, as expected for a binary system. We quantify the accuracy of the derived stellar properties by conducting a similar analysis of a Kepler-like data set for the Sun, and we investigate how the reliability of asteroseismic inference changes when fewer observational constraints are available or when different fitting methods are employed. We find that our estimates of the initial helium mass fraction are probably biased low by 0.02–0.03 from neglecting diffusion and settling of heavy elements, and we identify changes to our fitting method as the likely source of small shifts from our initial results in 2012. We conclude that in the best cases reliable stellar properties can be determined from asteroseismic analysis even without independent constraints on the radius and luminosity.

Nearly-uniform internal rotation of solar-like main-sequence stars revealed by space-based asteroseismology and spectroscopic measurements

Othman Benomar, Masao Takata, Hiromoto Shibahashi, Tugdual Ceillier, Rafael A. Garcia.
Full text

The rotation rates in the deep interior and at the surface of 22 main-sequence stars with masses between $1.0$ and $1.6\,{\rm M}_{\sun}$ are constrained by combining asteroseismological analysis with spectroscopic measurements. The asteroseismic data of each star are taken by the Kepler or CoRoT space mission. It is found that the difference between the surface rotation rate and the average rotation rate (excluding the convective core) of most of stars is small enough to suggest that an efficient process of angular momentum transport operates during and/or before the main-sequence stage of stars. If each of the surface convective zone and the underlying radiative zone, for individual stars, is assumed to rotate uniformly, the difference in the rotation rate between the two zones turns out to be no more than a factor of two in most of the stars independently of their ages.

K2P² - A photometry pipeline for the K2 mission

Mikkel N. Lund, Rasmus Handberg, Guy R. Davies, William J. Chaplin, Caitlin D. Jones.
Full text
NASA ADS
DOI

With the loss of a second reaction wheel, resulting in the inability to point continuously and stably at the same field of view, the NASA Kepler satellite recently entered a new mode of observation known as the K2 mission. The data from this redesigned mission present a specific challenge; the targets systematically drift in position on an ${\sim}6$ hr timescale, inducing a significant instrumental signal in the photometric time series—this greatly impacts the ability to detect planetary signals and perform asteroseismic analysis. Here we detail our version of a reduction pipeline for K2 target pixel data, which automatically defines masks for all targets in a given frame; extracts the target's flux and position time series; corrects the time series based on the apparent movement on the CCD (either in 1D or 2D), combined with the correction of instrumental and/or planetary signals via the Kepler Asteroseismic Science Operations Center (KASOC) filter, thus rendering the time series ready for asteroseismic analysis; computes power spectra for all targets; and identifies potential contaminations between targets. From a test of our pipeline on a sample of targets from the K2 campaign 0, the recovery of data for multiple targets increases the amount of potential light curves by a factor of ${\geq}10$. Our pipeline could be applied to the upcoming TESS and PLATO 2.0 missions.

Finding binaries among Kepler pulsating stars from phase modulation of their pulsations

Simon J. Murphy, Timothy R. Bedding, Hiromoto Shibahashi, Donald W. Kurtz, Hans Kjeldsen.
Full text

We present a method for finding binaries among pulsating stars that were observed by the Kepler Mission. We use entire four-year light curves to accurately measure the frequencies of the strongest pulsation modes, then track the pulsation phases at those frequencies in 10-d segments. This produces a series of time-delay measurements in which binarity is apparent as a periodic modulation whose amplitude gives the projected light travel time across the orbit. Fourier analysis of this time-delay curve provides the parameters of the orbit, including the period, eccentricity, angle of ascending node and time of periastron passage. Differentiating the time-delay curve yields the full radial-velocity curve directly from the Kepler photometry, without the need for spectroscopy. We show examples with $\delta$ Scuti stars having large numbers of pulsation modes, including one system in which both components of the binary are pulsating. The method is straightforward to automate, thus radial velocity curves can be derived for hundreds of non-eclipsing binary stars from Kepler photometry alone.

Asteroseismic estimate of helium abundance of a solar analog binary system

Kuldeep Verma, João P. Faria, H. M. Antia, Sarbani Basu, Anwesh Mazumdar, Mário J. P. F. G. Monteiro, Thierry Appourchaux, William J. Chaplin, Rafael A. García, Travis S. Metcalfe.
Full text
NASA ADS
DOI

16 Cyg A and B are among the brightest stars observed by Kepler. What makes these stars more interesting is that they are solar analogs. 16 Cyg A and B exhibit solar-like oscillations. In this work we use oscillation frequencies obtained using 2.5 years of Kepler data to determine the current helium abundance of these stars. For this we use the fact that the helium ionization zone leaves a signature on the oscillation frequencies and that this signature can be calibrated to determine the helium abundance of that layer. By calibrating the signature of the helium ionization zone against models of known helium abundance, we find that the helium abundance in the envelope of \SCA is $0.243\pm0.008$ and that of \SCB is $0.244\pm0.018$.

Asteroseismic surface gravity for evolved stars

S. Hekker, Y. Elsworth, B. Mosser, T. Kallinger, Sarbani Basu, W.J. Chaplin, D. Stello.
Full text

Context: Asteroseismic surface gravity values can be of importance in determining spectroscopic stellar parameters. The independent $\log(g)$ value from asteroseismology can be used as a fixed value in the spectroscopic analysis to reduce uncertainties due to the fact that $\log(g)$ and effective temperature can not be determined independently from spectra. Since 2012, a combined analysis of seismically and spectroscopically derived stellar properties is ongoing for a large survey with SDSS/APOGEE and Kepler. Therefore, knowledge of any potential biases and uncertainties in asteroseismic $\log(g)$ values is now becoming important. Aims: The seismic parameter needed to derive $\log(g)$ is the frequency of maximum oscillation power ($\nu_{\rm max}$). Here, we investigate the influence of $\nu_{\rm max}$ derived with different methods on the derived $\log(g)$ values. The large frequency separation between modes of the same degree and consecutive radial orders ($\Delta\nu$) is often used as an additional constraint for the determination of $\log(g)$. Additionally, we checked the influence of small corrections applied to $\Delta\nu$ on the derived values of $\log(g)$. Methods: We use methods extensively described in the literature to determine $\nu_{\rm max}$ and $\Delta\nu$ together with seismic scaling relations and grid-based modeling to derive $\log(g)$. Results:We find that different approaches to derive oscillation parameters give results for $\log(g)$ with small, but different, biases for red-clump and red-giant-branch stars. These biases are well within the quoted uncertainties of $\sim0.01$ dex (cgs). Corrections suggested in the literature to the $\Delta\nu$ scaling relation have no significant effect on $\log(g)$. However somewhat unexpectedly, method specific solar reference values induce biases of the order of the uncertainties, which is not the case when canonical solar reference values are used.

Asteroseismic inference on the spin-orbit misalignment and stellar parameters of HAT-P-7

Mikkel N. Lund, Mia Lundkvist, Victor Silva Aguirre, Günter Houdek, Luca Casagrande, Vincent Van Eylen, Tiago L. Campante, Christoffer Karoff, Hans Kjeldsen, Simon Albrecht, William J. Chaplin, Martin Bo Nielsen, Pieter Degroote, Guy R. Davies, Rasmus Handberg.
Full text
NASA ADS
DOI

The measurement of obliquities – the angle between the orbital and stellar rotation – in star-planet systems is of great importance for the understanding of planet system formation and evolution. The bright and well studied HAT-P-7 (Kepler-2) system is intriguing as several Rossiter-McLaughlin (RM) measurements found a large projected obliquity in this system, but it was so far not possible to determine if the orbit is polar and/or retrograde.
The goal of this study is to measure the stellar inclination and hereby the full 3D obliquity of the HAT-P-7 system instead of only the 2D projection as measured by the RM effect. In addition we provide an updated set of stellar parameters for the star.
We use the full set of available observations from Kepler spanning Q$0$-Q$17$ to produce the power spectrum of HAT-P-7. We extract oscillation mode frequencies via an MCMC peak-bagging routine, and use the results from this to estimate the stellar inclination angle. Combining this with the projected obliquity from RM and the inclination of the orbital plane allows us to determine the stellar obliquity. Furthermore, we use asteroseismology to model the star from the extracted frequencies using two different approaches to the modelling where either the MESA or the GARSTEC stellar evolution codes are adopted.
Using our updated asteroseismic modelling we find, a, the following stellar parameters for HAT-P-7: $M_{\star}=1.51^{+0.04}_{-0.05} \, M_{\odot}$, $R_{\star}=2.00^{+0.01}_{-0.02} \, R_{\odot}$, and age $= 2.07^{+0.28}_{-0.23}$ Gyr. Our asteroseismic modelling offers a high precision on the stellar parameters, for instance is the uncertainty on age of the order ${\sim}11\%$. For the stellar inclination we estimate $i_{\star}<36.5^{\circ}$, which translates to an obliquity of $83^{\circ}<\psi<111^{\circ}$. We find that the planet HAT-P-7b is likely retrograde in its orbit, and that the orbit is close to being polar. The new parameters for the star gives an updated planetary density of $\rho_p=0.65\pm 0.03\, \rm g\, cm^{-3}$, which is lower than previous estimates.

Rotation and magnetism of Kepler pulsating solar-like stars. Towards asteroseismically calibrated age-rotation relations

R. A. García, T. Ceillier, D. Salabert, S. Mathur, J. L. van Saders, M. Pinsonneault, J. Ballot, P. G. Beck, S. Bloemen, T. L. Campante, G. R. Davies, J.-D. do Nascimento Jr., S. Mathis, T. S. Metcalfe, M. B. Nielsen, J. C. Suárez, W. J. Chaplin and 2 coauthors.
Full text
NASA ADS
DOI

Kepler ultra-high precision photometry of long and continuous observations provides a unique dataset in which surface rotation and variability can be studied for thousands of stars. Because many of these old field stars also have independently measured asteroseismic ages, measurements of rotation and activity are particularly interesting in the context of age-rotation-activity relations. In particular, age-rotation relations generally lack good calibrators at old ages, a problem that this Kepler sample of old-field stars is uniquely suited to address. We study the surface rotation and photometric magnetic activity of a subset of 540 solar-like stars on the main-sequence and the subgiant branch for which stellar pulsations have been measured. The rotation period was determined by comparing the results from two different analysis methods: i) the projection onto the frequency domain of the time-period analysis, and ii) the autocorrelation function of the light curves. Reliable surface rotation rates were then extracted by comparing the results from two different sets of calibrated data and from the two complementary analyses. General photometric levels of magnetic activity in this sample of stars were also extracted by using a photometric activity index, which takes into account the rotation period of the stars. We report rotation periods for 310 out of 540 targets (excluding known binaries and candidate planet-host stars); our measurements span a range of 1 to 100 days. The photometric magnetic activity levels of these stars were computed, and for 61.5% of the dwarfs, this level is similar to the range, from minimum to maximum, of the solar magnetic activity. We demonstrate that hot dwarfs, cool dwarfs, and subgiants have very different rotation-age relationships, highlighting the importance of separating out distinct populations when interpreting stellar rotation periods. Our sample of cool dwarf stars with age and metallicity data of the highest quality is consistent with gyrochronology relations reported in the literature.

Impact on asteroseismic analyses of regular gaps in Kepler data

R.A. Garcia, S. Mathur, S. Pires, C. Regulo, B. Bellamy, P.L. Palle, J. Ballot, S. Barcelo Forteza, P.G.Beck, T.R. Bedding, T. Ceillier, T. Roca Cortes, D. Salabert, D. Stello.
Full text
NASA ADS
DOI

The NASA Kepler mission has observed more than 190,000 stars in the constellations of Cygnus and Lyra. Around 4 years of almost continuous ultra high-precision photometry have been obtained reaching a duty cycle higher than 90% for many of these stars. However, regular gaps due to nominal operations are present in the light curves at different time scales. In general, most of these gaps are not dealt with in any specific way and are just accepted.In this paper we want to highlight the impact of those regular gaps in asteroseismic analyses and we try to find a method that minimizes their impact in the frequency domain. To do so, we isolate the two main time scales of regular gaps in the data. We then interpolate the gaps and we compare the power density spectra of four different stars: two red giants at different stages of their evolution, a young F-type star, and a classical pulsator in the instability strip. The spectra obtained after filling the gaps in the selected solar-like stars show a net reduction in the overall background level, as well as a change in the background parameters. The inferred convective properties could change as much as ∼200% in the selected example, introducing a bias in the p-mode frequency of maximum power. When global asteroseismic scaling relations are used, this bias can lead up to a variation in the surface gravity of 0.05 dex. Finally, the oscillation spectrum in the classical pulsator is cleaner compared to the original one

Surface Activity and Oscillation Amplitudes of Red Giants in Eclipsing Binaries

Patrick Gaulme, Jason Jackiewicz, Thierry Appourchaux, Benoit Mosser.
Full text

Among the 19 red-giant stars belonging to eclipsing binary systems that have been identified in Kepler data, 15 display solar-like oscillations. We study whether the absence of mode detection in the remaining 4 is an observational bias or possibly evidence of mode damping that originates from tidal interactions. A careful analysis of the corresponding Kepler light curves shows that modes with amplitudes that are usually observed in red giants would have been detected if they were present. We observe that the mode depletion is strongly associated with short-period systems, in which stellar radii account for 16-24 % of the semi-major axis, that are likely locked in phase, and where red-giant surface activity is detected. We suggest that when close binary systems lock in phase, the red-giant component is spun up, so that a dynamo mechanism starts and generates magnetic field, leading to observable stellar activity. Pressure modes would then be damped as acoustic waves dissipate in these fields.

Oscillation mode linewidths and heights of 23 main-sequence stars observed by Kepler

T. Appourchaux, H. M. Antia, O. Benomar, T. L. Campante, G. R. Davies, R. Handberg, R. Howe, C. Régulo, K. Belkacem, G. Houdek, R. A. García, W. J. Chaplin.
Full text

Solar-like oscillations have been observed by Kepler and CoRoT in many solar-type stars, thereby providing a way to probe the stars using asteroseismology.
We provide the mode linewidths and mode heights of the oscillations of various stars as a function of frequency and of effective temperature.
We used a time series of nearly two years of data for each star. The 23 stars observed belong to the simple or F-like category. The power spectra of the 23 main-sequence and subgiant stars were analysed using both maximum likelihood estimators and Bayesian estimators, providing individual mode characteristics such as frequencies, linewidths, and mode heights. We study the source of systematic errors in the mode linewidths and mode heights, and we give a way to correct to a common reference.
Using the correction, we could explain all sources of systematic errors that could be reduced to less than $\pm$15% for mode linewidths and heights, and less than $\pm$5% for amplitude, when compared to a reference value. The effect of a different stellar background will provide frequency-dependent systematic errors that might affect the comparison with theoretical mode linewidth and mode height, therefore affecting the understanding of the physical nature of these parameters. All other sources of relative systematic errors are independent of frequency. We provide also the dependence of the so-called linewidth depression as a function of effective temperature. We show that the depth of the depression decreases with effective temperature. The dependence of the dip on effective temperature may imply that the mixing length parameter $\alpha$ may increase with effective temperature.

Rotation periods and ages of solar analogs and solar twins revealed by the Kepler mission

Jr. J.-D. do Nascimento, R. A. Garcia, S. Mathur, F. Anthony, S. A. Barnes, S. Meibom, J.S. da Costa, M. Castro, D. Salabert, T. Ceillier.
Full text
NASA ADS
DOI

A new sample of solar analogs and twin candidates have been constructed and studied, with particular attention to their light curves from NASA's Kepler mission. This letter aims to assess the evolutionary status, derive their rotation and ages and identify those solar analogs or solar twin candidates. We separate out the subgiants that compose a large fraction of the asteroseismic sample, and which show an increase in the average rotation period as the stars ascend the subgiant branch. The rotation periods of the dwarfs, ranging from 6 to 30 days, and averaged 19d, allow us to assess their individual evolutionary states on the main sequence, and to derive their ages using gyrochronology. These ages are found to be in agreement with a correlation coefficient of r = 0.79 with the independent asteroseismic ages, where available. As a result of this investigation, we are able to identify 34 stars as solar analogs and 22 of them as solar twin candidates

Kepler photometry of RRc stars: peculiar double-mode pulsations and period doubling

P. Moskalik, R. Smolec, K. Kolenberg, L. Molnár, D.W. Kurtz, R. Szabó, J.M. Benk, J. Nemec, M. Chadid, E. Guggenberger, C.-C. Ngeow, Y.-B. Jeon, G. Kopacki, S.M. Kanbur.
Full text

We present the analysis of four first overtone RR Lyrae (RRc) stars observed with the Kepler space telescope, based on data obtained over a time span of nearly 2.5 yr. All four RRc stars have been found to be multi-mode pulsators. The strongest secondary mode with frequency $f_2$ has an amplitude of a few mmag, $20-45$ times lower than the amplitude of the main radial mode with frequency $f_1$. The two oscillations have a period ratio of $P_2/P_1 = 0.612-0.632$ that cannot be reproduced by any two radial modes. Thus, the secondary mode must be nonradial. Secondary modes with similar period ratios have also recently been discovered in several other variables of the RRc and RRd types. These objects form a homogenous group and constitute a new class of multimode RR Lyrae pulsators, fully analogous to a similar class of multimode classical Cepheids in the Magellanic Clouds. Because a secondary mode with $P_2/P_1\!\sim\! 0.61$ is found in almost every RRc and RRd star observed from space, we argue that this form of multiperiodicity is common. In all four Kepler RRc stars studied, we find subharmonics of the secondary frequency at $\sim 1/2 f_2$ and at $\sim 3/2 f_2$. This is a signature of a period doubling of the secondary oscillation, and is the first detection of period doubling in RRc stars. The amplitudes and phases of $f_2$ and its subharmonics are variable on timescales of 10 to 200 d, depending on the star. The dominant radial modes also shows temporal variations on the same timescales, but with much smaller amplitude. In three of the four Kepler RRc stars we detect additional periodicities (beyond $f_1$, $f_2$ and its subharmonics), with amplitudes below 1 mmag. Some of these frequencies are lower than that of the radial fundamental mode, and must correspond to nonradial $g$-modes. Such modes never before have been observed in RR Lyrae variables.

Determining stellar macroturbulence using asteroseismic rotational velocities from Kepler

Amanda P. Doyle, Guy R. Davies, Barry Smalley, William J. Chaplin, Yvonne Elsworth.
Full text

The Rossiter-McLaughlin effect observed for transiting exoplanets often requires prior knowledge of the stellar sky-projected equatorial rotational velocity ($v \sin i$). This is usually provided by measuring the broadening of spectral lines, however this method has uncertainties as lines are also broadened by velocity fields in the stellar photosphere known as macroturbulence ($v_{\rm mac}$). We have estimated accurate $v \sin i$ values from asteroseismic analyses of main sequence stars observed by Kepler. The rotational frequency splittings of the detected solar-like oscillations of these stars are determined largely by the near-surface rotation. These estimates have been used to infer the $v_{\rm mac}$ values for 28 Kepler stars, and thus obtain a new calibration between $v_{\rm mac}$, effective temperature and surface gravity. As a result, $v_{\rm mac}$, and thus $v \sin i$, can now be determined with confidence for stars that do not have asteroseismic data available. We present new spectroscopic $v \sin i$ values for the WASP planet host stars, using high resolution HARPS spectra.

Asteroseismic inference on rotation, gyrochronology and planetary system dynamics of 16 Cygni

G.R.Davies, W.J.Chaplin, W.M.Farr, R.A.García, S.Mathis, T.S.Metcalfe, T.Appourchaux, S.Basu, O.Benomar, T.L.Campante, T.Ceillier, Y.Elsworth, R.Handberg, M.N.Lund, D.Salabert, D. Stello.
Full text

The solar analogs 16 Cyg A and 16 Cyg B are excellent asteroseismic targets in the \Kepler field of view and together with a red dwarf and a Jovian planet form an interesting system. The dynamics of this system have been studied previously but currently there is no reliable information on the rotation of the stellar components. Here, we use the technique of asteroseimology to determine the period of rotation and the angle of inclination for 16 Cyg A and B. We use the results on rotational period to demonstrate that asteroseismic analyses can provide constraints on mass-period-age relations. We perform a Bayesian model comparison between two gyrochronology relations that produces a decisive Bayes factor. Furthermore, with the excellent constraints available on mass, age, and rotational period we suggest that 16 Cyg A could be used in addition to the Sun as an anchor when calibrating gyrochronology relations. In addition, the results for the angle of inclination in 16 Cyg B do not contradict a low obliquity between the star and its eccentric planet. Given that the time scale to reach a low obliquity due to tidal interactions is much greater than the age of the system, we discuss the star-planet eccentricity and low obliquity which is consistent with Kozai cycling.

Determination of Three-dimensional Spin–orbit Angle with Joint Analysis of Asteroseismology, Transit Lightcurve, and the Rossiter–McLaughlin Effect: Cases of HAT-P-7 and Kepler-25

Othman Benomar, Kento Masuda, Hiromoto Shibahashi, Yasushi Suto.
Full text

We develop a detailed methodology of determining three-dimensionally the angle between the stellar spin and the planetary orbit axis vectors, $\psi$, for transiting planetary systems. The determination of $\psi$ requires the independent estimates of the inclination angles of the stellar spin axis and of the planetary orbital axis with respect to the line-of-sight, $i_\star$ and $i_{\rm orb}$,and the projection of the spin–orbit angle onto the plane of the sky, $\lambda$. These are mainly derived from asteroseismology, transit lightcurve and the Rossiter-McLaughlin effect, respectively. The detailed joint analysis of those three datasets enables an accurate and precise determination of the numerous parameters characterizing the planetary system, in addition to $\psi$.
We demonstrate the power of the joint analysis for the two specific systems, HAT-P-7 and Kepler-25. HAT-P-7b is the first exoplanet with significant misalignment $\lambda \approx 180^\circ$, and has been suspected as a retrograde (or polar) planet. Our joint analysis yields $\psi \approx 120^\circ$ with $i_\star \approx 30$ suggesting that its orbit is closer to polar rather than retrograde. Kepler-25 is one of the few multi-transiting planetary systems with measured $\lambda$, and hosts two short-period transiting planets and one outer non-transiting planet. The projected spin–orbit angle of the larger transiting planet, Kepler-25c, has been measured to be $\lambda \approx 0^\circ$, implying that the system is well-aligned. Due to the tight constraint from asteroseismology, we obtain $i_\star=65.4^{+10.6}_{-6.4}$ and $\psi=26.9^{+7.0}_{-9.2}$ to find that the system is actually mildly misaligned. This is the first detection of the spin–orbit misalignment for the multiple planetary system with a main-sequence host star, and points to mechanisms that tilt a stellar spin axis relative to its protoplanetary disk.

Asteroseismic measurement of surface-to-core rotation in a main sequence A star, KIC 11145123

Donald W. Kurtz, Hideyuki Saio, Masao Takata, Hiromoto Shibahashi, Simon J. Murphy, Takashi Sekii.
Full text
NASA ADS
DOI

We have discovered rotationally split core g-mode triplets and surface p-mode triplets and quintuplets in a terminal age main sequence A star, KIC 11145123, that shows both $\delta$ Sct p-mode pulsations and $\gamma$ Dor g-mode pulsations. This gives the first robust determination of the rotation of the deep core and surface of a main sequence star, essentially model-independently. We find its rotation to be nearly uniform with a period near 100 d, but we show with high confidence that the surface rotates slightly faster than the core. A strong angular momentum transfer mechanism must be operating to produce the nearly rigid rotation, and a mechanism other than viscosity must be operating to produce a more rapidly rotating surface than core. Our asteroseismic result, along with previous asteroseismic constraints on internal rotation in some B stars, and measurements of internal rotation in some subgiant, giant and white dwarf stars, has made angular momentum transport in stars throughout their lifetimes an observational science.

Kepler detection of a new extreme planetary system orbiting the subdwarf-B pulsator KIC 10001893

R. Silvotti, S. Charpinet, E. Green, G. Fontaine, J. H. Telting, R. H. stensen, V. Van Grootel, A. S. Baran, S. Schuh, L. Fox Machado.
Full text
NASA ADS
DOI

KIC 10001893 is one out of 19 subdwarf B (sdB) pulsators observed by the \kep spacecraft in its K1 phase. In addition to tens of pulsation frequencies in the $g$-mode domain, its Fourier spectrum shows three weak peaks at very low frequencies, too low to be explained in terms of $g$-modes. The most convincing explanation is that we are seeing the orbital modulation of three Earth-size planets (or planetary remnants) very close to their parent star, that are illuminated by the strong stellar radiation. The orbital periods are P$_1$=5.273, P$_2$=7.807 and P$_3$=19.48 hours and the period ratios P$_2$/P$_1$=1.481 and P$_3$/P$_2$=2.495 are very close to the 3:2 and 5:2 resonance respectively. One of the main pulsation modes of the star at 210.68 \muHz corresponds to the third harmonic of the orbital frequency of the inner planet, suggesting that we see, for the first time in an sdB star, g-mode pulsations tidally excited by a planetary companion. The extreme planetary system that emerge from the \kep data is very similar to the recent discovery of two Earth-size planets orbiting the sdB pulsator KIC 05807616 (Charpinet et al. 2011).

Mixed modes in red giants: a window on stellar evolution

B. Mosser, O. Benomar, K. Belkacem, M.J. Goupil, N. Lagarde, E. Michel, Y. Lebreton, D. Stello, M. Vrard, C. Barban, T.R. Bedding, W.J. Chaplin, S. Deheuvels, J. De Ridder, Y. Elsworth, J. Montalban, A. Noels and 4 coauthors.
Full text

Determining stellar ages is a central issue in astrophysics because it directly or indirectly impacts all fields, from deciphering exoplanetary structure and evolution, through reconstructing the dynamical and chemical evolution of galactic structures, to setting a lower limit to the age of the Universe. However, measuring ages is challenging. Even determining stellar evolutionary stages and characterizing the transitions between them has been difficult and often impossible for field stars, because there was no observable that would enable us to do so. The situation has dramatically changed with the space-borne missions CoRoT and Kepler. Here, we report seismic observations that directly probe the physical conditions inside the core of low-mass stars, during their evolution from the main sequence to the red giant branch, the commencement of core helium burning, and the asymptotic giant branch. With the detection of global gravity modes for hundreds of stars observed by the Kepler mission, we are able to identify all key transitions between the different stages of stellar evolution. For the first time, we have unambiguous markers of stellar evolution, indicating the end of the subgiant and helium-core burning stages. We also distinguish whether a star is on the red or starting to ascend the asymptotic giant branch. The precise determination of the evolutionary stage during their core helium phase opens new opportunities for the use of red clump stars as accurate distance indicators.

Non-radial oscillations in M-giant semi-regular variables: Stellar models and Kepler observations

Dennis Stello, Douglas L. Compton, Timothy R. Bedding, Jrgen Christensen-Dalsgaard, Laszlo L. Kiss, Hans Kjeldsen, Beau Bellamy, Rafael A. García, Savita Mathur.
Full text

The success of asteroseismology relies heavily on our ability to identify the frequency patterns of stellar oscillation modes. For stars like the Sun this is relatively easy because the mode frequencies follow a regular pattern described by a well-founded asymptotic relation. When a solar like star evolves off the main sequence and onto the red giant branch its structure changes dramatically resulting in changes in the frequency pattern of the modes. We follow the evolution of the adiabatic frequency pattern from the main sequence to near the tip of the red giant branch for a series of models. We find a significant departure from the asymptotic relation for the non-radial modes near the red giant branch tip, resulting in a triplet frequency pattern. To support our investigation we analyze almost four years of Kepler data of the most luminous stars in the field (late K and early M type) and find that their frequency spectra indeed show a triplet pattern dominated by dipole modes even for the most luminous stars in our sample. Our identification explains previous results from ground-based observations reporting fine structure in the Petersen diagram and sub ridges in the period-luminosity diagram. Finally, we find `new ridges' of non-radial modes with frequencies below the fundamental mode in our model calculations, and we speculate they are related to f modes.

The connection between stellar granulation and oscillation as seen by the Kepler mission

T. Kallinger, J. De Ridder, S. Hekker, S. Mathur, B. Mosser, M. Gruberbauer, R.A. Garcia, C. Karoff, J. Ballot.
Full text

The long and almost continuous observations by Kepler show clear evidence for a granulation background signal in a large sample of stars, which is interpreted to be the surface manifestation of convection. It has been shown that its characteristic time scale and rms intensity fluctuation scale with the peak frequency (\num ) of the solar-like oscillations. Various attempts have been made to quantify the observed signal, to determine scaling relations for its characteristic parameters, and to compare them to theoretical predictions. Even though consistent on a global scale, large systematic differences of unknown origin remain between different methods as well as between the observations and simulations. We aim to study different approaches to quantify the signature of stellar granulation and search for a unified model that reproduces the observed signal best in a large variety of stars. We then aim to define empirical scaling relations between the granulation properties and \num and various other stellar parameters. We use a probabilistic method to compare different approaches to extract the granulation signal. We fit the power density spectra of a large set of Kepler targets, determine the granulation and global oscillation parameter, and quantify scaling relations between them. We establish that a depression in power at about \num /2, known from the Sun and a few other main-sequence stars, is also statistically significant in red giants, and that a super-Lorentzian function with two components is suited best to reproduce the granulation signal in the broader vicinity of the pulsation power excess. We also establish that the specific choice of the background model can impact the determination of \num , introducing systematic uncertainties that can significantly exceed the random uncertainties. We find the characteristic frequency (i.e., inverse time scale) and amplitude of both background components to tightly scale with \num for a large variety of stars (about 2-2000\mh in \num ), and quantify a mass dependency of the latter. To enable comparison with theoretical predictions (which do not include the observed power depression) we compute effective time scales and bolometric intensity fluctuations and find them to approximately scale as $\tau_\mathrm{eff} \propto g^{-0.86}$ and $A_\mathrm{gran} \propto (g^2M)^{-1/4}$, respectively. Similarly, the bolometric pulsation amplitude approximately scales as $A_\mathrm{puls} \propto (g^2M)^{-1/3}$, which implicitly verifies a separate mass and luminosity dependence of $A_\mathrm{puls}$. We also check our scaling relations with solar reference values and find them in good agreement. We provide a thorough analysis of the granulation background signal in a large sample of stars, from which we establish a unified model that allows us to accurately extract the granulation and global oscillation parameter. The resulting scaling relations allow a simple estimate of the overall spectral shape of any solar-type oscillator and might serve as a starting point for future large-sample studies or as a reference for theoretical modelling of granulation.

HD183648: a Kepler eclipsing binary with anomalous ellipsoidal variations and a pulsating component

T. Borkovits, A. Derekas, J. Fuller, Gy. M. Szabó, K. Pavlovski, B. Csák, Á. Dózsa, J. Kovács, R. Szabó, K. M. Hambleton, K. Kinemuchi, V. Kolbas, D. W. Kurtz, F. Maloney, A. Pra, J. Southworth, J. Sztakovics and 2 coauthors.
Full text
NASA ADS
DOI

KIC 8560861 (HD 183648) is a marginally eccentric ($e=0.05$) eclipsing binary with an orbital period of $P_\mathrm{orb}=31.973$ d, exhibiting mmag amplitude pulsations on time scales of a few days. We present the results of the complex analysis of high and medium-resolution spectroscopic data and Kepler Q0 – Q16 long cadence photometry. The iterative combination of spectral disentangling, atmospheric analysis, radial velocity and eclipse timing variation studies, separation of pulsational features of the light curve, and binary light curve analysis led to the accurate determination of the fundamental stellar parameters. We found that the binary is composed of two main sequence stars with an age of $0.9\pm0.2$ Gyr, having masses, radii and temperatures of $M_1=1.93\pm0.12$ M$_{\odot}$, $R_1=3.30\pm0.07$ R$_{\odot}$, $T_\mathrm{eff1}=7650\pm100$ K for the primary, and $M_2=1.06\pm0.08$ M$_{\odot}$, $R_2=1.11\pm0.03$ R$_{\odot}$, $T_\mathrm{eff2}=6450\pm100$ K for the secondary. After subtracting the binary model, we found three independent frequencies, two of which are separated by twice the orbital frequency. We also found an enigmatic half orbital period sinusoidal variation that we attribute to an anomalous ellipsoidal effect. Both of these observations indicate that tidal effects are strongly influencing the luminosity variations of HD 183648. The analysis of the eclipse timing variations revealed both a parabolic trend, and apsidal motion with a period of $P_\mathrm{apse}^\mathrm{obs}=10\,400\pm3\,000$ y, which is three times faster than what is theoretically expected. These findings might indicate the presence of a distant, unseen companion.

Properties of 42 Solar-type Kepler Targets from the Asteroseismic Modeling Portal

T. S. Metcalfe, O. L. Creevey, G. Doan, S. Mathur, H. Xu, T. R. Bedding, W. J. Chaplin, J. Christensen-Dalsgaard, C. Karoff, R. Trampedach, O. Benomar, B. P. Brown, D. L. Buzasi, T. L. Campante, Z. elik, M. S. Cunha, G. R. Davies and 25 coauthors.
Full text
NASA ADS
DOI

Recently the number of main-sequence and subgiant stars exhibiting solar-like oscillations that are resolved into individual mode frequencies has increased dramatically. While only a few such data sets were available for detailed modeling just a decade ago, the Kepler mission has produced suitable observations for hundreds of new targets. This rapid expansion in observational capacity has been accompanied by a shift in analysis and modeling strategies to yield uniform sets of derived stellar properties more quickly and easily. We use previously published asteroseismic and spectroscopic data sets to provide a uniform analysis of 42 solar-type Kepler targets from the Asteroseismic Modeling Portal (AMP). We find that fitting the individual frequencies typically doubles the precision of the asteroseismic radius, mass and age compared to grid-based modeling of the global oscillation properties, and improves the precision of the radius and mass by about a factor of three over empirical scaling relations. We demonstrate the utility of the derived properties with several applications.

Automated preparation of Kepler time series of planet hosts for asteroseismic analysis

R. Handberg, M. N. Lund.
Full text
NASA ADS
DOI

One of the tasks of the Kepler Asteroseismic Science Operations Center (KASOC) is to provide asteroseismic analyses on Kepler Objects of Interest (KOIs). However, asteroseismic analysis of planetary host stars presents some unique complications with respect to data preprocessing, compared to pure asteroseismic targets. If not accounted for, the presence of planetary transits in the photometric time series often greatly complicates or even hinders these asteroseismic analyses. This drives the need for specialised methods of preprocessing data to make them suitable for asteroseismic analysis. In this paper we present the KASOC Filter, which is used to automatically prepare data from the Kepler/K2 mission for asteroseismic analyses of solar-like planet host stars. The methods are very effective at removing unwanted signals of both instrumental and planetary origins and produce significantly cleaner photometric time series than the original data. The methods are automated and can therefore easily be applied to a large number of stars. The application of the filter is not restricted to planetary hosts, but can be applied to any solar-like or red giant stars observed by Kepler/K2.

Long time-scale behavior of the Blazhko effect from rectified Kepler data

J.M. Benk, E. Plachy, R. Szabó, L. Molnár, Z. Kolláth.
Full text
NASA ADS
DOI

In order to benefit from the 4-year unprecedented precision of the Kepler data, we extracted light curves from the pixel photometric data of the Kepler space telescope for 15 Blazhko RR Lyrae stars. For collecting all the flux from a given target as accurately as possible, we defined tailor-made apertures for each star and quarter. In some cases the aperture finding process yielded sub-optimal result, because some flux have been lost even if the aperture contains all available pixels around the star. This fact stresses the importance of those methods that rely on the whole light curve instead of focusing on the extrema (O$-$C diagrams and other amplitude independent methods). We carried out detailed Fourier analysis of the light curves and the amplitude independent O$-$C diagram. We found 12 (80%) multiperiodically modulated stars in our sample. This ratio is much higher than previously found. Resonant coupling between radial modes, a recent theory to explain of the Blazhko effect, allows single, multiperiodic or even chaotic modulations. Among the stars with two modulations we found three stars (V355 Lyr, V366 Lyr and V450 Lyr) where one of the periods dominate in amplitude modulation, but the other period has larger frequency modulation amplitude. The ratio between the primary and secondary modulation periods is almost always very close to ratios of small integer numbers. It may indicate the effect of undiscovered resonances. Furthermore, we detected the excitation of the second radial overtone mode $f_2$ for three stars where this feature was formerly unknown. Our data set comprises the longest continuous, most precise observations of Blazhko RR Lyrae stars ever published. These data which is made publicly available will be unprecedented for years to come.

KIC 4544587: an Eccentric, Short Period Binary System with delta Scuti Pulsations and Tidally Excited Modes

K. Hambleton, D. W. Kurtz, A. Pra, J. A. Guzik, K. Pavlovski, S. Bloemen, J. Southworth, K. Conroy, S. P. Littlefair, J. Fuller.
Full text

We present Kepler photometry and ground based spectroscopy of KIC 4544587, a short-period eccentric eclipsing binary system with self-excited pressure and gravity modes, tidally excited modes, tidally influenced p modes, and rapid apsidal motion of 182 y per cycle. The primary and secondary components of KIC 4544587 reside within the delta Scuti and gamma Dor instability region of the Hurtzsprung-Russell diagram, respectively. By applying the binary modelling software phoebe to prewhitened Kepler photometric data and radial velocity data obtained using the William Herschel Telescope and 4-m Mayall telescope at KPNO, the fundamental parameters of this important system have been determined, including the stellar masses, 1.98 $\pm$0.07 Msun and 1.60 $\pm$ 0.06 Msun, and radii, 1.76 $\pm$ 0.03 Rsun and 1.42 $\pm$ 0.02 Rsun, for the primary and secondary components, respectively. Frequency analysis of the residual data revealed 31 modes, 14 in the gravity mode region and 17 in the pressure mode region. Of the 14 gravity modes 8 are orbital harmonics: a signature of tidal resonance. While the measured amplitude of these modes may be partially attributed to residual signal from binary model subtraction, we demonstrate through consideration of the folded light curve that these frequencies do infact correspond to tidally excited pulsations. Furthermore, we present an echelle diagram of the pressure mode frequency region (modulo the orbital frequency) and demonstrate that the tides are also influencing the p modes. A first look at asteroseismology hints that the secondary component is responsible for the p modes, which is contrary to our expectation that the hotter star should pulsate in higher radial overtone, higher frequency p modes.

Stroemgren for Asteroseismology and Galactic Archaeology: let the SAGA begin

L. Casagrande, V. Silva Aguirre, D. Stello, D. Huber, A.M. Serenelli, S. Cassisi, A. Dotter, A. Milone, S. Hodgkin, A.F. Marino, M.N. Lund, A. Pietrinferni, M. Asplund, S. Feltzing, C. Flynn, F. Grundahl, P.E. Nissen and 3 coauthors.
Full text
NASA ADS
DOI

Asteroseismology has the capability of precisely determining stellar properties which would otherwise be inaccessible, such as radii, masses and thus ages of stars. When coupling this information with classical determinations of stellar parameters, such as metallicities, effective temperatures and angular diameters, powerful new diagnostics for Galactic studies can be obtained. The ongoing Stroemgren survey for Asteroseismology and Galactic Archaeology (SAGA) has the goal of transforming the Kepler field into a new benchmark for Galactic studies, similarly to the solar neighborhood. Here we present first results from a stripe centred at Galactic longitude 74deg and covering latitude from about 8 to 20deg, which includes almost 1000 K-giants with seismic information and the benchmark open cluster NGC 6819. We describe the coupling of classical and seismic parameters, the accuracy as well as the caveats of the derived effective temperatures, metallicities, distances, surface gravities, masses, and radii. Confidence in the achieved precision is corroborated by the detection of the first and secondary clump in a population of field stars with a ratio of 2 to 1, and by the negligible scatter in the seismic distances among NGC 6819 member stars. An assessment of the reliability of stellar parameters in the Kepler Input Catalogue is also performed, and the impact of our results for population studies in the Milky Way is discussed, along with the importance of an all-sky Stroemgren survey.

Old puzzle, new insights: a lithium rich giant quietly burning helium in its core

V. Silva Aguirre, G.R. Ruchti, S. Hekker, S. Cassisi, J. Christensen-Dalsgaard, A. Datta, A. Jendreieck, J. Jessen-Hansen, A. Mazumdar, B. Mosser, D. Stello, P.G. Beck, J. de Ridder.
Full text

About 1% of giant stars have been shown to have large surface Li abundances, which is unexpected according to standard stellar evolution models. Several scenarios for lithium production have been proposed, but it is still unclear why these Li-rich giants exist. A missing piece in this puzzle is the knowledge of the exact stage of evolution of these stars. Using low-and-high-resolution spectroscopic observations, we have undertaken a survey of lithium-rich giants in the Kepler field. In this letter, we report the finding of the first confirmed Li-rich core-helium-burning giant, as revealed by asteroseismic analysis. The evolutionary timescales constrained by its mass suggest that Li-production most likely took place through non-canonical mixing at the RGB-tip, possibly during the helium flash.

CHARACTERIZING TWO SOLAR-TYPE KEPLER SUBGIANTS WITH ASTEROSEISMOLOGY: KIC 10920273 AND KIC 11395018

G. DOAN, T. S. METCALFE, S. DEHEUVELS, M. P. DI MAURO, P. EGGENBERGER, O. L. CREEVEY, M. J. P. F. G. MONTEIRO, M. PINSONNEAULT, A. FRASCA, C. KAROFF, S. MATHUR, S. G. SOUSA, I. M. BRANDÃO, T. L. CAMPANTE, R. HANDBERG, A.O. THYGESEN, K. BIAZZO and 13 coauthors.
Full text

Determining fundamental properties of stars through stellar modeling has improved substantially due to recent advances in asteroseismology. Thanks to the unprecedented data quality obtained by space missions, particularly CoRoT and Kepler, invaluable information is extracted from the high-precision stellar oscillation frequencies, which provide very strong constraints on possible stellar models for a given set of classical observations. In this work, we have characterized two relatively faint stars, KIC 10920273 and KIC 11395018, using oscillation data from Kepler photometry and atmospheric constraints from ground-based spectroscopy. Both stars have very similar atmospheric properties; however, using the individual frequencies extracted from the Kepler data, we have determined quite distinct global properties, with increased precision compared to that of earlier results. We found that both stars have left the main sequence and characterized them as follows: KIC 10920273 is a one-solar-mass star ($M=1.00\pm0.04M_{\odot}$), but much older than our Sun ($\tau=7.12\pm0.47$ Gyr), while KIC 11395018 is significantly more massive than the Sun ($M=1.27\pm0.04M_{\odot}$) with an age close to that of the Sun ($\tau=4.57\pm0.23$ Gyr). We confirm that the high lithium abundance reported for these stars should not be considered to represent young ages, as we precisely determined them to be evolved subgiants. We discuss the use of surface lithium abundance, rotation and activity relations as potential age diagnostics.

The historical vanishing of the Blazhko effect of RR Lyr from GEOS and Kepler surveys

J. F. Le Borgne, E. Poretti, A. Klotz, E. Denoux, H. A. Smith, K. Kolenberg, R. Szabo, S. Bryson, and other 13 authors.
Full text

RR Lyr is one of the most studied variable stars. Its light curve has been regularly monitored since the discovery of the periodic variability in 1899. Analysis of all observed maxima allows us to identify two primary pulsation states defined as pulsation over a “long” primary pulsation period (longer than 0.56684 d) and over a “short” one (shorter than 0.56682 d). These states alternate with intervals of 13-16 yr, and are well defined after 1943. The 40.8-d periodical modulations of the amplitude and the period (Blazhko effect) were noticed in 1916. We provide homogeneous determinations of the Blazhko period in the different primary pulsation states and we establish how it suddenly diminished to around 39.0 d in 1975. The monitoring of these periodicities deserved and deserves a continuous and intensive observational effort. For this purpose we have built dedicated, transportable, and autonomous small instruments (Very Tiny Telescopes, VTTs) to observe the times of maximum brightness of RR Lyr. As immediate results the VTTs recorded the last change of $P_0$ state in mid-2009 and extended the time coverage of the Kepler observations, thus recording a maximum O-C amplitude of the Blazhko effect in 2008, followed by the historical smallest O -C amplitude in late 2013. This phase is still ongoing and VTT instruments are ready to monitor the expected increase in the next few years.

Evidence of resonant mode coupling and the relationship between low and high frequencies in a rapidly rotating A star

M. Breger, M. Montgomery.
Full text

In the theory of resonant mode coupling, the parent and child modes are directly related in frequency and phase. The oscillations present in the fast rotating Delta Scuti star KIC 8054146 allow us to test the most general and generic aspects of such a theory. The only direct way to separate the parent and coupled (child) modes is to examine the correlations in amplitude variability between the different frequencies. For the dominant family of related frequencies, only a single mode and a triplet are the origins of nine dominant frequency peaks ranging from 2.93 to 66.30 cycles per day (as well as dozens of small-amplitude combination modes and a predicted and detected third high-frequency triplet).
The mode-coupling model correctly predicts the large amplitude variations of the coupled modes as a product of the amplitudes of the parent modes, while the phase changes are also correctly modeled. This differs from the behavior of "normal" combination frequencies in that the amplitudes are three orders of magnitude larger and may exceed even the amplitudes of the parent modes.
We show that two dominant low frequencies at 5.86 and 2.93 cycles per day in the gravity-mode region are not harmonics of each other, and their properties follow those of the almost equidistant high-frequency triplet. We note that the previously puzzling situation of finding two strong peaks in the low-frequency region related by nearly a factor of two in frequency has been seen in other Delta Scuti stars as well.

Asteroseismic Study on Cluster Distance Moduli for Red Giant Branch Stars in NGC 6791 and NGC 6819

T. Wu, Y. Li, and S. Hekker.
Full text
NASA ADS
DOI

Stellar distance is an important basic parameter in stellar astrophysics. Stars in a cluster are thought to be formed coevally from the same interstellar cloud of gas and dust. They are therefore expected to have common properties. These common properties strengthen our ability to constrain theoretical models and/or to determine fundamental parameters, such as stellar mass, metal fraction, and distance when tested against an ensemble of cluster stars. Here we derive a new relation based on solar-like oscillations, photometric observations, and the theory of stellar structure and evolution of red giant branch stars to determine cluster distance moduli through the global oscillation parameters \dnu and \numax and photometric data V. The values of \dnu and \numax are derived from \kepler observations. At the same time, it is used to interpret the trends between V and \dnu. From the analyses of this newly derived relation and observational data of NGC 6791 and NGC 6819 we devise a method in which all stars in a cluster are regarded as one entity to determine the cluster distance modulus. This approach fully reflects the characteristic of member stars in a cluster as a natural sample. From this method we derive true distance moduli of $13.09\pm0.10$ mag for NGC 6791 and $11.88\pm0.14$ mag for NGC 6819. Additionally, we find that the distance modulus only slightly depends on the metallicity [Fe/H] in the new relation. A change of 0.1 dex in [Fe/H] will lead to a change of 0.06 mag in the distance modulus.

Pulsating red giant stars in eccentric binary systems discovered from Kepler space-based photometry - A sample study and the analysis of KIC 5006817

P. G. Beck, K. Hambleton, J. Vos, T. Kallinger, S. Bloemen, A. Tkachenko, R. A. García, R. H. stensen, C. Aerts, D. W. Kurtz, J. De Ridder, S. Hekker, K. Pavlovski, S. Mathur, K. De Smedt, A. Derekas, E. Corsaro and 16 coauthors.
Full text
NASA ADS
DOI

The unparalleled photometric data obtained by NASA's Kepler space telescope led to an improved understanding of red giant stars and binary stars. Seismology allows us to constrain the properties of red giants. In addition to eclipsing binaries, eccentric non-eclipsing binaries, exhibiting ellipsoidal modulations, have been detected with Kepler.
We aim to study the properties of eccentric binary systems containing a red giant star and derive the parameters of the primary giant component.
We apply asteroseismic techniques to determine masses and radii of the primary component of each system. For a selected target, light and radial velocity curve modelling techniques are applied to extract the parameters of the system and its primary component. Stellar evolution and its effects on the evolution of the binary system are studied from theoretical models.
The paper presents the asteroseismic analysis of 18 pulsating red giants in eccentric binary systems, for which masses and radii were constrained. The orbital periods of these systems range from 20 to 440 days. The results of our ongoing radial velocity monitoring program with the HERMES spectrograph reveal an eccentricity range of $e$ = 0.2 to 0.76. As a case study we present a detailed analysis of KIC 5006817, whose rich oscillation spectrum allows for a detailed seismic analysis. From seismology we constrain the rotational period of the envelope to be at least 165 d, which is roughly twice the orbital period. The stellar core rotates 13 times faster than the surface. From the spectrum and radial velocities we expect that the Doppler beaming signal should have a maximum amplitude of 300 ppm in the light curve. Fixing the mass and radius to the asteroseismically determined values, from our binary modelling we find a value of the gravity darkening exponent that is significantly larger than expected. Through binary modelling, we determine the mass of the secondary component to be 0.29$\pm$0.03 $M$\sun.
For KIC 5006817 we exclude pseudo-synchronous rotation of the red giant with the orbit. The comparison of the results from seismology and modelling of the light curve shows a possible alignment of the rotational and orbital axis at the 2$\sigma$ level. Red giant eccentric systems could be progenitors of cataclysmic variables and hot subdwarf B stars.

Seismic constraints on the radial dependence of the internal rotation profiles of six Kepler subgiants and young red giants

S. Deheuvels, G. Doan, M. J. Goupil, T. Appourchaux, O. Benomar, H. Bruntt, T. L. Campante, L. Casagrande, T. Ceillier, G.  R. Davies, P. de Cat, J. N. Fu, R. A. García, A. Lobel, B. Mosser, D. R. Reese, C. Regulo and 11 coauthors.
Full text

Context: We still do not understand which physical mechanisms are responsible for the transport of angular momentum inside stars. The recent detection of mixed modes that contain the clear signature of rotation in the spectra of kepler subgiants and red giants gives us the opportunity to make progress on this issue.
Aims: Our aim is to probe the radial dependance of the rotation profiles for a sample of kepler targets. For this purpose, subgiants and early red giants are particularly interesting targets because their rotational splittings are more sensitive to the rotation outside the deeper core than is the case for their more evolved counterparts.
Method: We first extract the rotational splittings and frequencies of the modes for six young kepler red giants. We then perform a seismic modeling of these stars using the evolutionary codes Cesam2k and ASTEC. By using the observed splittings and the rotational kernels of the optimal models, we perform inversions of the internal rotation profiles of the six stars.
Results: We obtain estimates of the core rotation rates for these stars, and upper limits to the rotation in their convective envelope. We show that the rotation contrast between the core and the envelope increases during the subgiant branch. Our results also suggest that the core of subgiants spins up with time, while their envelope spins down. For two of the stars, we show that a discontinuous rotation profile with a deep discontinuity reproduces the observed splittings significantly better than a smooth rotation profile. Interestingly, the depths that are found most probable for the discontinuities roughly coincide with the location of the H-burning shell, which separates the layers that contract from those that expand.
Conclusions: We characterized the differential rotation pattern of six young giants with a range of metallicities, and with both radiative and convective cores on the main sequence. This will bring observational constraints to the scenarios of angular momentum transport in stars. Also, if the existence of sharp gradients in the rotation profiles of young red giants is confirmed, it should help discriminate between the physical processes that could transport angular momentum in the subgiant and red giant branches.

Study of KIC 8561221 observed by Kepler: an early red giant showing depressed dipolar modes

R. A. García, F. Pérez Hernández, O. Benomar, V. Silva Aguirre, J. Ballot, G. R. Davies, G. Dogan, D. Stello, J. Christensen-Dalsgaard, G. Houdek, F. Lignières, S. Mathur, M. Takata, T. Ceillier, W. J. Chaplin, S. Mathis, B. Mosser and 8 coauthors.
Full text
NASA ADS
DOI

The continuous high-precision photometric observations provided by the CoRoT and Kepler space missions have allowed us to better understand the structure and dynamics of red giants using asteroseismic techniques. A small fraction of these stars shows dipole modes with unexpectedly low amplitudes. The reduction in amplitude is more pronounced for stars with higher frequency of maximum power, nu_max. In this work we want to characterize KIC 8561221 in order to confirm that it is currently the least evolved star among this peculiar subset and to discuss several hypotheses that could help explain the reduction of the dipole mode amplitudes. We used Kepler short- and long-cadence data combined with spectroscopic observations to infer the stellar structure and dynamics of KIC 8561221. We then discussed different scenarios that could contribute to the reduction of the dipole amplitudes such as a fast rotating interior or the effect of a magnetic field on the properties of the modes. We also performed a detailed study of the inertia and damping of the modes. We have been able to characterize 37 oscillations modes, in particular, a few dipole modes above nu_max that exhibit nearly normal amplitudes. The frequencies of all the measured modes were used to determine the global properties and the internal structure of the star. We have inferred a surface rotation period of around 91 days and uncovered the existence of a variation in the surface magnetic activity during the last 4 years. The analysis of the convective background did not reveal any difference compared to “normal” red giants. As expected, the internal regions of the star probed by the l = 2 and 3 modes spin 4 to 8 times faster than the surface. With our grid of standard models we are able to properly fit the observed frequencies. Our model calculation of mode inertia and damping give no explanation for the depressed dipole modes. A fast rotating core is also ruled out as a possible explanation. Finally, we do not have any observational evidence of the presence of a strong deep magnetic field inside the star.

Atmospheric parameters and pulsational properties for a sample of δ Sct, γ Dor, and hybrid Kepler targets

G. Catanzaro, V. Ripepi, S. Bernabei, M. Marconi, L. Balona, D. W. Kurtz, B. Smalley, H. Bruntt, R. Szabó, A. Grigahcène, M. J. P. F. G. Monteiro, J.C. Suárez, K. Uytterhoeven, W. J. Borucki, D. G. Koch.
Full text

We report spectroscopic observations for 19 $\delta$ Sct candidates observed by the Kepler satellite both in long and short cadence mode. For all these stars, by using spectral synthesis, we derive the effective temperature, the surface gravity and the projected rotational velocity. An equivalent spectral type classification has been also performed for all stars in the sample. These determinations are fundamental for modelling the frequency spectra that will be extracted from the Kepler data for asteroseismic inference. For all the 19 stars, we present also periodograms obtained from Kepler data. We find that all stars show peaks in both low- ($\gamma$ Dor; g mode) and high-frequency ($\delta$ Sct; p mode) regions. Using the amplitudes and considering 5 c/d as a boundary frequency, we classified 3 stars as pure $\gamma$ Dor, 4 as $\gamma$ Dor - $\delta$ Sct hybrid, 5 as $\delta$ Sct - $\gamma$ Dor hybrid, and 6 as pure $\delta$ Sct. The only exception is the star KIC05296877 which we suggest could be a binary.

Measurement of acoustic glitches in solar-type stars from oscillation frequencies observed by Kepler

A. Mazumdar, M. J. P. F. G. Monteiro, J. Ballot, H. M. Antia, S. Basu, G. Houdek, S. Mathur, M. S. Cunha, V. Silva Aguirre, R. A. Garca, D. Salabert, G. A. Verner, J. Christensen-Dalsgaard, T. S. Metcalfe, D. T. Sanderfer, S. E. Seader, J. C. Smith and 1 coauthors.
Full text
NASA ADS
DOI

For the very best and brightest asteroseismic solar-type targets observed by Kepler, the frequency precision is sufficient to determine the acoustic depths of the surface convective layer and the helium ionization zone. Such sharp features inside the acoustic cavity of the star, which we call acoustic glitches, create small oscillatory deviations from the uniform spacing of frequencies in a sequence of oscillation modes with the same spherical harmonic degree. We use these oscillatory signals to determine the acoustic locations of such features in 19 solar-type stars observed by the Kepler mission. Four independent groups of researchers utilized the oscillation frequencies themselves, the second differences of the frequencies and the ratio of the small and large separation to locate the base of the convection zone and the second helium ionization zone. Despite the significantly different methods of analysis, good agreement was found between the results of these four groups, barring a few cases. These results also agree reasonably well with the locations of these layers in representative models of the stars. These results firmly establish the presence of the oscillatory signals in the asteroseismic data and the viability of several techniques to determine the location of acoustic glitches inside stars.

Magnetic activity of F stars observed by Kepler

S. Mathur, R.A. García, J. Ballot, T. Ceillier, D. Salabert, T. Metcalfe, C. Régulo, A. Jiménez, S. Bloemen.
Full text

The study of stellar activity is important because it can provide new constraints for dynamo models, when combined with surface rotation rates and the depth of the convection zone. We know that the dynamo mechanism, which is believed to be the main process to rule the magnetic cycle of solar-like stars at least, results from the interaction between (differential) rotation, convection, and magnetic field. The Kepler mission has already been collecting data for a large number of stars during four years allowing us to investigate magnetic stellar cycles. We investigated the Kepler light curves to look for magnetic activity or even hints of magnetic activity cycles. Based on the photometric data we also looked for new magnetic indexes to characterise the magnetic activity of the stars. We selected a sample of 22 solar-like F stars that have a rotation period smaller than 12 days. We performed a time-frequency analysis using the Morlet wavelet yielding a magnetic proxy for our sample of stars. We computed the magnetic index $S_{\rm ph}$ as the standard deviation of the whole time series and the index $<S_{\rm ph}>$ that is the mean of standard deviations measured in subseries of length five times the rotation period of the star. We defined new indicators to take into account the fact that complete magnetic cycles are not observed for all the stars, such as the contrast between high and low activity. We also inferred the Rossby number of the stars and studied their stellar background. With this analysis we obtained four categories of stars. Five of them show long-lived spots or active regions. We also observe stars that have a cycle-like behaviour. Two stars of our sample seem to have a decreasing or increasing trend in the temporal variation of the magnetic proxies. Finally the last group of stars show magnetic activity (with presence of spots) but no sign of cycle. We looked for correlation between our indexes and the stellar properties of the stars but did not find any clear correlation in our sample of F stars.

Accurate parameters of the oldest known rocky-exoplanet hosting system: Kepler-10 revisited

Alexandra Fogtmann-Schulz, Brian Hinrup, Vincent Van Eylen, Jrgen Christensen-Dalsgaard, Hans Kjeldsen, Víctor Silva Aguirre, Brandon Tingley.
Full text

Since the discovery of Kepler-10, the system has received considerable interest because it contains a small, rocky planet which orbits the star in less than a day. The system’s parameters, announced by the Kepler team and subsequently used in further research, were based on only five months of data. We have reanalyzed this system using the full span of 29 months of Kepler photometric data, and obtained improved information about its star and the planets. A detailed asteroseismic analysis of the extended time series provides a significant improvement on the stellar parameters: not only can we state that Kepler-10 is the oldest known rocky-planet-harboring system at $10.41\pm1.36$ Gyr, but these parameters combined with improved planetary parameters from new transit fits gives us the radius of Kepler-10b to within just 125 km. A new analysis of the full planetary phase curve leads to new estimates on the planetary temperature and albedo, which remain degenerate in the Kepler band. Our modeling suggests that the flux level during the occultation is slightly lower than at the transit wings, which would imply that the nightside of this planet has a non-negligible temperature.

Asteroseismology of evolved stars with Kepler: a new way to constrain stellar interiors using mode inertias

O. Benomar, K. Belkacem, T.R. Bedding, D. Stello, M.P. Di Mauro, R. Ventura, B. Mosser, M.J. Goupil, R. Samadi, R.A. Garcia.
Full text

The asteroseismology of evolved solar-like stars is experiencing growing interest due to the wealth of observational data from space-borne instruments such as the CoRoT and Kepler spacecraft. In particular, the recent detection of mixed modes, which probe both the innermost and uppermost layers of stars, paves the way for inferring the internal structure of stars along their evolution through the subgiant and red giant phases. Mixed modes can also place stringent constraints on the physics of such stars and on their global properties (mass, age, etc.). Here, using two Kepler stars (KIC 4351319 and KIC 6442183), we demonstrate that measurements of mixed mode characteristics allow us to estimate the mode inertias, providing a new and additional diagnostics on the mode trapping and subsequently on the internal structure of evolved stars. We however stress that the accuracy may be sensitive to non-adiabatic effects.

Asteroseismic fundamental properties of solar-type stars observed by the NASA Kepler Mission

W. J. Chaplin, S. Basu, D. Huber, A. Serenelli, L. Casagrande, V. Silva Aguirre, W. H. Ball, O. L. Creevey, L. Gizon, R. Handberg, C. Karoff, R. Lutz, J. P. Marques, A. Miglio, D. Stello, M. D. Suran, T. S. Metcalfe and 16 coauthors.
Full text

We use asteroseismic data obtained by the NASA Kepler Mission to estimate the fundamental properties of more than 500 main-sequence and sub-giant stars. Data obtained during the first 10 months of Kepler science operations, when these solar-type targets were observed for one month each in a survey mode, were used for this work. Stellar properties were estimated using two global asteroseismic parameters and complementary photometric and spectroscopic data. Homogeneous sets of effective temperatures, $T_{\rm eff}$, were available for the entire ensemble using available complementary photometry; spectroscopic estimates of $T_{\rm eff}$ and [Fe/H] were available from a homogeneous analysis of ground-based data on a subset totalling 89 stars. The median final uncertainties for the full ensemble (spectroscopic subset) are approximately $10.8$ % (5.5 %) in mass, 4.4 % (2.2 %) in radius, 0.017 dex (0.010 dex) in $\log\,g$, 4.3 % (2.8 %) in mean density, and about 34 % (25 %) in age.

Classifying stellar populations among 13 000 red giants observed by Kepler

Dennis Stello, Daniel Huber, Timothy R. Bedding, Othman Benomar, Lars Bildsten, Ronald L. Gilliland, Benoît Mosser, Bill Paxton, Timothy R. White.
Full text

Of the more than 150 000 targets followed by the Kepler Mission, about 10% were selected as red giants. Due to their high scientific value, in particular for Galaxy population studies and stellar structure and evolution, their Kepler light curves have been made public without a proprietary period since late 2011. More than 13 000 of these stars show intrinsic flux variability caused by solar-like oscillations making them ideal for large scale asteroseismic investigations. In a significant fraction we have been able to automatically extract individual frequencies and to measure the period spacings of the dipole modes. These measurements provide a way to classify the stars into various populations, such as the red giant branch, the low-mass ($1.0\lesssim M/$M$_\odot \lesssim 2.0$) helium-core-burning red clump, and the higher-mass ($2.0\lesssim M/$\M$_\odot \lesssim 3.0$) secondary clump. The period spacings also reveal that a large fraction of the stars show rotationally induced frequency splittings. This sample of stars provides an extremely valuable source for studying the stellar population in the direction of the Kepler field, in particular when combined with complementary spectroscopic surveys.

New asteroseismic scaling relations based on Hayashi track relation applied to red-giant branch stars in NGC 6791 and NGC 6819

Tao. Wu, Yan. Li, Saskia Hekker.
Full text
NASA ADS
DOI

Stellar mass $M$, radius $R$, and gravity $g$ are important basic parameters in stellar physics. Accurate values for these parameters can be obtained from the gravitational interaction between stars in multiple systems or from asteroseismology. Stars in a cluster are thought to be formed coevally from the same interstellar cloud of gas and dust. The cluster members are therefore expected to have some properties in common. These restrictions strengthen our ability to constrain stellar models and asteroseismically derived $M$, $R$ and $g$ when tested against an ensemble of cluster stars.
Here we derive new scaling relations based on a relation for stars on the Hayashi track ($\sqrt{T_{\rm eff}} \sim g^pR^q$) to determine the masses and metallicities of red giant branch stars in open clusters NGC 6791 and NGC 6819 from the global oscillation parameters \dnu (the large frequency separation) and \numax (frequency of maximum oscillation power). The \dnu and \numax values are derived from \kepler observations.
From the analysis of these new relations we derive: (1) direct observational evidence that the masses of red giant branch stars in a cluster are the same within their uncertainties. (2) new methods to derive $M$ and $z$ of the cluster in a self consistent way from \dnu and \numax. The resulting values for $M$ and $z$ in lower intrinsic uncertainties then previously derived scaling relations which depend on effective temperature. (3) the mass dependence in the \dnu - \numax relation for red giant branch stars.

Stellar granulation as seen in disk-integrated intensity. II. Theoretical scaling relations compared with observations

R. Samadi, K. Belkacem, H.-G. Ludwig, E. Caffau, T.L. Campante, G.R. Davies, T. Kallinger, M.N. Lund, B. Mosser, A. Baglin, S. Mathur, R. Garcia.
Full text
NASA ADS
DOI

A large set of stars observed by CoRoT and Kepler shows clear evidence for the presence of a stellar background, which is interpreted to arise from surface convection, i.e., granulation. These observations show that the characteristic time-scale ($\tau_{\rm eff}$) and the root-mean-square (rms) brightness fluctuations ($\sigma$) associated with the granulation scale as a function of the peak frequency ($\nu_{\rm max}$) of the solar-like oscillations. We aim at providing a theoretical background to the observed scaling relations based on a model developed in the companion paper (hereafter paper I). We computed for each 3D model the theoretical power density spectrum (PDS) associated with the granulation as seen in disk-integrated intensity on the basis of the theoretical model published in paper I. For each PDS we derived the associated characteristic time ($\tau_{\rm eff}$) and the rms brightness fluctuations ($\sigma$) and compared these theoretical values with the theoretical scaling relations derived from the theoretical model and the measurements made on a large set of Kepler targets. We derive theoretical scaling relations for $\tau_{\rm eff}$ and $\sigma$, which show the same dependence on $\nu_{\rm max}$ as the observed scaling relations. In addition, we show that these quantities also scale as a function of the turbulent Mach number (${\cal M}_{\rm a}$) estimated at the photosphere. The theoretical scaling relations for $\tau_{\rm eff}$ and $\sigma$ match the observations well on a global scale. Quantitatively, the remaining discrepancies with the observations are found to be much smaller than previous theoretical calculations made for red giants. Our modelling provides additional theoretical support for the observed variations of $\sigma$ and $\tau_{\rm eff}$ with $\nu_{\rm max}$. It also highlights the important role of ${\cal M}_{\rm a}$ in controlling the properties of the stellar granulation. However, the observations made with Kepler on a wide variety of stars cannot confirm the dependence of our scaling relations on ${\cal M}_{\rm a}$. Measurements of the granulation background and detections of solar-like oscillations in a statistically sufficient number of cool dwarf stars will be required for confirming the dependence of the theoretical scaling relations with ${\cal M}_{\rm a}$.

Period-luminosity relations in evolved red giants explained by solar-like oscillations

B. Mosser, W.A. Dziembowski, K. Belkacem, M.J.Goupil, E. Michel, R. Samadi, I. Soszyński, M. Vrard, Y. Elsworth, S. Hekker, S. Mathur.
Full text

Solar-like oscillations in red giants have been investigated with the space-borne missions CoRoT and Kepler, while pulsations in more evolved M giants have been studied with ground-based microlensing surveys. After 3.1 years of observation with Kepler, it is now possible to make a link between these different observations of the same type of stars.
We aim to identify period-luminosity sequences in evolved red giants and to interpret them in terms of solar-like oscillations. Then, we investigate the consequences of the comparison of ground-based and space-borne results.
We have first measured global oscillation parameters of evolved red giants observed with Kepler with the envelope autocorrelation function method. We then used an extended form of the universal red giant oscillation pattern, extrapolated to very low frequency, to fully identify their oscillations. The comparison with ground-based results was then used to obtain a relation between the large frequency separation and the stellar luminosity.
From the link between red giant oscillations observed by Kepler and period-luminosity sequences, we have identified these relations in evolved red giants as radial and non-radial solar-like oscillations. We were able to expand scaling relations at very low frequency (periods as high as 100 days, and large frequency separation less than 0.1 $\mu$Hz). This helped us to identify the different sequences of period-luminosity relations, and allowed us to propose an absolute calibration of the K magnitude with the observed frequency large separation.
Interpreting period-luminosity relations in red giants in terms of solar-like oscillations allows us to investigate, with a firm physical basis, the time series obtained from ground-based microlensing surveys. This can be done with the analytical expression that describe the low-frequency oscillation spectra. The different behavior of oscillations at low frequency, with frequency separations scaling only approximately with the square root of the mean stellar density, can be used to address precisely the physics of the most semi-regular variables. This will allow improved distance measurements and opens the way to extragalactic asteroseismology.

Metal Abundances, Radial Velocities and Other Physical Characteristics for the RR Lyrae Stars in the Kepler Field

James M. Nemec, Judith G. Cohen, Vincenzo Ripepi, Aliz Derekas, Pawel Moskalik, Branimir Sesar, Merieme Chadid, Hans Bruntt.
Full text
NASA ADS
DOI

Spectroscopic iron-to-hydrogen ratios, radial velocities, atmospheric parameters, and new photometric analyses are presented for 41 RR Lyrae stars (and one probable high-amplitude $\delta$ Sct star) located in the field-of-view of the Kepler space telescope. Thirty-seven of the RR Lyrae stars are fundamental-mode pulsators (i.e., RRab stars) of which 16 exhibit the Blazhko effect. Four of the stars are multiperiodic RRc pulsators oscillating primarily in the first-overtone mode. Spectroscopic [Fe/H] values for the 34 stars for which we were able to derive estimates range from $-2.54\pm0.13$ (NR Lyr) to $-0.05\pm0.13$ dex (V784 Cyg), and for the 19 Kepler-field non-Blazhko stars studied by Nemec et al. (2011) the abundances agree will with their photometric [Fe/H] values. Four non-Blazhko RR Lyrae stars that they identified as metal-rich (KIC 6100702, V2470 Cyg, V782 Cyg and V784 Cyg) are confirmed as such, and four additional stars (V839 Cyg, KIC 5520878, KIC 8832417, KIC 3868420) are also shown here to be metal-rich. Five of the non-Blazhko RRab stars are found to be more metal-rich than [Fe/H]$\sim -0.9$ dex while all of the 16 Blazhko stars are more metal-poor than this value. New $P$-$\phi_{\rm 31}^s$-[Fe/H] relationships are derived based on $\sim$970 days of quasi-continuous high-precison Q0-Q11 long- and short-cadence Kepler photometry. With the exception of some Blazhko stars, the spectroscopic and photometric [Fe/H] values are in good agreement. Several stars with unique photometric characteristics are identified, including a Blazhko variable with the smallest known amplitude and frequency modulations (V838 Cyg).

Period and light curve fluctuations of the Kepler Cepheid V1154 Cyg

A. Derekas, Gy. M. Szabo, L. Berdnikov, R. Szabo, R. Smolec, L. L. Kiss, L. Szabados, M. Chadid, N. R. Evans, K. Kinemuchi, J. M. Nemec, S. E. Seader, J. C. Smith, P. Tenenbaum.
Full text
NASA ADS
DOI

We present a detailed period analysis of the bright Cepheid-type variable star V1154 Cygni (V=9.1 mag, P$\approx$4.9 d) based on 600 days of continuous observations by the Kepler space telescope. The data reveal significant cycle-to-cycle fluctuations in the pulsation period, indicating that classical Cepheids may not be as accurate astrophysical clocks as commonly believed: regardless of the specific points used to determine the $O-C$ values, the cycle lengths do change by about 0.01 days (0.2 per cent) seemingly randomly over the 120 cycles covered by the observations. We compare the measurements with simulated light curves that were constructed to mimic V1154 Cyg as a perfect pulsator modulated only by the light travel time effect caused by low-mass companions. We show that the observed period noise in V1154 Cyg represents a serious limitation in search for binary companions. While the Kepler data are accurate enough to allow the detection of planetary bodies in close orbits around a Cepheid, the astrophysical noise can easily hide the signal of the light-time effect.

Detection of high-degree prograde sectoral mode sequences in the A-star KIC 8054146?

M. Breger, P. Lenz, A. A. Pamyatnykh.
Full text

This paper examines the 46 frequencies found in the $\delta$ Sct star KIC 8054146 involving a frequency spacing of exactly 2.814 cycles day$^{-1}$ (32.57 $\mu$Hz), which is also a dominant low-frequency peak near or equal to the rotational frequency. These 46 frequencies range from 7 to 146 cycles day$^{-1}$. We show from three years of Kepler data that these frequencies are comprised of distinct sequences including a basic sequence and side lobes associated with other dominant frequencies. The observed side-lobe sequences are interpreted as small amplitude modulations of the (other) dominant pulsation modes with the high-frequency basic sequence. The amplitudes of the basic sequence show a high-low pattern. The basic sequence follows the equation $f_m = 2.8519 + m * 2.81421$ cycles day$^{-1}$ with $m$ ranging from 25 to 35. The zero-point offset and the lack of low-order harmonics eliminate an interpretation in terms of a Fourier series of a non-sinusoidal light curve, while the exactness of the spacing eliminates high-order asymptotic pulsation. The frequency pattern is not compatible with simple hypotheses involving single or multiple spots, even with differential rotation.
The basic high-frequency sequence is interpreted in terms of prograde sectoral modes, which can be marginally unstable, while their corresponding low-degree counterparts are stable due to stronger damping. With a measured projected rotation velocity of $v \sin i$ of approximately 300 km s$^{-1}$ the star rotates with $\gtrapprox$ 70% of the Keplerian break-up velocity. The Keplerian break-up limit puts a lower constraint to the inclination angle, hence making a near equator-on view more likely. We qualitatively examine the visibility of prograde sectoral high-degree g-modes in integrated photometric light in such a geometrical configuration. We find that prograde sectoral modes can reproduce the frequencies and the odd-even amplitude pattern of the high-frequency sequence.

KIC 11285625: a double-lined spectroscopic binary with a gamma Dor pulsator discovered from Kepler space photometry

J. Debosscher, C. Aerts, A. Tkachenko, K. Pavlovski, C. Maceroni, D. Kurtz, P. Beck, S. Bloemen, P. Degroote, R. Lombaert, J. Southworth.
Full text

We present the first binary modelling results for the pulsating, eclipsing binary KIC 11285625 that was discovered by the Kepler mission. An automated method to disentangle the pulsation spectrum and the orbital variability in high quality light curves was developed and applied. The goal was to obtain accurate orbital and component properties in combination with essential information derived from spectroscopy. A binary model for KIC 11285625 was obtained, using a combined analysis of high-quality space-based Kepler light curves and ground-based high-resolution HERMES echelle spectra. The binary model was used to separate the pulsation characteristics from the orbital variability in the Kepler light curve in an iterative way. We used an automated procedure based on the JKTEBOP binary modelling code to perform this task, and adapted codes for frequency analysis and prewhitening of periodic signals. Using a disentangling technique applied to the composite HERMES spectra, we obtained a higher signal-to-noise mean spectrum for both the primary and the secondary components. A model grid search method for fitting synthetic spectra was used for fundamental parameter determination for both components. Accurate orbital and component properties of KIC 11285625 were derived, and we have obtained the pulsation spectrum of the gamma Dor pulsator in the system. Detailed analysis of the pulsation spectrum revealed amplitude modulation on a timescale of a hundred days, and strong indications of frequency splittings at both the orbital frequency and the rotational frequency derived from spectroscopy.

KIC 8410637: a 408-day period eclipsing binary containing a pulsating giant star.

S. Frandsen, H. Lehmann, S. Hekker, J. Southworth, J. Debosscher, P. Beck, M. Hartmann, A. Pigulski, G. Kopacki, Z. Koaczkowski, M. Stlicki, A. O. Thygesen, K. Brogaard, Y. Elsworth.
Full text

Detached eclipsing binaries (dEBs) are ideal targets for accurate measurement of mass $M$ and radius $R$ of the stars. If at least one of the stars has evolved off the main sequence (MS), the masses and radii give a strict constraint on the age of the stars. Several dEBs containing a bright K giant and a fainter MS star have been discovered by the \kepler satellite. The mass $M$ and the radius $R$ of a red giant (RG) star can be derived from its asteroseismic signal. The parameters determined in this way may depend on stellar models and may contain systematic errors. Additional strong constraints on stellar parameters are invaluable in order to narrow down the parameter space for the asteroseismic analysis. Such constraints can be obtained when stars are members of stellar clusters or from dEBs. This paper presents an analysis of the dEB system KIC 8410637, which consists of a RG and a MS star. The aim is to derive accurate masses and radii for both components and provide the foundation for a strong test of the asteroseismic method and the accuracy of the parameters $M$, $R$ and age. We analyse high-resolution, high-signal-to-noise spectra from three different spectrographs for the dEB KIC 8410637. We also calculate a fit to the Kepler lightcurve and use groundbased photometry to determine the flux ratios between the component stars in the $BVRI$ passbands. We measured masses and radii for the stars in the dEB, and the classical parameters $T_{\rm eff}$,$\log g$ and [Fe/H] from the spectra and groundbased photometry. The giant component of KIC 8410637 is most likely in the core helium-burning red clump phase of evolution and has an age and composition very similar to the stars in the open cluster NGC 6819. The mass of the giant component of KIC 8410637 should therefore be similar to the mass of giant stars in NGC 6819, thus lending support to the most up-to-date version of the asteroseismic scaling relations. We find excellent agreement between $\log g$ values derived from the binary analysis and asteroseismic scaling relations, thus providing the first verification that $\log g$ values for giant stars from asteroseismic scaling relations are not only precise, but also accurate.

Atmospheric Parameters of 169 F, G, K and M-type Stars in the Kepler Field

J. Molenda-akowicz, S. G. Sousa, A. Frasca, K. Uytterhoeven, M. Briquet, H. Van Winckel, D. Drobek, E. Niemczura, P. Lampens, J. Lykke, S. Bloemen, J. F. Gameiro, C. Jean, D. Volpi, N. Gorlova, A. Mortier, M. Tsantaki and 1 coauthors.
Full text
NASA ADS
DOI

Both the asteroseismic and planetary studies need precise and accurate atmospheric parameters of the stars as input. We aim at deriving the effective temperature ($T_{\rm eff}$), the surface gravity ($\log g$), the metallicity ([Fe/H]), the projected rotational velocity ($v\sin i$) and the MK type for 169 F, G, K, and M-type Kepler targets which were observed spectroscopically from the ground with five different instruments. We use two different spectroscopic methods to analyse 189 high-resolution, high-signal-to-noise spectra acquired for those 169 stars. For 69, $T_{\rm eff}$, $\log g$, [Fe/H], $v\sin i$, and the MK type are derived for the first time. KIC 9025370, 9693187 and 11179629 are discovered to be double-lined spectroscopic binary systems. The results obtained for those stars for which independent determinations of the atmospheric parameters are available in the literature are used for a comparative analysis. As a result, we show that for the solar-type stars the accuracy of the present determinations of $T_{\rm eff}$ is $\pm$ 150 K, $\pm$ 0.15 dex in [Fe/H], and $\pm$ 0.3 dex in $\log g$. Finally, we confirm that the analysis of the curve-of-growth and the method of the spectral synthesis yield systematically different results when they are applied to stars of $T_{\rm eff}$ ranging from 6,000 to 7,000K.

Asteroseismic surface gravity for evolved stars

S. Hekker, Y. Elsworth, B. Mosser, T. Kallinger, Sarbani Basu, W.J. Chaplin, D. Stello.
Full text

Context: Asteroseismic surface gravity values can be of importance in determining spectroscopic stellar parameters. The independent $\log(g)$ value from asteroseismology can be used as a fixed value in the spectroscopic analysis to reduce uncertainties due to the fact that $\log(g)$ and effective temperature can not be determined independently from spectra. Since 2012, a combined analysis of seismically and spectroscopically derived stellar properties is ongoing for a large survey with SDSS/APOGEE and Kepler. Therefore, knowledge of any potential biases and uncertainties in asteroseismic $\log(g)$ values is now becoming important. Aims: The seismic parameter needed to derive $\log(g)$ is the frequency of maximum oscillation power ($\nu_{\rm max}$). Here, we investigate the influence of $\nu_{\rm max}$ derived with different methods on the derived $\log(g)$ values. The large frequency separation between modes of the same degree and consecutive radial orders ($\Delta\nu$) is often used as an additional constraint for the determination of $\log(g)$. Additionally, we checked the influence of small corrections applied to $\Delta\nu$ on the derived values of $\log(g)$. Methods: We use methods extensively described in the literature to determine $\nu_{\rm max}$ and $\Delta\nu$ together with seismic scaling relations and grid-based modeling to derive $\log(g)$. Results: We find that different approaches to derive oscillation parameters give results for $\log(g)$ with small, but different, biases for red-clump and red-giant-branch stars. These biases are well within the quoted uncertainties of $\sim0.01$ dex (cgs). Corrections suggested in the literature to the $\Delta\nu$ scaling relation have no significant effect on $\log(g)$. However somewhat unexpectedly, method specific solar reference values induce biases of the order of the uncertainties, which is not the case when canonical solar reference values are used.

Accurate fundamental parameters and detailed abundance patterns from spectroscopy of 93 solar-type Kepler targets

H. Bruntt, S. Basu, B. Smalley, W. J. Chaplin, G. A. Verner, T. R. Bedding, C. Catala, J.-C. Gazzano, J. Molenda-Zakowicz, A. O. Thygesen, K. Uytterhoeven, S. Hekker, D. Huber, C. Karoff, S. Mathur, B. Mosser, T. Appourchaux and 15 coauthors.
Full text
NASA ADS
DOI

We present a detailed spectroscopic study of 93 solar-type stars that are targets of the NASA Kepler mission and provide detailed chemical composition of each target. We find that the overall metallicity is well-represented by Fe lines. Relative abundances of light elements (CNO) and $\alpha$ elements are generally higher for low-metallicity stars. Our spectroscopic analysis benefit from the accurately measured surface gravity from the asteroseismic analysis of the Kepler light curves. The $\log g$ parameter is known to better than 0.03 dex and is held fixed in the analysis. We compare our $T_{\rm eff}$ determination with a recent colour calibration of $V_{\rm T}-K_{\rm S}$ (TYCHO $V$ magnitude minus 2MASS $K_{\rm S}$ magnitude) and find very good agreement and a scatter of only 80 K, showing that for other nearby Kepler targets this index can be used. The asteroseismic $\log g$ values agree very well with the classical determination using Fei minus Feii balance, although we find a small systematic offset of $0.08$ dex (asteroseismic $\log g$ values are lower). The abundance patterns of metals, $\alpha$ elements, and the light elements (CNO) show that a simple scaling by [Fe/H] is adequate to represent the metallicity of the stars, except for the stars with metallicity below $-0.3$, where $\alpha$-enhancement becomes important. However, this is only important for a very small fraction of the Kepler sample. We therefore recommend that a simple scaling with [Fe/H] be employed in the asteroseismic analyses of large ensembles of solar-type stars.

Discovery of a new AM CVn system with the Kepler satellite

G. Fontaine, P. Brassard, E.M. Green, S. Charpinet, P. Dufour, I. Hubeny, D. Steeghs, C. Aerts, S.K. Randall, P. Bergeron, B. Guvenen, C.J. O'Malley, V. Van Groote, R.H. stensen, S. Bloemen, R. Silvotti, S.B. Howell and 11 coauthors.
Full text

We report the discovery of a new AM CVn system on the basis of broadband photometry obtained with the Kepler satellite supplemented by ground-based optical spectroscopy. Initially retained on Kepler target lists as a potential compact pulsator, the blue object SDSS J190817.07+394036.4 (KIC 004547333) has turned out to be a high-state AM CVn star showing the He-dominated spectrum of its accretion disk significantly reddened by interstellar absorption. We constructed new grids of NLTE synthetic spectra for accretion disks in order to analyze our spectroscopic observations. These models are highly idealized and we use them in an exploratory spirit. On this basis, we find that the accretion disk is seen under a fairly well-constrained angle of 10$-$20$^{\rm o}$. We also estimate that the current rate of mass transfer in the system is in the range 3.5$-$8.5 $\times 10^{-9} M_\odot \mathrm{yr}^{-1}$. By comparing the measured energy distribution with those obtained from our model disk spectra, we further infer a reddening index $E(g-r)$ = 0.26$-$0.29, an absorption coefficient $A_r$ = 0.70$-$0.76, and a distance to the system in the range 250$-$330 pc. The AM CVn nature of the system is also evident in the Kepler light curve, from which we extracted 11 secure periodicities with amplitudes well above 4.3 times the mean local noise. These are found in the range of periods from 212 s to 953 s, and with amplitudes from $\sim0.03$% to $\sim0.31$% of the mean brightness of the star in the Kepler bandpass. The luminosity variations are dominated by a basic periodicity of 938.507 s, likely to correspond to a superhump modulation. In this scenario, the orbital period of the system would be equal to 953.262 s, corresponding to another strong modulation found close to the 938.507 s variation in Fourier space. The light curve folded on the period of 938.507 s exhibits a pulse shape that is very similar to the superhump wavefront seen in AM CVn itself, which is a high-state system and the prototype of the class. Our Fourier analysis also suggests the likely presence of a quasi-periodic oscillation (QPO) similar to those already observed in some high-state AM CVn systems. Furthermore, some very low-frequency, low-amplitude aperiodic photometric activity is likely present, which is in line with what is expected in accreting binary systems. Inspired by previous work, we further looked for and found some intriguing numerical relationships between the 11 secure detected frequencies, in the sense that we can account for all of them in terms of only 3 basic clocks. This is further evidence in favor of the AM CVn nature of the system. Given that the photometric properties of AM CVn systems are still not fully understood, and given the particular importance of such stars as laboratories for testing binary star evolution, it is hoped that SDSS J190817.07+394036.4 will be retained as a long-term target since Kepler can play a vital role for progress on this front.

Kepler's view of KPD1946+4340, a relativistic binary star

S. Bloemen, T. R. Marsh, R. H. stensen, S. Charpinet, P. Degroote, S. D. Kawaler, C. Aerts, G. Fontaine, E. M. Green, J. Telting, P. Brassard, B. T. Gänsicke, G. Handler, U. Heber, D. W. Kurtz, R. Silvotti, V. Van Grootel and 10 coauthors.
Full text
NASA ADS
DOI

The Kepler Mission has acquired 33.5 d of continuous one-minute photometry of KPD1946+4340, a short-period binary system that consists of a subdwarf B star (sdB) and a white dwarf. In the light curve, eclipses are clearly seen, with the deepest occurring when the compact white dwarf crosses the disc of the sdB (0.4%) and the more shallow ones (0.1%) when the sdB eclipses the white dwarf. As expected, the sdB is deformed by the gravitational field of the white dwarf, which produces an ellipsoidal modulation of the light curve. Spectacularly, a very strong Doppler beaming (also known as Doppler boosting) effect is also clearly evident at the 0.1% level. This originates from the sdB’s orbital velocity, which we measure to be $164.0 \pm 1.9 km s^{-1}$ from supporting spectroscopy. We present light curve models that account for all these effects, as well as gravitational lensing, which decreases the apparent radius of the white dwarf by about 6% when it eclipses the sdB. We derive system parameters and uncertainties from the light curve using Markov Chain Monte Carlo simulations. Adopting a theoretical white dwarf mass-radius relation, the mass of the subdwarf is found to be $0.47\pm0.03 M_\odot$ and the mass of the white dwarf $0.59\pm0.02 M_\odot$. The effective temperature of the white dwarf is 15 900 ± 300K. With a spectroscopic effective temperature of $T_\mathrm{eff} = 34 730 \pm 250K$ and a surface gravity of $\log g = 5.43 \pm 0.04$, the subdwarf has most likely exhausted its core helium, and is in a shell He burning stage. The detection of Doppler beaming in Kepler light curves potentially allows one to measure radial velocities without the need of spectroscopic data. For the first time, a photometrically observed Doppler beaming amplitude is compared to a spectroscopically established value. For unknown reasons, the radial velocity derived from the photometry is $7 \pm 2\%$ larger than the spectroscopic value. After subtracting our best model for the orbital effects, we searched the residuals for stellar oscillations but did not find any significant pulsation frequencies.

A large sample of calibration stars for Gaia: logg from Kepler and CoRoT fields

O. L. Creevey, F. Thevenin, S. Basu, W. J. Chaplin, L. Bigot, Y. Elsworth, D. Huber, M. J. P. F. G. Monteiro, A. Serenelli.
Full text
NASA ADS
DOI

Asteroseismic data can be used to determine stellar surface gravities with precisions of $< 0.05$ dex by using the global seismic quantites $\langle \Delta \nu \rangle$ and $\nu_{\rm max}$ along with standard atmospheric data such as $T_{\rm eff}$ and metallicity. Surface gravity is also one of the four stellar properties to be derived by automatic analyses for 1 billion stars from Gaia data (workpackage GSP_phot). In this paper we explore seismic data from main sequence F, G, K stars (solar-like stars) observed by the Kepler spacecraft as a potential calibration source for the methods that Gaia will use for object characterisation (ogg). We compile a list of bright nearby stars for which radii and masses are known (e.g. from interferometry or binaries), and we compare their $\log g$ with those derived from asteroseismic data using a grid-based method. Agreement to within 1$\sigma$ of the accepted $\log g$ values validates our method. We also find that errors in adopted atmospheric parameters (mainly [Fe/H]) can, however, cause systematic errors on the order of 0.02 dex. We then apply our method to a list of 41 stars to deliver precise values of surface gravity, and we find agreement with recent literature values. Finally we explore the typical precision that we expect in a sample of 400+ Kepler stars which have their global seismic quantities measured. We find typical uncertainties (precision) on the order of better than 0.02 dex in $\log g$. We study sources of systematic errors in $\log g$ and we find possible biases on the order of 0.04 dex in $\log g$ which accounts for errors in the $T_{\rm eff}$ and [Fe/H] measurements, as well as from using a different grid-based method. We conclude that Kepler stars provide a wealth of reliable information that can help to calibrate methods that Gaia will use, in particular, for source characterisation where excellent precision and accuracy in $\log g$ is obtained from Kepler data and this is one of the four stellar parameters to be extracted by GSP_phot.

Kepler photometry of the prototypical Blazhko star RR Lyr: An old friend seen in a new light

K. Kolenberg, S. Bryson, R. Szabó, D. W. Kurtz, R. Smolec, J. M. Nemec, E. Guggenberger, P. Moskalik, J. M. Benk, M. Chadid, Y.-B. Jeon, L. Kiss, G. Kopacki, J. Nuspl, M. Still, J. Christensen-Dalsgaard, H. Kjeldsen and 4 coauthors.
Full text
NASA ADS
DOI

We present our analysis of the long cadence Kepler data for the well-studied Blazhko star RR Lyr, gathered during the first two quarters of the satellite's observations and covering a total of 127 d. Besides being of great importance for our understanding of RR Lyrae stars in general, these RR Lyr data can be regarded as a case study for observations of bright stars with Kepler. Kepler can perform high-precision photometry on targets like RR Lyr, as the saturated flux is conserved to a very high degree. The Kepler data on RR Lyr are revolutionary in several respects. Even with long-cadence sampling (one measurement per 29.4 min), the unprecedented precision ( $<$ mmag) of the Kepler photometry allows the study of the star's extreme light curve variations in detail. The multiplet structures at the main frequency and its harmonics, typical for Blazhko stars, are clearly detected up to the quintuplets. For the first time, photometric data of RR Lyr reveal the presence of half-integer frequencies, linked to a period doubling effect. This phenomenon may be connected to the still unexplained Blazhko modulation. Moreover, with three observed Blazhko cycles at our disposal, we observe that there is no exact repetition in the light curve changes from one modulation cycle to the next for RR Lyr. This may be due to additional periodicities in the star, or to transient or quasi-periodic changes.

Solar-like oscillations in red giants observed with Kepler: how to improve knowledge of the global oscillation parameters

S. Hekker, Y. Elsworth, B. Mosser, T.Kallinger, W.J Chaplin, J. De Ridder, R.A. García, D. Stello, B.D. Clarke, J.R. Hall, K.A. Ibrahim.
Full text

The length of the asteroseismic timeseries obtained from the Kepler satellite analysed here span 19 months. Kepler provides the longest continuous timeseries currently available, which calls for a study of the influence of the increased timespan on the accuracy and precision of the obtained results. We aim to investigate how the increased timespan influences the detectability of the oscillation modes, and the absolute values and uncertainties of the global oscillation parameters, i.e., frequency of maximum oscillation power, $\nu_{\rm max}$, and large frequency separation between modes of the same degree and consecutive orders, $\Delta \nu$. We use published methods to derive $\nu_{\rm max}$ and $\Delta \nu$ for timeseries ranging from 50 to 600 days and compare these results as a function of method, timespan and $\Delta \nu$. We find that in general a minimum of the order of 200 day long timeseries are necessary to obtain reliable results for the global oscillation parameters, but that this does depend on $\Delta \nu$. In a statistical sense the quoted uncertainties seem to provide a reasonable indication of the precision of the obtained results in short (50-day) runs, they do however seem to be overestimated for results of longer runs. Furthermore, the different definitions of the global parameters used in the different methods have non-negligible effects on the obtained values. We show that this can be used to identify the evolutionary state of the star using global oscillation parameters determined with high precision. Additionally, we show that there is a correlation between $\nu_{\rm max}$ and the flux variance. We conclude that longer timeseries improve the likelihood to detect oscillations with automated codes and the precision of the obtained global oscillation parameters. The trends suggest that the improvement will continue for even longer timeseries than the 600 days considered here. This work shows that global parameters determined with high precision - thus from long datasets - using different definitions can be used to identify the evolutionary state of the stars. The variance of the flux provides a reliable indication of $\nu_{\rm max}$, which can be used as an additional constraint.

Asteroseismic inferences on red giants in open clusters NGC 6791, NGC 6819 and NGC 6811 using Kepler

S. Hekker, S. Basu, D. Stello, T. Kallinger, F. Grundahl, S. Mathur, R.A. Garcia, B. Mosser, D. Huber, T.R. Bedding, R. Szabo, J. De Ridder, W.J. Chaplin, Y. Elsworth, S.J. Hale, J. Christensen-Dalsgaard, R.L. Gilliland and 3 coauthors.
Full text

Context: Four open clusters are present in the Kepler field of view and timeseries of nearly a year in length are now available. These timeseries allow us to derive asteroseismic global oscillation parameters of red-giant stars in the three open clusters NGC 6791, NGC 6819 and NGC 6811. From these parameters and effective temperatures, we derive mass, radii and luminosities for the cluster as well as field red giants.
Aims: We study the influence of evolution and metallicity on the observed red-giant populations.
Methods: The global oscillation parameters are derived using different published methods and the effective temperatures are derived from 2MASS colours. The observational results are compared with BaSTI evolution models. Results: We find that the mass has significant influence on the asteroseismic quantities $\Delta \nu$ vs. $\nu_{\rm max}$ relation, while the influence of metallicity is negligible, under the assumption that the metallicity does not affect the excitation / damping of the oscillations. The position of the stars in the H-R diagram depends on both mass and metallicity. Furthermore, the stellar masses derived for the field stars are bracketed by those of the cluster stars.
Conclusions: Both the mass and metallicity contribute to the observed difference in locations in the H-R diagram of the old metal-rich cluster NGC 6791 and the middle-aged solar-metallicity cluster NGC 6819. %Roughly 40% of this difference in location can be attributed to the mass difference between the clusters and $\sim$60% to the difference in metallicity. For the young cluster NGC 6811, the explanation of the position of the stars in the H-R diagram challenges the assumption of solar metallicity, and this open cluster might have significantly lower metallicity [Fe/H] in the range $-$0.3 to $-$0.7 dex. Also, all the observed field stars seem to be older than NGC 6811 and younger than NGC 6791.

Characterisation of red-giant stars in the public Kepler data

S. Hekker, R.L. Gilliland, Y. Elsworth, W.J. Chaplin, J. De Ridder, D. Stello, A. Miglio, T. Kallinger, K.A. Ibrahim, T.C. Klaus, J. Li.
Full text

The first public release has now been made of long-cadence stellar photometric data collected by the NASA Kepler mission. In this paper we aim to characterise the red-giant (G-K) stars in this large sample in terms of their solar-like oscillations. We use published methods and well-known scaling relations in the analysis. Just over 70% of the red giants in the sample show detectable solar-like oscillations, and from these oscillations we are able to estimate the fundamental properties of the stars. This asteroseismic analysis reveals different populations: low-luminosity H-shell burning red-giant branch stars, cool high-luminosity red giants on the red-giant branch and He-core burning clump and secondary-clump giants. Furthermore, the detection of solar-like oscillations in red giants does not depend on the long-term variability of the stars.

Solar-like oscillations in red giants observed with Kepler: comparison of global oscillation parameters from different methods

S. Hekker, Y. Elsworth, J. De Ridder, B. Mosser, R.A. García, T. Kallinger, S. Mathur, D. Huber, D.L. Buzasi, H.L. Preston, S.J. Hale, J. Ballot, W.J. Chaplin, C. Régulo, T.R. Bedding, D. Stello, W.J. Borucki and 6 coauthors.
Full text

The large number of stars for which uninterrupted high-precision photometric timeseries data are being collected with Kepler and CoRoT initiated the development of automated methods to analyse the stochastically excited oscillations in main-sequence, subgiant and red-giant stars. We investigate the differences in results for global oscillation parameters of G and K red-giant stars due to different methods and definitions. We also investigate uncertainties originating from the stochastic nature of the oscillations. For this investigation we use Kepler data obtained during the first four months of operation. These data have been analysed by different groups using already published methods and the results are compared. We also performed simulations to investigate the uncertainty on the resulting parameters due to different realizations of the stochastic signal. We obtain results for the frequency of maximum oscillation power ($\nu_{\rm max}$) and the mean large separation ($\langle\Delta\nu\rangle$) from different methods for over one thousand red-giant stars. The results for these parameters agree within a few percent and seem therefore robust to the different analysis methods and definitions used here. The uncertainties for $\nu_{\rm max}$ and $\langle\Delta\nu\rangle$ due to differences in realization noise are not negligible and should be taken into account when using these results for stellar modeling.

Orbital properties of an unusually low-mass sdB star in a close binary system with a white dwarf

R. Silvotti, R. H. stensen, S. Bloemen, J. H. Telting, U. Heber, R. Oreiro, M. D. Reed, L. E. Farris, S. J. O’Toole, L. Lanteri, P. Degroote, H. Hu, A. S. Baran, J. J. Hermes, L. Althaus, T. R. Marsh, S. Charpinet and 3 coauthors.
Full text
NASA ADS
DOI

We have used 605 days of photometric data from the Kepler spacecraft to study KIC6614501, a close binary system with an orbital period of 0.15749747(25) days (3.779939 hours), that consists of a low-mass subdwarf B (sdB) star and a white dwarf. As seen in many other similar systems, the gravitational field of the white dwarf produces an ellipsoidal deformation of the sdB which appears in the light curve as a modulation at two times the orbital frequency. The ellipsoidal deformation of the sdB implies that the system has a maximum inclination of  40 degrees, with i≈20° being the most likely. The orbital radial velocity of the sdB star is high enough to produce a Doppler beaming effect with an amplitude of 432±5 ppm, clearly visible in the folded light curve. The photometric amplitude that we obtain, K1=85.8 km/s, is  12 per cent less than the spectroscopic RV amplitude of 97.2±2.0 km/s. The discrepancy is due to the photometric contamination from a close object at about 5 arcsec North West of KIC6614501, which is difficult to remove. The atmospheric parameters of the sdB star, Teff=23700±500 K and logg=5.70±0.10, imply that it is a rare object below the Extreme Horizontal Branch (EHB), similar to HD188112. The comparison with different evolutionary tracks suggests a mass between  0.18 and  0.25 M⊙, too low to sustain core helium burning. If the mass was close to 0.18-0.19 M⊙, the star could be already on the final He-core WD cooling track. A higher mass, up to  0.25 M⊙, would be compatible with a He-core WD progenitor undergoing a cooling phase in a H-shell flash loop. A third possibility, with a mass between  0.32 and  0.40 M⊙, can not be excluded and would imply that the sdB is a “normal” (but with an unusually low mass) EHB star burning He in its core. In all these different scenarios the system is expected to merge in less than 3.1 Gyr due to gravitational wave radiation.

A uniform asteroseismic analysis of 22 solar-type stars observed by Kepler

S. Mathur, T. S. Metcalfe, M. Woitaszek, H. Bruntt, G. A. Verner, J. Christensen-Dalsgaard, O. L. Creevey, G. Doan, S. Basu, C. Karoff, D. Stello, T. Appourchaux, T. L.Campante, W. J. Chaplin, R. A. García, T. R. Bedding, O. Benomar and 25 coauthors.
Full text

Asteroseismology with the Kepler space telescope is providing not only an improved characterization of exoplanets and their host stars, but also a new window on stellar structure and evolution for the large sample of solar-type stars in the field. We perform a uniform analysis of 22 of the brightest asteroseismic targets with the highest signal-to-noise ratio observed for 1 month each during the first year of the mission, and we quantify the precision and relative accuracy of asteroseismic determinations of the stellar radius, mass, and age that are possible using various methods. We present the properties of each star in the sample derived from an automated analysis of the individual oscillation frequencies and other observational constraints using the Asteroseismic Modeling Portal (AMP), and we compare them to the results of model-grid-based methods that fit the global oscillation properties. We find that fitting the individual frequencies typically yields asteroseismic radii and masses to $\sim$1% precision, and ages to $\sim$2.5% precision (respectively 2, 5, and 8 times better than fitting the global oscillation properties). The absolute level of agreement between the results from different approaches is also encouraging, with model-grid-based methods yielding slightly smaller estimates of the radius and mass and slightly older values for the stellar age relative to AMP, which computes a large number of dedicated models for each star. The sample of targets for which this type of analysis is possible will grow as longer data sets are obtained during the remainder of the mission.

Evolutionary influences on the fine structure of red-giant acoustic oscillation spectra from 600d of Kepler observations

T. Kallinger, S. Hekker, B. Mosser, J. De Ridder, T. R. Bedding, Y. P. Elsworth, M. Gruberbauer, D. B. Guenther, D. Stello, S. Basu, R. A. García, W. J. Chaplin, F. Mullaly, M. Still, S. E. Thompson.
Full text

Context: It was recently discovered that the period spacings of mixed pressure/gravity dipole modes in red giants permit a distinction between the otherwise unknown evolutionary stage of these stars. The Kepler space mission is reaching continuous observing times long enough to also start studying the fine structure of the observed pressure-mode spectra.
Aims: In this paper, we aim to study the signature of stellar evolution on the radial and pressure-dominated $l$ = 2 modes in an ensemble of red giants that show solar-type oscillations.
Methods: We use established methods to automatically identify the mode degree of $l$ = 0 and 2 modes and measure a local large ($\Delta\nu_c$) and small ($\delta\nu_{02}$) frequency separation around the central radial mode. We then determine the phase shift \epsc of the central radial mode, i.e. the local linear offset in the asymptotic development of acoustic modes. Furthermore we use a straightforward approach to measure the individual frequencies of the radial modes and investigate their average curvature.
Results: We find that $\epsilon_c$ is significantly different for red giants at a given $\Delta\nu_c$ but which burn only H in a shell (RGB) and those that have already ignited core He burning. Even though not directly probing the stellar core the pair of local seismic observables ($\Delta\nu_c$, $\epsilon_c$) can be used as an evolutionary stage discriminator that turned out to be just as reliable as the period spacing of the mixed dipole modes. We find a tight correlation between $\epsilon_c$ and $\Delta\nu_c$ for RGB stars and no indication that their $\epsilon_c$ depends on other properties of the star. It appears that the difference in $\epsilon_c$ between the two populations becomes smaller and eventually undistinguishable if we use a global, i.e. an average of several radial orders, instead of a local, i.e. only around the central radial modes, large separation to determine $\epsilon$. This indicates that the information on the evolutionary stage is encoded locally, more precisely in the shape of the radial mode sequence. This turns out to be approximately symmetric around the central radial mode for RGB stars but asymmetric for core He burning stars. We computed radial mode frequencies for a sequence of red-giant models and find them to qualitatively confirm our findings. We also find a clear signature of the evolutionary stage in $\delta\nu_{02}$ and quantify the mass dependency of this seismic parameter.

Seismic evidence for non-synchronization in two close sdB+dM binaries from Kepler photometry

Herbert Pablo, Steven D. Kawaler, M. D. Reed, S. Bloemen, S. Charpinet, H. Hu., J. Telting, R. H. stensen, A. S. Baran, E. M. Green, J.J. Hermes, T. Barclay, S. J. O'Toole, D. W. Kurtz, J. Christensen-Dalsgaard, Fergal Mullally, Douglas A. Caldwell and 2 coauthors.
Full text

We report on extended photometry of two pulsating sdB stars in close binaries. In both cases, we show that the sdB component rotates much too slowly to be in synchronous rotation. We use the theory of tidal interaction in binary stars to place upper limits on the mass ratios that are independent of estimates based on the radial velocity curves. The companions have masses below 0.26 $M_{\odot}$. The pulsation spectra show the signature of high–overtone $g$-mode pulsation. One star, KIC 11179657, has a clear sequence of $g$-modes with equal period spacings as well as several periodicities that depart from that trend. KIC 02991403 shows a similar sequence, but has many more modes that do not fit the simple pattern.

Preparation of Kepler lightcurves for asteroseismic analyses

R.A. García, S. Hekker, D. Stello, J. Gutiérrez-Soto, R. Handberg, D. Huber, C. Karoff, K. Uytterhoeven, T. Appourchaux, W. J. Chaplin, Y. Elsworth, S. Mathur, J. Ballot, J. Christensen-Dalsgaard, R. L. Gilliland, G. Houdek, J. M. Jenkins and 8 coauthors.
Full text
NASA ADS
DOI

The Kepler mission is providing photometric data of exquisite quality for the asteroseismic study of different classes of pulsating stars. These analyzes place particular demands on the pre-processing of the data, over a range of timescales from minutes to months. Here, we describe processing procedures developed by the Kepler Asteroseismic Science Consortium (KASC) to prepare light curves that are optimized for the asteroseismic study of solar-like oscillating stars in which outliers, jumps and drifts are corrected.

Detection of stochastic gravity modes in the massive binary V380 Cyg from Kepler customized-mask space photometry and high-resolution spectroscopy

A. Tkachenko, C. Aerts, K. Pavlovski, J. Southworth, P. Degroote, J. Debosscher, M. Still, S. Bryson, G. Molenberghs, S. Bloemen, B. L. de Vries, Y. Frémat, M. Hrudkova, R. Lombaert, P. Neyskens, P. Pápics, G. Raskin and 4 coauthors.
Full text

We report the discovery of low-amplitude stochastic gravity-mode oscillations in the massive binary V380 Cyg, from 180 d of Kepler custom-aperture space photometry and 5 months of high-resolution high signal-to-noise spectroscopy. The new data are of unprecedented quality and allowed the deduction of improved values of the orbital and fundamental parameters for this important binary. The orbital model was subtracted from the photometric data and seismic signals due to gravity modes were detected in the residual light curve. Spectral disentangling allowed the detection of line-profile variability in the primary. With our discovery of gravity modes, V380 Cyg becomes an important laboratory for future tuning of the near-core physics for B-type stars. The binary is therefore the subject of ongoing and future Kepler measurements.

Early asteroseismic results from Kepler: Structural and core parameters of the hot B subdwarf KPD 1943+4058 as inferred from g-mode oscillations

V. Van Grootel, S. Charpinet, G. Fontaine, P. Brassard, E. M. Green, S. K. Randall, R. Silvotti, R. H. Østensen, H. Kjeldsen, J. Christensen-Dalsgaard, W. J. Borucki, D. Koch.
Full text
NASA ADS
DOI

We present a seismic analysis of the pulsating hot B subdwarf KPD 1943+4058 (KIC 005807616) on the basis of the long-period, gravity-mode pulsations recently uncovered by Kepler. This is the first time that g-mode seismology can be exploited quantitatively for stars on the extreme horizontal branch, all previous successful seismic analyses having been confined so far to short-period, p-mode pulsators. We demonstrate that current models of hot B subdwarfs can explain quite well the observed g-mode periods, while being consistent with independent constraints provided by spectroscopy. We identify the 18 pulsations retained in our analysis as low-degree ($\ell$ = 1 and 2), intermediate-order ($k$ = -9 through -58) g-modes. The periods (frequencies) are recovered, on the average, at the 0.22% level, which is comparable to the best results obtained for p-mode pulsators. We infer the following structural and core parameters for KPD 1943+4058 : $T_\mathrm{eff} = 28,050 \pm 470 K$, $\log g = 5.520 \pm 0.029$, $M_* = 0.4964 \pm 0.0013 M_\odot$, $\log(M_\mathrm{env}/M_{*}) = -2.552 \pm 0.070$, $\log(1-M_\mathrm{core}/M_{*}) = -0.366 \pm 0.010$, and $X_\mathrm{core}(C+O) = 0.2612 \pm 0.0080$. We additionally derive the age of the star since the Zero-Age EHB $18.4 \pm 1.0$ Myr, the radius $R = 0.2026 \pm 0.0070 R_\odot$, the luminosity $L = 22.92\pm3.13 L_\odot$, the absolute magnitude $M_V = 4.21 \pm 0.11$, the reddening index $E(B-V) = 0.094 \pm 0.017$, and the distance $d = 1183 \pm 93$ pc.

First Kepler results on compact pulsators VII. Pulsating subdwarf B stars detected in the second half of the survey phase

A. S. Baran, S. D. Kawaler, M. D. Reed, A. C. Quint, S. J. O'Toole, R. H. stensen, J. H. Telting, R. Silvotti, S. Charpinet, J. Christensen-Dalsgaard, M. Still, J.R. Hall, K. Uddin.
Full text

We present five new pulsating subdwarf B (sdB) stars discovered by the Kepler spacecraft during the asteroseismology survey phase. We perform time-series analysis on the nearly continuous month-long Kepler datasets of these 5 objects; these datasets provide nearly alias–free time–series photometry at unprecedented precision. Following an iterative prewhitening process we derive the pulsational frequency spectra of these stars, separating out artefacts of known instrumental origin. We find that these new pulsating sdB stars are multiperiodic long-period pulsators of the V1093 Her type, with the number of periodicities ranging from 8 (KIC 8302197) to 53 (KIC 11558725). The frequencies and amplitudes are typical of $g$-mode pulsators of this type. We do not find any evidence for binarity in the five stars from their observed pulsation frequencies. As these are $g-$mode pulsators, we briefly looked for period spacings for mode identification, and found average spacings about 260 s and 145 s. This may indicate $l$ = 1 and 2 patterns. Some modes may show evidence of rotational splitting. These discoveries complete the list of compact pulsators found in the survey phase. Of the 13 compact pulsators, only one star was identified as a short-period ($p$-mode) V361 Hya pulsator, while all other new pulsators turned out to be V1093 Her class objects. Among the latter objects, two of them seemed to be pure V1093 Her while the others show additional low amplitude peaks in the $p-$mode frequency range, suggesting their hybrid nature. Authenticity of these peaks will be tested with longer runs currently under analysis.

A pulsation zoo in the hot subdwarf B star KIC 10139564 observed by Kepler

A. S. Baran, M. D. Reed, D. Stello, R.H. stensen, J.H. Telting, E. Paktien, S. J. O'Toole, R. Silvotti, P. Degroote, S. Bloemen, H. Hu, V. Van Grootel, B.D. Clarke, J. Van Cleve, S.E. Thompson, S. Kawaler.
Full text

We present our analyses of 15 months of Kepler data on KIC 10139564. We detected 57 periodicities with a variety of properties not previously observed all together in one pulsating subdwarf B star. Ten of the periodicities were found in the low-frequency region, and we associate them with nonradial g-modes. The other periodicities were found in the high-frequency region, which are likely p-modes. We discovered that most of the periodicities are components of multiplets with a common spacing. Assuming that multiplets are caused by rotation, we derive a rotation period of 25.6$\pm$1.8 days. The multiplets also allow us to identify the pulsations to an unprecedented extent for this class of pulsator. We also detect $l\geq2$ multiplets, which are sensitive to the pulsation inclination and can constrain limb darkening via geometric cancellation factors. While most periodicities are stable, we detected several regions that show complex patterns. Detailed analyses showed these regions are complicated by several factors. Two are combination frequencies that originate in the superNyquist region and were found to be reflected below the Nyquist frequency. The Fourier peaks are clear in the superNyquist region, but the orbital motion of Kepler smears the Nyquist frequency in the barycentric reference frame and this effect is passed on to the subNyquist reflections. Others are likely multiplets but unstable in amplitudes and/or frequencies. The density of periodicities also make KIC 10139564 challenging to explain using published models. This menagerie of properties should provide tight constraints on structural models, making this subdwarf B star the most promising for applying asteroseismology.
To support our photometric analysis we have obtained spectroscopic radial-velocity measurements of KIC 10139564 using low-resolution spectra in the Balmer-line region. We did not find any radial-velocity variation. We used our high S/N average spectrum to improve the atmospheric parameters of the sdB star, deriving T$_{\rm eff}$ = 27,910 K and $\log g$ = 5.41 dex.

Solar-like oscillations from the depths of the red-giant star KIC 4351319 observed with Kepler

M. P. Di Mauro, D. Cardini, G. Catanzaro, R. Ventura, C. Barban, T. R. Bedding, J. Christensen-Dalsgaard, J. De Ridder, S. Hekker, D. Huber, T. Kallinger, A. Miglio, J. Montalban, B. Mosser, D. Stello, K. Uytterhoeven, K. Kinemuchi and 3 coauthors.
Full text
NASA ADS
DOI

We present the results of the asteroseismic analysis of the red-giant star KIC 4351319 (TYC 3124-914-1), observed for 30 days with the Kepler satellite. The analysis has allowed us to determine the large and small frequency separations, $\Delta \nu_{0} = 24.6 \pm 0.2\, \mu$Hz and $\delta \nu_{02} = 2.2 \pm 0.3 \,\mu$Hz respectively, and the frequency of maximum oscillation power, $\nu_{\rm max} = 386.5 \pm 4.0\, \mu$Hz. The high signal-to-noise ratio of the observations allowed us to identify $25$ independent pulsation modes whose frequencies range approximately from $300$ to $500\,\mu$Hz.
The observed oscillation frequencies together with the accurate determination of the atmospheric parameters (effective temperature, gravity and metallicity), provided by additional ground-based spectroscopic observations, enabled us to theoretically interpret the observed oscillation spectrum. KIC 4351319 appears to oscillate with a well defined solar-type p-modes pattern due to radial acoustic modes and non-radial nearly pure p modes. In addition, several non-radial mixed modes have been identified. Theoretical models well reproduce the observed oscillation frequencies and indicate that this star, located at the base of the ascending red-giant branch, is in the hydrogen-shell burning phase, with a mass of $\sim 1.3\, {\rm M}_{\odot}$, a radius of $\sim 3.4\,R_{\odot}$ and an age of $\sim5.6$ Gyr. The main parameters of this star have been determined with an unprecedent level of precision for a red-giant star, with uncertainties of $2\,\%$ for mass, $7\,\%$ for age, $1\,\%$ for radius, and $5\,\%$ for luminosity.

Calibrating Convective properties of Solar-like Stars in the Kepler Field of View

Ana Bonaca, Joel D. Tanner, Sarbani Basu, William J. Chaplin, Travis S. Metcalfe, Mario J. P. F. G. Monteiro, Jerome Ballot, Timothy R. Bedding, Alfio Bonanno, Anne-Marie Broomhall, Hans Bruntt, Tiago Campante, Joergen Christensen-Dalsgaard, Enrico Corsaro, Yvonne Elsworth, Rafael A. Garcia, Saskia Hekker and 10 coauthors.
Full text
NASA ADS
DOI

Stellar models generally use simple parametrizations to treat convection. The most widely used parametrization is the so-called “Mixing Length Theory” where the convective eddy sizes are described using a single number, $\alpha$, the mixing-length parameter. This is a free parameter, and the general practice is to calibrate $\alpha$ using the known properties of the Sun and apply that to all stars. Using data from NASA's Kepler mission we show that using the solar-calibrated $\alpha$ is not always appropriate, and that in many cases it would lead to estimates of initial helium abundances that are lower than the primordial helium abundance. Kepler data allow us to calibrate $\alpha$ for many other stars and we show that for the sample of stars we have studied, the mixing-length parameter is generally lower than the solar value. We studied the correlation between $\alpha$ and stellar properties, and we find that $\alpha$ increases with metallicity. We therefore conclude that results obtained by fitting stellar models or by using population-synthesis models constructed with solar values of $\alpha$ are likely to have large systematic errors. Our results also confirm theoretical expectations that the mixing-length parameter should vary with stellar properties.

Asteroseismology of old open clusters with Kepler: direct estimate of the integrated RGB mass loss in NGC6791 and NGC6819

A. Miglio, K. Brogaard, D. Stello, W. J. Chaplin, F. D'Antona, J. Montalbán, S. Basu, A. Bressan, F. Grundahl, M. Pinsonneault, A. M. Serenelli, Y. Elsworth, S. Hekker, T. Kallinger, B. Mosser, P. Ventura, A. Bonanno and 3 coauthors.
Full text
NASA ADS
DOI

The integrated and instantaneous mass loss of red giant branch (RGB) stars is still poorly determined and understood, despite its crucial role in the chemical enrichment of galaxies. Thanks to the recent detection of solar-like oscillations in G-K giants in open clusters with Kepler, we can now directly determine stellar masses for a statistically significant sample of stars in the old open clusters NGC6791 and NGC6819. The aim of this work is to provide constraints on the integrated RGB mass loss by comparing the average mass of stars in the red clump (RC) with that of stars in the low-luminosity portion of the RGB (i.e. stars with $L\lesssim L({\rm RC})$). Stellar masses are determined by combining the available seismic parameters $\nu_{\rm max}$ and $\Delta\nu$ with additional photometric constraints and with independent distance estimates. We measure the mass of 40 (19) stars in the RGB (RC) of the old metal-rich cluster NGC6791. We find that the difference between the average mass of RGB and RC stars is small, but significant ($\Delta \bar{M}=0.09\pm 0.03$ (random) $\pm 0.04$ (systematic) $M_\odot$). Interestingly, such a small $\Delta\bar{M}$ does not support scenarios of an extreme mass loss for this metal-rich cluster. If we describe the mass-loss rate with Reimers' prescription, a first comparison with isochrones suggests that the observed $\Delta\bar M$ is compatible with a mass-loss efficiency parameter in the range $0.1 \lesssim \eta \lesssim 0.3$. Constraints on the RGB mass-loss rate set by the analysis of the $\sim 2$ Gyr-old NGC6819 are less stringent, largely due to the lower mass loss expected for this cluster, and to the lack of an independent and accurate distance determination. In the near future, additional constraints which comprise frequencies of individual pulsation modes and spectroscopic effective temperatures, will allow further stringent tests of the $\Delta\nu$ and $\nu_{\rm max}$ scaling relations, which provide a novel, and potentially very accurate, means of determining stellar radii and masses.

The complex case of V445 Lyr: Two Blazhko modulations, a non-radial mode, possible triple mode RR Lyrae pulsation, and more

E. Guggenberger, K. Kolenberg, J. Nemec, R. Smolec, J.M. Benk, C.-C. Ngeow, R. Szabó, M. Catelan, K. Kinemuchi, S. E. Seader, J. C. Smith, P. Tenenbaum, H. Kjeldsen.
Full text

Rapid and strong changes in the Blazhko modulation of RR Lyrae stars, as they have recently been detected in high precision satellite data, have become a crucial topic in finding an explanation of the long-standing mystery of the Blazhko effect. We present here an analysis of the most extreme case detected so far, the RRab star V445 Lyr (KIC 6186029) which was observed with the Kepler space mission. V445 Lyr shows very strong cycle-to-cycle changes in its Blazhko modulation, which are caused both by a secondary long-term modulation period as well as irregular variations. In addition to the complex Blazhko modulation, V445 Lyr also shows a rich spectrum of additional peaks in the frequency range between the fundamental pulsation and the first harmonic. Among those peaks, the second radial overtone could be identified, which, combined with a metallicity estimate of [Fe/H]=-1.5 dex from spectroscopy, allowed to constrain the mass (0.65-0.75 $M_{\odot}$) and luminosity (40-50 $L_{\odot}$) of V445 Lyr through theoretical Petersen diagrams. A non-radial mode as well as possibly the first overtone are also excited. Furthermore, V445 Lyr shows signs of the period doubling phenomenon and a long term period change. A detailed Fourier analysis along with a study of the O-C variation of V445 Lyr is presented, and the origin of the additional peaks and possible causes of the changes in the Blazhko modulation are discussed. The results are then put into context with those of the only other star with a variable Blazhko effect for which a long enough set of high precision continuous satellite data has been published so far, the CoRoT star 105288363.

Kepler photometry of KIC 10661783: a binary star with total eclipses and $\delta\,$Scuti pulsations

John Southworth, W. Zima, C. Aerts, H. Bruntt, H. Lehmann, S.-L. Kim, D. W. Kurtz, K. Pavlovski, A. Prsa, B. Smalley, R. Gilliland, S. Kawaler, J. Christensen-Dalsgaard, H. Kjeldsen.
Full text

We present Kepler satellite photometry of KIC 10661783, a short-period binary star system which shows total eclipses and multi-periodic $\delta$ Scuti pulsations. A frequency analysis of the eclipse-subtracted light curve reveals at least 75 frequencies of which 64 or more can be attributed to pulsation modes. The main limitation on this analysis is the frequency resolution within the 27-day short-cadence light curve. Most of the variability signal lies between in the frequency range 18 to 31 $\mathrm{d}^{-1}$, with amplitudes between 0.1 and 4 mmag. One harmonic term ($2 \times f$) and a few combination frequencies ($f_i + f_j$) have been detected. From a plot of the residuals versus orbital phase we assign the pulsations to the primary star in the system. The pulsations were removed from the short-cadence data and the light curve was modelled using the Wilson-Devinney code. We are unable to get a perfect fit due to the residual effects of pulsations and also to the treatment of reflection and reprocessing in the light curve model. A model where the secondary star fills its Roche lobe is favoured, which means that KIC 10661783 can be classified as an oEA system. Further photometric and spectroscopic observations will allow the masses and radii of the two stars to be measured to high precision and hundreds of $\delta$ Scuti pulsation frequencies to be resolved, leading to unique constraints on theoretical models of $\delta$ Scuti stars.

Oscillation mode frequencies of 61 main-sequence and subgiant stars observed by Kepler

T. Appourchaux, W. J. Chaplin, R. A. García, M. Gruberbauer, G. A. Verner, H. M. Antia, O. Benomar, T. L. Campante, G. R. Davies, S. Deheuvels, R. Handberg, S. Hekker, R. Howe, C. Régulo, D. Salabert, T. R. Bedding, T. R. White and 11 coauthors.
Full text
NASA ADS
DOI

Solar-like oscillations have been observed by Kepler and CoRoT in several solar-type stars, thereby providing a way to probe the stars using asteroseismology.
We provide the mode frequencies required for performing a comparison with those obtained from stellar modelling.
Time series of 9 months of data have been used. The 61 stars observed were categorised in three groups: simple, F-like, mixed-mode. The simple group includes stars for which the identification of the degree is the mode is obvious. The F-like group includes stars for which the identification of the degree is ambiguous. The mixed-mode group includes evolved stars for which the modes do not follow the asymptotic relation of low-degree frequencies. Following the categorisation, the power spectra of the 61 main-sequence and subgiant stars have been analysed using both Maximum Likelihood Estimators and Bayesian estimators, providing individual mode characteristics such as frequencies, linewidths, and mode heights. We derived and describe a methodology for extracting a single set of mode frequencies from multiple sets derived by different methods and individuals. We report on how one can assess the quality of the fitted parameters using the likelihood ratio test and the posterior probabilities.
Here we give the mode frequencies of 61 stars (with their 1-$\sigma$ error bars), as well as their associated échelle diagrams.

Deep asteroseismic sounding of the compact hot B subdwarf pulsator KIC02697388 from Kepler time series photometry

S. Charpinet, V. Van Grootel, G. Fontaine, E.M. Green, P. Brassard, S.K. Randall, R. Silvotti, R.H. Ostensen, H. Kjeldsen, J. Christensen-Dalsgaard, S.D. Kawaler, B.D. Clarke, J. Li, B. Wohler.
Full text

Contemporary high precision photometry from space provided by the CoRoT and Kepler satellites generates significant breakthroughs in terms of exploiting the long period, g-mode pulsating hot B subdwarf (sdBV$_{{\rm s}}$) stars with asteroseismology. In this paper, we present a detailed asteroseismic study of the sdBV$_{{\rm s}}$ star KIC02697388 monitored with Kepler, using the rich pulsation spectrum uncovered during the $\sim27$ day long exploratory run Q2.3. We analyse new high-S/N spectroscopy of KIC02697388 using appropriate NLTE model atmospheres in order to provide accurate atmospheric parameters for this star. We also reanalyse the Kepler light curve using standard prewhitening techniques. On this basis, we apply a forward modeling technique using our latest generation of sdB models. The simultaneous match of the independent periods observed in KIC02697388 with those of models leads objectively to the identification of the pulsation modes and, more importantly, to the determination of some of the parameters of the star. The light curve analysis reveals 43 independent frequencies that can be associated with oscillation modes. All the modulations observed in this star correspond to $g$-mode pulsations except one high-frequency signal, which is typical of a $p$-mode oscillation. Although the presence of this $p$-mode is surprising considering the atmospheric parameters that we derive for this cool sdB star ($T_{{\rm eff}}=$25 395 $\pm$ 227 K, $\log g=5.500\pm0.031$, and $\log N({\rm He})/N({\rm H})=-2.767\pm0.122$), we show that this mode can be accounted for particularly well by our optimal seismic models, both in terms of frequency match and nonadiabatic properties. The seismic analysis leads us to identify two model solutions that can both account for the observed pulsation properties of KIC02697388. Despite this remaining ambiguity, several key parameters of the star can be derived with stringent constraints, such as its mass, its H-rich envelope mass, its radius, and its luminosity. Information on the core is also obtained and suggests a relatively young sdB star having burnt less than $\sim 34 \%$ (in mass) of its central helium and having a relatively large mixed He/C/O core. This latter measurement is in line with the trend already uncovered for two other $g$-mode sdB pulsators analysed with asteroseismology and suggests that extra mixing is occurring quite early in the evolution of He cores on the horizontal branch. Additional monitoring with Kepler of this particularly interesting sdB star should further reveal the inner properties of KIC02697388 and provide precious information on the mode driving mechanism and the helium core properties.

Stellar ages and convective cores in field main-sequence stars: first asteroseismic application to two Kepler targets

V. Silva Aguirre, S. Basu, I. M. Brandão, J. Christensen-Dalsgaard, S. Deheuvels, G. Doan, T. S. Metcalfe, A. M. Serenelli, J. Ballot, W. J. Chaplin, M. S. Cunha, A. Weiss, T. Appourchaux, T. R.Bedding, L. Casagrande, S. Cassisi, O. L. Creevey and 8 coauthors.
Full text

Using asteroseismic data and stellar evolution models we make the first detection of a convective core in a Kepler field main-sequence star, putting a stringent constraint on the total size of the mixed zone and showing that extra mixing beyond the formal convective boundary exists. In a slightly less massive target the presence of a convective core cannot be conclusively discarded, and thus its remaining main-sequence life time is uncertain. Our results reveal that best-fit models found solely by matching individual frequencies of oscillations corrected for surface effects do not always properly reproduce frequency combinations. Moreover, slightly different criteria to define what the best-fit model is can lead to solutions with similar global properties but very different interior structures. We argue that the use of frequency ratios is a more reliable way to obtain accurate stellar parameters, and show that our analysis in field main-sequence stars can yield an overall precision of 1.5%, 4%, and 10% in radius, mass and age, respectively. We compare our results with those obtained from global oscillation properties, and discuss the possible sources of uncertainties in asteroseismic stellar modeling where further studies are still needed.

The unusual roAp star KIC 8677585

L. A. Balona, G. Catanzaro, L. Crause, M. S. Cunha, D. Gandolfi, A. Hatzes, P. Kabath, K. Uytterhoeven, P. De Cat.
Full text

KIC 8677585 is a roAp star in the Kepler field which is unique in that there are four low-frequency variations of unknown origin in addition to more than twenty high-frequency roAp modes. We analysed all available spectroscopy and conclude that the star has a constant radial velocity and most likely not a binary. We estimate its effective temperature to be $T_{\rm eff} = 7300 \pm 200$ K from high-dispersion spectra. We present an analysis of 829 d of Kepler short-cadence data which shows clear frequency and amplitude variations with a timescale of months. The dominant low-frequency peak at 3.142 d$^{-1}$ has the same frequency and amplitude variation as one of the roAp modes. We therefore conclude that the low frequencies are oscillations in the roAp star itself, but the driving mechanism is unknown. We find several frequency spacings among the roAp modes equal to the dominant low frequency, suggestive of nonlinear interactions. There is also a clear spacing of $37.2$ $\mu$Hz which we interpret as the large separation and deduce that $\log g = 3.90 \pm 0.03$. Models with these parameters which take into account the effect of the magnetic field on the oscillations are able to reproduce the observed range of roAp frequencies, but not the observed large separation. It is found that the properties of the oscillations are sensitive to the assumed stellar parameters and that a more detailed analysis is required. The fact that low frequencies are closely coupled to the roAp frequencies calls into question our current understanding of pulsation in these stars.

Kepler photometry and optical spectroscopy of the ZZ Lep central star of the Planetary Nebula NGC 6826: rotational and wind variability

G. Handler, R. K. Prinja, M. A. Urbaneja, V. Antoci, J. D. Twicken, T. Barclay.
Full text
NASA ADS
DOI

We present three years of long-cadence and over one year of short-cadence photometry of the central star of the Planetary Nebula NGC 6826 obtained with the Kepler spacecraft, and temporally coinciding optical spectroscopy. The light curves are dominated by incoherent variability on time scales of several hours, but contain a lower-amplitude periodicity of 1.23799 d. The temporal amplitude and shape changes of this signal are best explicable with a rotational modulation, and are not consistent with a binary interpretation. We argue that we do not observe stellar pulsations within the limitations of our data, and show that a binary central star with an orbital period less than seven days could only have escaped our detection in the case of low orbital inclination. Combining the photometric and spectroscopic evidence, we reason that the hourly variations are due to a variable stellar wind, and are global in nature. The physical cause of the wind variability of NGC 6826 and other ZZ Leporis stars is likely related to the mechanism responsible for wind variations in massive hot stars.

Gravity modes as a way to distinguish between hydrogen- and helium-burning red giant stars

Timothy R. Bedding, Benoit Mosser, Daniel Huber, Josefina Montalbán, Paul Beck, Jrgen Christensen-Dalsgaard, Yvonne P. Elsworth, Rafael A. García, Andrea Miglio, Dennis Stello, Timothy R. White, Joris De Ridder, Saskia Hekker, Conny Aerts, Caroline Barban, Kevin Belkacem, Anne-Marie Broomhall and 17 coauthors.
Full text
NASA ADS
DOI

Red giants are evolved stars that have exhausted the supply of hydrogen in their cores and instead burn hydrogen in a surrounding shell. Once a red giant is sufficiently evolved, the helium in the core also undergoes fusion. Outstanding issues in our understanding of red giants include uncertainties in the amount of mass lost at the surface prior to helium ignition and the amount of internal mixing from rotation and other processes. Progress is hampered by our inability to distinguish between red giants burning helium in the core and those still only burning hydrogen in a shell. Asteroseismology offers a way forward, being a powerful tool for probing the internal structures of stars using their natural oscillation frequencies. Here we report observations of gravity-mode period spacings in red giants that permit a distinction between evolutionary stages to be made. We use high-precision photometry obtained with the Kepler spacecraft over more than a year to measure oscillations in several hundred red giants. We find many stars whose dipole modes show sequences with approximately regular period spacings. These stars fall into two clear groups, allowing us to distinguish unambiguously between hydrogen-shell-burning stars and those that are also burning helium.

Super-Nyquist asteroseismology with the Kepler Space Telescope

Simon J. Murphy, Hiromoto Shibahashi, Donald W. Kurtz.
Full text

Barycentric corrections made to the timing of \kepler observations, necessitated by variations in light arrival time at the satellite, break the regular time-sampling of the data – the time stamps are periodically modulated. A consequence is that Nyquist aliases are split into multiplets that can be identified by their shape. Real pulsation frequencies are distinguishable from these aliases and their frequencies are completely recoverable, even in the super-Nyquist regime, that is, when the sampling interval is longer than half the pulsation period. We provide an analytical derivation of the phenomenon, alongside demonstrations with simulated and real \kepler data for $\delta$ Sct, roAp, and sdBV stars. For Kepler data sets spanning more than one Kepler orbital period (372.5 d), there are no Nyquist ambiguities on the determination of pulsation frequencies, which are the fundamental data of asteroseismology.

Fourier analysis of non-Blazhko ab-type RR Lyr stars observed with the Kepler space telescope

J. M. Nemec, R. Smolec, J. M. Benk, P. Moskalik, K. Kolenberg, R. Szabó, D. W. Kurtz, S. Bryson, E. Guggenberger, M. Chadid, Y.-B. Jeon, A. Kunder, A. C. Layden, K. Kinemuchi, L. L. Kiss, E. Poretti, J. Christensen-Dalsgaard and 3 coauthors.
Full text
NASA ADS
DOI

Nineteen of the $\sim$40 RR Lyr stars in the Kepler field have been identified as candidate non-Blazhko (or unmodulated) stars. In this paper we present the results of Fourier decomposition of the time-series photometry of these stars acquired during the first 417 days of operation (Q0-Q5) of the Kepler telescope. Fourier parameters based on $\sim$18400 long-cadence observations per star (and $\sim$150000 short-cadence observations for FN Lyr and for AW Dra) are derived. None of the stars shows the recently discovered `period-doubling' effect seen in Blazhko variables; however, KIC 7021124 has been found to pulsate simultaneously in the fundamental and second overtone modes with a period ratio $P_2/P_0 \sim 0.59305$ and is similar to the double-mode star V350 Lyr. Period change rates are derived from O-C diagrams spanning, in some cases, over 100 years; these are compared with high-precision periods derived from the Kepler data alone. Extant Fourier correlations by Kovács, Jurcsik et al. (with minor transformations from the $V$- to the $Kp$ passband) have been used to derive underlying physical characteristics for all the stars. This procedure seems to be validated through comparisons of the Kepler variables with galactic and LMC field RR Lyr stars and with RR Lyr stars in galactic and LMC globular clusters. The most metal-poor star in the sample is NR Lyr, with [Fe/H]$=-2.25\pm0.11$ dex; fourteen of the stars have metallicities between $-1.3$ and $-2.1$ dex; and the four lowest amplitude stars (KIC 6100702, V2470 Cyg, V782 Cyg and V784 Cyg) are found to be quite metal-rich with [Fe/H] between $-0.4$ and $-0.9$ dex. The metal-rich stars also have the lowest luminosities and masses. Pulsational luminosities and masses are found to be systematically smaller than $L$ and ${\cal M}$ values derived from stellar evolution models, but are favoured over the evolutionary values when periods are computed with the Warsaw non-linear hydrodynamics code. Finally, the Fourier parameters are compared with theoretical values derived using the Warsaw convective pulsation codes (linaer and nonlinear).

Low-frequency variations of unknown origin in the Kepler $\delta$ Scuti star KIC 5988140 = HD188774

P. Lampens, A. Tkachenko, H. Lehmann, J. Debosscher, C. Aerts, P. G. Beck, S. Bloemen, N. Kochiashvili, A. Derekas, J. C. Smith, P. Tenenbaum, J. D. Twicken.
Full text

We used high-quality Kepler photometry and spectroscopic data to investigate the Kepler target and binary candidate KIC 5988140. We aim to interpret the observed variations of KIC 5988140 by considering three possible scenario's: binarity, co-existence of $\delta$ Scuti- and $\gamma$ Dor-type oscillations, and rotational modulation due to stellar surface spots. We used the spectrum synthesis method to derive the fundamental parameters T$_{\rm eff}$, log $g$, [M/H], and $v$ sin $i$ from the newly obtained high-resolution, high S/N spectra. Frequency analyses of both the photometric and the spectroscopic data were performed. The star has a spectral type of A7.5 IV-III and a metallicity slightly lower than that of the Sun. Both Fourier analyses reveal the same two dominant frequencies $\mathrm{f}_1=2\mathrm{f}_2=0.688$ and $\mathrm{f}_2$=0.344 c\d. We also detected in the photometry the signal of nine more, significant frequencies located in the typical range of $\delta$ Sct pulsation. The light and radial velocity curves, which are not exactly in anti-phase but show a relative phase shift of about 0.1 period between the moment of minimum velocity and that of maximum light, follow a similar, stable double-wave pattern. Such findings are incompatible with the star being a binary system. We next show that, for all possible (limit) configurations of a spotted surface, the predicted light-to-velocity amplitude ratio is almost two orders larger than the observed value, which pleads against rotational modulation. The same argument also invalidates the explanation in terms of pulsations of type $\gamma$ Dor (i.e. hybrid pulsations). We confirm the occurrence of various independent $\delta$ Sct-type pressure modes in the Kepler light curve. With respect to the low-frequency content, however, we argue that the physical cause of the remaining light and radial velocity variations of this late A-type star remains unexplained by any of the presently considered scenarios.

Regularities in frequency spacings of Delta Scuti stars: The Kepler star KIC 9700322

M. Breger, L. Balona, P. Lenz, J. K. Hollek, D. W. Kurtz, G. Catanzaro, M. Marconi, A. A. Pamyatnykh, B. Smalley, J. C. Suarez, R. Szabo, K. Uytterhoeven, V. Ripepi, J. Christensen-Dalsgaard, H. Kjeldsen, M. N. Fanelli, K. A. Ibrahim and 1 coauthors.
Full text

In the faint star KIC 9700322 observed by the Kepler satellite, 76 frequencies with amplitudes from 14 to 29000 ppm were detected. The two dominant frequencies at 9.79 and 12.57 d$^{-1}$ (113.3 and 145.5 $\mu$Hz), interpreted to be radial modes, are accompanied by a large number of combination frequencies. A small additional modulation with a 0.16 d$^{-1}$ frequency is also seen; this is interpreted to be the rotation frequency of the star. The corresponding prediction of slow rotation is confirmed by a spectrum from which $v \sin i = 19 \pm 1$ km s$^{-1}$ is obtained. The analysis of the spectrum shows that the star is one of the coolest $\delta$ Sct variables. We also determine T$_{\rm eff}$ = 6700 $\pm$ 100 K and $\log g$ = 3.7 $\pm$ 0.1, compatible with the observed frequencies of the radial modes. Normal solar abundances are found. An $\ell=2$ frequency quintuplet is also detected with a frequency separation consistent with predictions from the measured rotation rate. A remarkable result is the absence of additional independent frequencies down to an amplitude limit near 14 ppm, suggesting that the star is stable against most forms of nonradial pulsation. The frequency spectrum of this star emphasizes the need for caution in interpreting low frequencies in $\delta$ Sct stars as independent gravity modes. A low frequency peak at 2.7763 d$^{-1}$ in KIC 9700322 is, in fact, the frequency difference between the two dominant modes and is repeated over and over in various frequency combinations involving the two dominant modes. The relative phases of the combination frequencies show a strong correlation with frequency, but the physical significance of this result is not clear.

High frequencies in the $\delta$ Scuti star KIC 4840675

L. A. Balona, M. Breger, G. Catanzaro, M. S. Cunha, G. Handler, Z. Koaczkowski, D. W. Kurtz, S. Murphy, E. Niemczura, M. Paparo, B. Smalley, R. Szabo, K. Uytterhoeven, J. L. Christiansen, K. Uddin, M. C. Stumpe.
Full text

We show that the star KIC 4840675 observed by Kepler is a triple system with a rapidly-rotating A-type star and two solar-type fainter companions. The A-type star is a $\delta$ Scuti variable with a dominant mode and many other modes of lower amplitude, including several low-frequency variations. The low-frequency variation with highest amplitude can be interpreted as rotational modulation with the light curve changing with time. However, the most interesting aspect of this star is a triplet of independent modes in the range 118–129 d$^{-1}$ (1.4–1.5 mHz), which is far outside the range of typical $\delta$ Scuti frequencies. We discuss the possibility that these modes could be solar-like oscillations, oscillations of the roAp type or due to an unseen pulsating compact companion.

Fundamental Properties of Stars using Asteroseismology from Kepler & CoRoT and Interferometry from the CHARA Array

D. Huber, M. J. Ireland, T. R. Bedding, I. M. Brandão, L. Piau, V. Maestro, T. R. White, H. Bruntt, L. Casagrande, J. Molenda-akowicz, V. Silva Aguirre, S. G. Sousa, T. Barclay, C. J. Burke, W. J. Chaplin, J. Christensen-Dalsgaard, M. S. Cunha and 22 coauthors.
Full text

We present results of a long-baseline interferometry campaign using the PAVO beam combiner at the CHARA Array to measure the angular sizes of five main-sequence stars, one subgiant and four red giant stars for which solar-like oscillations have been detected by either Kepler or CoRoT. By combining interferometric angular diameters, Hipparcos parallaxes, asteroseismic densities, bolometric fluxes and high-resolution spectroscopy we derive a full set of near model-independent fundamental properties for the sample. We first use these properties to test asteroseismic scaling relations for the frequency of maximum power (nu_max) and the large frequency separation (Delta_nu). We find excellent agreement within the observational uncertainties, and empirically show that simple estimates of asteroseismic radii for main-sequence stars are accurate to < 4%. We furthermore find good agreement of our measured effective temperatures with spectroscopic and photometric estimates with mean deviations for stars between T_eff = 4600-6200 K of -22+/-32 K (with a scatter of 97K) and -58+/-31 K (with a scatter of 93 K), respectively. Finally we present a first comparison with evolutionary models, and find differences between observed and theoretical properties for the metal-rich main-sequence star HD173701. We conclude that the constraints presented in this study will have strong potential for testing stellar model physics, in particular when combined with detailed modelling of individual oscillation frequencies.

Spin down of the core rotation in red giants

B. Mosser, M.J. Goupil, K. Belkacem, J.P. Marques, P. Beck, J. De Ridder, C. Barban, S. Deheuvels, Y. Elsworth, S. Hekker, T. Kallinger, R.M. Ouazzani, M. Pinsonneault, R. Samadi, D. Stello, T.C. Klaus, J. Li and 1 coauthors.
Full text

The space mission Kepler provides us with long and uninterrupted photometric time series of red giants. Thanks to the observation of mixed modes, we can probe their deep interior and have now access to the rotation in the innermost region. We aim to measure the rotational splittings in red giants and to derive scaling relations related to seismic and fundamental stellar parameters. We have developed a dedicated method for measuring in an automated way the rotational splittings in a large number of red giants. We have also used the measurements provided by the full identification of the mixed-mode pattern. We then performed ensemble asteroseismology: the examination of a large number of red giants allows us to derive information on their evolution. We have shown that the rotational splittings are in all cases dominated by the core rotation. For the first time, we directly measure this rotation in a large sample of red giants (about 300). We observe a significant change in the core rotation associated with internal angular momentum transfer in stars ascending the red giant branch. This transfer is boosted when the stellar radius reaches $\simeq \Rdredge$, where the first dredge-up is supposed to occur. We also show that, at fixed stellar radius, the specific angular momentum increases with increasing stellar mass, and is smaller for stars in the red clump compared to stars in the red giant branch. Ensemble asteroseismology directly proves what has been indirectly suspected for a while: the core rotation significantly slows down during the red giant phase. It occurs when the convection envelope expands and gives rise to the first dredge-up. This spinning down explains, for instance, the long rotation periods measured in white dwarfs.

Relationship between low and high frequencies in Delta Scuti stars: Photometric Kepler and spectroscopic analyses of the rapid rotator KIC 8054146

M. Breger, L. Fossati, L. Balona, D. W. Kurtz, P. Robertson, D. Bohlender, P. Lenz, I. Mueller, Th. Lueftinger, B. D. Clarke, J. R. Hall, A. I. Khadeejah.
Full text

Two years of Kepler data of KIC 8054146 ($\delta$ Sct/$\gamma$ Dor hybrid) revealed 349 statistically significant frequencies between 0.54 and 191.36 cd$^{-1}$ (6.3 $\mu$Hz to 2.21 mHz). The 117 low frequencies cluster in specific frequency bands, but do not show the equidistant period spacings predicted for gravity modes of successive radial order, $n$, and reported for at least one other hybrid pulsator. The four dominant low frequencies in the 2.8 to 3.0 cd$^{-1}$ (32 to 35 $\mu$Hz) range show strong amplitude variability with timescales of months and years. These four low frequencies also determine the spacing of the higher frequencies in and beyond the $\delta$ Sct pressure-mode frequency domain. In fact, most of the higher frequencies belong to one of three families with spacings linked to a specific dominant low frequency. In the Fourier spectrum, these family regularities show up as triplets, high-frequency sequences with absolutely equidistant frequency spacings, side lobes (amplitude modulations) and other regularities in frequency spacings. Furthermore, within two families the amplitude variations between the low and high frequencies are related. We conclude that the low frequencies (gravity modes, rotation) and observed high frequencies (mostly pressure modes) are physically connected. This unusual behavior may be related to the very rapid rotation of the star: from a combination of high and low-resolution spectroscopy we determined that KIC 8054146 is a very fast rotator (ini = 300 $\pm$ 20 \kms) with an effective temperature of 7600 $\pm$ 200 K and a surface gravity ogg of 3.9 $\pm$ 0.3. Several astrophysical ideas explaining the origin of the relationship between the low and high frequencies are explored.

Asteroseismology of the Open Clusters NGC 6791, NGC 6811, and NGC 6819 from 19 Months of Kepler Photometry

Enrico Corsaro, Dennis Stello, Daniel Huber, Timothy R. Bedding, Alfio Bonanno, Karsten Brogaard, Thomas Kallinger, Othman Benomar, Timothy R. White, Benoit Mosser, Sarbani Basu, William J. Chaplin, Jrgen Christensen-Dalsgaard, Yvonne P. Elsworth, Rafael A. García, Saskia Hekker, Hans Kjeldsen and 5 coauthors.
Full text
NASA ADS
DOI

We studied solar-like oscillations in 115 red giants in the three open clusters, NGC 6791, NGC 6811, and NGC 6819, based on photometric data covering more than 19 months with NASA's Kepler space telescope. We present the asteroseismic diagrams of the asymptotic parameters $\delta\nu_{02}$, $\delta\nu_{01}$, and epsilon, which show clear correlation with fundamental stellar parameters such as mass and radius. When the stellar populations from the clusters are compared, we see evidence for a difference in mass of the red giant branch stars and possibly a difference in structure of the red clump stars, from our measurements of the small separations $\delta\nu_{02}$ and $\delta\nu_{01}$. Ensemble échelle diagrams and upper limits to the linewidths of $\ell = 0$ modes as a function of $\Delta\nu$ of the clusters NGC 6791 and NGC 6819 are also shown, together with the correlation between the $\ell = 0$ ridge width and the $T_{eff}$ of the stars. Lastly, we distinguish between red giant branch and red clump stars through the measurement of the period spacing of mixed dipole modes in 53 stars among all the three clusters to verify the stellar classification from the color-magnitude diagram. These seismic results also allow us to identify a number of special cases, including evolved blue stragglers and binaries, as well as stars in late He-core burning phases, which can be potentially interesting targets for detailed theoretical modeling.

Verifying asteroseismically determined parameters of Kepler stars using Hipparcos parallaxes: self-consistent stellar properties and distances

V. Silva Aguirre, L. Casagrande, S. Basu, T. L. Campante, W. J. Chaplin, D. Huber, A. Miglio, A. M. Serenelli, J. Ballot, T. R. Bedding, J. Christensen-Dalsgaard, O. L. Creevey, Y. Elsworth, R. A. García, R. L. Gilliland, S. Hekker, H. Kjeldsen and 9 coauthors.
Full text

Accurately determining the properties of stars is of prime importance for characterizing stellar populations in our Galaxy. The field of asteroseismology has been thought to be particularly successful in such an endeavor for stars in different evolutionary stages. However, to fully exploit its potential, robust methods for estimating stellar parameters are required and independent verification of the results is mandatory. With this purpose, we present a new technique to obtain stellar properties by coupling asteroseismic analysis with the InfraRed Flux Method. By using two global seismic observables and multi-band photometry, the technique allows us to obtain masses, radii, effective temperatures, bolometric fluxes, and hence distances for field stars in a self-consistent manner. We apply our method to 22 solar-like oscillators in the Kepler short-cadence sample, that have accurate Hipparcos parallaxes. Our distance determinations agree to better than 5%, while measurements of spectroscopic effective temperatures and interferometric radii also validate our results. We briefly discuss the potential of our technique for stellar population analysis and models of Galactic Chemical Evolution.

Constraining the core structure and evolution of red giants using gravity-dominated mixed modes observed with Kepler

B. Mosser, MJ. Goupil, K. Belkacem, D. Stello, J.P. Marques, Y. Elsworth, C. Barban, P. Beck, J. De Ridder, R.A. García, S. Hekker, T. Kallinger, R. Samadi, M.C. Stumpe, T. Barclay, C.J. Burke.
Full text

There are now more than 19 months of long-cadence data available for some of the red giants observed with the Kepler space mission. Consequently we are able to clearly resolve fine detail in their oscillation spectra and see many components of the so-called mixed modes that probe the radiative regions of the stellar core. In this paper, we report for the first time a parametric fit to the pattern of the $\ell=1$ mixed modes in red giants which is a powerful tool in the identification of gravity-dominated mixed modes. With these modes that share the characteristics of both pressure and gravity modes, we are able to probe directly the helium core and the surrounding shell where hydrogen is burning. We propose two ways for describing the so-called mode bumping that affects the locations of the mixed modes. A phenomenological approach is used to describe the main feature of the mode bumping. Alternatively a quasi-asymptotic mixed-mode relation provides a powerful link between seismic observations and the stellar interior structure. We use period échelle diagrams to emphasize the detection of the gravity-dominated mixed modes. The asymptotic law for mixed modes is confirmed. It allows us to measure the unperturbed gravity-mode period spacings in more than hundred red giant stars. The identification of the gravity-dominated mixed modes allows us to complete the identification of all major peaks in a red giant oscillation spectrum, with significant consequences on the true identification of $\ell=3$ modes, of $\ell=2$ mixed modes, and of the mode widths and amplitudes. The accurate measurement of the gravity-mode period spacing provides an effective probe of the inner, g-mode cavity. The derived value of the coupling coefficient between the cavities is different for RGB and clump stars. This provides a probe of the hydrogen-shell burning region which surrounds the helium core. Core contraction as red giants ascend the red giant branch can be explored using the variation of the gravity-mode spacing as a function of the mean large separation.

Nonlinear asteroseismology of RR Lyrae

L. Molnár, Z. Kolláth, R. Szabó, S. Bryson, K. Kolenberg, F. Mullally, S. E. Thompson.
Full text
NASA ADS
DOI

The observations of the Kepler space telescope revealed that fundamental-mode RR Lyrae stars may show various radial overtones. The presence of multiple radial modes may allow us to conduct nonlinear asteroseismology: comparison of mode amplitudes and frequency shifts between observations and models. Here we report the detection of three radial modes in the star RR Lyr, the eponym of the class, using the Kepler short cadence data: besides the fundamental mode, both the first and the ninth overtones can be derived from the data set. RR Lyrae shows period doubling, but switches occasionally to a state where a pattern of six pulsation cycles repeats instead of two. We found hydrodynamic models that show the same three modes and the period-six state, allowing for comparison with the observations.

Stochastic Brightness Variations in the Central Star of Planetary Nebula NGC 6826

N. Jevtic, P. Stine, W. Nilsen, J. S. Schweitzer, Jon M Jenkins, Todd C. Klaus, Jie Li, Sean McCauliff.
Full text

One year of continuous Kepler Space Telescope data for the central object of planetary nebula NGC 6826 (KIC 12071221) were analyzed. KIC 12071221 has long-cadence light curves with stochastic brightness variations and broadband power spectra. The exception is a one-month long period during which variability was detected with a period of 14.8 hr. In search of the origin of this periodicity and in an attempt to better understand the complex NGC 6826 system, long-cadence KIC 12071221 data were analyzed using nonlinear time series methods. Nonlinear projective noise reduction from a phase-space dimension of six lowered the tail of the power spectra at the higher frequencies by factors of 38 to 133, an unexpected result for light curves with no obvious dominant frequencies. Even more notable is the identification of a structured distribution of trajectories, or coherence, in the reconstructed phase space for vectors spanning an interval of 1.5 hr. Organization weakens as the delay increases and is lost for phase-space vectors spanning times longer than 10 hr. This may be indicative of a binary object which shares a common envelope and/or pulsations but for a complete understanding one or both have to be combined with wind structure. Thus, an estimate is made of the timescale of the response to what we propose is stochastic driving. This estimate of the time scale of the organized response is made by monitoring the loss of structure in the distribution of trajectories in phase space, a novel application of nonlinear methodology that may be used on data sampled with any sampling time.

Seismic evidence for a rapidly rotating core in a lower-giant-branch star observed with Kepler

S. Deheuvels, R. A. García, W. J. Chaplin, S. Basu, H. M. Antia, T. Appourchaux, O. Benomar, G. R. Davies, Y. Elsworth, L. Gizon, M. J. Goupil, D. R. Reese, C. Regulo, J. Schou, T. Stahn, L. Casagrande, J. Christensen-Dalsgaard and 10 coauthors.
Full text

Rotation is expected to have an important influence on the structure and the evolution of stars. However, the mechanisms of angular momentum transport in stars remain theoretically uncertain and very complex to take into account in stellar models. To achieve a better understanding of these processes, we desperately need observational constraints on the internal rotation of stars, which until very recently were restricted to the Sun. In this paper, we report the detection of mixed modes — i.e. modes that behave both as g modes in the core and as p modes in the envelope — in the spectrum of the early red giant KIC7341231, which was observed during one year with the Kepler spacecraft. By performing an analysis of the oscillation spectrum of the star, we show that its non-radial modes are clearly split by stellar rotation and we are able to determine precisely the rotational splittings of 18 modes. We then find a stellar model that reproduces very well the observed atmospheric and seismic properties of the star. We use this model to perform inversions of the internal rotation profile of the star, which enables us to show that the core of the star is rotating at least five times faster than the envelope. This will shed new light on the processes of transport of angular momentum in stars. In particular, this result can be used to place constraints on the angular momentum coupling between the core and the envelope of early red giants, which could help us discriminate between the theories that have been proposed over the last decades.

Atmospheric parameters of 82 red giants in the Kepler field.

A. O. Thygesen, S. Frandsen, H. Bruntt, T. Kallinger, M. F. Andersen, Y. P. Elsworth, S. Hekker, C. Karoff, D. Stello, K. Brogaard, C. Burke, D. A. Caldwell, J. L. Christiansen.
Full text
NASA ADS
DOI

Context: Accurate fundamental parameters of stars are essential for the asteroseismic analysis of data from the NASA kepler mission. Aims: We aim at determining accurate atmospheric parameters and the abundance pattern for a sample of 82 red giants that are targets for the Kepler mission. Methods: We have used high-resolution, high signal-to-noise spectra from three different spectrographs. We used the iterative spectral synthesis method VWA to derive the fundamental parameters from carefully selected high-quality iron lines. After determination of the fundamental parameters abundances of 13 elements were measured using equivalent widths of the spectral lines. Results: We identify discrepancies in log (g) and [Fe/H], compared to the parameters based on photometric indices in the Kepler Input Catalogue (larger than 2.0 dex for log (g) and [Fe/H] for individual stars). The $T_{eff}$ found from spectroscopy and photometry shows good agreement within the uncertainties. We find good agreement between the spectroscopic log (g) and the log (g) derived from asteroseismology. Also we see indications of a potential metallicity effect on the stellar oscillations. Conclusions: We have determined the fundamental parameters and element abundances of 82 red giants. The large discrepancies between the spectroscopic log (g) and [Fe/H] and values in the Kepler Input catalogue emphasize the need for further detailed spectroscopic follow-up of the Kepler targets in order to produce reliable results from the asteroseismic analyses.

Solving the mode identification problem in asteroseismology of F stars observed with Kepler

Timothy R. White, Timothy R. Bedding, Michael Gruberbauer, Othman Benomar, Dennis Stello, Thierry Appourchaux, William J. Chaplin, Jrgen Christensen-Dalsgaard, Yvonne P. Elsworth, Rafael A. García, Saskia Hekker, Daniel Huber, Hans Kjeldsen, Benoît Mosser, Karen Kinemuchi, Fergal Mullally, Martin Still.
Full text
NASA ADS
DOI

Asteroseismology of F-type stars has been hindered by an ambiguity in identification of their oscillation modes. The regular mode pattern that makes this task trivial in cooler stars is masked by increased linewidths. The absolute mode frequencies, encapsulated in the asteroseismic variable $\epsilon$, can help solve this impasse because the values of $\epsilon$ implied by the two possible mode identifications are distinct. We find that the correct $\epsilon$ can be deduced from the effective temperature and the linewidths and we apply these methods to a sample of solar-like oscillators observed with Kepler.

Testing Scaling Relations for Solar-Like Oscillations from the Main-Sequence to Red Giants using Kepler Data

D. Huber, T. R. Bedding, D. Stello, S. Hekker, S. Mathur, B. Mosser, G. A. Verner, A. Bonnano, D. L. Buzasi, T. L. Campante, Y. P. Elsworth, S. J. Hale, T. Kallinger, V. Silva Aguirre, W. J. Chaplin, J. De Ridder, R. A. García and 15 coauthors.
Full text

We have analyzed solar-like oscillations in $\sim$1700 stars observed by the Kepler Mission, spanning from the main-sequence to the red clump. Using evolutionary models, we test asteroseismic scaling relations for the frequency of maximum power ($\nu_{\rm max}$), the large frequency separation ($\Delta\nu$) and oscillation amplitudes. We show that the difference of the $\Delta\nu$-$\nu_{\rm max}$ relation for unevolved and evolved stars can be explained by different distributions in effective temperature and stellar mass, in agreement with what is expected from scaling relations. For oscillation amplitudes, we show that neither $(L/M)^s$ scaling nor the revised scaling relation by Kjeldsen & Bedding (2011) are accurate for red-giant stars, and demonstrate that a revised scaling relation with a separate luminosity-mass dependence can be used to calculate amplitudes from the main-sequence to red-giants to a precision of $\sim$25%. The residuals show an offset particularly for unevolved stars, suggesting that an additional physical dependency is necessary to fully reproduce the observed amplitudes. We investigate correlations between amplitudes and stellar activity, and find evidence that the effect of amplitude suppression is most pronounced for subgiant stars. Finally, we test the location of the cool edge of the instability strip in the Hertzsprung-Russell diagram using solar-like oscillations and find the detections in the hottest stars compatible with a domain of hybrid stochastically excited and opacity driven pulsation.

Kepler observations of the high-amplitude delta Scuti star V2367 Cyg

L. A. Balona, P. Lenz, V. Antoci, S. Bernabei, G. Catanzaro, J. Daszyńska-Daszkiewicz, M. Di Criscienzo, A. Grigahcène, G. Handler, D. W. Kurtz, M. Marconi, J. Molenda-akowicz, A. Moya, J. M. Nemec, A. Pigulski, D. Pricopi, V. Ripepi and 6 coauthors.
Full text

We analyse Kepler observations of the high-amplitude $\delta$ Scuti (HADS) star V2367 Cyg (KIC 9408694). The variations are dominated by a mode with frequency $f_1 = 5.6611$ d$^{-1}$. Two other independent modes with $f_2 = 7.1490$ d$^{-1}$ and $f_3 = 7.7756$ d$^{-1}$ have amplitudes an order of magnitude smaller than $f_1$. Nearly all the light variation is due to these three modes and their combination frequencies, but several hundred other frequencies of very low amplitude are also present. The amplitudes of the principal modes may vary slightly with time. The star has twice the projected rotational velocity of any other HADS star, which makes it unusual. We find a correlation between the phases of the combination frequencies and their pulsation frequencies, which is not understood. Since modes of highest amplitude in HADS stars are normally radial modes, we assumed that this would also be true in this star. However, attempts to model the observed frequencies as radial modes without mode interaction were not successful. For a star with such a relatively high rotational velocity, it is important to consider the effect of mode interaction. Indeed, when this was done we were able to obtain a model in which a good match with $f_1$ and $f_2$ is obtained, with $f_1$ being the fundamental radial mode.

Asteroseismology of the solar analogs 16 Cyg A and B from Kepler observations

T. S. Metcalfe, W. J. Chaplin, T. Appourchaux, R. A. García, S. Basu, I. Brandão, O. L. Creevey, S. Deheuvels, G. Doan, P. Eggenberger, C. Karoff, A. Miglio, D. Stello, M. Yldz, Z. elik, H. M. Antia, O. Benomar and 20 coauthors.
Full text
NASA ADS
DOI

The evolved solar-type stars 16 Cyg A and B have long been studied as solar analogs, yielding a glimpse into the future of our own Sun. The orbital period of the binary system is too long to provide meaningful dynamical constraints on the stellar properties, but asteroseismology can help because the stars are among the brightest in the Kepler field. We present an analysis of three months of nearly uninterrupted photometry of 16 Cyg A and B from the Kepler space telescope. We extract a total of 46 and 41 oscillation frequencies for the two components respectively, including a clear detection of octupole ($l$=3) modes in both stars. We derive the properties of each star independently using the Asteroseismic Modeling Portal, fitting the individual oscillation frequencies and other observational constraints simultaneously. We evaluate the systematic uncertainties from an ensemble of results generated by a variety of stellar evolution codes and fitting methods. The optimal models derived by fitting each component individually yield a common age ($t=6.8\pm0.4$ Gyr) and initial composition ($Z_{\rm i}=0.024\pm0.002, Y_{\rm i}=0.25\pm0.01$) within the uncertainties, as expected for the components of a binary system, bolstering our confidence in the reliability of asteroseismic techniques. The longer data sets that will ultimately become available will allow future studies of differential rotation, convection zone depths, and long-term changes due to stellar activity cycles.

The Kepler view of gamma Doradus stars

L. A. Balona, J. A. Guzic, K. Uytterhoeven, J. C. Smith, P. Tenenbaum, J. D. Twicken.
Full text

Visual classification of over 10 000 stars in the Kepler database has revealed a class of stars with almost mono-periodic light variations and characteristic beating. A subset of these stars have a larger light amplitude and asymmetric light curves with larger variation in maximum brightness than in minimum brightness. The beating is mostly a result of two dominant, closely-spaced frequencies. A third group of stars shows multiple low frequencies of comparable amplitudes. All three types of star fall in the region of the HR diagram where $\gamma$ Dor stars are found and we therefore identify them as $\gamma$ Dor variables. However, stars with migrating starspots also have symmetric light curves with beats, so it is likely that the sample is contaminated by non-pulsating stars of this type. If we assume that the dominant frequency in stars with beats is the rotational frequency, the resulting distribution of equatorial rotational velocities matches that of field stars of similar temperature and luminosity. We therefore conclude that the pulsation periods of these stars must be close to their rotational periods. The third group with multiple frequencies may be slowly-rotating $\gamma$ Dor stars. This investigation is closely related to the presence of low-frequencies in $\delta$ Scuti stars which we briefly discuss.

First Kepler results on compact pulsators V: Pulsating subdwarf B stars in short-period binaries

S. D. Kawaler, M. D. Reed, R. H. Østensen, S. Bloemen, D. W. Kurtz, A. C. Quint, R. Silvotti, A. S. Baran, E. M. Green, S. Charpinet, J. Telting, C. Aerts, G. Handler, H. Kjeldsen, J. Christensen-Dalsgaard, W. J. Borucki, D.G. Koch.
Full text
NASA ADS
DOI

The survey phase of the Kepler Mission includes a number of hot subdwarf B (sdB) stars to search for nonradial pulsations. We present our analysis of two sdB stars that are found to be $g$-mode pulsators of the V1093 Her class. These two stars also display the distinct irradiation effect typical of sdB stars with a close M-dwarf companion with orbital periods of less than half a day. Because the orbital period is so short, the stars should be in synchronous rotation, and if so, the rotation period should imprint itself on the multiplet structure of the pulsations. However, we do not find clear evidence for such rotational splitting. Though the stars do show some frequency spacings that are consistent with synchronous rotation, they also display multiplets with splittings that are much smaller. Longer-duration time series photometry will be needed to determine if those small splittings are in fact rotational splitting, or caused by slow amplitude or phase modulation. Further data should also improve the signal-to-noise, perhaps revealing lower amplitude periodicities that would confirm the expectation of synchronous rotation. The pulsation periods seen in these stars show period spacings that are suggestive of high-overtone $g-$mode pulsations.

First Kepler results on compact pulsators II: KIC 010139564, a new pulsating subdwarf B (V361 Hya) star with an additional low-frequency mode

S. D. Kawaler, M. D. Reed, A. C. Quint, R. H. Østensen, R. Silvotti, A. Baran, S. Charpinet, S. Bloemen, D. W. Kurtz, J. Telting, G. Handler, H. Kjeldsen, J. Christensen-Dalsgaard, W. J. Borucki, D. G. Koch.
Full text
NASA ADS
DOI

We present the discovery of nonradial pulsations in a hot subdwarf B star based on 30.5 days of nearly continuous time-series photometry using the Kepler spacecraft. KIC 010139564 is found to be a short–period pulsator of the V361 Hya (EC 14026) class with more than 10 independent pulsation modes whose periods range from 130 to 190 seconds. It also shows one periodicity at a period of 3165 seconds. If this periodicity is a high order g-mode, then this star may be the hottest member of the hybrid DW Lyn stars. In addition to the resolved pulsation frequencies, additional periodic variations in the light curve suggest that a significant number of additional pulsation frequencies may be present. The long duration of the run, the extremely high duty cycle, and the well–behaved noise properties allow us to explore the stability of the periodic variations, and to place strong constraints on how many of them are independent stellar oscillation modes. We find that most of the identified periodicities are indeed stable in phase and amplitude, suggesting a rotation period of 2-3 weeks for this star, but further observations are needed to confirm this suspicion.

Characterization of the power excess of solar-like oscillations in red giants with Kepler

B. Mosser, Y. Elsworth, S. Hekker, D. Huber, T. Kallinger, S. Mathur, K. Belkacem, M.J. Goupil, R. Samadi, C. Barban, T.R. Bedding, W.J. Chaplin, R.A. García, D. Stello, J. De Ridder, C.K. Middour, R.L. Morris and 1 coauthors.
Full text
NASA ADS
DOI

The space mission Kepler provides us with long and uninterrupted photometric time series of red giants. This allows us to examine their seismic global properties and to compare them with theoretical predictions. We aim to describe the oscillation power excess observed in red-giant oscillation spectra with global seismic parameters, and to propose empirical scaling relations governing these parameters. From these scalings relations, we derive new physical properties of red-giant oscillation. Various different methods were compared in order to validate the processes and to derive reliable output values. Then, for consistency, a single method was used to determine scaling relations concerning the relevant global asteroseismic parameters: mean mode height, mean height of the background signal superimposed on the oscillation excess power, width of the excess power, bolometric amplitude of the radial modes and visibility of non-radial modes. A method for deriving oscillation amplitudes has been proposed, which relies on the complete identification of the red-giant oscillation spectrum. The comparison of the different methods has shown the important role of the modelling of the background. The convergence reached by the collaborative work enables us to derive significant results concerning the oscillation power excess. We obtained several scaling relations, and identify the influence of the stellar mass and/or of the evolutionary status. We found that none of the amplitude scaling relations motivated from physics considerations predicts the observed mode amplitudes of red-giant stars. In parallel, the degree-dependent mode visibility exhibit important variations. Both effects seem related to the significant influence of the high mode mass of non-radial mixed modes. The clear correlation between the power densities of the background signal and of the stellar oscillation induces important consequences to be considered for deriving a reliable theoretical relation of the mode amplitude. As a by-product of this work, we have verified that red-giant asteroseismology delivers new insights for stellar and Galactic physics, given the evidence for mass loss at the tip of the red-giant branch.

Fundamental properties of five Kepler stars using global seismic quantities and ground-based observations

Orlagh L. Creevey, Gülnur Doan, Antonio Frasca, Anders Overaa Thygesen, Sarbani Basu, Jishnu Bhattacharya, Katia Biazzo, Isa Maria Bernardo Brandão, Hans Bruntt, Anwesh Mazumdar, Ewa Niemczura, Tushar Shrotriya, Sérgio G. Sousa, Dennis Stello, Akshay Subramaniam, Tiago L. Campante, Rasmus Handberg and 19 coauthors.
Full text
NASA ADS
DOI

We present an asteroseismic study of the solar-like stars KIC 11395018, KIC 10273246, KIC 10920273, KIC 10339342, and KIC 11234888 using short-cadence time series of more than eight months from the Kepler satellite. For four of these stars, we derive atmospheric parameters from spectra acquired with the Nordic Optical Telescope. The global seismic quantities (average large frequency separation and frequency of maximum power), combined with the atmospheric parameters, yield the mean density and surface gravity with precisions of 2% and ∼0.03 dex, respectively. We also determine the radius, mass, and age with precisions of 2–5%, 7–11%, and $\sim$35%, respectively, using grid-based analyses. Coupling the stellar parameters with photometric data yields an asteroseismic distance with a precision better than 10%. A v sin i measurement provides a rotational period-inclination correlation, and using the rotational periods from the recent literature, we constrain the stellar inclination for three of the stars. An Li abundance analysis yields an independent estimate of the age, but this is inconsistent with the asteroseismically determined age for one of the stars. We assess the performance of five grid-based analysis methods and find them all to provide consistent values of the surface gravity to $\sim$0.03 dex when both atmospheric and seismic constraints are at hand. The different grid-based analyses all yield fitted values of radius and mass to within 2.4$\sigma$, and taking the mean of these results reduces it to 1.5$\sigma$. The absence of a metallicity constraint when the average large frequency separation is measured with a precision of 1% biases the fitted radius and mass for the stars with non-solar metallicity (metal-rich KIC 11395018 and metal-poor KIC 10273246), while including a metallicity constraint reduces the uncertainties in both of these parameters by almost a factor of two. We found that including the average small frequency separation improves the determination of the age only for KIC 11395018 and KIC 11234888, and for the latter this improvement was due to the lack of strong atmospheric constraints.

Oscillation mode linewidths of main-sequence and subgiant stars observed by Kepler

T. Appourchaux, O. Benomar, M. Gruberbauer, W. J. Chaplin, R.A. García, R. Handberg, G.A. Verner, H.M. Antia, G.R. Davies, S. Deheuvels, S. Hekker, R. Howe, D. Salabert, T.R. Bedding, T.R. White, G. Houdek, V. Silva Aguirre and 5 coauthors.
Full text
NASA ADS
DOI

Solar-like oscillations have been observed by Kepler and CoRoT in several solar-type stars.
We study the variations of stellar p-mode linewidth as a function of effective temperature..
Time series of 9 months of Kepler data have been used. The power spectra of 42 cool main-sequence stars and subgiants have been analysed using both Maximum Likelihood Estimators and Bayesian estimators, providing individual mode characteristics such as frequencies, linewidths and mode heights.
Here we report on the mode linewidth at maximum power and at maximum mode height for these 42 stars as a function of effective temperature.
We show that the mode linewidth at either maximum mode height or maximum amplitude follows a scaling relation with effective temperature, which is a combination of a power law plus a lower bound. The typical power law index is about 13 for the linewidth derived from the maximum mode height, and about 16 for the linewidth derived from the maximum amplitude while the lower bound is about 0.3 $\mu$Hz and 0.7 $\mu$Hz, respectively. We stress that this scaling relation is only valid for the cool main-sequence stars and subgiants, and does not have predictive power outside the temperature range of these stars.

Fast core rotation in red-giant stars revealed by gravity-dominated mixed modes

Paul G. Beck, Josefina Montalban, Thomas Kallinger, Joris De Ridder, Conny Aerts, Rafael A. García, Saskia Hekker, Marc-Antoine Dupret, Benoit Mosser, Patrick Eggenberger, Dennis Stello, Yvonne Elsworth, Sren Frandsen, Fabien Carrier, Michel Hillen, Michael Gruberbauer, Jrgen Christensen-Dalsgaard and 7 coauthors.
Full text
NASA ADS
DOI

When the core hydrogen is exhausted during stellar evolution, the central region of a star contracts and the outer envelope expands and cools, giving rise to a red giant, in which convection occupies a large fraction of the star. Conservation of angular momentum requires that the cores of these stars rotate faster than their envelopes, and indirect evidence supports this. Information about the angular momentum distribution is inaccessible to direct observations, but it can be extracted from the effect of rotation on oscillation modes that probe the stellar interior. Here, we report the detection of non-rigid rotation in the interiors of red-giant stars by exploiting the rotational frequency splitting of recently detected mixed modes. We demonstrate an increasing rotation rate from the surface of the star to the stellar core. Comparing with theoretical stellar models, we conclude that the core must rotate at least ten times faster than the surface. This observational result confirms the theoretical prediction of a steep gradient in the rotation profile towards the deep stellar interior.

First Kepler results on compact pulsators VIII: Mode identifications via period spacings in g−mode pulsating Subdwarf B stars

M. D. Reed, A. Baran, A. C. Quint, S. D. Kawaler, S. J. O’Toole, J. Telting, S. Charpinet, C. Rodríguez-López, R. H. Østensen, J. L. Provencal, E. S. Johnson, S. E. Thompson, C. Allen, C. K. Middour, H. Kjeldsen, J. Christensen-Dalsgaard.
Full text
NASA ADS
DOI

We investigate the possibility of nearly-equally spaced periods in 13 hot subdwarf B (sdB) stars observed with the Kepler spacecraft and one observed with CoRoT. Asymptotic limits for gravity ($g-$)mode pulsations provide relationships between equal period spacings of modes with differing degrees $\ell$ and relationships between periods of the same radial order $n$ but differing degrees $\ell$. Period transforms, Kolmogorov-Smirnov tests, and linear least-squares fits have been used to detect and determine the significance of equal period spacings. We have also used Monte Carlo simulations to estimate the likelihood that the detected spacings could be produced randomly.
Period transforms for nine of the Kepler stars indicate $\ell =1$ period spacings, with five also showing peaks for $\ell =2$ modes. 12 stars indicate $\ell =1$ modes using the Kolmogorov-Smirnov test while another shows solely $\ell =2$ modes. Monte Carlo results indicate that equal period spacings are significant in 10 stars above 99% confidence and 13 of the 14 are above 94% confidence. For 12 stars, the various methods find consistent regular period spacing values to within the errors, two others show some inconsistencies, likely caused by binarity, and the last has significant detections but the mode assignment disagrees between methods.
We use asymptotic period spacing relationships to associate observed periods of variability with pulsation modes for $\ell =1$ and $2$. From the Kepler first year survey sample of 13 multiperiodic $g-$mode pulsators, five stars have several consecutive overtones making period spacings easy to detect, six others have fewer consecutive overtones but period spacings are readily detected, %regular period spacings are easily found %in five stars, while an additional six stars show period %spacings that are not as regular, and two stars show marginal indications of equal period spacings. We also examine a $g-$mode sdB pulsator observed by CoRoT with a rich pulsation spectrum and our tests detect regular period spacings.
We use Monte Carlo simulations to estimate the significance of the detections in individual stars. From the simulations it is determined that regular period spacings in 10 of the 14 stars is very unlikely to be random, another two are moderately unlikely to be random and two are mostly unconstrained.
We find a common $\ell =1$ period spacing spanning a range from 231 to 272 s allowing us to correlate pulsation modes with 222 periodicities and that the $\ell =2$ period spacings are related to the $\ell =1$ spacings by the asymptotic relationship $1/\sqrt{3}$. We briefly discuss the impact of equal period spacings which indicate low-degree modes with a lack of significant mode trappings.

First Kepler results on compact pulsators III: Subdwarf B stars with V1093 Her and hybrid (DW Lyn) type pulsations.

M. D. Reed, S. D. Kawaler, R. H. Østensen, S. Bloemen, A. Baran, J. H. Telting, R. Silvotti, S. Charpinet, A. C. Quint, G. Handler, R. L. Gilliland, W. J. Borucki, D. G. Koch, H. Kjeldsen, J. Christensen-Dalsgaard.
Full text
NASA ADS
DOI

We present the discovery of nonradial pulsations in hot subdwarf B (sdB) stars based on 27 days of nearly continuous time-series photometry using the Kepler spacecraft. We find that every sdB star cooler than $\approx 27\,500\,$K is a long-period pulsator of the V1093 Her (PG 1716) class or a hybrid star with both short and long periods. The apparently single long-period and hybrid pulsators are described here. The V1093 Her periods range from one to 4.5 h and are associated with $g-$mode pulsations. Three stars also exhibit short periods indicative of $p-$modes with periods of 2 to 5 m and in addition, these stars exhibit periodicities between both classes from 15 to 45 m. We detect the coolest and longest-period V1093 Her-type pulsator to date, KIC010670103 ($T_{\rm eff}\approx 20\,900\,$K, $P_{\rm max}\approx 4.5$ h) as well as an extremely cool hybrid pulsator, KIC002697388 ($T_{\rm eff}\approx 23\,900\,$K) and for the first time hybrid pulsators which have larger $g-$mode amplitudes than $p-$mode ones. We find that all of these pulsators are quite rich with many frequencies and apply asymptotic relationships to associate periodicities with modes for KIC010670103. Kepler data are particularly well-suited for these studies as they are long-duration, extremely high duty cycle observations with well-behaved noise properties.

Modelling Kepler observations of solar-like oscillations in the red-giant star HD 186355

C. Jiang, B. W. Jiang, J. Christensen-Dalsgaard, T. R. Bedding, D. Stello, D. Huber, S. Frandsen, H. Kjeldsen, B. Mosser, P. Demarque, M. N. Fanelli, K. Kinemuchi, F. Mullally.
Full text
NASA ADS
DOI

We have analysed oscillations of the red giant star HD 186355 observed by the NASA Kepler satellite. The data consist of the first five quarters of science operations of Kepler, which covers around 13 months. The high-precision time-series data allow us to accurately extract the oscillation frequencies from the power spectrum. We find the frequency of the maximum oscillation power, $\nu_{\rm max}$, and the mean large frequency separation, $\Delta \nu$, are around 106 and 9.4 $\mu$Hz respectively. A regular pattern of radial and non-radial oscillation modes is identified by stacking the power spectra in an échelle diagram. We use the scaling relations of $\Delta \nu$ and $\nu_{\rm max}$ to estimate the preliminary asteroseismic mass, which is confirmed with the modelling result (M = 1.43 $\pm$ 0.02 $M_{\sun}$) using the Yale Rotating stellar Evolution Code (YREC7). In addition, we constrain the effective temperature, luminosity and radius from comparisons between observational constraints and models. A number of mixed $l = 1$ modes are also detected and taken into account in our model comparisons. We find a mean observational period spacing for these mixed modes of about 58 sec, suggesting that this red giant branch star is in the shell hydrogen-burning phase.

Granulation in Red Giants: Observations by the Kepler Mission and Three-dimensional Convection Simulations

S. Mathur, S. Hekker, R. Trampedach, J. Ballot, T. Kallinger, D. Buzasi, R. A. García, D. Huber, A. Jiménez, B. Mosser, T. R. Bedding, Y. Elsworth, C. Régulo, D. Stello, J. De Ridder, S. J. Hale, K. Kinemuchi and 3 coauthors.
Full text
NASA ADS
DOI

The granulation pattern that we observe on the surface of the Sun is due to hot plasma from the interior rising to the photosphere where it cools down, and descends back into the interior at the edges of granules. This is the visible manifestation of convection taking place in the outer part of the solar convection zone. Because red giants have deeper convection zones and more extended atmospheres than the Sun, we cannot a priori assume that granulation in red giants is a scaled version of solar granulation. Until now, neither observations nor 1D analytical convection models could put constraints on granulation in red giants. However, thanks to asteroseismology, this study can now be performed. The resulting parameters yield physical information about the granulation. We analyze $\sim$ 1000 red giants that have been observed by Kepler during 13 months. We fit the power spectra with Harvey-like profiles to retrieve the characteristics of the granulation (time scale $\tau_{\rm gran}$ and power $P_{\rm gran}$). We also introduce a new time scale, $\tau_{\rm eff}$, which takes into account that different slopes are used in the Harvey functions. We search for a correlation between these parameters and the global acoustic-mode parameter (the position of maximum power, $\nu_{\rm max}$) as well as with stellar parameters (mass, radius, surface gravity (log $g$) and effective temperature ($T_{\rm eff}$)). We show that $\tau_{\rm eff} \propto \nu_{\rm max}^{-0.89}$ and $P_{\rm gran} \propto \nu_{\rm max}^{-1.90}$, which is consistent with the theoretical predictions. We find that the granulation time scales of stars that belong to the red clump have similar values while the time scales of stars in the red-giant branch are spread in a wider range. Finally, we show that realistic 3D simulations of the surface convection in stars, spanning the ($T_{\rm eff}$, log $g$)-range of our sample of red giants, match the Kepler observations well in terms of trends.

Asteroseismic Diagrams from a Survey of Solar-like Oscillations with Kepler

Timothy R. White, Timothy R. Bedding, Dennis Stello, Thierry Appourchaux, Jérôme Ballot, Othman Benomar, Alfio Bonanno, Anne-Marie Broomhall, Tiago L. Campante, William J. Chaplin, Jrgen Christensen-Dalsgaard, Enrico Corsaro, Gülnur Doan, Yvonne P. Elsworth, Stephen T. Fletcher, Rafael A. García, Patrick Gaulme and 16 coauthors.
Full text
NASA ADS
DOI

Photometric observations made by the NASA Kepler Mission have led to a dramatic increase in the number of main-sequence and subgiant stars with detected solar-like oscillations. We present an ensemble asteroseismic analysis of 78 solar-type stars. Using frequencies determined from the Kepler time-series photometry, we have measured three asteroseismic parameters that characterize the oscillations: the large frequency separation ($\Delta\nu$), the small frequency separation between modes of $l=0$ and $l=2$ ($\delta\nu_{02}$), and the dimensionless offset ($\epsilon$). These measurements allow us to construct asteroseismic diagrams, namely the so-called C-D diagram of $\delta\nu_{02}$ versus $\Delta\nu$, and the recently introduced $\epsilon$ diagram. We compare the Kepler results with previously observed solar-type stars and with theoretical models. The positions of stars in these diagrams places constraints on their masses and ages.

The Kepler characterization of the variability amongst A- and F-type stars I. General overview

K. Uytterhoeven, A. Moya, A. Grigahcène, J.A. Guzik, J. Gutiérrez-Soto, B. Smalley, G. Handler, L.A. Balona, E. Niemczura, L. Fox Machado, S. Benatti, E. Chapellier, A. Tkachenko, R. Szabó, J.C. Suárez, V. Ripepi, J. Pascual and 24 coauthors.
Full text
NASA ADS
DOI

Context. The Kepler spacecraft is providing time series of photometric data with micromagnitude precision for hundreds of A-F type stars.
Aims. We present a first general characterization of the pulsational behaviour of A-F type stars as observed in the Kepler light curves of a sample of 750 candidate A-F type stars, and observationally investigate the relation between $\gamma$ Doradus ($\gamma$ Dor), $\delta$ Scuti ($\delta$ Sct), and hybrid stars.
Methods. We compile a database of physical parameters for the sample stars from the literature and new ground-based observations. We analyse the Kepler light curve of each star and extract the pulsational frequencies using different frequency analysis methods. We construct two new observables, 'energy' and 'efficiency', related to the driving energy of the pulsation mode and the convective efficiency of the outer convective zone, respectively.
Results. We propose three main groups to describe the observed variety in pulsating A-F type stars: $\gamma$ Dor, $\delta$ Sct, and hybrid stars. We assign 63% of our sample to one of the three groups, and identify the remaining part as rotationally modulated/active stars, binaries, stars with a different spectral type than A or F, or stars that show no clear periodic variability. 23% of the stars (172 stars) are hybrid stars, which is a much larger fraction than what has been observed before. We characterize for the first time a large number of A-F type stars (475 stars) in terms of number of detected frequencies, frequency range, and typical pulsation amplitudes. The majority of hybrid stars show frequencies with all kinds of periodicities within the $\gamma$ Dor and $\delta$ Sct range, also between 5 and 10 d$^{-1}$, which is a challenge for the current models. The location of gdor and dsct classes in the (Teff, logg)-diagram has been extended. We find indications for the existence of $\delta$ Sct and $\gamma$ Dor stars beyond the edges of the current observational instability strips. The hybrid stars occupy the entire region within the $\delta$ Sct, and $\gamma$ Dor instability strips and beyond. Non-variable stars seem to exist within the instability strips. We investigate two newly constructed variables 'efficiency' and 'energy' as a means to explore the relation between $\gamma$ Dor and $\delta$ Sct stars.
Conclusions. Our results suggest a revision of the current observational instability strips of $\delta$ Sct and $\gamma$ Dor stars, and imply an investigation of pulsation mechanisms to supplement the $\kappa$ mechanism and convective blocking effect to drive hybrid pulsations. Accurate physical parameters for all stars are needed to confirm these findings.

Constructing a one-solar-mass evolutionary sequence using asteroseismic data from Kepler

V. Silva Aguirre, W. J. Chaplin, J. Ballot, S. Basu, T. R.Bedding, A. M. Serenelli, G. Verner, A. Miglio, M. J. P. F. G. Monteiro, A. Weiss, T. Appourchaux, A. Bonanno, A. M. Broomhall, H. Bruntt, T. L. Campante, L. Casagrande, E. Corsaro and 25 coauthors.
Full text
NASA ADS
DOI

Asteroseismology of solar-type stars has entered a new era of large surveys with the success of the NASA Kepler mission, which is providing exquisite data on oscillations of stars across the Hertzprung-Russell (HR) diagram. From the time-series photometry, the two seismic parameters that can be most readily extracted are the large frequency separation ($\Delta\nu$) and the frequency of maximum oscillation power ($\nu_\mathrm{max}$). After the survey phase, these quantities are available for hundreds of solar-type stars. By considering scaling relations from solar values, we use these two asteroseismic observables to identify for the first time an evolutionary sequence of 1-M$_\odot$ field stars, without the need of further information from stellar models. Comparison of our determinations with the few available spectroscopic results shows an excellent level of agreement. We discuss the potential of the method for differential analysis throughout the main-sequence evolution, and the possibility of detecting twins of very well-known stars.

Asteroseismology from multi-month Kepler photometry: the evolved Sun-like stars KIC 10273246 and KIC 10920273

T. L. Campante, R. Handberg, S. Mathur, T. Appourchaux, T. R. Bedding, W. J. Chaplin, R. A. García, B. Mosser, O. Benomar, A. Bonanno, E. Corsaro, S. T. Fletcher, P. Gaulme, S. Hekker, C. Karoff, C. Régulo, D. Salabert and 19 coauthors.
Full text
NASA ADS
DOI

The evolved main-sequence Sun-like stars KIC 10273246 (F-type) and KIC 10920273 (G-type) were observed with the NASA Kepler satellite for approximately ten months with a duty cycle in excess of 90%. Such continuous and long observations are unprecedented for solar-type stars other than the Sun.
We aimed mainly at extracting estimates of p-mode frequencies – as well as of other individual mode parameters – from the power spectra of the light curves of both stars, thus providing scope for a full seismic characterization.
The light curves were corrected for instrumental effects in a manner independent of the Kepler science pipeline. Estimation of individual mode parameters was based both on the maximization of the likelihood of a model describing the power spectrum and on a classic prewhitening method. Finally, we employed a procedure for selecting frequency lists to be used in stellar modeling.
A total of 30 and 21 modes of degree $l=0,1,2$ – spanning at least eight radial orders – have been identified for KIC 10273246 and KIC 10920273, respectively. Two avoided crossings ($l=1$ ridge) have been identified for KIC 10273246, whereas one avoided crossing plus another likely one have been identified for KIC 10920273. Good agreement is found between observed and predicted mode amplitudes for the F-type star KIC 10273246, based on a revised scaling relation. Estimates are given of the rotational periods, the parameters describing stellar granulation and the global asteroseismic parameters $\Delta\nu$ and $\nu_{\rm{max}}$.

Solar-like oscillations in KIC 11395018 and KIC 11234888 from 8 months of Kepler data

S. Mathur, R. Handberg, T. L. Campante, R. A. García, T. Appourchaux, T. R. Bedding, B. Mosser, W. J. Chaplin, J. Ballot, O. Benomar, A. Bonanno, E. Corsaro, P. Gaulme, S. Hekker, C. Régulo, D. Salabert, G. Verner and 17 coauthors.
Full text
NASA ADS
DOI

We analyze the photometric short-cadence data obtained with the Kepler Mission during the first eight months of observations of two solar-type stars of spectral types G and F: KIC 11395018 and KIC 11234888 respectively, the latter having a lower signal-to-noise ratio compared to KIC 11395018. We estimate global parameters of the acoustic (p) modes such as the average large and small frequency separations, the frequency of the maximum of the p-mode envelope and the average linewidth of the acoustic modes. We were able to identify and to measure 22 p-mode frequencies for the first star and 16 for the second one even though the signal-to-noise ratios of these stars are rather low. We also derive some information about the stellar rotation periods from the analyses of the low-frequency parts of the power spectral densities. We emphasize the importance of continued observations for the stars with low signal-to-noise ratio for an improved characterization of the oscillation modes. Our results offer a preview of what will be possible for many stars with the long data sets obtained during the remainder of the mission.

Amplitudes of solar-like oscillations: constraints from red giants in open clusters observed by Kepler

Dennis Stello, Daniel Huber, Thomas Kallinger, Sarbani Basu, Benoît Mosser, Saskia Hekker, Savita Mathur, Rafael A. García, Timothy R. Bedding, Hans Kjeldsen, Ronald L. Gilliland, William J. Chaplin, Othman Benomar, Sren Meibom, Frank Grundahl, Yvonne P. Elsworth, Joanna Molenda-akowicz and 1 coauthors.
Full text
NASA ADS
DOI

Scaling relations that link asteroseismic quantities to global stellar properties are important for gaining understanding of the intricate physics that underpins stellar pulsation. The common notion that all stars in an open cluster have essentially the same distance, age, and initial composition, implies that the stellar parameters can be measured to much higher precision than what is usually achievable for single stars. This makes clusters ideal for exploring how quantities of solar-like oscillations such as the mode amplitude depend on the global stellar properties. We have analyzed data obtained with NASA's Kepler space telescope to study solar-like oscillations in 100 red giant stars located in either of the three open clusters, NGC 6791, NGC 6819, and NGC 6811. By fitting the measured amplitudes to predictions from simple scaling relations that depend on luminosity, mass, and effective temperature, we find that the data cannot be described by any power of the luminosity-to-mass ratio as previously assumed. As a result we provide a new improved empirical relation which treats luminosity and mass separately. This relation turns out to also work remarkably well for main-sequence and subgiant stars. In addition, the measured amplitudes reveal the presence of a number of previously unknown unresolved binaries in the red clump in NGC 6791 and NGC 6819, pointing to an interesting new application for asteroseismology as a probe into the formation history of open clusters.

An Asteroseismic Membership Study of the Red Giants in Three Open Clusters Observed by Kepler: NGC 6791, NGC 6819, and NGC 6811

Dennis Stello, Sren Meibom, Ronald L. Gilliland, Frank Grundahl, Saskia Hekker, Benoît Mosser, Thomas Kallinger, Savita Mathur, Rafael A. García, Daniel Huber, Sarbani Basu, Timothy R. Bedding, Karsten Brogaard, William J. Chaplin, Yvonne P. Elsworth, Joanna Molenda-akowicz, Robert Szabó and 5 coauthors.
Full text
NASA ADS
DOI

Studying star clusters offers significant advances in stellar astrophysics due to the combined power of having many stars with essentially the same distance, age, and initial composition. This makes clusters excellent test benches for verification of stellar evolution theory. To fully exploit this potential, it is vital that the star sample is uncontaminated by stars that are not members of the cluster. Techniques for determining cluster membership therefore play a key role in the investigation of clusters. We present results on three clusters in the Kepler field of view based on a newly established technique that uses asteroseismology to identify fore- or background stars in the field. Four previously identified seismic non-members in NGC 6819 are confirmed in this study, and three additional non-members are found – two in NGC 6819 and one in NGC6791. We further highlight which stars are, or might be, affected by blending, which needs to be taken into account when analysing these Kepler data.

Verification of the Kepler Input Catalog from asteroseismology of solar-type stars

G. A. Verner, W. J. Chaplin, S. Basu, T. M. Brown, S. Hekker, D. Huber, C. Karoff, S. Mathur, T. S. Metcalfe, B. Mosser, P-O. Quirion, T. Appourchaux, T. R. Bedding, H. Bruntt, T. Campante, Y. Elsworth, R. A. García and 11 coauthors.
Full text
NASA ADS
DOI

We calculate precise stellar radii and surface gravities from the asteroseismic analysis of over 500 solar-type pulsating stars observed by the Kepler space telescope. These physical stellar properties are compared with those given in the Kepler Input Catalog (KIC), determined from ground-based multi-colour photometry. For the stars in our sample, we find general agreement but we detect an average overestimation bias of 0.23 dex in the KIC determination of $\log(g)$ for stars with $\log(g)_\mathrm{KIC}>4.0$ dex, and a resultant underestimation bias of up to 50 % in the KIC radii estimates for stars with $R_\mathrm{KIC}<2$ $R_\odot$. Part of the difference may arise from selection bias in the asteroseismic sample; nevertheless, this result implies there may be fewer stars characterised in the KIC with $R\sim 1$ $R_\odot$ than is suggested by the physical properties in the KIC. Furthermore, if the radius estimates are taken from the KIC for these affected stars and then used to calculate the size of transiting planets, a similar underestimation bias will be applied to the planetary radii.

Variable turbulent convection as the cause of the Blazhko effect - testing the Stothers model

R. Smolec, P. Moskalik, K. Kolenberg, S. Bryson, M. T. Cote, R. L. Morris.
Full text, Online material
NASA ADS
DOI

The amplitude and phase modulation observed in a significant fraction of the RR Lyrae variables - the Blazhko effect - represents a long-standing enigma in stellar pulsation theory. No satisfactory explanation for the Blazhko effect has been proposed so far. In this paper we focus on the Stothers (2006) idea, in which modulation is caused by changes in the structure of the outer convective zone, caused by a quasi-periodically changing magnetic field. However, up to this date no quantitative estimates were made to investigate whether such a mechanism can be operational and whether it is capable of reproducing the light variation we observe in Blazhko variables. We address the latter problem. We use a simplified model, in which the variation of turbulent convection is introduced into the non-linear hydrodynamic models in an ad hoc way, neglecting interaction with the magnetic field. We study the light curve variation through the modulation cycle and properties of the resulting frequency spectra. Our results are compared with Kepler observations of RR Lyr. We find that reproducing the light curve variation, as is observed in RR Lyr, requires a huge modulation of the mixing length, of the order of $\pm$50 per cent, on a relatively short time-scale of less than 40 days. Even then, we are not able to reproduce all the observed relations between modulation components present in the frequency spectrum and the relations between Fourier parameters describing the shape of the instantaneous light curves.

Global asteroseismic properties of solar-like oscillations observed by Kepler: Comparison of complementary analysis methods

G. A. Verner, Y. Elsworth, W. J. Chaplin, T. L. Campante, E. Corsaro, P. Gaulme, S. Hekker, D. Huber, C. Karoff, S. Mathur, B. Mosser, T. Appourchaux, J. Ballot, T. R. Bedding, A. Bonanno, A-M. Broomhall, R. A. García and 10 coauthors.
Full text
NASA ADS
DOI

We present the asteroseismic analysis of 1948 F-, G- and K-type main-sequence and subgiant stars observed by the NASA Kepler Mission. We detect and characterise solar-like oscillations in 642 of these stars. This represents the largest cohort of main-sequence and subgiant solar-like oscillators observed to date. The photometric observations are analysed using the methods developed by nine independent research teams. The results are combined to validate the determined global asteroseismic parameters and calculate the relative precision by which the parameters can be obtained. We correlate the relative number of detected solar-like oscillators with stellar parameters from the Kepler Input Catalog and find a deficiency for stars with effective temperatures in the range $5300 \lesssim T_\mathrm{eff} \lesssim 5700$ K and a drop-off in detected oscillations in stars approaching the red edge of the classical instability strip. We compare the power-law relationships between the frequency of peak power, $\nu_\mathrm{max}$, the mean large frequency separation, $\Delta\nu$, and the maximum mode amplitude, $A_\mathrm{max}$, and show that there are significant method-dependent differences in the results obtained. This illustrates the need for multiple complementary analysis methods to be used to assess the robustness and reproducibility of results derived from global asteroseismic parameters.

Cepheid investigations using the Kepler space telescope

R. Szabó, L. Szabados, C.-C. Ngeow, R. Smolec, A. Derekas, P. Moskalik, J. Nuspl, H. Lehmann, G. Frész, J. Molenda-akowicz, S. T. Bryson, A. A. Henden, D. W. Kurtz, D. Stello, J. M. Nemec, J. M. Benk, L. Berdnikov and 21 coauthors.
Full text
NASA ADS
DOI

We report results of initial work done on selected candidate Cepheids to be observed with the Kepler space telescope. Prior to the launch 40 candidates were selected from previous surveys and databases. The analysis of the first 322 days of Kepler photometry, and recent follow-up ground-based multicolour photometry and spectroscopy allowed us to confirm that one of these stars, V1154 Cyg (KIC 7548061), is indeed a 4.9-d Cepheid. Using the phase lag method we show that this star pulsates in the fundamental mode. New radial velocity data are consistent with previous measurements, suggesting that a long-period binary component is unlikely. No evidence is seen in the ultra-precise, nearly uninterrupted Kepler photometry for nonradial or stochastically excited modes at the micromagnitude level. The other candidates are not Cepheids but an interesting mix of possible spotted stars, eclipsing systems and flare stars.

Ensemble asteroseismology of solar-type stars with the NASA Kepler Mission

W. J. Chaplin, H. Kjeldsen, J. Christensen-Dalsgaard, S. Basu, A. Miglio, T. Appourchaux, T. R. Bedding, Y. Elsworth, R. A. García, R. L. Gilliland, L. Girardi, G. Houdek, C. Karoff, S. D. Kawaler, T. S. Metcalfe, J. Molenda-Zakowicz, M. J. P. F. G. Monteiro and 36 coauthors.
Full text
NASA ADS
DOI

In addition to its search for extrasolar planets, the NASA Kepler mission provides exquisite data on stellar oscillations. We report the detections of oscillations in 500 solar-type stars in the Kepler field of view, an ensemble that is large enough to allow statistical studies of intrinsic stellar properties (such as mass, radius, and age) and to test theories of stellar evolution. We find that the distribution of observed masses of these stars shows intriguing differences to predictions from models of synthetic stellar populations in the Galaxy.

Predicting the detectability of oscillations in solar-type stars observed by Kepler

W. J. Chaplin, H. Kjeldsen, T. R. Bedding, J. Christensen-Dalsgaard, R. L. Gilliland, S. D. Kawaler, T. Appourchaux, Y. Elsworth, R. A. Garcia, G. Houdek, C. Karoff, T. S. Metcalfe, J. Molenda-Zakowicz, M. J. P. F. G. Monteiro, M. J. Thompson, G. A. Verner, N. Batalha and 32 coauthors.
Full text
NASA ADS
DOI

Asteroseismology of solar-type stars has an important part to play in the exoplanet program of the NASA Kepler Mission. Precise and accurate inferences on the stellar properties that are made possible by the seismic data allow very tight constraints to be placed on the exoplanetary systems. Here, we outline how to make an estimate of the detectability of solar-like oscillations in any given Kepler target, using rough estimates of the temperature and radius, and the Kepler apparent magnitude.

The impact of stellar activity on the detectability of solar-like oscillations observed by Kepler

W. J. Chaplin, T. R. Bedding, A. Bonanno, A.-M. Broomhall, R. A. Garcia, S. Hekker, D. Huber, G. A. Verner, S. Basu, Y. Elsworth, G. Houdek, S. Mathur, B. Mosser, R. New, I. R. Stevens, T. Appourchaux, C. Karoff and 20 coauthors.
Full text
NASA ADS
DOI

We use photometric observations of solar-type stars, made by the NASA Kepler Mission, to conduct a statistical study of the impact of stellar surface activity on the detectability of solar-like oscillations. We find that the number of stars with detected oscillations fall significantly with increasing levels of activity. The results present strong evidence for the impact of magnetic activity on the properties of near-surface convection in the stars, which appears to inhibit the amplitudes of the stochastically excited, intrinsically damped solar-like oscillations.

HD 181068: A Red Giant in a Triply Eclipsing Compact Hierarchical Triple System

A. Derekas, L.L. Kiss, T. Borkovits, D. Huber, H. Lehmann, J. Southworth, T.R. Bedding, D. Balam, M. Hartmann, M. Hrudkova, J. Kovács, Gy. Mez, A. Moór, E. Niemczura, G. Sarty, Gy.M. Szabó, R. Szabó and 27 coauthors.
Full text
NASA ADS
DOI

Hierarchical triple systems comprise a close binary and a more distant component. They are important for testing theories of star formation and of stellar evolution in the presence of nearby companions. We obtained 218 days of Kepler photometry of HD 181068 (magnitude of 7.1), supplemented by ground-based spectroscopy and interferometry, which show it to be a hierarchical triple with two types of mutual eclipses. The primary is a red giant that is in a 45-day orbit with a pair of red dwarfs in a close 0.9-day orbit. The red giant shows evidence for tidally induced oscillations that are driven by the orbital motion of the close pair. HD 181068 is an ideal target for studies of dynamical evolution and testing tidal friction theories in hierarchical triple systems.

Kepler-Detected Gravity-Mode Period Spacings in a Red Giant Star

P. G. Beck, T. R. Bedding, B. Mosser, D. Stello, R. A. García, T. Kallinger, S. Hekker, Y. Elsworth, S. Frandsen, F. Carrier, J. De Ridder, C. Aerts, T. R. White, D. Huber, M.-A. Dupret, A. Miglio, A. Noels and 6 coauthors.
Full text
NASA ADS
DOI

Stellar interiors are inaccessible through direct observations. For this reason, helioseismologists made use of the Sun's acoustic oscillation modes to tune models of its structure. The quest to detect modes which probe the solar core has been ongoing for decades. We report the detection of mixed modes penetrating all the way to the core of an evolved star from 320 days of observations with the Kepler satellite. The period spacings of these mixed modes are directly dependent on the density gradient between the core region and the convective envelope

Atmospheric parameters and pulsational properties for a sample of $\delta$ Scuti, $\gamma$ Doradus, and hybrid Kepler targets

G. Catanzaro, V. Ripepi, S. Bernabei, M. Marconi, L. Balona, D. W. Kurtz, B. Smalley, W. J. Borucki, H. Bruntt, J. Christensen-Dalsgaard, A. Grigahcène, H. Kjeldsen, D. G. Koch, M. J. P. F. G. Monteiro, J.C. Suárez, R. Szabó, K. Uytterhoeven.
Full text
NASA ADS
DOI

We report spectroscopic observations for 19 $\delta$ Scuti (Sct) candidates observed by the Kepler satellite in both long and short cadence mode. For all these stars, by using spectral synthesis, we derive the effective temperature, the surface gravity and the projected rotational velocity. An equivalent spectral type classification has been also performed for all stars in the sample. These determinations are fundamental for modelling the frequency spectra that will be extracted from the Kepler data for asteroseismic inference. For all the 19 stars, we present also periodograms obtained from Kepler data. We find that all stars show peaks in both low- ($\gamma$ Dor; g mode) and high-frequency ($\delta$ Sct; p mode) ranges. Using the amplitudes and considering 5 c/d as a boundary frequency, we classified 3 stars as pure $\gamma$ Dor, 4 as $\gamma$ Dor - $\delta$ Sct hybrid, 5 as $\delta$ Sct - $\gamma$ Dor hybrid, and 6 as pure $\delta$ Sct. The only exception is the star KIC 05296877 which we suggest could be a binary.

Kepler observations of Am stars

L. A. Balona, V. Ripepi, G. Catanzaro, D. W. Kurtz, B. Smalley, P. De Cat, L. Eyer, A. Grigahcene, S. Leccia, J. Southworth, K. Uytterhoeven, H. Van Winckel, W. J. Borucki, J. Christensen-Dalsgaard, H. Kjeldsen, D. G. Koch.
Full text
NASA ADS
DOI

We present an analysis of high-resolution spectra for two pulsating Am stars in the Kepler field. The stellar parameters derived in this way are important because parameters derived from narrow-band photometry may be affected by the strong metal lines in these stars. We analyse the Kepler time series of ten known Am stars and find that six of them clearly show $\delta$ Scuti pulsations. The other four appear to be non-pulsating. We derive fundamental parameters for all known pulsating Am stars from ground-based observations and also for the Kepler Am stars to investigate the location of the instability strip for pulsating Am stars. We find that there is not much difference between the Am-star instability strip and the $\delta$ Scuti instability strip. We find that the observed location of pulsating Am stars in the HR diagram does not agree with the location predicted from recent diffusion calculations.

Kepler observations of variability in B-type stars

L. A. Balona, A. Pigulski, P. De Cat, G. Handler, J. Gutiérrez-Soto, C. A. Engelbrecht, F. Frescura, M. Briquet, J. Cuypers, J. Daszyńska-Daszkiewicz, P. Degroote, R. J. Dukes, R. A. Garcia, E. M. Green, U. Heber, S. D. Kawaler, H. Lehmann and 19 coauthors.
Full text
NASA ADS
DOI

The analysis of the light curves of 48 B-type stars observed by Kepler is presented. Among these are 15 pulsating stars, all of which show low frequencies characteristic of SPB stars. Seven of these stars also show a few weak, isolated high frequencies and they could be considered as SPB/$\beta$ Cep hybrids. In all cases the frequency spectra are quite different from what is seen from ground-based observations. We suggest that this is because most of the low frequencies are modes of high degree which are predicted to be unstable in models of mid-B stars. We find that there are non-pulsating stars within the $\beta$ Cep and SPB instability strips. Apart from the pulsating stars, we can identify stars with frequency groupings similar to what is seen in Be stars but which are not Be stars. The origin of the groupings is not clear but may be related to rotation. We find periodic variations in other stars which we attribute to proximity effects in binary systems. The variations in other stars can be understood as proximity effects in a binary system or possibly rotational modulation. We find no evidence for pulsating stars between the cool edge of the SPB and the hot edge of the $\delta$ Sct instability strips. None of the stars show the broad features which can be attributed to stochastically-excited modes as recently proposed. Among our sample of B stars are two chemically peculiar stars, one of which is a HgMn star showing rotational modulation in the light curve.

Rotation and oblique pulsation in Kepler observations of the roAp star KIC 10483436

L. A. Balona, M. S. Cunha, M. Gruberbauer, D. W. Kurtz, H. Saio, T. R. White, J. Christensen-Dalsgaard, H. Kjeldsen, J. L. Christiansen, J. R. Hall, S. E. Seader.
Full text
NASA ADS
DOI

Photometry of KIC 10483436 was obtained continuously with 1-min exposures over a 27-d period from the Kepler satellite. The light curve shows rotational variations from surface spots with a period of $4.303 \pm 0.002$ d, an amplitude of about 6 mmag, and eight pulsation frequencies typical of roAp stars. The high frequency pattern consists of a quintuplet of equally spaced peaks where the frequency of the dominant central peak (68 $\mu$mag amplitude) is 1353.00 $\mu$Hz ($P = 12.32$ min). A second set of three peaks of lower amplitude is also visible. These appear to form part of a quintuplet centered on 1511.6 $\mu$Hz with the central peak and one side peak missing. The equidistant frequency spacing is 2.69 $\mu$Hz, which corresponds to the 4.303 d rotation period. However, the amplitudes (12 $\mu$mag) of these peaks are too close to the detection level to allow definite identification of the multiplets. Although no spectrum is available, the character of the pulsations shows that this is a roAp star with two high frequency modes modulated in amplitude in accordance with the oblique pulsator model. The 4.303-d variation in the light curve, which is interpreted as rotational modulation, shows harmonics as high as the 26$^{\rm th}$. These harmonics are probably a result of many patches of varying surface brightness associated with surface abundance variations characteristic of Ap stars.

The first evidence for multiple pulsation axes: a new roAp star in the Kepler field, KIC 10195926

D.W. Kurtz, M.S. Cunha, H. Saio, L. Bigot, L. A. Balona, V.G. Elkin, H. Shibahashi, I.M. Brandão, K. Uytterhoeven, S. Frandsen, S. Frimann, A. Hatzes, T. Lueftinger, M. Gruberbauer, H. Kjeldsen, J. Christensen-Dalsgaard, S.D. Kawaler.
Full text
NASA ADS
DOI

We have discovered a new rapidly oscillating Ap star among the Kepler Mission target stars, KIC 10195926. This star shows two pulsation modes with periods that are amongst the longest known for roAp stars at 17.1 min and 18.1 min, indicating that the star is near the terminal age main sequence. The principal pulsation mode is an oblique dipole mode that shows a rotationally split frequency septuplet that provides information on the geometry of the mode. The secondary mode also appears to be a dipole mode with a rotationally split triplet, but we are able to show within the improved oblique pulsator model that these two modes cannot have the same axis of pulsation. This is the first time for any pulsating star that evidence has been found for separate pulsation axes for different modes. The two modes are separated in frequency by 55 $\mu$Hz, which we model as the large separation. The star is an $\alpha^2$ CVn spotted magnetic variable that shows a complex rotational light variation with a period of $P_{\rm rot} = 5.68459$ d. For the first time for any spotted magnetic star of the upper main sequence, we find clear evidence of light variation with a period of twice the rotation period; i.e. a subharmonic frequency of $\nu_{\rm rot}/2$. We propose that this and other subharmonics are the first observed manifestation of torsional modes in an roAp star. From high resolution spectra we determine $T_{\rm eff} = 7400$ K, $\log g = 3.6$ and $v \sin i = 21$ km s$^{-1}$. We have found a magnetic pulsation model with fundamental parameters close to these values that reproduces the rotational variations of the two obliquely pulsating modes with different pulsation axes. The star shows overabundances of the rare earth elements, but these are not as extreme as most other roAp stars. The spectrum is variable with rotation, indicating surface abundance patches.

Sounding open clusters: asteroseismic constraints from Kepler on the properties of NGC 6791 and NGC 6819

Sarbani Basu, Frank Grundahl, Dennis Stello, Thomas Kallinger, Saskia Hekker, Benoit Mosser, Rafael A. García, Savita Mathur, Karsten Brogaard, Hans Bruntt, William J. Chaplin, Ning Gai, Yvonne Elsworth, Lisa Esch, Jerome Ballot, Timothy R. Bedding, Michael Gruberbauer and 9 coauthors.
Full text
NASA ADS
DOI

We present initial results on some of the properties of open clusters NGC 6791 and NGC 6819 derived from asteroseismic data obtained by NASA's Kepler mission. In addition to estimating the mass, radius and ogg of stars on the red-giant branch of these clusters, we estimate the distance to the clusters and their ages. Our model-independent estimate of the distance modulus of NGC 6791 is $(m-M)_0= 13.11\pm 0.06$. We find $(m-M)_0= 11.85\pm 0.05$ for NGC 6819. The average mass of stars on the red-giant branch of NGC 6791 is $1.20 \pm 0.01 M_\odot$, while that of NGC 6819 is $1.68\pm 0.03M_\odot$. It should be noted that we do not have data that cover the entire red-giant branch and the actual mass will be somewhat lower. We have determined model-dependent estimates of ages of these clusters. We find ages between 6.8 and 8.6 Gyr for NGC 6791, however, most sets of models give ages around 7Gyr. We obtain ages between 2 and 2.4 Gyr for NGC 6819.

Kepler observations of a roAp star: $\delta$ Scuti and $\gamma$ Doradus pulsations in Ap stars

L. A. Balona, M. S. Cunha, D. W. Kurtz, I. M. Brandão, M. Gruberbauer, H. Saio, R. stensen, V.G. Elkin, W. J. Borucki, J. Christensen-Dalsgaard, H. Kjeldsen, D. G. Koch.
Full text
NASA ADS
DOI

Observations of the A5p star KIC 8677585 obtained during the Kepler 10-d commissioning run with 1-min time resolution show that it is a roAp star with several frequencies with periods near 10 min. In addition, a low frequency at $3.142 d^{-1}$ is also clearly present. Multiperiodic $\gamma$ Dor and $\delta$ Sct pulsations, never before seen in any Ap star, are present in Kepler observations of at least three other Ap stars. Since $\gamma$ Dor pulsations are seen in Ap stars, it is likely that the low-frequency in KIC 8677585 is also a $\gamma$ Dor pulsation. The simultaneous presence of both $\gamma$ Dor and roAp pulsations and the unexpected detection of $\delta$ Sct and $\gamma$ Dor pulsations in Ap stars present new opportunities and challenges for the interpretation of these stars.

Does Kepler unveil the mystery of the Blazhko effect? First detection of period doubling in Kepler Blazhko RR Lyrae stars

R. Szabó, Z. Kolláth, L. Molnár, K. Kolenberg, D. W. Kurtz, S. T. Bryson, J. M. Benkő, J. Christensen-Dalsgaard, H. Kjeldsen, W. J. Borucki, D. Koch, J. D. Twicken, M. Chadid, M. Di Criscienzo, Y-B. Jeon, P. Moskalik, J. M. Nemec and 1 coauthors.
Full text
NASA ADS
DOI

The first detection of the period doubling phenomenon is reported in the Kepler RR Lyrae stars RR Lyr, V808 Cyg and V355 Lyr. Interestingly, all these pulsating stars show Blazhko modulation. The period doubling manifests itself as alternating maxima and minima of the pulsational cycles in the light curve, as well as through the appearance of half-integer frequencies located halfway between the main pulsation period and its harmonics in the frequency spectrum. The effect was found to be stronger during certain phases of the modulation cycle. We were able to reproduce the period doubling bifurcation in our nonlinear RR Lyrae models computed by the Florida-Budapest hydrocode. This enabled us to trace the origin of this instability in RR Lyrae stars to a resonance, namely a 9:2 resonance between the fundamental mode and a high-order (9th) radial overtone showing strange mode characteristics. We discuss the connection of this new type of variation to the mysterious Blazhko effect and argue that it may give us fresh insights to solve this century-old enigma.

Asteroseismology of Red Giants from the first four months of Kepler data: Global oscillation parameters for 800 stars

D. Huber, T. R. Bedding, D. Stello, B. Mosser, S. Mathur, T. Kallinger, S. Hekker, Y. P. Elsworth, D. L. Buzasi, J. De Ridder, R. L. Gilliland, H. Kjeldsen, W. J. Chaplin, R. A. García, S. J. Hale, H. L. Preston, T. R. White and 5 coauthors.
Full text
NASA ADS
DOI

We studied solar-like oscillations in $\sim$800 red-giant stars using Kepler long-cadence photometry. The sample includes stars ranging in evolution from the lower part of the red-giant branch to the Helium main sequence. We investigate the relation between the large frequency separation ($\Delta\nu$) and the frequency of maximum power ($\nu_\mathrm{max}$) and show that it is different for red giants than for main-sequence stars, which is consistent with evolutionary models and scaling relations. The distributions of $\nu_\mathrm{max}$ and $\Delta\nu$ are in qualitative agreement with a simple stellar population model of the Kepler field, including first evidence for a secondary clump population characterized by $M\gtrsim2\,M_{\odot}$ and $\nu_\mathrm{max}\simeq40-110\,\mu\mathrm{Hz}$. We measured the small frequency separations $\delta\nu_{02}$ and $\delta\nu_{01}$ in over 400 red giants and $\delta\nu_{03}$ in over 40 stars. We present C-D diagrams for $l=1$, 2 and 3 and show that the frequency separation ratios $\delta\nu_{02}/\Delta\nu$ and $\delta\nu_{01}/\Delta\nu$ have opposite trends as a function of $\Delta\nu$. The data show a narrowing of the $l=1$ ridge towards lower $\nu_\mathrm{max}$, in agreement with models predicting more efficient mode trapping in stars with higher luminosity. We investigate the offset $\epsilon$ in the asymptotic relation and find a clear correlation with $\Delta\nu$, demonstrating that it is related to fundamental stellar parameters. Finally, we present first results for the amplitude-$\nu_\mathrm{max}$ relation for Kepler red giants. We observe a lack of low amplitude stars for $\nu_\mathrm{max}\gtrsim110\,\mu\mathrm{Hz}$, and find that for a given $\nu_\mathrm{max}$ between $40-110\,\mu\mathrm{Hz}$ stars with lower $\Delta\nu$ (and consequently higher mass) tend to show lower amplitudes than stars with higher $\Delta\nu$.

A precise asteroseismic age and radius for the evolved Sun-like star KIC 11026764

T. S. Metcalfe, M. J. P. F. G. Monteiro, M. J. Thompson, J. Molenda-Zakowicz, T. Appourchaux, W. J. Chaplin, G. Dogan, P. Eggenberger, T. R. Bedding, H. Bruntt, O. L. Creevey, P.-O. Quirion, D. Stello, A. Bonanno, V. Silva Aguirre, S. Basu, L. Esch and 41 coauthors.
Full text
NASA ADS
DOI

The primary science goal of the Kepler Mission is to provide a census of exoplanets in the solar neighborhood, including the identification and characterization of habitable Earth-like planets. The asteroseismic capabilities of the mission are being used to determine precise radii and ages for the target stars from their solar-like oscillations. Chaplin et al. (2010) published observations of three bright G-type stars, which were monitored during the first 33.5 d of science operations. One of these stars, the subgiant KIC 11026764, exhibits a characteristic pattern of oscillation frequencies suggesting that it has evolved significantly. We have derived asteroseismic estimates of the properties of KIC 11026764 from Kepler photometry combined with ground-based spectroscopic data. We present the results of detailed modeling for this star, employing a variety of independent codes and analyses that attempt to match the asteroseismic and spectroscopic constraints simultaneously. We determine both the radius and the age of KIC 11026764 with a precision near 1%, and an accuracy near 2% for the radius and 15% for the age. Continued observations of this star promise to reveal additional oscillation frequencies that will further improve the determination of its fundamental properties.

Asteroseismology of red giants from the first four months of Kepler data: Fundamental stellar parameters

T. Kallinger, B. Mosser, S. Hekker, D. Huber, D. Stello, S. Mathur, S. Basu, T. R. Bedding, W. J. Chaplin, J. De Ridder, Y. P. Elsworth, S. Frandsen, R. A. García, M. Gruberbauer, J. M. Matthews, W. J. Borucki, J. Christensen-Dalsgaard and 3 coauthors.
Full text
NASA ADS
DOI

Context: Clear power excess in a frequency range typical for solar-type oscillations in red giants has been detected in more than 1000 stars, which have been observed during the first 138 days of science operation of the NASA Kepler satellite. The sample includes stars in a large mass and radius range with spectral type G and K, extending in luminosity from the bottom of the giant branch up to high-luminous red giants including the red bump and clump. The high-precision asteroseismic observations with Kepler provide a perfect source for testing stellar structure and evolutionary models as well as investigating the stellar population in our Galaxy.
Aims: We aim to extract accurate seismic parameters from the Kepler time series and use them to infer asteroseismic fundamental parameters from scaling relations and a comparison with red-giant models.
Methods: We fit a global model to the observed power density spectra which allows us to accurately estimate the granulation background signal and the global oscillation parameters, such as the frequency of maximum oscillation power. We find regular patterns of radial and non-radial oscillation modes and use a new technique to automatically identify the mode degree and the characteristic frequency separations between consecutive modes of the same spherical degree. In most cases we can also measure the small separation between $\ell$ = 0, 1, and 2 modes. Subsequently, the seismic parameters are used to estimate stellar masses and radii and to place the stars in a H-R diagram by using an extensive grid of stellar models that covers a large parameter range. Using Bayesian techniques throughout our entire analysis allows us to determine reliable uncertainties for all parameters.
Results: We provide accurate seismic parameters and their uncertainties for a large sample of red giants and determine their asteroseismic fundamental parameters. We investigate the influence of the stars’ metallicities on their positions in the H-R diagram. Finally, we study the red-giant populations in the red clump and bump and compare them to a synthetic population. We find a mass and metallicity gradient in the red clump and clear evidence for a secondary-clump population.

Flavours of variability: 29 RR Lyrae stars observed with Kepler

J. M. Benkő, K. Kolenberg, R. Szabó, D. W. Kurtz, S. Bryson, J. Bregman, M. Still, R. Smolec, J. Nuspl, J. Nemec, P. Moskalik, G. Kopacki, Z. Kolláth, E. Guggenberger, M. Di Criscienzo, J. Christensen-Dalsgaard, H. Kjeldsen and 4 coauthors.
Full text
NASA ADS
DOI

We present our analysis of Kepler observations of 29 RR Lyrae stars, based on 138-d of observation. We report precise pulsation periods for all stars. Nine of these stars had incorrect or unknown periods in the literature. Fourteen of the stars exhibit both amplitude and phase Blazhko modulations, with Blazhko periods ranging from 27.7 to more than 200 days. For V445 Lyr, a longer secondary variation is also observed in addition to its 53.2-d Blazhko period. The unprecedented precision of the Kepler photometry has led to the discovery of the the smallest modulations detected so far. Moreover, additional frequencies beyond the well-known harmonics and Blazhko multiplets have been found. These frequencies are located around the half-integer multiples of the main pulsation frequency for at least three stars. In four stars, these frequencies are close to the first and/or second overtone modes. The amplitudes of these periodicities seem to vary over the Blazhko cycle. V350 Lyr, a non-Blazhko star in our sample, is the first example of a double mode RR Lyrae star that pulsates in its fundamental and second overtone modes.

Detection of Solar-like Oscillations from Kepler Photometry of the Open Cluster NGC 6819

Dennis Stello, Sarbani Basu, Hans Bruntt, Benoît Mosser, Ian R. Stevens, Timothy M. Brown, Jørgen Christensen-Dalsgaard, Ronald L. Gilliland, Hans Kjeldsen, Torben Arentoft, J´erˆome Ballot, Caroline Barban, Timothy R. Bedding, William J. Chaplin, Yvonne P. Elsworth, Rafael A. García, Marie-Jo Goupil and 34 coauthors.
Full text
NASA ADS
DOI

Asteroseismology of stars in clusters has been a long-sought goal because the assumption of a common age, distance, and initial chemical composition allows strong tests of the theory of stellar evolution. We report results from the first 34 days of science data from the Kepler Mission for the open cluster NGC 6819 - one of the four clusters in the field of view. We obtain the first clear detections of solar-like oscillations in the cluster red giants and are able to measure the large frequency separation, $\Delta\nu$, and the frequency of maximum oscillation power, $\nu_\mathrm{max}$. We find that the asteroseismic parameters allow us to test cluster membership of the stars, and even with the limited seismic data in hand, we can already identify four possible non-members despite their having a better than 80% membership probability from radial velocity measurements. We are also able to determine the oscillation amplitudes for stars that span about 2 orders of magnitude in luminosity and find good agreement with the prediction that oscillation amplitudes scale as the luminosity to the power of 0.7. These early results demonstrate the unique potential of asteroseismology of the stellar clusters observed by Kepler.

First Kepler results on RR Lyrae stars

K. Kolenberg, R. Szabó, D. W. Kurtz, R. L. Gilliland, J. Christensen-Dalsgaard, H. Kjeldsen, T. M. Brown, J. M. Benko, M. Chadid, A. Derekas, M. Di Criscienzo, E. Guggenberger, K. Kinemuchi, A. Kunder, Z. Kolláth, G. Kopacki, P. Moskalik and 7 coauthors.
Full text
NASA ADS
DOI

We present the first results of our analyses of selected RR Lyrae stars for which data have been obtained by the Kepler Mission. As expected, we find a significant fraction of the RRab stars to show the Blazhko effect, a still unexplained phenomenon that manifests itself as periodic amplitude and phase modulations of the light curve, on timescales of typically tens to hundreds of days. The long time span of the Kepler Mission of 3.5 yr and the unprecedentedly high precision of its data provide a unique opportunity for the study of RR Lyrae stars. Using data of a modulated star observed in the first roll as a showcase, we discuss the data, our analyses, findings, and their implications for our understanding of RR Lyrae stars and the Blazhko effect. With at least 40% of the RR Lyrae stars in our sample showing modulation, we confirm the high incidence rate that was only found in recent high-precision studies. Moreover, we report the occurrence of additional frequencies, beyond the main pulsation mode and its modulation components. Their half-integer ratio to the main frequency is reminiscent of a period doubling effect caused by resonances, observed for the first time in RR Lyrae stars.

Solar-like oscillations in low-luminosity red giants: first results from Kepler

T. R. Bedding, D. Huber, D. Stello, Y. P. Elsworth, S. Hekker, T. Kallinger, S. Mathur, B. Mosser, H. L. Preston, J. Ballot, C. Barban, A. M. Broomhall, D. L. Buzasi, W. J. Chaplin, R. A. García, M. Gruberbauer, S. J. Hale and 33 coauthors.
Full text
NASA ADS
DOI

We have measured solar-like oscillations in red giants using time-series photometry from the first 34 days of science operations of the Kepler Mission. The light curves, obtained with 30-minute sampling, reveal clear oscillations in a large sample of G and K giants, extending in luminosity from the red clump down to the bottom of the giant branch. We confirm a strong correlation between the large separation of the oscillations ($\Delta \nu$) and the frequency of maximum power ($\nu_{\rm max}$). We focus on a sample of 50 low-luminosity stars ($\nu_{\rm max} > 100\,\mu$Hz, $L \lesssim 30\,L_\odot$) having high signal-to-noise ratios and showing the unambiguous signature of solar-like oscillations. These are H-shell-burning stars, whose oscillations should be valuable for testing models of stellar evolution and for constraining the star-formation rate in the local disk. We use a new technique to compare stars on a single échelle diagram by scaling their frequencies and find well-defined ridges corresponding to radial and non-radial oscillations, including clear evidence for modes with angular degree $l=3$. Measuring the small separation between $l=0$ and $l=2$ allows us to plot the so-called C-D diagram of $\delta\nu_{02}$ versus $\Delta \nu$. The small separation $\delta\nu_{01}$ of $l=1$ from the midpoint of adjacent $l=0$ modes is negative, contrary to the Sun and solar-type stars. The ridge for $l=1$ is notably broadened, which we attribute to mixed modes, confirming theoretical predictions for low-luminosity giants. Overall, the results demonstrate the tremendous potential of Kepler data for asteroseismology of red giants.

Hybrid γ Doradus-δ Scuti Pulsators: New Insights into the Physics of the Oscillations from Kepler Observations

A. Grigahcène, V. Antoci, L. Balona, G. Catanzaro, J. Daszyńska-Daszkiewicz, J. A. Guzik, G. Handler, G. Houdek, D. W. Kurtz, M. Marconi, M. J. P. F. G. Monteiro, A. Moya, V. Ripepi, J.-C. Suárez, K. Uytterhoeven, W. J. Borucki, T. M. Brown and 25 coauthors.
Full text
NASA ADS
DOI

Observations of the pulsations of stars can be used to infer their interior structure and test theoretical models. The main-sequence γ Doradus (Dor) and δ Scuti (Sct) stars with masses 1.2-2.5 M sun are particularly useful for these studies. The γ Dor stars pulsate in high-order g-modes with periods of order 1 day, driven by convective blocking at the base of their envelope convection zone. The δ Sct stars pulsate in low-order g- and p-modes with periods of order 2 hr, driven by the κ mechanism operating in the He II ionization zone. Theory predicts an overlap region in the Hertzsprung-Russell diagram between instability regions, where "hybrid" stars pulsating in both types of modes should exist. The two types of modes with properties governed by different portions of the stellar interior provide complementary model constraints. Among the known γ Dor and δ Sct stars, only four have been confirmed as hybrids. Now, analysis of combined Quarter 0 and Quarter 1 Kepler data for hundreds of variable stars shows that the frequency spectra are so rich that there are practically no pure δ Sct or γ Dor pulsators, i.e., essentially all of the stars show frequencies in both the δ Sct and the γ Dor frequency range. A new observational classification scheme is proposed that takes into account the amplitude as well as the frequency and is applied to categorize 234 stars as δ Sct, γ Dor, δ Sct/γ Dor or γ Dor/δ Sct hybrids.

The asteroseismic potential of Kepler: first results for solar-type stars

W. J. Chaplin, T. Appourchaux, Y. Elsworth, R. A. García, G. Houdek, C. Karoff, T. S. Metcalfe, J. Molenda-Zakowicz, M. J. P. F. G. Monteiro, M. J. Thompson, T. M. Brown, J. Christensen-Dalsgaard, R. L. Gilliland, H. Kjeldsen, W. J. Borucki, D. Koch, J. M. Jenkins and 91 coauthors.
Full text
NASA ADS
DOI

We present preliminary asteroseismic results from Kepler on three G-type stars. The observations, made at one-minute cadence during the first 33.5 d of science operations, reveal high signal-to-noise solar-like oscillation spectra in all three stars: About 20 modes of oscillation may be clearly distinguished in each star. We discuss the appearance of the oscillation spectra, use the frequencies and frequency separations to provide first results on the radii, masses and ages of the stars, and comment in the light of these results on prospects for inference on other solar-type stars that Kepler will observe.