Red-giant and solar-like oscillators
Asteroseismology is the study of oscillating stars. Because the stellar interior is obscured by the luminous atmosphere, the deep interior is hidden to direct measurements. Through analyzing the oscillation waves, we can reconstruct the stellar structure.
Binary systems are gravitationally-bound pairs of stars that orbit around a common center of mass. Unless created in a rare capturing event, both stellar components are born from the same interstellar cloud. Because both stars are located at the same distance and have similar initial conditions and stellar age, binaries allow us to draw significant constraints on input parameters for the stellar models.
STARS LIKE OUR SUN
Stellar Activity, Rotation, & Lithium
Stars with masses similar to the Sun, provide an excellent opportunity to study our own host star. From such investigations, we can learn how our stars and planets are formed and better understand, how life started on this planet.
TOOLS AND TECHNIQUES
Observational tools for modern astrophysics
The rapid advances of the field of asteroseismology, stellar physics and exoplanet research are driven by the rapid developments in the field of space and ground-based telescopes. The space missions NASA Kepler/K2 and TESS missions have provided a vast amount of photometric data in unprecedented quality provided. High-resolution, ground-based telescopes such as MERCATOR or SONG provide accurate velocity measurements.
My research line is the study of the structure and internal dynamics in stars resembling the Sun at its present, past, and particularly its future as a red giant. My interest is to understand better the structural changes and the connecting mechanisms, such as internal rotation, mixing, and stellar surface activity in the evolution of solar-like stars.
To provide vital input for improved modeling of the star, I follow three lines of research, which represent different analysis approaches: (1) infer the properties of the star and the dynamics (particularly the internal rotation) of its core from studying individual oscillation frequencies, (2) study internal-mixing processes and surface activity, and (3) performing seismic studies of gravitationally bound stars in binary systems.
FIRST AUTHOR PUBLICATION IN REFEREED FIRST-QUARTILE JOURNALS
a full list of all publications can be retrieved from the my
electronic library at the NASA Astrophysical Data Service (ADS), my Google Scholar and my ORCID profile.
Because I am convinced that Open Access ensures equity and inclusion in modern academia,
my papers are also available as preprint on ArXiv.
99 OSCILLATING RED-GIANT STARS IN BINARY SYSTEMS WITH NASA TESS AND NASA KEPLER IDENTIFIED FROM THE SB9-CATALOGUE
Beck, P. G., Mathur, S., Hambleton, K., García, R. A., Steinwender, L., Eisner, N. L., do Nascimento, J. -D., Gaulme, P., and Mathis, S.
Oscillating red-giant stars in binary systems are an ideal testbed for investigating the structure and evolution of stars in the advanced phases of evolution. With 83 known red-giants in binary systems, of which only about 40 have determined global seismic parameters and orbital parameters, the sample is slender compared to the numerous known oscillating stars. The detection of red-giant binary systems is typically obtained from the signature of stellar binarity in space photometry. The time base of such data biases the detection towards systems with shorter periods and orbits of insufficient size to allow a red-giant to fully extend as it evolves up the red-giant branch. Consequently, the sample shows an excess of H-shell burning giants while being sparse of stars in the He-core burning phase. From the ninth catalogue of spectroscopic binary orbits (SB9), we identified candidate systems hosting a red-giant primary component. Searching space photometry from the NASA missions Kepler, K2, and TESS (Transiting Exoplanet Survey Satellite) as well as the BRITE (BRIght Target Explorer) constellation mission, we found 99 systems, which were previously unknown of hosting an oscillating giant component. The revised search strategy allowed us to extend the range of orbital periods of systems hosting oscillating giants up to 26000 days. Such wide orbits allow for a rich population of He-core burning primaries, which are required for a complete view on stellar evolution from binary studies. Tripling the size of known oscillating red-giant stars in binary systems is an important step towards an ensemble approach for seismology and tidal studies. While for non-eclipsing binaries the inclination is unknown, such a seismically well-characterized sample will be a treasure trove in combination with Gaia astrometric orbits for binary systems.
TESTING TIDAL THEORY FOR EVOLVED STARS BY USING RED GIANT BINARIES OBSERVED BY KEPLER
Monthly Notices of the Royal Astronomical Society: Letters, Volume 479, Issue 1, p.L123-L128 (2018).
Beck, P. G., Mathis, S., Gallet, F., Charbonnel, C., Benbakoura, M., García, R. A., and do Nascimento, J. -D.
Tidal interaction governs the redistribution of angular momentum in close binary stars and planetary systems and determines the systems evolution towards the possible equilibrium state. Turbulent friction acting on the equilibrium tide in the convective envelope of low-mass stars is known to have a strong impact on this exchange of angular momentum in binaries. Moreover, theoretical modelling in recent literature as well as presented in this paper suggests that the dissipation of the dynamical tide, constituted of tidal inertial waves propagating in the convective envelope, is weak compared to the dissipation of the equilibrium tide during the red giant phase. This prediction is confirmed when we apply the equilibrium-tide formalism developed by Zahn, Verbunt & Phinney, and Remus, Mathis & Zahn on to the sample of all known red giant binaries observed by the NASA Kepler mission. Moreover, the observations are adequately explained by only invoking the equilibrium tide dissipation. Such ensemble analysis also benefits from the seismic characterization of the oscillating components and surface rotation rates. Through asteroseismology, previous claims of the eccentricity as an evolutionary state diagnostic are discarded. This result is important for our understanding of the evolution of multiple star and planetary systems during advanced stages of stellar evolution.
SEISMIC PROBING OF THE FIRST DREDGE-UP EVENT THROUGH THE ECCENTRIC RED-GIANT AND RED-GIANT SPECTROSCOPIC BINARY KIC 9163796. HOW DIFFERENT ARE RED-GIANT STARS WITH A MASS RATIO OF 1.015?
Beck, P. G., Kallinger, T., Pavlovski, K., Palacios, A., Tkachenko, A., Mathis, S., García, R. A., Corsaro, E., Johnston, C., Mosser, B., Ceillier, T., do Nascimento, J. -D., and Raskin, G.
Context. 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 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. Aim. In this work, we aim to study the eccentric binary system KIC 9163796, whose two components are very close in mass and both are low-luminosity red-giant stars.
Methods: We analysed four years of Kepler space photometry and we obtained high-resolution spectroscopy with the Hermes instrument. The orbital elements and the spectra of both components were determined using spectral disentangling methods. The effective temperatures, and metallicities were extracted from disentangled spectra of the two stars. Mass and radius of the primary were 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 derived the internal rotational gradient, while for a grid of models, the measured lithium abundance is compared with theoretical predictions.
Results: From seismology the primary of KIC 9163796 is a star of 1.39 ± 0.06 M⊙, while the spectroscopic mass ratio between both components can be determined with much higher precision by spectral disentangling to be 1.015 ± 0.005. With such mass and a difference in effective temperature of 600 K from spectroscopy, the secondary and primary are, respectively, 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 ten days. The radial rotational gradient between the surface and core in KIC 9163796 is found to be 6.9-1.0+2.0. 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.
Conclusions: 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.
LITHIUM ABUNDANCE AND ROTATION OF SEISMIC SOLAR ANALOGUES. SOLAR AND STELLAR CONNECTION FROM KEPLER AND HERMES OBSERVATIONS
Beck, P. G.; do Nascimento, J. -D., Jr.; Duarte, T.; Salabert, D.; Tkachenko, A.; Mathis, S.; Mathur, S.; García, R. A.; Castro, M.; Pallé, P. L.; Egeland, R.; Montes, D.; Creevey, O.; Andersen, M. F.; Kamath, D.; and van Winckel, H.
Context. Lithium abundance A(Li) and surface rotation are good diagnostic tools to probe the internal mixing and angular momentum transfer in stars.
Aims: We explore the relation between surface rotation, A(Li), and age in a sample of seismic solar-analogue stars, and we study their possible binary nature.
Methods: We selected a sample of 18 solar-analogue stars observed by the NASA Kepler satellite for an in-depth analysis. Their seismic properties and surface rotation rates are well constrained from previous studies. About 53 h of high-resolution spectroscopy were obtained to derive fundamental parameters from spectroscopy and A(Li). These values were combined and compared with seismic masses, radii, and ages, as well as with surface rotation periods measured from Kepler photometry.
Results: Based on radial velocities, we identify and confirm a total of six binary star systems. For each star, a signal-to-noise ratio of 80 ≲ S/N ≲ 210 was typically achieved in the final spectrum around the lithium line. We report fundamental parameters and A(Li). Using the surface rotation period derived from Kepler photometry, we obtained a well-defined relation between A(Li) and rotation. The seismic radius translates the surface rotation period into surface velocity. With models constrained by the characterisation of the individual mode frequencies for single stars, we identify a sequence of three solar analogues with similar mass ( 1.1 M⊙) and stellar ages ranging between 1 to 9 Gyr. Within the realistic estimate of 7% for the mass uncertainty, we find a good agreement between the measured A(Li) and the predicted A(Li) evolution from a grid of models calculated with the Toulouse-Geneva stellar evolution code, which includes rotational internal mixing, calibrated to reproduce solar chemical properties. We found a scatter in ages inferred from the global seismic parameters that is too large when compared with A(Li).
Conclusions: We present the Li-abundance for a consistent spectroscopic survey of solar-analogue stars with a mass of 1.00 ± 0.15 M⊙ that are characterised through asteroseismology and surface rotation rates based on Kepler observations. The correlation between A(Li) and Prot supports the gyrochronological concept for stars younger than the Sun and becomes clearer when the confirmed binaries are excluded. The consensus between measured A(Li) for solar analogues with model grids, calibrated on the Sun's chemical properties, suggests that these targets share the same internal physics. In this light, the solar Li and rotation rate appear to be normal for a star like the Sun.
THE HERMES SOLAR ATLAS AND THE SPECTROSCOPIC ANALYSIS OF THE SEISMIC SOLAR ANALOGUE KIC 3241581
Beck, P. G.; Allende Prieto, C.; Van Reeth, T.; Tkachenko, A.; Raskin, G.; van Winckel, H.; do Nascimento, J. -D., Jr.; Salabert, D.; Corsaro, E.; and García, R. A.
Context. Solar-analogue stars provide an excellent resource to study the Sun's evolution, I.e. the changes with time in stellar structure, activity, or rotation for solar-like stars. The unparalleled photometric data from the NASA space telescope Kepler allows us to study and characterise solar-like stars through asteroseismology.
Aims: We aim to spectroscopically investigate the fundamental parameter and chromospheric activity of solar analogues and twins, based on observations obtained with the HERMES spectrograph and combine them with asteroseismology. Therefore, we need to build a solar atlas for the spectrograph, to provide accurate calibrations of the spectroscopically determined abundances of solar- and late-type stars observed with this instrument and thus perform differential spectroscopic comparisons.
Methods: We acquire high-resolution and high signal-to-noise (S/N) spectroscopy to construct three solar reference spectra by observing the reflected light of the asteroids Vesta and Victoria and the jovian moon Europa (100 ≲ S/N ≲ 450) with the HERMES spectrograph. We then observe the Kepler solar analogue KIC 3241581 (S/N ~ 170). For this star, the fundamental spectral parameters are extracted using a differential analysis. Sufficient S/N in the near ultraviolet allows us to investigate the chromospheric magnetic activity in both objects.
Results: We constructed three solar spectrum atlases from 385 to 900 nm, obtained with the HERMES spectrograph from observations of two bright asteroids and a jovian moon. A comparison between our solar spectra atlas to the Kurucz and HARPS solar spectrum shows an excellent agreement. KIC 3241581 was found to be a long-periodic binary system. The fundamental parameter for the stellar primary component are Teff = 5689 ± 11 K, log g = 4.385 ± 0.005, [Fe/H] = + 0.22 ± 0.01, being in agreement with the published global seismic values, which confirms its status as solar analogue. The chromospheric activity level is compatible with the solar magnetic activity observed during 2014 and 2015.
Conclusions: Our solar atlas is an essential tool for the analysis of solar-like stars and to characterise solar analogues and twins with HERMES. The differential analysis, using the presented solar atlas from HERMES observations allows us to obtain the fundamental parameters with very high accuracy. KIC 3241581 is a metal-rich solar analogue with a solar-like activity level in a binary system of unknown period.
DETECTION OF SOLAR-LIKE OSCILLATIONS IN THE BRIGHT RED GIANT STARS Γ PISCIUM AND Θ1 TAURI FROM A 190-DAY HIGH-PRECISION SPECTROSCOPIC MULTI-SITE CAMPAIGN
Beck, P. G.; Kambe, E.; Hillen, M.; Corsaro, E.; Van Winckel, H.; Moravveji, E.; De Ridder, J.; Bloemen, S.; Saesen, S.; Mathias, P.; Degroote, P.; Kallinger, T.; Verhoelst, T.; Ando, H.; Carrier, F.; Acke, B.; Oreiro, R.; Miglio, A.; Eggenberger, P.; Sato, B.; Zwintz, K.; Pápics, P. I.; Marcos-Arenal, P.; Sans Fuentes, S. A.; Schmid, V. S.; Waelkens, C.; Østensen, R.; Matthews, J. M.; Yoshida, M.; Izumiura, H.; Koyano, H.; Nagayama, S.; Shimizu, Y.; Okada, N.; Okita, K.; Sakamoto,
A.; Yamamuro, T.; and Aerts, C.
Context. Red giants are evolved stars that exhibit solar-like oscillations. Although a multitude of stars have been observed with space telescopes, only a handful of red giant stars were targets of spectroscopic asteroseismic observing projects.
Aims: We search for solar-like oscillations in the two bright red giant stars γ Psc and θ1 Tau from a time series of ground-based spectroscopy and determine the frequency of the excess of oscillation power νmax and the mean large frequency separation Δν for both stars. Seismic constraints on the stellar mass and radius will provide robust input for stellar modelling.
Methods: The radial velocities of γ Psc and θ1 Tau were monitored for 120 and 190 days, respectively. Nearly 9000 spectra were obtained. To reach accurate radial velocities, we used simultaneous thorium-argon and iodine-cell calibration of our optical spectra. In addition to the spectroscopy, we acquired interferometric observations of γ Psc for an independent estimate of the radius. We also analysed 22 days of observations of θ1 Tau with the MOST satellite.
Results: The frequency analysis of the radial velocity data of γ Psc revealed an excess of oscillation power around 32 μHz and a large frequency separation of 4.1 ± 0.1 μHz. θ1 Tau exhibits oscillation power around 90 μHz, with a large frequency separation of 6.9 ± 0.2 μHz. Scaling relations indicate that γ Psc is a star of about 1 M⊙ and 10 R⊙. The object θ1 Tau appears to be a massive star of about 2.7 M⊙ and 10 R⊙. The radial velocities of both stars were found to be modulated on timescales much longer than the oscillation periods.
Conclusions: The estimated radii from seismology are in agreement with interferometric observations and also with estimates based on photometric data. While the mass of θ1 Tau is in agreement with results from dynamical parallaxes, we find a lower mass for γ Psc than is found in the literature. The long periodic variability agrees with the expected timescales of rotational modulation.
PULSATING RED GIANT STARS IN ECCENTRIC BINARY SYSTEMS DISCOVERED FROM KEPLER SPACE-BASED PHOTOMETRY. A SAMPLE STUDY AND THE ANALYSIS OF KIC 5006817
Beck, P. G.; Hambleton, K.; Vos, J.; Kallinger, T.; Bloemen, S.; Tkachenko, A.; García, R. A.; Østensen, R. H.; Aerts, C.; Kurtz, D. W.; De Ridder, J.; Hekker, S.; Pavlovski, K.; Mathur, S.; De Smedt, K.; Derekas, A.; Corsaro, E.; Mosser, B.; Van Winckel, H.; Huber, D.; Degroote, P.; Davies, G. R.; Prša, A.; Debosscher, J.; Elsworth, Y.; Nemeth, P.; Siess, L.; Schmid, V. S.; Pápics, P. I.; de Vries, B. L.;
van Marle, A. J.; Marcos-Arenal, P.; and Lobel, A.
Context. The unparalleled photometric data obtained by NASA's Kepler Space Telescope has led to improved understanding of red giant stars and binary stars. Seismology allows us to constrain the properties of red giants. In addition to eclipsing binaries, eccentric non-eclipsing binaries that exhibit ellipsoidal modulations have been detected with Kepler.
Aims: We aim to study the properties of eccentric binary systems containing a red giant star and to derive the parameters of the primary giant component.
Methods: We applied asteroseismic techniques to determine the masses and radii of the primary component of each system. For a selected target, light and radial velocity curve modelling techniques were applied to extract the parameters of the system and its primary component. Stellar evolution and its effects on the evolution of the binary system were studied from theoretical models.
Results: The paper presents the asteroseismic analysis of 18 pulsating red giants in eccentric binary systems, for which masses and radii were constrained. The orbital periods of these systems range from 20 to 440 days. The results of our ongoing radial velocity monitoring programme with the Hermes spectrograph reveal an eccentricity range of e = 0.2 to 0.76. As a case study we present a detailed analysis of KIC 5006817, whose rich oscillation spectrum allows for detailed seismic analysis. From seismology we constrain the rotational period of the envelope to be at least 165 d, which is roughly twice the orbital period. The stellar core rotates 13 times faster than the surface. From the spectrum and radial velocities we expect that the Doppler beaming signal should have a maximum amplitude of 300 ppm in the light curve. Fixing the mass and radius to the asteroseismically determined values, we find from our binary modelling a value of the gravity darkening exponent that is significantly larger than expected. Through binary modelling, we determine the mass of the secondary component to be 0.29 ± 0.03 M⊙.
Conclusions: For KIC 5006817 we exclude pseudo-synchronous rotation of the red giant with the orbit. The comparison of the results from seismology and modelling of the light curve shows a possible alignment of the rotational and orbital axis at the 2σ level. Red giant eccentric systems could be progenitors of cataclysmic variables and hot subdwarf B stars.
FAST CORE ROTATION IN RED-GIANT STARS AS REVEALED BY GRAVITY-DOMINATED MIXED MODES
Beck, Paul G.; Montalban, Josefina; Kallinger, Thomas; De Ridder, Joris; Aerts, Conny; García, Rafael A.; Hekker, Saskia; Dupret, Marc-Antoine; Mosser, Benoit; Eggenberger, Patrick; Stello, Dennis; Elsworth, Yvonne; Frandsen, Søren; Carrier, Fabien; Hillen, Michel; Gruberbauer, Michael; Christensen-Dalsgaard, Jørgen; Miglio, Andrea; Valentini, Marica; Bedding, Timothy R.; Kjeldsen, Hans; Girouard, Forrest R.; Hall, Jennifer R.; and Ibrahim, Khadeejah A.
When the core hydrogen is exhausted during stellar evolution, the central region of a star contracts and the outer envelope expands and cools, giving rise to a red giant. Convection takes place over much of the star's radius. Conservation of angular momentum requires that the cores of these stars rotate faster than their envelopes; indirect evidence supports this. Information about the angular-momentum distribution is inaccessible to direct observations, but it can be extracted from the effect of rotation on oscillation modes that probe the stellar interior. Here we report an increasing rotation rate from the surface of the star to the stellar core in the interiors of red giants, obtained using the rotational frequency splitting of recently detected `mixed modes'. By comparison with theoretical stellar models, we conclude that the core must rotate at least ten times faster than the surface. This observational result confirms the theoretical prediction of a steep gradient in the rotation profile towards the deep stellar interior.
KEPLER DETECTED GRAVITY-MODE PERIOD SPACINGS IN A RED GIANT STAR
Beck, P. G.; Bedding, T. R.; Mosser, B.; Stello, D.; Garcia, R. A.; Kallinger, T.; Hekker, S.; Elsworth, Y.; Frandsen, S.; Carrier, F.; De Ridder, J.; Aerts, C.; White, T. R.; Huber, D.; Dupret, M. -A.; Montalbán, J.; Miglio, A.; Noels, A.; Chaplin, W. J.; Kjeldsen, H.; Christensen-Dalsgaard, J.; Gilliland, R. L.; Brown, T. M.; Kawaler, S. D.; Mathur, S.; and Jenkins, J. M.
Stellar interiors are inaccessible through direct observations. For this reason, helioseismologists made use of the Sun’s acoustic oscillation modes to tune models of its structure. The quest to detect modes that probe the solar core has been ongoing for decades. We report the detection of mixed modes penetrating all the way to the core of an evolved star from 320 days of observations with the Kepler satellite. The period spacings of these mixed modes are directly dependent on the density gradient between the core region and the convective envelope.
First author oral and poster contributions
THE BRITE-SONG OF ALDEBARAN - STELLAR MUSIC IN THREE VOICES
Proceedings of the conference Stars and their Variability Observed from Space, held in Vienna on August 19-23, 2019. Eds.: C. Neiner, W. W. Weiss, D. Baade, R. E. Griffin, C. C. Lovekin, A. F. J. Moffat. University of Vienna, 2020, pp.75-79.
Beck, P. G., Kuschnig, R., Houdek, G., Kallinger, T., Weiss, W. W., Palle, P. L., Grundahl, F., Hatzes, A., Parviainen, H., Allende Prieto, C., Deeg, H. J., Jiménez, A., Mathur, S., Garcia, R. A., White, T. R., Bedding, T. R., Grossmann, D. H., Janisch, S., Zaqarashvili, T., Hanslmeier, A., and Zwintz, K.
Solar-like oscillations in red-giant stars are now commonly detected in thousands of stars with space telescopes such as Kepler. Parallel radial-velocity and photometric measurements would help us understand better the physics governing the amplitudes of solar-like oscillators, but most stars targetted for space photometry are too faint for light-demanding ground-based spectroscopy. The BRITE-Constellation provides a unique opportunity of monitoring in two colours the flux variations of bright luminous red giants. Those stars are also bright enough to be monitored with high-resolution spectrographs on small telescopes, such as the SONG Network. This contribution provided a first overview of our comprehensive, multi-year campaign to use both BRITE and SONG to characterize Aldebaran (one of the brightest red giants in the sky) seismically. Because luminous red giants can be seen at large distances, when characterized well they will serve as valuable benchmark stars for Galactic archeology.
OBSERVATIONS OF TIDES AND CIRCULARIZATION IN RED-GIANT BINARIES FROM KEPLER PHOTOMETRY
EAS Publications Series, Volume 82, 2019, pp.119-125. Proceedings paper for the J-P Zahn Symposion, Paris, 6 Pages, 2 Figures (2019)
Beck, P. G.; Mathis, S.; Kallinger, T.; García, R. A.; and Benbakoura, M.
Binary stars are places of complex stellar interactions. While all binaries are in principle converging towards a state of circularization, many eccentric systems are found even in advanced stellar phases. In this work we discuss the sample of binaries with a red-giant component, discovered from observations of the NASA Kepler space mission. We first discuss which effects and features of tidal interactions are detectable in photometry, spectroscopy and the seismic analysis. In a second step, the sample of binary systems observed with Kepler, is compared to the well studied sample of Verbunt & Phinney (1995, hereafter VP95). We find that this study of circularization of systems hosting evolving red-giant stars with deep convective envelopes is also well applicable to the red-giant binaries in the sample of Kepler stars.
CONSTRAINING STELLAR PHYSICS FROM RED-GIANT STARS IN BINARIES - STELLAR ROTATION, MIXING PROCESSES AND STELLAR ACTIVITY
Seismology of the Sun and the Distant Stars - Using Today's Successes to Prepare the Future - TASC2 & KASC9 Workshop - SPACEINN & HELAS8 Conference, Azores Islands, Portugal, Edited by Monteiro, M.J.P.F.G.; Cunha, M.S.; Ferreira, J.M.T.S.; EPJ Web of Conferences, Volume 160, id.05008
Beck, P. G.; Kallinger, T.; Pavlovski, K.; Palacios, A.; Tkachenko, A.; García, R. A.; Mathis, S.; Corsaro, E.; Johnston, C.; Mosser, B.; Ceillier, T.; do Nascimento, J. -D.; and Raskin, G.
The unparalleled photometric data obtained by NASA's Kepler Space Telescope has led to an improved understanding of stellar structure and evolution - in particular for solar-like oscillators in this context. Binary stars are fascinating objects. Because they were formed together, binary systems provide a set of two stars with very well constrained parameters. Those can be used to study properties and physical processes, such as the stellar rotation, dynamics and rotational mixing of elements and allows us to learn from the differences we find between the two components. In this work, we discussed a detailed study of the binary system KIC 9163796, discovered through Kepler photometry. The ground-based follow-up spectroscopy showed that this system is a double-lined spectroscopic binary, with a mass ratio close to unity. However, the fundamental parameters of the components of this system as well as their lithium abundances differ substantially. Kepler photometry of this system allows to perform a detailed seismic analysis as well as to derive the orbital period and the surface rotation rate of the primary component of the system. Indications of the seismic signature of the secondary are found. The differing parameters are best explained with both components located in the early and the late phase of the first dredge up at the bottom of the red-giant branch. Observed lithium abundances in both components are in good agreement with prediction of stellar models including rotational mixing. By combining observations and theory, a comprehensive picture of the system can be drawn.
PROBING SEISMIC SOLAR ANALOGUES THROUGH OBSERVATIONS WITH THE NASA KEPLER SPACE TELESCOPE AND HERMES HIGH-RESOLUTION SPECTROGRAPH
The 19th Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun (CS19), Uppsala, Sweden, 06-10 June 2016, id.42
Beck, P. G.; Salabert, D.; Garcia, R. A.; do Nascimento, J., Jr.; Duarte, T. S. S.; Mathis, S.; Regulo, C.; Ballot, J.; Egeland, R.; Castro, M.; Pérez-Herńandez, F.,; Creevey, O.; Tkachenko, A.; van Reeth, T.; Bigot, L.; Corsaro, E.; Metcalfe, T.; Mathur, S.; Palle, P. L.; Allende Prieto, C.; Montes, D.; Johnston, C.; Andersen, M. F.; and van Winckel, H.
Stars similar to the Sun, known as solar analogues, provide an excellent opportunity to study the preceding and following evolutionary phases of our host star. The unprecedented quality of photometric data collected by the Kepler NASA mission allows us to characterise solar-like stars through asteroseismology and study diagnostics of stellar evolution, such as variation of magnetic activity, rotation and the surface lithium abundance. In this project, presented in a series of papers by Salabert et al (2016ab) and Beck et al. (2016ab), we investigate the link between stellar activity, rotation, lithium abundance and oscillations in a group of 18 solar-analogue stars through space photometry, obtained with the NASA Kepler space telescope and from currently 50+ hours of ground-based, high-resolution spectroscopy with the Hermes instrument. In these proceedings, we first discuss the selection of the stars in the sample, observations and calibrations and then summarise the main results of the project.
By investigating the chromospheric and photospheric activity of the solar analogues in this sample, it was shown that for a large fraction of these stars the measured activity levels are compatible to levels of the 11-year solar activity cycle 23. A clear correlation between the lithium abundance and surface rotation was found for rotation periods shorter than the solar value. Comparing the lithium abundance measured in the solar analogues to evolutionary models with the Toulouse-Geneva Evolutionary Code (TGEC), we found that the solar models calibrated to the Sun also correctly describe the set of solar/stellar analogs showing that they share the same internal mixing physics. Finally, the star KIC3241581 and KIC10644353 are discussed in more detail.
OSCILLATING RED-GIANT STARS IN ECCENTRIC BINARIES
The Space Photometry Revolution - CoRoT Symposium 3, Kepler KASC-7 Joint Meeting, Toulouse, France, Edited by R.A. García; J. Ballot; EPJ Web of Conferences, Volume 101, id.06004
Beck, P. G.; Hambleton, K.; Vos, J.; Kallinger, T.; Garcia, R. A.; Mathur, S.; and Houmani, K.
The unparalleled photometric data obtained by NASA's Kepler Space Telescope has led to improved understanding of red-giant stars and binary stars. We discuss the characterization of known eccentric system, containing a solar-like oscillating red-giant primary component. We also report several new binary systems that are candidates for hosting an oscillating companion. A powerful approach to study binary stars is to combine asteroseimic techniques with light curve fitting. Seismology allows us to deduce the properties of red giants. In addition, by modeling the ellipsoidal modulations we can constrain the parameters of the binary system. An valuable independent source are ground-bases, high-resolution spectrographs.
CONSTRAINING THE CORE-ROTATION RATE IN RED-GIANT STARS FROM KEPLER SPACE PHOTOMETRY
Astronomische Nachrichten, Vol.333, Issue 10, p.967, proceedings of "The New Era of Helio- and Asteroseismology", 2012. 05. 20-25, Obergurgl (AT)
Beck, P. G.; De Ridder, J.; Aerts, C.; Kallinger, T.; Hekker, S.; García, R. A.; Mosser, B.; and Davies , G. R.
Rotation plays a key role in stellar structure and its evolution. Through transport processes which induce rotational mixing of chemical species and the redistribution of angular momentum, internal stellar rotation influences the evolutionary tracks in the Hertzsprung-Russell diagram. In turn, evolution influences the rotational properties. Therefore, information on the rotational properties of the deep interior would help to better understand the stellar evolution. However, as the internal rotational profile cannot be measured directly, it remains a major unknown leaving this important aspect of models unconstrained. We can test for nonrigid rotation inside the stars with asteroseismology. Through the effect of rotational splitting of non-radial oscillation modes, we investigate the internal rotation profile indirectly. Red giants have very slow rotation rates leading to a rotational splitting on the level of a few tenth of a \muHz. Only from more than 1.5 years of consecutive data from the NASA Kepler space telescope, these tiny variations could be resolved. A qualitative comparison to theoretical models allowed constraining the core-to-surface rotation rate for some of these evolved stars. In this paper, we report on the first results of a large sample study of splitting of individual dipole modes.
TOWARDS PROBING THE INTERNAL ANGULAR MOMENTUM DISTRIBUTION IN RED GIANTS FROM SOLAR-LIKE OSCILLATIONS
Astronomische Nachrichten, Vol. 331, Issue 9/10, P32
[ ADS ]
Beck, P. G., Carrier, F., and Aerts, C.
As red giants show very low surface velocities, rotational splitting has not been detected so far in their comb-like structure from solar-like oscillations. We propose to study the rotational splitting of l=3 modes to discriminate between the competing effects of rotational splitting and finite mode lifetime. Modes of different radial orders probe different depth. The information of rotational splitting with respect to the radial order could lead to an estimate of the magnitude of differential rotation inside their convective layers. This is a first attempt to measure the magnitude of the differential rotation in these evolved stars. An extensive spectroscopic multi-site campaign to study solar-like oscillations in two promising targets, gamma Psc and theta1 Tau is being set up for the second half of 2010. The observational result will be confronted with state-of-the-art models. Independent from the outcome on differential rotation, strong constraints on the models of red giants will be set. gamma Psc and theta1 Tau will provide an optimal case to test the underlying physics.
HD 210111 COMPARED TO FG VIR AND Θ2 TAU: A TEST TO THE Λ BOOTIS PHENOMENON
Beck, P. G.
The λ Bootis stars are a spectroscopic subgroup of chemically peculiar Population I A-type stars. They show significant underabundances of Fe-peak elements and solar abundances of lighter elements. Theoretical explanations include surface phenomena and binarity. We compared the pulsating λ Bootis star HD 210111 with two normal δ Scuti pulsators. This comparison shows that the pulsational behaviour of HD 210111 is compatible with the λ Bootis phenomenon being a surface effect only.