## Scientific Publications

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

### Insights from the APOKASC Determination of the Evolutionary State of Red-Giant Stars by consolidation of different methods

Yvonne Elsworth, Saskia Hekker, Jennifer A. Johnson, Thomas Kallinger, Benoit Mosser, Marc Pinsonneault, Marc Hon, James Kuszlewicz, Aldo Serenelli, Dennis Stello, Jamie Tayar, Mathieu Vrard.
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The internal working of low-mass stars is of great significance to both the study of stellar structure and the history of the Milky Way. Asteroseismolgy has the power to directly sense the internal structure of stars and allows for the determination of the evolutionary state – i.e. has helium burning commenced or is the energy generated by the fusion of hydrogen in the core? We use observational data from red-giant stars in a combination (known as APOKASC) of asteroseismology (from the Kepler mission) and spectroscopy (from SDSS/APOGEE). The new feature of the analysis is that the APOKASC evolutionary state determination is based on the comparison of diverse approaches to the investigation of the frequency-power spectrum. The high level of agreement between the methods is a strong validation of the approaches. Stars for which there is not a consensus view are readily identified. The comparison also facilitates the identification of unusual stars including those that show evidence for very strong coupling between p and g cavities. The comparison between the classification based on the spectroscopic data and asteroseismic data have led to a new value for the statistical uncertainty in APOGEE temperatures. These consensus evolutionary states will be used as an input for methods that derive masses and ages for these stars based on comparison of observables with stellar evolutionary models (‘grid-based modeling’) and as a training set for machine-learning and data-driven methods of evolutionary state determination.

### Testing stellar evolution with asteroseismic inversions of a main sequence star harboring a small convective core

Earl P. Bellinger, Sarbani Basu, Saskia Hekker, Jørgen Christensen-Dalsgaard.
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The goal of stellar evolution theory is to predict the structure of stars throughout their lifetimes. Usually, these predictions can be assessed only indirectly, for example by comparing predicted and observed effective temperatures and luminosities. Thanks now to asteroseismology, which can reveal the internal structure of stars, it becomes possible to compare the predictions from stellar evolution theory to actual stellar structures. In this work, we present an inverse analysis of the oscillation data from the solar-type star KIC 6225718, which was observed by the Kepler space observatory during its nominal mission. As its mass is about 20% greater than solar, this star is predicted to transport energy by convection in its nuclear-burning core. We find significant differences between the predicted and actual structure of the star in the radiative interior near to the convective core. In particular, the predicted sound speed is higher than observed in the deep interior of the star, and too low at a fractional radius of 0.25 and beyond. The cause of these discrepancies is unknown, and is not remedied by known physics in the form of convective overshooting or elemental diffusion.

### FliPer: A global measure of power density to estimate surface gravities of Solar-like stars

L. Bugnet, R. A. García, G. R. Davies, S. Mathur, E. Corsaro, O. J. Hall, B. M. Rendle.
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Asteroseismology provides global stellar parameters such as masses, radii or surface gravities using the mean global seismic parameters as well as the effective temperature for thousands of low-mass stars ($0.8 M_\odot <M<3 M_\odot$). This methodology can only be applied to stars in which acoustic modes excited by turbulent convection are measured. Other techniques such as the Flicker can also be used to determine stellar surface gravities, but only works for $\log{g}$ above $2.5$ dex. In this work, we present a new metric called FliPer (Flicker in Power, in opposition to the standard Flicker measurement which is computed in the time domain) that is able to extend the range for which reliable surface gravities can be obtained ($0.1<\log{g}<4.6$ dex) without performing any seismic analysis. FliPer takes into account the average variability of a star measured in the power density spectrum in a given range of frequencies. However, FliPer values calculated on several ranges of frequency are required to better characterize a star. Using a large set of asteroseismic targets it is possible to calibrate the behavior of surface gravity with FliPer through machine learning. This calibration made with a random forest regressor covers a wide range of surface gravities from main-sequence stars to subgiants and red giants, with very small uncertainties from $0.04$ to $0.1$ dex. FliPer values can be inserted in automatic global seismic pipelines to either give estimation of the stellar surface gravity or to assess the quality of the seismic results by detecting any outliers in the obtained surface gravities. FliPer also constrain the surface gravities of main-sequence dwarfs using only long cadence data for which the Nyquist frequency is too low to measure the acoustic-mode properties. This is the first seismic-independent method that allows the estimation of surface gravities below $2.5$ dex with good precision.

### Helium settling in F stars: constraining turbulent mixing using observed helium glitch signature

Kuldeep Verma, Víctor Silva Aguirre.
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Recent developments in asteroseismology – thanks to space-based missions such as CoRoT and Kepler – provide handles on those properties of stars that were either completely inaccessible in the past or only poorly measured. Among several such properties is the surface helium abundance of F and G stars. We used the oscillatory signature introduced by the ionization of helium in the observed oscillation frequencies to constrain the amount of helium settling in F stars. For this purpose, we identified three promising F stars for which the standard models of atomic diffusion predict large settling (or complete depletion) of surface helium. Assuming turbulence at the base of envelope convection zone slows down settling of the helium and heavy elements, we found an envelope mixed mass of approximately $5 \times 10^{-4}$M$_\odot$ necessary to reproduce the observed amplitude of helium signature for all the three stars. This is much larger than the mixed mass of the order of $10^{-6}$M$_\odot$ found in the previous studies performed using the measurements of the heavy element abundances. This demonstrates the potential of using the helium signature together with measurements of the heavy element abundances to identify the most important physical processes competing against atomic diffusion, allowing eventually to correctly interpret the observed surface abundances of hot stars, consistent use of atomic diffusion in modelling both hot and cool stars, and shed some light on the long-standing cosmological lithium problem.

### Influence of magnetic activity on the determination of stellar parameters through asteroseismology

F. Pérez Hernández, R. A. García, S. Mathur, A. R. G. Santos, C. Régulo.
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Magnetic activity changes the gravito-acoustic modes of solar-like stars and in particular their frequencies. There is an angular-degree dependence that is believed to be caused by the non-spherical nature of the magnetic activity in the stellar convective envelope. These changes in the mode frequencies could modify the small separation of low-degree modes (i.e. frequency difference between consecutive quadrupole and radial modes), which is sensitive to the core structure and hence to the evolutionary stage of the star. Determining global stellar parameters such as the age using mode frequencies at a given moment of the magnetic activity cycle could lead to biased results. Our estimations show that in general these errors are lower than other systematic uncertainties, but in some circumstances they can be as high as $10\%$ in age and of a few percent in mass and radius. In addition, the frequency shifts caused by the magnetic activity are also frequency dependent. In the solar case this is a smooth function that will mostly be masked by the filtering of the so-called surface effects. However the observations of other stars suggest that there is an oscillatory component with a period close to the one corresponding to the acoustic depth of the He$\,$II zone. This could give rise to a misdetermination of some global stellar parameters, such as the helium abundance. Our computations show that the uncertainties introduced by this effect are lower than the $3\%$ level.

### Asteroseismology of solar-type stars

Rafael A. García, Jérôme Ballot.
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Until the last few decades, investigations of stellar interiors had been restricted to theoretical studies only constrained by observations of their global properties and external characteristics. However, in the last thirty years the field has been revolutionized by the ability to perform seismic investigations of stellar interiors. This revolution begun with the Sun, where helioseismology has been yielding information competing with what can be inferred about the Earth's interior from geoseismology. The last two decades have witnessed the advent of asteroseismology of solar-like stars, thanks to a dramatic development of new observing facilities providing the first reliable results on the interiors of distant stars. The coming years will see a huge development in this field. In this review we focus on solar-type stars, i.e., cool main-sequence stars where oscillations are stochastically excited by surface convection. After a short introduction and a historical overview of the discipline, we review the observational techniques generally used, and we describe the theory behind stellar oscillations in cool main-sequence stars. We continue with a complete description of the normal mode analyses through which it is possible to extract the physical information about the structure and dynamics of the stars. We then summarize the lessons that we have learned and discuss unsolved issues and questions that are still unanswered.

### Fundamental properties of Kepler and CoRoT targets: IV. Masses and radii from frequencies of minimum $\Delta \nu$ and their implications

M. Yildiz, Z. Celik Orhan, C. Kayhan.
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Recently, by analysing the oscillation frequencies of 90 stars, Yıldız, C¸ elik Orhan & Kayhan have shown that the reference frequencies (νmin0, νmin1, and νmin2) derived from glitches due to He II ionization zone have very strong diagnostic potential for the determination of their effective temperatures. In this study, we continue to analyse the same stars and compute their mass, radius, and age from different scaling relations including relations based on νmin0, νmin1, and νmin2. For most of the stars, the masses computed using νmin0 and νmin1 are very close to each other. For 38 stars, the difference between these masses is less than 0.024 M. The radii of these stars from νmin0 and νmin1 are even closer, with differences of less than 0.007 R. These stars may be the most well known solar-like oscillating stars and deserve to be studied in detail. The asteroseismic expressions we derive for mass and radius show slight dependence on metallicity. We therefore develop a new method for computing initial metallicity from this surface metallicity by taking into account the effect of microscopic diffusion. The time dependence of initial metallicity shows some very interesting features that may be important for our understanding of chemical enrichment of Galactic Disc. According to our findings, every epoch of the disc has its own lowest and highest values for metallicity. It seems that rotational velocity is inversely proportional to 1/2 power of age as given by the Skumanich relation.

### Bolometric corrections of stellar oscillation amplitudes as observed by the Kepler, CoRoT, and TESS missions

Mikkel N. Lund.
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A better understanding of the amplitudes of stellar oscillation modes and surface granulation is essential for improving theories of mode physics and the properties of the outer convection zone of solar-like stars. A proper prediction of these amplitudes is also essential for appraising the detectability of solar-like oscillations for asteroseismic analysis. Comparisons with models, or between different photometric missions, are enabled by applying a bolometric correction, which converts mission-specific amplitudes to their corresponding bolometric (full light) values. We derive the bolometric correction factor for amplitudes of radial oscillation modes and surface granulation as observed by the Kepler, CoRoT, and TESS missions. The calculations are done assuming a stellar spectrum given by a black-body as well as by synthetic spectral flux densities from 1D model atmospheres. We derive a power-law and polynomial relations for the bolometric correction as a function of temperature from the black-body approximation and evaluate the deviations from adopting a more realistic spectrum. Across the full temperature range from $4000-7500$ K, the amplitudes from TESS are in the black-body approximation predicted to be a factor ${\sim}0.83-0.84$ times those observed by Kepler. We find that using more realistic flux spectra over the black-body approximation can change the bolometric correction by as much as ${\sim}30\%$ at the lowest temperatures, but with a change typically within ${\sim}5-10 \%$ around a $T_{\rm eff}$ of $5500-6000$ K. We find that after $T_{\rm eff}$, the bolometric correction most strongly depends on $\rm [M/H]$, which could have an impact on reported metallicity dependencies of amplitudes reported in the literature.

### Angular Momentum Transport in Stellar Interiors

Conny Aerts, Stéphane Mathis, Tamara M. Rogers.
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Dear KASC/TASC community,
several of you asked me during conferences over the summer if it would be possible to get free access to the final published version of our ARAA on "Angular Momentum Transport in Stellar Interiors". The paper has come out and you can download it free of charge via my personal ARAA e-link.
The review represents a major community effort in that it discusses the asteroseismic rotation and log g estimates of 1210 Kepler/ K2 targets across stellar evolution (assembled until 1 August 2018). You can now download the PDF of the published paper, the figures and supplemental material, and the electronic data file to reproduce Figure 4, from the ARAA website via this link:
http://www.annualreviews.org/eprint/PTFT2M89NRMEAWTEHVE9/full/10.1146/annurev-astro-091918-104359
In order to comply with the ARAA regulations, may I please ask you not to distribute the PDF yourself, but rather to send the above link to the free-of-charge manuscript download to interested readers.
Many thanks, Conny Aerts, also on behalf of Stéphane Mathis and Tami Rogers

### Deep Learning Applied to the Asteroseismic Modeling of Stars with Coherent Oscillation Modes

Luc Hendriks, Conny Aerts.
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We develop a novel method based on machine learning principles to achieve optimal initiation of CPU-intensive computations for forward asteroseismic modeling in a multi-D parameter space. A deep neural network is trained on a precomputed asteroseismology grid containing about 62 million coherent oscillation-mode frequencies derived from stellar evolution models. These models are representative of the core-hydrogen burning stage of intermediate-mass and high-mass stars. The evolution models constitute a 6D parameter space and their predicted low-degree pressure- and gravity-mode oscillations are scanned, using a genetic algorithm. A software pipeline is created to find the best fitting stellar parameters for a given set of observed oscillation frequencies. The proposed method finds the optimal regions in the 6D parameters space in less than a minute, hence providing the optimal starting point for further and more detailed forward asteroseismic modeling in a high-dimensional context. We test and apply the method to seven pulsating stars that were previously modeled asteroseismically by classical grid-based forward modeling based on a $\chi^2$ statistic and obtain good agreement with past results. Our deep learning methodology opens up the application of asteroseismic modeling in +6D parameter space for thousands of stars pulsating in coherent modes with long lifetimes observed by the Kepler space telescope and to be discovered with the TESS and PLATO space missions, while applications so far were done star-by-star for only a handful of cases. Our method is open source and can be used by anyone freely.

### The subgiant HR 7322 as an asteroseismic benchmark star

Amalie Stokholm, Poul Erik Nissen, Víctor Silva Aguirre, Timothy R. White, Mikkel N. Lund, Jakob Rørsted Mosumgaard, Daniel Huber, Jens Jessen-Hansen.
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We present an in-depth analysis of the bright subgiant star HR 7322 (KIC 10005473) using Kepler short-cadence photometry, optical interferometry from CHARA, high-resolution spectroscopy using SONG spectra, and stellar modelling using garstec grids and the Bayesian grid-fitting algorithm basta. HR 7322 is only the second subgiant with high-quality Kepler asteroseismology for which we also have interferometric data. We find a limb-darkened angular diameter of $0.443 \pm 0.007$ mas, which combined with a distance derived from the parallax from the second data release of Gaia and a bolometric flux yields a linear radius of $2.00 \pm 0.03$ R$_{\odot}$ and an effective temperature of $6350 \pm 90$ K. HR 7322 exhibits solar-like oscillations and using the asteroseismic scaling relations and revisions thereof, we find excellent agreement between asteroseismic and interferometric stellar radius. The level of precision reached by the careful modelling is to a great extent due to the presence of an avoided crossing in the dipole oscillation mode pattern of HR 7322. We find that the standard models predict radii systematically smaller than the observed interferometric one and that a sub-solar mixing length parameter is needed to achieve an excellent fit to individual oscillation frequencies, interferometric temperature, and spectroscopic metallicity.

### Six new rapidly oscillating Ap stars in the Kepler long-cadence data using super-Nyquist asteroseismology

Daniel R. Hey, Daniel L. Holdsworth, Timothy R. Bedding, Simon J. Murphy, Margarida S. Cunha, Donald W. Kurtz, Daniel Huber, Benjamin Fulton, Andrew W. Howard.
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We perform a search for rapidly oscillating Ap stars in the Kepler long-cadence data, where true oscillations above the Nyquist limit of 283.21 $\mu$Hz can be reliably distinguished from aliases as a consequence of the Barycentric time corrections applied to the Kepler data. We find evidence for rapid oscillations in six stars: KIC 6631188, KIC 7018170, KIC 11031749, KIC 10685175, KIC 11296437 and KIC 11409673, and identify each star as chemically peculiar through either pre-existing classifications, or a combination of LAMOST spectroscopy and characteristic spot-based modulation of the light curve. For each star, we identify the principal pulsation mode, and are able to observe several additional pulsation modes in KIC 7018170, allowing for a lower constraint on the large frequency separation $\Delta \nu$. We find that KIC 7018170 and KIC 11409673 both oscillate above their theoretical acoustic cutoff frequency, whilst KIC 11031749 oscillates at the cutoff frequency within uncertainty. All but KIC 11031749 exhibit strong amplitude modulation consistent with the oblique pulsator model, yielding confirmation of their mode geometry and periods of rotation.

### gamma Doradus stars as test of angular momentum transport models

Rhita-Maria Ouazzani, J.P. Marques, M-J. Goupil, S. Christophe, V. Antoci, S.J.A.J. Salmon, J. Ballot.
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Helioseismology and asteroseismology of red giant stars have shown that the distribution of angular momentum in stellar interiors, and its evolution with time remains an open issue in stellar physics. Owing to the unprecedented quality of Kepler photometry, we are able to seismically infer internal rotation rates in gamma Doradus stars, which provide the MS counterpart to the red-giants puzzle. We confront these internal rotation rates to stellar evolution models with rotationally induced transport of angular momentum, in order to test angular momentum transport mechanisms. We used a stellar model-independent method developed by Christophe et al. in order to obtain seismically inferred, buoyancy radii and near-core rotation for 37 gamma Doradus stars observed by Kepler. We show that the buoyancy radius can be used as a reliable evolution indicator for field stars on the MS. We computed rotating evolutionary models including transport of angular momentum in radiative zones, following Zahn and Maeder, with the CESTAM code. This code calculates the rotational history of stars from the birth line to the tip of the RGB. The initial angular momentum content has to be set initially, which is done by fitting rotation periods in young stellar clusters. We show a clear disagreement between the near-core rotation rates measured in the sample and the rotation rates obtained from evolutionary models including rotationally induced transport following Zahn (1992). These results show a disagreement similar to that of the Sun and red giant stars. This suggests the existence of missing mechanisms responsible for the braking of the core before and along the MS. The efficiency of the missing mechanisms is investigated. The transport of angular momentum as formalized by Zahn and Maeder cannot explain the measurements of near-core rotation in main-sequence intermediate-mass stars we have at hand.

### The Second APOKASC Catalog: The Empirical Approach

Marc H. Pinsonneault, Yvonne P. Elsworth, Jamie Tayar, Aldo Serenelli, Dennis Stello, Joel Zinn, Savita Mathur, Rafael A. García, Jennifer A. Johnson, Saskia Hekker, Daniel Huber, Thomas Kallinger, Szabolcs Mészáros, Benoit Mosser, Keivan Stassun, Léo Girardi, Thaíse S. Rodrigues and 19 coauthors.
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We present a catalog of stellar properties for a large sample of 6676 evolved stars with APOGEE spectroscopic parameters and Kepler asteroseismic data analyzed using five independent techniques. Our data includes evolutionary state, surface gravity, mean density, mass, radius, age, and the spectroscopic and asteroseismic measurements used to derive them. We employ a new empirical approach for combining asteroseismic measurements from different methods, calibrating the inferred stellar parameters, and estimating uncertainties. With high statistical significance, we find that asteroseismic parameters inferred from the different pipelines have systematic offsets that are not removed by accounting for differences in their solar reference values. We include theoretically motivated corrections to the large frequency spacing ($\Delta \nu$) scaling relation, and we calibrate the zero point of the frequency of maximum power ($\nu_{\rm max}$) relation to be consistent with masses and radii for members of star clusters. For most targets, the parameters returned by different pipelines are in much better agreement than would be expected from the pipeline-predicted random errors, but 22% of them had at least one method not return a result and a much larger measurement dispersion. This supports the usage of multiple analysis techniques for asteroseismic stellar population studies. The measured dispersion in mass estimates for fundamental calibrators is consistent with our error model, which yields median random and systematic mass uncertainties for RGB stars of order 4%. Median random and systematic mass uncertainties are at the 9% and 8% level respectively for RC stars.
Data tables are online.

### The period–luminosity relation for δ Scuti stars using Gaia DR2 parallaxes

Elham Ziaali, Timothy R. Bedding, Simon J. Murphy, Timothy Van Reeth, Daniel R. Hey.
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We have examined the period–luminosity (P–L) relation for $\delta$ Scuti stars using Gaia DR2 parallaxes. We included 228 stars from the catalogue of Rodriguez et al. (2000), as well as 1124 stars observed in the four-year \kepler mission. For each star we considered the dominant pulsation period, and used DR2 parallaxes and extinction corrections to determine absolute $V$ magnitudes. Many stars fall along a sequence in the P–L relation coinciding with fundamental-mode pulsation, while others pulsate in shorter-period overtones. The latter stars tend to have higher effective temperatures, consistent with theoretical calculations. Surprisingly, we find an excess of stars lying on a ridge with periods half that of the fundamental. We suggest this may be due to a 2:1 resonance between the third or fourth overtone and the fundamental mode.

### The period-luminosity relation for delta Scuti stars using Gaia DR2 parallaxes

Elham Ziaali, Timothy R. Bedding, Simon J. Murphy, Timothy Van Reeth, Daniel R. Hey.
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We have examined the period–luminosity (P–L) relation for $\delta$ Scuti stars using Gaia DR2 parallaxes. We included 228 stars from the catalogue of Rodriguez et al. (2000), as well as 1124 stars observed in the four-year Kepler mission. For each star we considered the dominant pulsation period, and used DR2 parallaxes and extinction corrections to determine absolute $V$ magnitudes. Many stars fall along a sequence in the P–L relation coinciding with fundamental-mode pulsation, while others pulsate in shorter-period overtones. The latter stars tend to have higher effective temperatures, consistent with theoretical calculations. Surprisingly, we find an excess of stars lying on a ridge with periods half that of the fundamental. We suggest this may be due to a 2:1 resonance between the third or fourth overtone and the fundamental mode.

### Revised Stellar Properties of Kepler Targets for the Q1-17 (DR25) Transit Detection Run

Savita Mathur, Daniel Huber, Natalie M. Batalha, David R. Ciardi, Fabienne A. Bastien, Allyson Bieryla, Lars A. Buchhave, William D. Cochran, Michael Endl, Gilbert A. Esquerdo, Elise Furlan, Andrew Howard, Steve B. Howell, Howard Isaacson, David W. Latham, Phillip J. MacQueen, David R. Silva.
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The determination of exoplanet properties and occurrence rates using Kepler data critically depends on our knowledge of the fundamental properties (such as temperature, radius and mass) of the observed stars. We present revised stellar properties for 197,096 Kepler targets observed between Quarters 1–17 (Q1–17), which were used for the final transiting planet search run by the Kepler Mission (Data Release 25, DR25). Similar to the Q1–16 catalog by Huber et al. the classifications are based on conditioning published atmospheric parameters on a grid of Dartmouth isochrones, with significant improvements in the adopted methodology and over 29,000 new sources for temperatures, surface gravities or metallicities. In addition to fundamental stellar properties the new catalog also includes distances and extinctions, and we provide posterior samples for each stellar parameter of each star. Typical uncertainties are $\sim$ 27% in radius, $\sim$ 17% in mass, and $\sim$ 51% in density, which is somewhat smaller than previous catalogs due to the larger number of improved ogg constraints and the inclusion of isochrone weighting when deriving stellar posterior distributions. On average, the catalog includes a significantly larger number of evolved solar-type stars, with an increase of 43.5% in the number of subgiants. We discuss the overall changes of radii and masses of Kepler targets as a function of spectral type, with particular focus on exoplanet host stars.

### The seismic performance

B. Mosser, E. Michel, R. Samadi, A. Miglio, G.R. Davies, L. Girardi, MJ. Goupil.
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The space missions CoRoT and Kepler have demonstrated the capability of asteroseismology to deliver unique information for stellar physics, stellar evolution, and Galactic archaeology. Many ongoing large-scale surveys use seismology, for instance for measuring the mass and radius of stars hosting exoplanets; future projects seek to use the seismic information too. We aim at predicting the information we can derive from the seismic observation of a low-mass star showing solar-like oscillations in a way as simple as possible, in order to assess what we call the seismic performance: the capability to predict and quantify which information can be inferred, with which precision, from the seismic signal. Previous seismic observations with CoRoT and Kepler are used to define and calibrate a seismic index that can be used as a performance index. This index defined with simple analytical computations is used to build a seismic simulator and to characterize the seismic performance. Two regimes dominate the seismic performance. For evolved stars, the stellar regime is dominated by stellar properties only; neither the stellar magnitude nor the instrumental properties play any role in this regime. In the instrumental regime, the instrumental noise and the photon shot noise dominate; the performance index crucially depends on the stellar magnitude. Main-sequence stars are usually in this instrumental regime. In both regimes, the performance index increases linearly with the observation time. Different thresholds are defined, for the detection of the oscillations and for the measurement of different stellar parameters. The method shows a spread of about 21 %, related to multiple sources as stellar variability or binarity. This new way for considering solar-like oscillations allows us to estimate the seismic performance, calibrated by previous observations, of space missions such as TESS. It can help elaborating the seismic yield of future mission programs as the forthcoming runs of PLATO, in complement to light-curve simulations.

### Damping rates and frequency corrections of Kepler LEGACY stars

G. Houdek, M. N. Lund, R. Trampedach, J. Christensen-Dalsgaard, R. Handberg, T. Appourchaux.
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Linear damping rates and modal frequency corrections of radial oscillation modes in selected LEGACY main-sequence stars are estimated by means of a nonadiabatic stability analysis. The selected stellar sample covers stars observed by Kepler with a large range of surface temperatures and surface gravities. A nonlocal, time-dependent convection model is perturbed to assess stability against pulsation modes. The mixing-length parameter is calibrated to the surface-convection-zone depth of a stellar model obtained from fitting adiabatic frequencies to the LEGACY observations, and two of the nonlocal convection parameters are calibrated to the corresponding LEGACY linewidth measurements. The remaining nonlocal convection parameters in the 1D calculations are calibrated so as to reproduce profiles of turbulent pressure and of the anisotropy of the turbulent velocity field of corresponding 3D hydrodynamical simulations. The atmospheric structure in the 1D stability analysis adopts a temperature-optical-depth relation derived from 3D hydrodynamical simulations. Despite the small number of parameters to adjust, we find good agreement with detailed shapes of both turbulent pressure profiles and anisotropy profiles with depth, and with damping rates as a function of frequency. Furthermore, we find the absolute modal frequency corrections, relative to a standard adiabatic pulsation calculation, to increase with surface temperature and surface gravity.

### Stellar ages, masses and radii from asteroseismic modeling are robust to systematic errors in spectroscopy

Earl P. Bellinger, Saskia Hekker, George C. Angelou, Amalie Stokholm, Sarbani Basu.
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Context. The search for twins of the Sun and Earth relies on accurate characterization of stellar and the exoplanetary parameters age, mass, and radius. In the modern era of asteroseismology, parameters of solar-like stars are derived by fitting theoretical models to observational data, which include measurements of their oscillation frequencies, metallicity [Fe/H], and effective temperature $T_{\text{eff}}$. Furthermore, combining this information with transit data yields the corresponding parameters for their associated exoplanets.
Aims. While values of [Fe/H] and $T_{\text{eff}}$ are commonly stated to a precision of $\sim$ 0.1 dex and $\sim$ 100 K, the impact of systematic errors in their measurement has not been studied in practice within the context of the parameters derived from them. Here we seek to quantify this.
Methods. We used the Stellar Parameters in an Instant (SPI) pipeline to estimate the parameters of nearly 100 stars observed by Kepler and Gaia, many of which are confirmed planet hosts. We adjusted the reported spectroscopic measurements of these stars by introducing faux systematic errors and, separately, artificially increasing the reported uncertainties of the measurements, and quantified the differences in the resulting parameters.
Results. We find that a systematic error of 0.1 dex in [Fe/H] translates to differences of only 4%, 2%, and 1% on average in the resulting stellar ages, masses, and radii, which are well within their uncertainties ($\sim$ 11%, 3.5%, 1.4%) as derived by SPI. We also find that increasing the uncertainty of [Fe/H] measurements by 0.1 dex increases the uncertainties of the ages, masses, and radii by only 0.01 Gyr, 0.02 $\text{M}_\odot$, and 0.01 $\text{R}_\odot$, which are again well below their reported uncertainties ($\sim$ 0.5 Gyr, 0.04 $\text{M}_\odot$, 0.02 $\text{R}_\odot$). The results for $T_{\text{eff}}$ at 100 K are similar.
Conclusions. Stellar parameters from SPI are unchanged within uncertainties by errors of up to 0.14 dex or 175 K. They are even more robust to errors in $T_{\text{eff}}$ than the seismic scaling relations. Consequently, the parameters for their exoplanets are also robust.

### Understanding the Limitations of Gyrochronology for Old Field Stars

Travis S. Metcalfe, Ricky Egeland.
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Nearly half a century has passed since the initial indications that stellar rotation slows while chromospheric activity weakens with a power-law dependence on age, the so-called Skumanich relations. Subsequent characterization of the mass-dependence of this behavior up to the age of the Sun led to the advent of gyrochronology, which uses the rotation rate of a star to infer its age from an empirical calibration. The efficacy of the method relies on predictable angular momentum loss from a stellar wind entrained in the large-scale magnetic field produced by global dynamo action. Recent observational evidence suggests that the global dynamo begins to shut down near the middle of a star's main-sequence lifetime, leading to a disruption in the production of large-scale magnetic field, a dramatic reduction in angular momentum loss, and a breakdown of gyrochronology relations. For solar-type stars this transition appears to occur near the age of the Sun, when rotation becomes too slow to imprint Coriolis forces on the global convective patterns, reducing the shear induced by differential rotation, and disrupting the large-scale dynamo. We use data from Barnes (2007) to reveal the signature of this transition in the observations that were originally used to validate gyrochronology. We propose that chromospheric activity may ultimately provide a more reliable age indicator for older stars, and we suggest that asteroseismology can be used to help calibrate activity-age relations for field stars beyond the middle of their main-sequence lifetimes.

### Asteroseismology of main-sequence F stars with Kepler: overcoming short mode lifetimes

Douglas L. Compton, Timothy R. Bedding, Dennis Stello.
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Asteroseismology is a powerful way of determining stellar parameters and properties of stars like the Sun. However, main-sequence F-type stars exhibit short mode lifetimes relative to their oscillation frequency, resulting in overlapping radial and quadrupole modes. The goal of this paper is to use the blended modes for asteroseismology in place of the individual separable modes. We used a peak-bagging method to measure the centroids of radial-quadrupole pairs for 66 stars from the Kepler LEGACY sample, as well as $\theta$ Cyg, HD 49933, HD 181420, and Procyon. We used the relative quadrupole-mode visibility to estimate a theoretical centroid frequency from a grid of stellar oscillation models. The observed centroids were matched to the modelled centroids with empirical surface correction to calculate stellar parameters. We find that the stellar parameters returned using this approach agree with the results using individual mode frequencies for stars, where those are available. We conclude that the unresolved centroid frequencies can be used to perform asteroseismology with an accuracy similar to that based on individual mode frequencies.

### Helium abundance in a sample of cool stars: measurements from asteroseismology

Kuldeep Verma, Keyuri Raodeo, Sarbani Basu, Victor Silva Aguirre, Anwesh Mazumdar, Jakob Rorsted Mosumgaard, Mikkel N. Lund, Pritesh Ranadive.
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The structural stratification of a solar-type main sequence star primarily depends on its mass and chemical composition. The surface heavy element abundances of the solar-type stars are reasonably well determined using conventional spectroscopy, however the second most abundant element helium is not. This is due to the fact that the envelope temperature of such stars is not high enough to excite helium. Since the helium abundance of a star affects its structure and subsequent evolution, the uncertainty in the helium abundance of a star makes estimates of its global properties (mass, radius, age etc.) uncertain as well. The detections of the signatures of the acoustic glitches from the precisely measured stellar oscillation frequencies provide an indirect way to estimate the envelope helium content. We use the glitch signature caused by the ionization of helium to determine the envelope helium abundance of 38 stars in the Kepler seismic LEGACY sample. Our results confirm that atomic diffusion does indeed take place in solar-type stars. We use the measured surface abundances in combination with the settling predicted by the stellar models to determine the initial abundances. The initial helium and metal mass fractions have subsequently been used to get the preliminary estimates of the primordial helium abundance, $Y_p = 0.244\pm0.019$, and the galactic enrichment ratio, $\Delta Y / \Delta Z = 1.226\pm0.849$. Although the current estimates have large errorbars due to the limited sample size, this method holds great promises to determine these parameters precisely in the era of upcoming space missions.

### Butterfly diagram of a Sun-like star observed using asteroseismology

M. Bazot, M. B. Nielsen, D. Mary, J. Christensen-Dalsgaard, O. Benomar, P. Petit, L. Gizon, K. R. Sreenivasan, T. R. White.
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Stellar magnetic fields are poorly understood but are known to be important for stellar evolution and exoplanet habitability. They drive stellar activity, which is the main observational constraint on theoretical models for magnetic field generation and evolution. Starspots are the main manifestation of the magnetic fields at the stellar surface. In this study we measure the variation of their latitude with time, called a butterfly diagram in the solar case, for the solar analogue HD 173701 (KIC 8006161). To that effect, we use Kepler data, to combine starspot rotation rates at different epochs and the asteroseismically determined latitudinal variation of the stellar rotation rates. We observe a clear variation of the latitude of the starspots. It is the first time such a diagram is constructed using asteroseismic data.

### Chaotic dynamics in the pulsation of DF Cygni, as observed by Kepler

E. Plachy, A. Bódi, Z. Kolláth.
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Pulsations of RV Tauri-type variable stars can be governed by chaotic dynamics. However, observational evidence for this happening is usually hard to come by. Here we use the continuous, 4-year-long observations of the Kepler space telescope to search for the signs of chaos in the RVb-type pulsating supergiant, DF Cygni. We use the Global Flow Reconstruction method to estimate the quantitative properties of the dynamics driving the pulsations of the star. The secondary, long-term light variation, i.e., the RVb phenomenon was removed in the analysis with the Empirical Mode Decomposition method. Our analysis revealed that the pulsation of DF Cyg could be described as a chaotic signal with a Lyapunov dimension of $\sim$2.8. DF Cyg is only the third RV Tau star, and the first of the RVb subtype, where the nonlinear analysis indicates that low-dimensional chaos may explain the peculiarities of the pulsation.

### Forward asteroseismic modeling of stars with a convective core from gravity-mode oscillations: parameter estimation and stellar model selection

C. Aerts, G. Molenberghs, M. Michielsen, M. G. Pedersen, R. Björklund, C. Johnston, J. S. G. Mombarg, D. M. Bowman, B. Buysschaert, P. I. Pápics, S. Sekaran, J. O. Sundqvist, A. Tkachenko, K. Truyaert, T. Van Reeth, E. Vermeyen.
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We propose a methodological framework to perform forward asteroseismic modeling of stars with a convective core, based on gravity-mode oscillations. These probe the near-core region in the deep stellar interior. The modeling relies on a set of observed high-precision oscillation frequencies of low-degree coherent gravity modes with long lifetimes and their observational uncertainties. Identification of the mode degree and azimuthal order is assumed to be achieved from rotational splitting and/or from period spacing patterns. This paper has two major outcomes. The first is a comprehensive list and discussion of the major uncertainties of theoretically predicted gravity-mode oscillation frequencies based on linear pulsation theory, caused by fixing choices of the input physics for evolutionary models. Guided by a hierarchy among these uncertainties of theoretical frequencies, we subsequently provide a global methodological scheme to achieve forward asteroseismic modeling. We properly take into account correlations amongst the free parameters included in stellar models. Aside from the stellar mass, metalicity and age, the major parameters to be estimated are the near-core rotation rate, the amount of convective core overshooting, and the level of chemical mixing in the radiative zones. This modeling scheme allows for maximum likelihood estimation of the stellar parameters for fixed input physics of the equilibrium models, followed by stellar model selection considering various choices of the input physics. Our approach uses the Mahalanobis distance instead of the often used $\chi^2$ statistic and includes heteroscedasticity. It provides estimation of the unknown variance of the theoretically predicted oscillation frequencies.

### Coefficients of variation for detecting solar-like oscillations

K. J. Bell, S. Hekker, J. S. Kuszlewicz.
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Detecting the presence and characteristic scale of a signal is a common problem in data analysis. We develop a fast statistical test of the null hypothesis that a Fourier-like power spectrum is consistent with noise. The null hypothesis is rejected where the local “coefficient of variation” (CV)—the ratio of the standard deviation to the mean—in a power spectrum deviates significantly from expectations for pure noise ($\mathrm{CV}\approx1.0$ for a $\chi^2$ 2-degrees-of-freedom distribution). This technique is of particular utility for detecting signals in power spectra with frequency-dependent noise backgrounds, as it is only sensitive to features that are sharp relative to the inspected frequency bin width. We develop a CV-based algorithm to quickly detect the presence of solar-like oscillations in photometric power spectra that are dominated by stellar granulation. This approach circumvents the need for background fitting to measure the frequency of maximum solar-like oscillation power, $\nu_\textrm{max}$. In this paper, we derive the basic method and demonstrate its ability to detect the pulsational power excesses from the well-studied APOKASC-2 sample of oscillating red giants observed by Kepler. We recover the cataloged $\nu_\textrm{max}$ values with an average precision of 2.7% for 99.5% of the stars with 4 years of Kepler photometry. Our method produces false positives for $<1\%$ of dwarf stars with $\nu_\textrm{max}$ well above the long-cadence Nyquist frequency. The algorithm also flags spectra that exhibit astrophysically interesting signals in addition to single, solar-like oscillation power excesses, which we catalog as part of our characterization of the Kepler light curves of APOKASC-2 targets.

### Nonlinear seismic scaling relations

T. Kallinger, P. G. Beck, D. Stello, R. A. Garcia.
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Context: In recent years the so-called seismic scaling relations for the frequency of maximum power, $\nu_\mathrm{max} \propto g/\sqrt{T_\mathrm{eff}}$, and for the large frequency separation, $\Delta\nu \propto \sqrt{\bar\rho}$, have caught the attention of various fields of astrophysics. This is because these relations can be used to estimate parameters, such as mass and radius of stars that show solar-like oscillations. With the exquisite photometry of Kepler, the uncertainties in the seismic observables are small enough to estimate masses and radii with a precision of only a few per cent. Even though this seems to work quite well for main-sequence stars, there is empirical evidence, mainly from studies of eclipsing binary systems, that the seismic scaling relations systematically overestimate the mass and radius of red giants by about 5 and 15%, respectively. Various model-based corrections of the $\Delta\nu -$scaling reduce the problem but do not solve it. Aims: Our goal is to define revised seismic scaling relations that account for the know systematic mass and radius discrepancies in a completely model-independent way. Methods: We use probabilistic methods to analyse the seismic data and to derive nonlinear scaling relations based on a sample of six red-giant branch (RGB) stars that are members of eclipsing binary systems and about 60 red giants on the RGB as well as in the core-helium burning red clump (RC) in the two open clusters NGC 6791 and NGC 6819. Results: We re-examine the global oscillation parameters of the giants in the binary systems in order to determine their seismic fundamental parameters and find them to agree with the dynamic parameters from the literature if we adopt nonlinear scalings. We note that a curvature and glitch corrected $\Delta\nu_\mathrm{cor}$ should be preferred over a local or average value of $\Delta\nu$. We then compare the observed seismic parameters of the cluster giants to those scaled from independent measurements and find the same nonlinear behaviour as for the eclipsing binaries. Our final proposed scaling relations are based on both samples and cover a broad range of evolutionary stages from RGB to RC stars: $g/\sqrt{T_\mathrm{eff}} = (\nu_\mathrm{max}/\nu_\mathrm{max,\odot})^{1.0075\pm0.0021}$ and $\sqrt{\bar\rho} = (\Delta\nu_\mathrm{cor}/\Delta\nu_\mathrm{cor,\odot})[\eta - (0.0085\pm0.0025) \log^2 (\Delta\nu_\mathrm{cor}/\Delta\nu_\mathrm{cor,\odot})]^{-1}$, where $g$, $T_\mathrm{eff}$, and $\bar\rho$ are in solar units, $\nu_\mathrm{max,\odot}=3140\pm5$\mh and $\Delta\nu_\mathrm{cor,\odot}=135.08\pm0.02$\mh , and $\eta$ is equal to one in case of RGB stars and $1.04\pm0.01$ for RC stars. Conclusions: A direct consequence of these new scaling relations is that the average mass of stars on the ascending giant branch reduces to $1.10\pm0.03M$\sun in \na and $1.45\pm0.06M$\sun in \nb, allowing us to revise the clusters' distance modulus to $13.00\pm0.02$ and $11.82\pm0.02$, respectively. We also find strong evidence that both clusters are significantly older than concluded from previous seismic investigations.

### Period spacings in red giants IV. A complete description of the mixed-mode pattern

B. Mosser, C. Gehan, K. Belkacem, R. Samadi, E. Michel, M-J. Goupil.
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Oscillation modes with a mixed character, as observed in evolved low-mass stars, are highly sensitive to the physical properties of the innermost regions. Measuring their properties is therefore extremely important to probe the core, but requires some care, due to the complicated nature of the mixed-mode pattern.
This work aims at providing a complete and consistent description of the mixed-mode pattern, based on the asymptotic expansion, among which the gravity offset $\epsg$. We revisit previous findings and empirically test how period spacings, rotational splittings, mixed-mode widths and amplitudes can be estimated in a consistent view, based on the properties of the mode inertia ratios.
From the asymptotic fit of the mixed-mode pattern of red giants at various evolutionary stages, we derive asymptotic parameters that are not only very precise, but also accurate. We decipher the complex pattern in a rapidly rotating star, and explain how asymmetrical splittings can be inferred. The variation of the asymptotic gravity offsets along stellar evolution is investigated in detail. We also derive generic properties that explain under which conditions mixed modes can be observed.

### The rotational shear layer inside the early red-giant star KIC 4448777

Maria Pia Di Mauro, Rita Ventura, Enrico Corsaro, Bruno Lustosa de Moura.
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We present the asteroseismic study of the early red-giant star KIC 4448777, complementing and integrating a previous work \citep[][Paper I]dimauro2016, aimed at characterizing the dynamics of its interior by analyzing the overall set of data collected by the Kepler satellite during the four years of its first nominal mission. We adopted the Bayesian inference code DIAMONDS \citepCorsaro14 for the peak bagging analysis and asteroseismic splitting inversion methods to derive the internal rotational profile of the star. The detection of new splittings of mixed modes, more concentrated in the very inner part of the helium core, allowed us to reconstruct the angular velocity profile deeper into the interior of the star and to disentangle the details better than in Paper I: the helium core rotates almost rigidly about 6 times faster than the convective envelope, while part of the hydrogen shell seems to rotate at a constant velocity about 1.15 times lower than the He core. In particular, we studied the internal shear layer between the fast-rotating radiative interior and the slow convective zone and we found that it lies partially inside the hydrogen shell above $r \simeq 0.05R$ and extends across the core-envelope boundary. Finally, we theoretically explored the possibility for the future to sound the convective envelope in the red-giant stars and we concluded that the inversion of a set of splittings with only low-harmonic degree $l\leq 3$, even supposing a very large number of modes, will not allow to resolve the rotational profile of this region in detail.

### Predicting radial-velocity jitter induced by stellar oscillations based on Kepler data

Jie Yu, Daniel Huber, Timothy R. Bedding, Dennis Stello.
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Radial velocity jitter due to intrinsic stellar variability introduces challenges when charactering exoplanet systems, in particular, when studying the exoplanetary signals of small (sub Neptune-sized) planets orbiting solar-type stars. In this Letter we aim to predict for dwarfs and giants the jitter due to stellar oscillations, which in velocity have much larger amplitudes than noise introduced by granulation on longer timescales. Based on global oscillation parameters measured with Kepler data we fit the jitter in terms of the following sets of stellar parameters: (1) Luminosity, mass, and effective temperature: the fit returns precisions of 17.9% and 27.1% for dwarfs and giants, respectively. (2) Luminosity, effective temperature, and surface gravity: The precisions are the same as using the previous parameter set. (3) Surface gravity and effective temperature: we obtain a precision of 22.6% for dwarfs and 27.1% for giants. (4): Luminosity and effective temperature: the precision is 47.8% for dwarfs and 27.5% for giants. Our method will be valuable for anticipating the potential radial-velocity stellar noise level of exoplanet host stars to be found by the TESS andPLATO space missions, and thus can be useful for their follow-up spectroscopic observations. We provide publicly available code (https://github.com/Jieyu126/Jitter) which can also be used to set a prior for the jitter term as a component when modeling the Keplerian orbits of the exoplanets.

### Core rotation braking on the red giant branch for various mass ranges

C. Gehan, B. Mosser, E. Michel.
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Asteroseismology allows us to probe stellar interiors. In the case of red giant stars, conditions in the stellar interior are such to allow for the existence of mixed modes, consisting in a coupling between gravity waves in the radiative interior and pressure waves in the convective envelope. Mixed modes can thus be used to probe the physical conditions in red giant cores. However, we still need to identify the physical mechanisms that transport angular momentum inside red giants, leading to the slow-down observed for the red giant core rotation. Thus large-scale measurements of the red giant core rotation are of prime importance to obtain tighter constraints on the efficiency of the internal angular momentum transport, and to study how this efficiency changes with stellar parameters. This work aims at identifying the components of the rotational multiplets for dipole mixed modes in a large number of red giant oscillation spectra observed by Kepler. Such identification provides us with a direct measurement of the red giant mean core rotation. We compute stretched spectra that mimic the regular pattern of pure dipole gravity modes. Mixed modes with same azimuthal order are expected to be almost equally spaced in stretched period, with a spacing equals to the pure dipole gravity mode period spacing. The departure from this regular pattern allows us to disentangle the various rotational components and therefore to determine the mean core rotation rates of red giants. We automatically identify the rotational multiplet components of 1183 stars on the red giant branch with a success rate of 69 % with respect to our initial sample. As no information on the internal rotation can be deduced for stars seen pole-on, we obtain mean core rotation measurements for 875 red giant branch stars. This large sample includes stars with a mass as large as 2.5 $M_{\odot}$, allowing us to test the dependence of the core slow-down rate on the stellar mass. Disentangling rotational splittings from mixed modes is now possible in an automated way for stars on the red giant branch, even for the most complicated cases, where the rotational splittings exceed half the mixed-mode period spacing. This work on a large sample allows us to refine previous measurements of the evolution of the mean core rotation on the red giant branch. Rather than a slight slow down, our results suggest rotation to be constant along the red giant branch, with values independent on the mass.

### K2 space photometry reveals rotational modulation and stellar pulsations in chemically peculiar A and B stars

D. M. Bowman, B. Buysschaert, C. Neiner, P. I. P\' apics, M. E. Oksala, C. Aerts.
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Context. The physics of magnetic hot stars and how a large-scale magnetic field affects their interior properties is largely unknown. Few studies have combined high-quality observations and modelling of magnetic pulsating stars, known as magneto-asteroseismology, primarily because of the dearth of detected pulsations in stars with a confirmed and well-characterised large-scale magnetic field. Aims. We aim to characterise observational signatures of rotation and pulsation in chemically peculiar candidate magnetic stars using photometry from the K2 space mission. Thus, we identify the best candidate targets for ground-based, optical spectropolarimetric follow-up observations to confirm the presence of a large-scale magnetic field. Methods. We employed customised reduction and detrending tools to process the K2 photometry into optimised light curves for a variability analysis. We searched for the periodic photometric signatures of rotational modulation caused by surface abundance inhomogeneities in 56 chemically peculiar A and B stars. Furthermore, we searched for intrinsic variability caused by pulsations (coherent or otherwise) in the amplitude spectra of these stars. Results. The rotation periods of 38 chemically peculiar stars are determined, 16 of which are the first determination of the rotation period in the literature. We confirm the discovery of high-overtone roAp pulsation modes in HD 177765 and find an additional 3 Ap and Bp stars that show evidence of high-overtone pressure modes found in roAp stars in the form of possible Nyquist alias frequencies in their amplitude spectra. Furthermore, we find 6 chemically peculiar stars that show evidence of intrinsic variability caused by gravity or pressure pulsation modes. Conclusions. The discovery of pulsations in a non-negligible fraction of chemically peculiar stars make these stars high-priority targets for spectropolarimetric campaigns to confirm the presence of their expected large-scale magnetic field. The ultimate goal is to perform magneto-asteroseismology and probe the interior physics of magnetic pulsating stars.

### A weak modulation effect detected in the light curves of KIC 5950759: intrinsic or instrumental effect?

Tao-Zhi Yang, Ali Esamdin, Fang-Fang Song, Hu-Biao Niu, Guo-Jie Feng, Peng Zong, Xiangyun Zeng, Junhui Liu, Jin-Zhong Liu, Lu Ma, Fei Zhao.
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In this paper, the high-precision light curves of the Kepler target KIC 5950759 are analyzed. The Fourier analysis of the long cadence (LC) light curve reveals three independent frequencies. Two of them are main pulsation modes: F0 = 14.221373(21) $\rm{d^{-1}}$ and F1 = 18.337249(44) $\rm{d^{-1}}$. The third independent frequency, $f_m$ = 0.3193 d$^{-1}$, is found in LC data with a signal-to-noise ratio of 6.2. A weak modulation of $f_m$ to F0 and F1 modes (triplet structures centered on F0 and F1) are detected both in long and short cadence data. This is the first detection of the modulation effect in a double-mode high-amplitude $\delta$ Scuti (HADS) star. The most possible cause of the modulation effect in the light curves is amplitude modulation with the star's rotation frequency of 0.3193 d$^{-1}$. The preliminary analysis suggests that KIC 5950759 is in the bottom of the HADS instability strip and likely situated in the main sequence. Spectroscopic observations are necessary to verify the true nature of the modulation terms.

### Surface gravities for 15,000 Kepler stars measured from stellar granulation and validated with Gaia DR2 parallaxes

Durlabh Pande, Timothy R. Bedding, Daniel Huber, Hans Kjeldsen.
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We have developed a method to estimate surface gravity ($\log g$) from light curves by measuring the granulation background, similar to the “flicker” method by Bastien et al. (2016) but working in the Fourier power spectrum. We calibrated the method using Kepler stars for which asteroseismology has been possible with short-cadence data, demonstrating a precision in $\log g$ of about 0.05 dex. We also derived a correction for white noise as a function of Kepler magnitude by measuring white noise directly from observations. We then applied the method to the same sample of long-cadence stars as Bastien et al. We provide a catalogue of $\log g$ values for about 15,000 stars having uncertainties better than 0.5 dex. We use Gaia DR2 parallaxes to validate that granulation is a powerful method to measure $\log g$ from light curves. Our method can also be applied to the large number of light curves collected by K2 and TESS.

### The mass and age of the first SONG target: the red giant 46 LMi.

S. Frandsen, M. Fredslund Andersen, K. Brogaard, C. Jiang, T. Arentoft, F. Grundahl, H. Kjeldsen, J. Christensen-Dalsgaard, E. Weiss, P. Pallé, V. Antoci, P. Kjærgaard, A. N. Srensen, J. Skottfelt, U. G. Jrgensen.
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The Stellar Observation Network Group (SONG) is an initiative to build a worldwide network of 1m telescopes with high-precision radial-velocity spectrographs. Here we analyse the first radial-velocity time series of a red-giant star measured by the SONG telescope at Tenerife. The asteroseismic results demonstrate a major increase in the achievable precision of the parameters for red-giant stars obtainable from ground-based observations. Reliable tests of the validity of these results are needed, however, before the accuracy of the parameters can be trusted. We analyse the first SONG time series for the star 46 LMi, which has a precise parallax and an angular diameter measured from interferometry, and therefore a good determination of the stellar radius. We use asteroseismic scaling relations to obtain an accurate mass, and modelling to determine the age. A 55-day time series of high-resolution, high S/N spectra were obtained with the first SONG telescope. We derive the asteroseismic parameters by analysing the power spectrum. To give a best guess on the large separation of modes in the power spectrum, we have applied a new method which uses the scaling of Kepler red-giant stars to 46 LMi. Several methods have been applied: classical estimates, seismic methods using the observed time series, and model calculations to derive the fundamental parameters of 46 LMi. Parameters determined using the different methods are consistent within the uncertainties. We find the following values for the mass $M$ (scaling), radius $R$ (classical), age (modelling), and surface gravity (combining mass and radius): $M = 1.09\pm0.04$ Msun $R = 7.95\pm0.11$ Rsun, age $t = 8.2\pm1.9$ Gy, and $\log g = 2.674 \pm 0.013$. The exciting possibilities for ground-based asteroseismology of solar-like oscillations with a fully robotic network have been illustrated with the results obtained from just a single site of the SONG network. The window function is still a severe problem which will be solved when there are more nodes in the network.

### Surface correction of main sequence solar-like oscillators with the it Kepler LEGACY sample

D. L. Compton, T. R. Bedding, W. H. Ball, D. Stello, D. Huber, T. R. White, H. Kjeldsen.
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Poor modelling of the surface regions of solar-like stars causes a systematic discrepancy between the observed and model pulsation frequencies. We aim to characterise this frequency discrepancy for main sequence solar-like oscillators for a wide range of initial masses and metallicities. We fit stellar models to the observed mode frequencies of the 67 stars, including the Sun, in the Kepler LEGACY sample, using three different empirical surface corrections. The three surface corrections we analyse are a frequency power-law, a cubic frequency term divided by the mode inertia, and a linear combination of an inverse and cubic frequency term divided by the mode inertia. We construct a grid of stellar evolution models using the stellar evolution code MESA and calculate mode frequencies using GYRE. Along with the surface correction coefficients, we calculate an empirical homology scaling factor to the model frequencies, which greatly improves the robustness of our grid. We calculate accurate stellar and surface correction parameters for each star using the average of the best-fitting models from each evolutionary track, weighted by the likelihood of each model. The resulting model stellar parameters agree well with an independent reference, the BASTA pipeline. We find that scaling the frequencies by the mode inertia improves the fit between the models and observations. The inclusion of the inverse frequency term produces substantially better model fits to lower surface gravity stars. However, the extra free parameter can cause over-fitting resulting and increased uncertainties for some of the more evolved stars in the sample.

### Seismic probing of the first dredge-up event through the eccentric red-giant/red-giant spectroscopic binary KIC9163796

P. G. Beck, T. Kallinger, K. Pavlovski, A. Palacios, A. Tkachenko, S. Mathis, R. A. García, E. Corsaro, C. Johnston, B. Mosser, T. Ceillier, Jr. J.-D. do Nascimento, G. Raskin.
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Binaries in double-lined spectroscopic systems (SB2) provide a homogeneous set of stars. Differences of parameters, such as age or initial conditions, which otherwise would have strong impact on the stellar evolution, can be neglected. The observed differences are then determined by the difference in stellar mass between the two components. The mass ratio can be determined with much higher accuracy than the actual stellar mass. In this work, we aim to study the eccentric binary system KIC9163796, whose two components are very close in mass and both are low-luminosity red-giant stars. We analyse four years of Kepler space photometry and we obtain high-resolution spectroscopy with the Hermes instrument. The orbital elements and the spectra of both components are determined using spectral disentangling methods. The effective temperatures, and metallicities are extracted from disentangled spectra of the two stars. Mass and radius of the primary are determined through asteroseismology. The surface rotation period of the primary is determined from the Kepler light curve. From representative theoretical models of the star, we derive the internal rotational gradient, while for a grid of models, the measured lithium abundance is confronted with theoretical predictions. From seismology the primary of KIC9163796 is a star of 1.39$\pm$0.06 M$_\odot$, while the spectroscopic mass ratio between both components can be determined with much higher precision by spectral disentangling to be 1.015$\pm$0.005. With such mass and a difference in effective temperature of 600 K from spectroscopy, the secondary and primary are in the early and advanced stage of the first dredge-up event on the red-giant branch. The period of the primary's surface rotation resembles the orbital period within 10 days. The radial rotational gradient between the surface and core in KIC9163796 is found to be 6.9$^{+2.0}_{-1.0}$. This is a low value but not exceptional if compared to the sample of typical single field stars. The seismic average of the envelope's rotation agrees with the surface rotation rate. The lithium abundance is in agreement with quasi rigidly-rotating models. The agreement between the surface rotation with the seismic result indicates that the full convective envelope is rotating quasi-rigidly. The models of the lithium abundance are compatible with a rigid rotation in the radiative zone during the main sequence. Because of the many constraints offered by oscillating stars in binary systems, such objects are important test beds of stellar evolution.

### Amplitude and lifetime of radial modes in red giant star spectra observed by Kepler

M. Vrard, T. Kallinger, B. Mosser, C. Barban, F. Baudin, K. Belkacem, M. S. Cunha.
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The space-borne missions CoRoT and Kepler have provided photometric observations of unprecedented quality. The study of solar-like oscillations observed in red giant stars by these satellites allows a better understanding of the different physical processes occurring in their interiors. In particular, the study of the mode excitation and damping is a promising way to improve our understanding of stellar physics that has, so far, been performed only on a limited number of targets. The recent asteroseismic characterization of the evolutionary status for a large number of red giants allows us to study the physical processes acting in the interior of red giants and how they are modify during stellar evolution. In this work, we aim to obtain information on the excitation and damping of pressure modes through the measurement of the stars’ pressure mode widths and amplitudes and to analyze how they are modified with stellar evolution. The objective is to bring observational constraints on the modeling of the physical processes behind mode excitation and damping. We fit the frequency spectra of red giants with well defined evolutionary status using Lorentzians functions to derive the pressure mode widths and amplitudes. To strengthen our conclusions, we used two different fitting techniques. Pressure mode widths and amplitudes were determined for more than 5000 red giants. We put into light a variation of the mode width with stellar evolution as well as a dependence of this parameter with the stellar mass and temperature. We also confirm observationally the influence of the stellar metallicity on the mode amplitudes, as predicted by models.

### Asteroseismology of 16000 Kepler red giants: Global oscillation parameters, Masses, and Radii

Jie Yu, Daniel Huber, Timothy R. Bedding, Dennis Stello, Marc Hon, Simon J. Murphy, Shourya Khanna.
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The Kepler mission has provided exquisite data to perform an ensemble asteroseismic analysis on evolved stars. In this work we systematically characterize solar-like oscillations and granulation for 16,094 oscillating red giants, using end-of-mission long-cadence data. We produced a homogeneous catalog of the frequency of maximum power (typical uncertainty $\sigma_{\nu_{\rm max}}$=1.6%), the mean large frequency separation ($\sigma_{\Delta\nu}$=0.6%), oscillation amplitude ($\sigma_{\rm A}$=4.7%), granulation power ($\sigma_{\rm gran}$=8.6%), power excess width ($\sigma_{\rm width}$=8.8%), seismically-derived stellar mass ($\sigma_{\rm M}$=7.8%), radius ($\sigma_{\rm R}$=2.9%), and thus surface gravity ($\sigma_{\log g}$=0.01 dex). Thanks to the large red giant sample, we confirm that red-giant-branch (RGB) and helium-core-burning (HeB) stars collectively differ in the distribution of oscillation amplitude, granulation amplitude, and width of power excess, which is mainly due to the mass difference. The distribution of oscillation amplitudes shows an extremely sharp upper edge at fixed $\nu_{max}$, which might hold clues to understand the excitation and damping mechanisms of the oscillation modes. We find both oscillation amplitude and granulation power depend on metallicity, causing a spread of 15% in oscillation amplitudes and a spread of 25% in granulation power from [Fe/H]=-0.7 to 0.5 dex. Our asteroseismic stellar properties can be used as reliable distance indicators and age proxies for mapping and dating galactic stellar populations observed by Kepler. They will also provide an excellent opportunity to test asteroseismology using Gaia parallaxes, and lift degeneracies in deriving atmospheric parameters in large spectroscopic surveys such as APOGEE and LAMOST.

### Detecting Solar-like Oscillations in Red Giants with Deep Learning

Marc Hon, Dennis Stello, Joel C. Zinn.
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Time-resolved photometry of tens of thousands of red giant stars from space missions like Kepler and K2 has created the need for automated asteroseismic analysis methods. The first and most fundamental step in such analysis, is to identify which stars show oscillations. It is critical that this step can be performed with no, or little, detection bias, particularly when performing subsequent ensemble analyses that aim to compare properties of observed stellar populations with those from galactic models. Yet, an automated and efficient solution to this initial detection step has still not been found, meaning that expert visual inspection of data from each star is required to obtain the highest level of detections. Hence, to mimic how an expert eye analyses the data, we use supervised deep learning to not only detect oscillations in red giants, but also predict the location of the frequency at maximum power, $\nu_{\mathrm{max}}$, by observing features in 2D images of power spectra. By training on Kepler data, we achieve a detection accuracy of 98% on K2 Campaign 6 stars and a detection accuracy of 99% on K2 Campaign 3 stars. We further find that the estimated uncertainty of our deep learning-based $\nu_{\mathrm{max}}$ predictions is about 5%. This is comparable to human-level performance using visual inspection. When examining outliers we find that the deep learning results are more likely to provide robust $\nu_{\mathrm{max}}$ estimates than the classical model-fitting method.

### Deep Learning Classification in Asteroseismology Using an Improved Neural Network: Results on 15000 Kepler Red Giants and Applications to K2 and TESS Data

Marc Hon, Dennis Stello, Jie Yu.
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Deep learning in the form of 1D convolutional neural networks have previously been shown to be capable of efficiently classifying the evolutionary state of oscillating red giants into red giant branch stars and helium-core burning stars by recognizing visual features in their asteroseismic frequency spectra. We elaborate further on the deep learning method by developing an improved convolutional neural network classifier. To make our method useful for current and future space missions such as K2, TESS and PLATO, we train classifiers that are able to classify the evolutionary states of lower frequency resolution spectra expected from these missions. Additionally, we provide new classifications for 8633 Kepler red giants, out of which 426 have previously not been classified using asteroseismology. This brings the total to 14983 Kepler red giants classified with our new neural network. We also verify that our classifiers are remarkably robust to suboptimal data, including low signal-to-noise and incorrect training truth labels.

### 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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