## Scientific Publications

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

### The Impact of Gaia DR1 on Asteroseismic Inferences from Kepler

Travis Metcalfe, Orlagh Creevey, Jennifer van Saders.
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The Kepler mission has been fantastic for asteroseismology of solar-type stars, but the targets are typically quite distant. As a consequence, the reliability of asteroseismic modeling has been limited by the precision of additional constraints from high-resolution spectroscopy and parallax measurements. A precise luminosity is particularly important to minimize potential biases due to the intrinsic correlation between stellar mass and initial helium abundance. We have applied the latest version of the Asteroseismic Modeling Portal (AMP) to the complete Kepler data sets for 30 stars with known rotation rates and chromospheric activity levels. We compare the stellar properties derived with and without the measured parallaxes from the first data release of Gaia. We find that in most cases the masses and ages inferred from asteroseismology shift within their uncertainties. For a few targets that show larger shifts, the updated stellar properties only strengthen previous conclusions about anomalous rotation in middle-aged stars.

### Gravity mode offset and properties of the evanescent zone in red-giant stars

S. Hekker, Y. Elsworth, G.C. Angelou.
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Context:The wealth of asteroseismic data for red-giant stars and the precision with which these data have been observed over the last decade calls for investigations to further understand the internal structures of these stars. Aims: The aim of this work is to validate a method to measure the underlying period spacing, coupling term and mode offset of pure gravity modes that are present in the deep interiors of red-giant stars. We subsequently investigate the physical conditions of the evanescent zone between the gravity mode cavity and the pressure mode cavity. Methods: We implement an alternative mathematical description, compared to what is used in the literature, to analyse observational data and to extract the underlying physical parameters that determine the frequencies of mixed modes. This description takes the radial order of the modes explicitly into account, which reduces its sensitivity to aliases. Additionally, and for the first time, this method allows us to constrain the gravity mode offset $\epsilon_{\rm g}$ for red-giant stars. Results: We find that this alternative mathematical description allows us to determine the period spacing $\Delta\Pi$ and the coupling term $q$ for the dipole modes within a few percent of literature values. Additionally, we find that $\epsilon_{\rm g}$ varies on a star by star basis and should not be kept fixed in the analysis. Furthermore, we find that the coupling factor is logarithmically related to the physical width of the evanescent region normalised by the radius at which the evanescent zone is located. Finally, the local density contrast at the edge of the core of red giant branch models shows a tentative correlation with the offset $\epsilon_{\rm g}$. Conclusions: We are continuing to exploit the full potential of the mixed modes to investigate the internal structures of red-giant stars; in this case we focus on the evanescent zone. It remains, however, important to perform comparisons between observations and models with great care as the methods employed are sensitive to the range of input frequencies.

### K2 photometry and HERMES spectroscopy of the blue supergiant rho Leo: rotational wind modulation and low-frequency waves

C. Aerts, D. Bowman, S. Sımon-Dıaz, B. Buysschaert, C. C. Johnston, E. Moravveji, P. G. Beck, P. De Cat, S. Triana, S. Aigrain, N. Castro, D. Huber, T. R. White.
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We present an 80-day long uninterrupted high-cadence K2 light curve of the B1Iab supergiant $\rho\,$Leo, deduced with the method of halo photometry. This light curve reveals a dominant frequency of $f_{\rm rot}=0.0373$ d$^{-1}$ and its harmonics, corresponding with a rotation period of 26.8 d and subject to amplitude and phase modulation. The K2 photometry additionally reveals low-frequency variability ($<1.5\,$d$^{-1}$) and is in full agreement with low-cadence high-resolution spectroscopy assembled during 1800 days. The spectroscopy reveals rotational wind modulation of about 20 km s$^{-1}$ and photospheric velocity variations of a few km s$^{-1}$ at frequencies in the range 0.2 to 0.6 d$^{-1}$. Given the large macroturbulence needed to explain the spectral line broadening of the star, we interpret the detected photospheric velocity as due to travelling super-inertial gravity waves with dominant tangential amplitude.

### Model-independent measurement of internal stellar structure in 16 Cygni A & B

Earl P. Bellinger, Sarbani Basu, Saskia Hekker, Warrick H. Ball.
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We present a method for measuring internal stellar structure based on asteroseismology that we call "inversions-for-agreement." The method accounts for imprecise estimates of stellar mass and radius as well as the relatively limited oscillation mode sets that are available for distant stars. By construction, the results of the method are independent of stellar models. We apply this method to measure the isothermal sound speeds in the cores of the solar-type stars 16 Cyg A and B using asteroseismic data obtained from Kepler observations. We compare the asteroseismic structure that we deduce against best-fitting evolutionary models and find that the sound speeds in the cores of these stars exceed those of the models.

### Robo-AO Kepler Asteroseismic Survey. I. Adaptive optics imaging of 99 asteroseismic Kepler dwarfs and subgiants

Jessica S. Schonhut-Stasik, Christoph Baranec, Daniel Huber, Carl Ziegler, Dani Atkinson, Eric Gaidos, Nicholas M. Law, Reed Riddle, Janis Hagelberg, Nienke van der Marel, Klaus W. Hodapp.
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We used the Robo-AO laser adaptive optics system to image 99 main sequence and subgiant stars that have Kepler-detected asteroseismic signals. Robo-AO allows us to resolve blended secondary sources at separations as close as $\sim$0$.\!\!^{\prime\prime}$15 that may contribute to the measured Kepler light curves and affect asteroseismic analysis and interpretation. We report 8 new secondary sources within 4$.\!\!^{\prime\prime}$0 of these Kepler asteroseismic stars. We used Subaru and Keck adaptive optics to measure differential infrared photometry for these candidate companion systems. Two of the secondary sources are likely foreground objects while the remaining 6 are background sources; however we cannot exclude the possibility that three of the objects may be physically associated. We measured a range of i'-band amplitude dilutions for the candidate companion systems from 0.43% to 15.4%. We find that the measured amplitude dilutions are insufficient to explain the previously identified excess scatter in the relationship between asteroseismic oscillation amplitude and the frequency of maximum power.

### KIC 9533489: a genuine $\gamma$ Doradus – $\delta$ Scuti Kepler hybrid pulsator with transit events

Zs. Bognár, P. Lampens, Y. Frémat, J. Southworth, Á. Sódor, P. De Cat, H. T. Isaacson, G. W. Marcy, D. R. Ciardi, R. L. Gilliland, P. Martín-Fernández.
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Context. Several hundred candidate hybrid pulsators of type A–F have been identified from space-based observations. Their large number allows both statistical analyses and detailed investigations of individual stars. This offers the opportunity to study the full interior of the genuine hybrids, in which both low-radial-order p- and high-order g-modes are self-excited at the same time. However, a few other physical processes can also be responsible for the observed hybrid nature, related to binarity or to surface inhomogeneities. The finding that most $\delta$ Scuti stars also show long-period light variations represents a real challenge for theory. Aims: We aim at determining the pulsation frequencies of KIC 9533489, to search for regular patterns and spacings among them, and to investigate the stability of the frequencies and the amplitudes. An additional goal is to study the serendipitously detected transit events: is KIC 9533489 the host star? What are the limitations on the physical parameters of the involved bodies? Methods: Fourier analysis of all the available Kepler light curves. Investigation of the frequency and period spacings. Determination of the stellar physical parameters from spectroscopic observations. Modelling of the transit events. Results: The Fourier analysis of the Kepler light curves revealed 55 significant frequencies clustered into two groups, which are separated by a gap between 15 and 27 d$^{-1}$. The light variations are dominated by the beating of two dominant frequencies located at around 4 d$^{-1}$. The amplitudes of these two frequencies show a monotonic long-term trend. The frequency spacing analysis revealed two possibilities: the pulsator is either a highly inclined moderate rotator ($v\approx 70$ km s$^{-1}$, $i > 70^\circ$) or a fast rotator ($v\approx 200$ km s$^{-1}$) with $i\approx20^\circ$. The transit analysis disclosed that the transit events which occur with a $\approx197$ d period may be caused by a $1.6\,R_{\rm Jup}$ body orbiting a fainter star, which would be spatially coincident with KIC 9533489.

### Automated asteroseismic peak detections

A. García Saravia Ortiz de Montellano, S. Hekker, N. Themeßl.
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Space observatories such as Kepler have provided data that can potentially revolutionise our understanding of stars. Through detailed asteroseismic analyses we are capable of determining fundamental stellar parameters and reveal the stellar internal structure with unprecedented accuracy. However, such detailed analyses, known as peak bagging, have so far been obtained for only a handful of stars while most of the scientific potential of the available data remains unexplored. One of the major challenges in peak bagging is identifying how many solar-like oscillation modes are visible in a power density spectrum. Identification of oscillation modes is usually done by visual inspection which is time-consuming and has a degree of subjectivity. Here, we present a peak detection algorithm specially suited for the detection of solar-like oscillations. It reliably characterises the solar-like oscillations in a power density spectrum and estimates their parameters without human intervention. Furthermore, we provide a metric to characterise the false positive and false negative rates to provide further information about the reliability of a detected oscillation mode or the significance of a lack of detected oscillation modes. The algorithm presented here opens the possibility for detailed and automated peak bagging of the thousands of solar-like oscillators observed by Kepler.

### Finding binaries from phase modulation of pulsating stars with Kepler: V. Orbital parameters, with eccentricity and mass-ratio distributions of 341 new binaries

Simon J. Murphy, Maxwell Moe, Donald W. Kurtz, Timothy R. Bedding, Hiromoto Shibahashi, Henri M. J. Boffin.
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The orbital parameters of binaries at intermediate periods ($10^2$$10^3$ d) are difficult to measure with conventional methods and are very incomplete. We have undertaken a new survey, applying our pulsation timing method to Kepler light curves of 2224 main-sequence A/F stars and found 341 non-eclipsing binaries. We calculate the orbital parameters for 317 PB1 systems (single-pulsator binaries) and 24 PB2s (double-pulsators), tripling the number of intermediate-mass binaries with full orbital solutions. The method reaches down to small mass ratios $q \approx 0.02$ and yields a highly homogeneous sample. We parametrize the mass-ratio distribution using both inversion and MCMC forward-modelling techniques, and find it to be skewed towards low-mass companions, peaking at $q \approx 0.2$. While solar-type primaries exhibit a brown dwarf desert across short and intermediate periods, we find a small but statistically significant (2.6$\sigma$) population of extreme-mass-ratio companions ($q < 0.1$) to our intermediate-mass primaries. We find a large fraction of companions (21% $\pm$ 6%) are white dwarfs in post-mass-transfer systems with primaries that are now blue stragglers, some of which are the progenitors of Type Ia supernovae, barium stars, symbiotics, and related phenomena. Excluding these white dwarfs, we determine the binary fraction of A/F primaries to be 13.9% $\pm$ 2.1% at $q>0.1$ and periods of 100 – 1500 d. Combining our measurements with those in the literature, we find the binary fraction across these periods is a constant 5% for primaries $M_1 < 0.8$ M$_{\odot}$, but then increases linearly with $\log M_1$, demonstrating that natal discs around more massive protostars $M_1 \gtrsim 1$ M$_{\odot}$ become increasingly more prone to fragmentation. Finally, we find the eccentricity distribution of the main-sequence pairs to be much less eccentric than the thermal distribution.

### 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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