## Scientific Publications

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

### Asteroseismic inference of the central structure in a subgiant star

Earl P. Bellinger, Sarbani Basu, Saskia Hekker, Joergen Christensen-Dalsgaard, Warrick H. Ball.
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Asteroseismic measurements enable inferences of the underlying stellar structure, such as the density and the speed of sound at various points within the interior of the star. This provides an opportunity to test stellar evolution theory by assessing whether the predicted structure of a star agrees with the measured structure. Thus far, this kind of inverse analysis has only been applied to the Sun and three solar-like main-sequence stars. Here we extend the technique to stars on the subgiant branch, and apply it to one of the best-characterized subgiants of the Kepler mission, HR 7322. The observation of mixed oscillation modes in this star facilitates inferences of the conditions of its inert helium core, nuclear-burning hydrogen shell, and the deeper parts of its radiative envelope. We find that despite significant differences in the mode frequencies, the structure near to the center of this star does not differ significantly from the predicted structure.

### Revisiting the impact of stellar magnetic activity on the detectability of solar-like oscillations by Kepler

Savita Mathur, Rafael A. García, Lisa Bugnet, \^Angela R. G. Santos, Netsha Santiago, Paul G. Beck.
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Over 2,000 stars were observed for one month with a high enough cadence in order to look for acoustic modes during the survey phase of the \kep mission. Solar-like oscillations have been detected in about 540 stars. The question of why no oscillations were detected in the remaining stars is still open. Previous works explained the non-detection of modes with the high level of magnetic activity of the stars. However, the sample of stars studied contained some classical pulsators and red giants that could have biased the results. In this work, we revisit this analysis on a cleaner sample of main-sequence solar-like stars that consists of 1,014 stars. First we compute the predicted amplitude of the modes of that sample and for the stars with detected oscillation and compare it to the noise at high frequency in the power spectrum. We find that the stars with detected modes have an amplitude to noise ratio larger than 0.94. We measure reliable rotation periods and the associated photometric magnetic index for 684 stars out of the full sample and in particular for 323 stars where the amplitude of the modes is predicted to be high enough to be detected. We find that among these 323 stars 32% of them have a level of magnetic activity larger than the Sun during its maximum activity, explaining the non-detection of acoustic modes. Interestingly, magnetic activity cannot be the primary reason responsible for the absence of detectable modes in the remaining 68$\%$ of the stars without acoustic modes detected and with reliable rotation periods. Thus, we investigate metallicity, inclination angle of the rotation axis, and binarity as possible causes of low mode amplitudes. Using spectroscopic observations for a subsample, we find that a low metallicity could be the reason for suppressed modes. No clear correlation with binarity nor inclination is found. We also derive the lower limit for our photometric activity index (of 20-30 ppm) below which rotation and magnetic activity are not detected. Finally, with our analysis we conclude that stars with a photometric activity index larger than 2,000 ppm have 98.3% probability of not having oscillations detected.

### Internal mixing of rotating stars inferred from dipole gravity modes

May G. Pedersen, Conny Aerts, Péter I. Pápics, Mathias Michielsen, Sarah Gebruers, Tamara M. Rogers, Geert Molenberghs, Siemen Burssens, Stefano Garcia, Dominic M. Bowman.
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During most of their life, stars fuse hydrogen into helium in their cores. The mixing of chemical elements in the radiative envelope of stars with a convective core is able to replenish the core with extra fuel. If effective, such deep mixing allows stars to live longer and change their evolutionary path. Yet localized observations to constrain internal mixing are absent so far. Gravity modes probe the deep stellar interior near the convective core and allow us to calibrate internal mixing processes. Here we provide core-to-surface mixing profiles inferred from observed dipole gravity modes in 26 rotating stars with masses between 3 and 10 solar masses. We find a wide range of internal mixing levels across the sample. Stellar models with stratified mixing profiles in the envelope reveal the best asteroseismic performance. Our results provide observational guidance for three-dimensional hydrodynamical simulations of transport processes in the deep interiors of stars.
(You can also view the paper here: https://rdcu.be/ckjRm)

### Orbital solutions derived from radial velocities and time delays for four Kepler systems with A/F-type hybrid pulsations

P. Lampens, L. Vermeylen, Y. Frémat, Á. Sódor, M. Skarka, A. Samadi-Ghadim, Zs. Bognár, H. Lehmann, P. De Cat, A. Goswami, L. Dumortier.
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The presence of A/F-type Kepler hybrid stars extending across the entirety of the delta Scuti -gamma Doradus instability strips and beyond remains largely unexplained. In order to better understand these particular stars, we performed a multi-epoch spectroscopic study of a sample of 49 candidate A/F-type hybrid stars and one cool(er) hybrid object detected by the Kepler mission. We determined a lower limit for the multiplicity fraction of 27 %. For six spectroscopic systems, we also reported long-term variations in the time delays (TDs). For four of them, the TD variations are fully coherent with those of the RVs and can be attributed to orbital motion. We aim to improve the orbital solutions for those spectroscopic systems with long orbital periods (order of 4-6 years) among the Kepler hybrid stars that we continued to observe. The orbits are computed based on a simultaneous modelling of the RVs obtained with high-resolution spectrographs and the photometric TDs derived from time-dependent frequency analyses of the Kepler light curves. We refined the orbital solutions of four spectroscopic systems with A/F-type Kepler hybrid component stars: KIC 4480321,  5219533,  8975515 and KIC 9775454. Simultaneous modelling of both data types analysed together enabled us to improve the orbital solutions (all), obtain more robust and accurate information on the mass ratio (some for the first time), and identify the component with short-period delta Sct-type pulsations (all). The information gained is maximized when one of the components, generally the one exhibiting the delta Sct-type pulsations, is a fast rotator. In several cases, we were also able to derive new constraints for the minimum component masses. From a search for regular frequency patterns in the high-frequency regime of the Fourier transforms of each system, we found no evidence of tidal splitting among the triple systems with close (inner) companions. However, some systems exhibit frequency spacings which can be explained by the mechanism of rotational splitting.

### Asteroseismology of luminous red giants with Kepler. II. Dependence of mass loss on pulsations and radiation

Jie Yu, Saskia Hekker, Timothy R. Bedding, Dennis Stello, Daniel Huber, Laurent Gizon, Shourya Khanna, Shaolan Bi.
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Mass loss by red giants is an important process to understand the final stages of stellar evolution and the chemical enrichment of the interstellar medium. Mass-loss rates are thought to be controlled by pulsation-enhanced dust-driven outflows. Here we investigate the relationships between mass loss, pulsations, and radiation, using 3213 luminous Kepler red giants and 135000 ASAS–SN semiregulars and Miras. Mass-loss rates are traced by infrared colours using 2MASS and WISE and by observed-to-model WISE fluxes, and are also estimated using dust mass-loss rates from literature assuming a typical gas-to-dust mass ratio of 400. To specify the pulsations, we extract the period and height of the highest peak in the power spectrum of oscillation. Absolute magnitudes are obtained from the 2MASS $K_s$ band and the Gaia DR2 parallaxes. Our results follow. (i) Substantial mass loss sets in at pulsation periods above $\sim$60 and $\sim$100 days, corresponding to Asymptotic-Giant-Branch stars at the base of the period-luminosity sequences C$'$ and C. (ii) The mass-loss rate starts to rapidly increase in semiregulars for which the luminosity is just above the Red-Giant-Branch tip and gradually plateaus to a level similar to that of Miras. (iii) The mass-loss rates in Miras do not depend on luminosity, consistent with pulsation-enhanced dust-driven winds. (iv) The accumulated mass loss on the Red Giant Branch consistent with asteroseismic predictions reduces the masses of red-clump stars by $6.3$%, less than the typical uncertainty on their asteroseismic masses. Thus mass loss is currently not a limitation of stellar age estimates for galactic archaeology studies.

### Automated approach to measure stellar inclinations: validation through large-scale measurements on the red giant branch

C. Gehan, B. Mosser, E. Michel, M. S. Cunha.
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Measuring stellar inclinations is fundamental to understand planetary formation and dynamics as well as physical conditions during star formation. Oscillation spectra of red giant stars exhibit mixed modes that have both a gravity component from the radiative interior and a pressure component from the convective envelope. Gravity-dominated (g-m) mixed modes split by rotation are well separated inside frequency spectra, making possible accurate measurements of stellar inclinations.
This work aims at developing an automated and general approach to measure stellar inclinations, that can be applied to any solar-type pulsator for which oscillation modes are identified, and at validating it using red giant branch stars observed by Kepler.
The stellar inclination impacts the visibility of oscillation modes with azimuthal orders $m=\lbrace -1, 0, +1 \rbrace$. We use the mean height-to-background ratio of dipole mixed modes with different azimuthal orders to measure stellar inclinations. The underlying statistical distribution of inclinations is recovered in an unbiased way using a probability density function for the stellar inclination angle.
We derive stellar inclination measurements for 1139 stars on the red giant branch, for which Gehan et al. (2018) have identified the azimuthal order of dipole g-m mixed modes. Raw measured inclinations exhibit strong deviation with respect to isotropy which is expected for random inclinations over the sky. When taking uncertainties into account, the reconstructed distribution of inclinations actually follows the expected isotropic distribution of the rotational axis.
This work highlights the biases that affect inclination measurements and provides the way to infer their underlying statistical distribution. When the star is seen either pole-on or equator-on, measurements are challenging and result in a biased distribution. Correcting biases that appear at the low- and high inclination regimes allows us to recover the underlying inclination distribution.

### Testing the intrinsic scatter of the asteroseismic scaling relations with Kepler red giants

Yaguang Li, Timothy R. Bedding, Dennis Stello, Sanjib Sharma, Daniel Huber and, Simon J. Murphy.
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Asteroseismic scaling relations are often used to derive stellar masses and radii, particulaly for stellar, exoplanet, and Galactic studies. It is therefore important that their precisions are known. Here we measure the intrinsic scatter of the underlying seismic scaling relations for $\Delta\nu$ and $\nu_{\rm max}$, using two sharp features that are formed in the H-R diagram (or related diagrams) by the red giant populations. These features are the edge near the zero-age core-helium-burning phase, and the strong clustering of stars at the so-called red giant branch bump. The broadening of those features is determined by factors including the intrinsic scatter of the scaling relations themselves, and therefore it is capable of imposing constraints on them. We modelled Kepler stars with a Galaxia synthetic population, upon which we applied the intrinsic scatter of the scaling relations to match the degree of sharpness seen in the observation. We found that the random errors from measuring $\Delta\nu$ and $\nu_{\rm max}$ provide the dominating scatter that blurs the features. As a consequence, we conclude that the scaling relations have intrinsic scatter of $\sim0.5\%$ ($\Delta\nu$), $\sim1.1\%$ ($\nu_{\rm max}$), $\sim1.7\%$ (mass) and $\sim0.4\%$ (radius), for the SYD pipeline measured $\Delta\nu$ and $\nu_{\rm max}$. This confirms that the scaling relations are very powerful tools. In addition, we show that standard evolution models fail to predict some of the structures in the observed population of both the HeB and RGB stars. Further stellar model improvements are needed to reproduce the exact distributions.

### On the first dSct–roAp hybrid pulsator and the stability of p and g modes in chemically peculiar A/F stars

Simon J. Murphy, Hideyuki Saio, Masahide Takada-Hidai, Donald W. Kurtz, Hiromoto Shibahashi, Masao Takata, Daniel R. Hey.
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### Modelling Kepler Red Giants in Eclipsing Binaries: Calibrating the Mixing-Length Parameter with Asteroseismology

Tanda Li, Timothy R. Bedding, Daniel Huber, Warrick H. Ball, Dennis Stello, Simon J. Murphy, Joss Bland-Hawthorn.
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Stellar models rely on a number of free parameters. High-quality observations of eclipsing binary stars observed by Kepler offer a great opportunity to calibrate model parameters for evolved stars. Our study focuses on six Kepler red giants with the goal of calibrating the mixing-length parameter of convection as well as the asteroseismic surface term in models. We introduce a new method to improve the identification of oscillation modes which exploits theoretical frequencies to guide the mode identification ('peak-bagging') stage of the data analysis. Our results indicate that the convective mixing-length parameter ($\alpha$) is $\approx$14% larger for red giants than for the Sun, in agreement with recent results from modelling the APOGEE stars. We found that the asteroseismic surface term (i.e. the frequency offset between the observed and predicted modes) correlates with stellar parameters ($T_{\rm{eff}}$, $\log g$) and the mixing-length parameter. This frequency offset generally decreases as giants evolve. The two coefficients $a_{-1}$ and $a_3$ for the inverse and cubic terms that have been used to describe the surface term correction are found to correlate linearly. The effect of the surface term is also seen in the p-g mixed modes, however, established methods for correcting the effect are not able to properly correct the g-dominated modes in late evolved stars.

### Surface rotation of Kepler red giant stars

T. Ceillier, J. Tayar, S. Mathur, D. Salabert, R. A. García, D. Stello, M. H. Pinsonneault, J. van Saders, P. G. Beck, S. Bloemen.
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The long and continuous photometric observations collected by the Kepler satellite of a large number of stars allows the study of the surface rotation and activity variability of thousands of field stars. Such information complements the asteroseismic measurements that constrain the interiors of stars and provides good calibration possibilities for the age-rotation-activity relations. Here, we study the light curves of a large number of red giant stars observed by the Kepler satellite to identify the ones exhibiting surface modulations due to the presence of star spots crossing the visible surface of the star and determine their rotational periods. We use optimized corrections to treat the Kepler data to retrieve the intrinsic modulations present in these light curves. Two different methods based on a wavelet decomposition and on the autocorrelation function of the light curve were then used to get estimates of the rotation period of each star. We also present a new tool which is a combination of the two previous methods, called Composite Spectrum. The results of these various methods are then compared to identify the stars showing clear signs of surface rotation. Out of a sample of 17, 377 red giants, we isolate 361 with a validated rotation rate. This represents 2.08% of our sample, which is consistent with the expectations from spectroscopic measurements. Among the 4881 intermediate mass stars ($M>2M_\odot$), we find a smaller rate of rapid rotators than expected, 1.92%, suggesting enhanced loss or differential rotation in those stars. Finally, we find that 15% of the 575 low-mass clump stars ($M<1.1M_\odot$) are rotating rapidly, which is indicative of a recent interaction.

### Multi-technique investigation of the binary fraction among A-F type candidate hybrid variable stars discovered by Kepler

P. Lampens, \and, Y. Frémat, \and, L. Vermeylen, \and, Á. Sódor, \and, M. Skarka, \and, P. De Cat, \and, Zs. Bognár, et al.\.
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Context. Hundreds of candidate hybrid pulsators of intermediate type A-F have been revealed by the recent space missions. Hybrid pulsators offer the advantage to study the full stellar interiors, where both low-order p- and high-order g-modes are simultaneously excited. The true hybrid stars must be identified since other processes, related to stellar multiplicity or rotation, might also explain the presence of (some) low frequencies observed in the periodograms of these pulsating stars. \
Aims. We measured the radial velocities of 50 candidate $\delta$ Scuti - $\gamma$ Doradus hybrid stars from the Kepler mission with the Hermes and Ace spectrographs over a time span of months to years. We aim to derive the fraction of binary and multiple systems, to provide an independent determination of the atmospheric properties and v sini, and to identify the (probable) physical cause of the low frequencies. \
Methods. We computed 1-D cross-correlation functions (CCFs) in order to find the best set of model parameters in terms of the number of components, spectral type(s) and v sini for each target. Radial velocities were measured from spectrum synthesis and by using a 2-D cross-correlation technique in the case of double- and triple-lined systems. Fundamental parameters were determined by fitting (in casu composite) synthetic spectra to the normalised median spectra corrected for the appropriate Doppler shifts. \
Results. We report on the analysis of 478 high-resolution Hermes and 41 Ace spectra of A/F-type candidate hybrid pulsating stars from the Kepler field. We determined their radial velocities, projected rotational velocities, atmospheric properties and classified our targets based on the shape of the CCFs and the temporal behaviour of the radial velocities. We derived orbital solutions for seven systems.Three long-period preliminary orbital solutions are confirmed by a photometric time-delay analysis. Finally, we determined a global multiplicity fraction of 27% in our sample of candidate hybrid pulsators. \

### Deep Learning Classification in Asteroseismology

Marc Hon, Dennis Stello, Jie Yu.
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In the power spectra of oscillating red giants, there are visually distinct features defining stars ascending the red giant branch from those that have commenced helium core burning. We train a one-dimensional convolutional neural network by supervised learning to automatically learn these visual features from images of folded oscillation spectra. By training and testing on Kepler red giants, we achieve an accuracy of up to 99% in separating helium-burning red giants from those ascending the red giant branch. The convolutional neural network additionally shows capability in accurately predicting the evolutionary states of 5379 previously unclassified Kepler red giants, by which we now have greatly increased the number of classified stars.

### NGC 6819: testing the asteroseismic mass scale, mass loss, and evidence for products of non-standard evolution

R. Handberg, K. F. Brogaard, A. Miglio, D. Bossini, Y. Elsworth, D. Slumstrup, G. R. Davies, W. J. Chaplin.
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We present an extensive peakbagging effort on Kepler light curves of $\sim$50 red giant stars in the open star cluster NGC 6819. By employing sophisticated pre-processing of the time series and Markov Chain Monte Carlo (MCMC) techniques we extracted individual frequencies, heights and linewidths for hundreds of oscillation modes in the sample of stars.
We show that the ‘average’ asteroseismic parameter $\delta\nu_{02}$, derived from these, can be used to distinguish the stellar evolutionary state between the red giant branch (RGB) stars and red clump (RC) stars.
The masses and radii of the giants are estimated using asteroseismic scaling relations, both empirically corrected to obtain self-consistency as well as agreement with independent measures of distance and age, and, alternatively, using updated theoretical corrections. Remarkable agreement is found, allowing the evolutionary state of the giants to be determined exclusively from the empirical correction to the scaling relations. We find a mean mass of the RGB stars and RC stars in NGC 6819 to be $1.61\pm0.02\,\mathrm{M}_\odot$ and $1.64\pm0.02\,\mathrm{M}_\odot$, respectively. The difference $\Delta M=-0.03\pm0.01\,\mathrm{M}_\odot$ is almost insensitive to systematics, suggesting very little RGB mass loss, if any.
Stars that are outliers relative to the ensemble reveal overmassive members that likely evolved via mass-transfer in a blue straggler phase. We also suggest that KIC 4937011, a low-mass Li-rich giant previously studied in the literature, is a cluster member in the RC phase that experienced very high mass-loss during its evolution. Such over- and undermassive stars need to be considered when studying field giants, since the true age of such stars cannot be known and there is currently no way to distinguish them from normal stars.

### 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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.

### Hybrid Î³ Doradus-Î´ Scuti Pulsators: New Insights into the Physics of the Oscillations from Kepler Observations

A. GrigahcÃ¨ne, V. Antoci, L. Balona, G. Catanzaro, J. DaszyÅ„ska-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. SuÃ¡rez, K. Uytterhoeven, W. J. Borucki, T. M. Brown and 25 coauthors.
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