Stellar acoustic radii, mean densities and ages from seismic inversion techniques

Context. Determining stellar characteristics such as the radius, the mass or the age is crucial when studying stellar evolution, exoplanetary systems or characterising stellar populations in the Galaxy. Asteroseismology is the golden path to accurately obtain these characteristics. In this context, a key question is how to make these methods less model-dependant. Aims. Building on the work of Reese et al. (2012), we wish to extend the SOLA inversion technique to new stellar global characteristics in addition to the mean density. The goal is to provide a general framework in which to estimate these characteristics as accurately as possible in low mass main sequence stars. Methods. First, we describe our framework and discuss the reliability of the inversion technique and the possible sources of error.We then apply this methodology to the acoustic radius, an age indicator based on the sound speed derivative and the mean density and compare it to estimates based on the average large and small frequency separations. These inversions are carried out for several test cases which include: various metallicities, different mixing-lengths, non-adiabatic effects and turbulent pressure. Results. We observe that the SOLA method yields accurate results in all test cases whereas results based on the large and small frequency separations are less accurate and more sensitive to surface effects and structural differences in the models. If we include the surface corrections of Kjeldsen et al. (2008), we obtain results of comparable accuracy for the mean density. Overall, the mean density and acoustic radius inversions are more robust than the inversions for the age indicator. Moreover, the current approach is limited to relatively young stars with radiative cores. Increasing the number of observed frequencies improves the reliability and accuracy of the method

Seismic constraints on rotation of Sun-like star and mass of exoplanet

Rotation is thought to drive cyclic magnetic activity in the Sun and Sun-like stars. Stellar dynamos, however, are poorly understood owing to the scarcity of observations of rotation and magnetic fields in stars. Here, inferences are drawn on the internal rotation of a distant Sun-like star by studying its global modes of oscillation. We report asteroseismic constraints imposed on the rotation rate and the inclination of the spin axis of the Sun-like star HD 52265, a principal target observed by the CoRoT satellite that is known to host a planetary companion. These seismic inferences are remarkably consistent with an independent spectroscopic observation (rotational line broadening) and with the observed rotation period of star spots. Furthermore, asteroseismology constrains the mass of exoplanet HD 52265b. Under the standard assumption that the stellar spin axis and the axis of the planetary orbit coincide, the minimum spectroscopic mass of the planet can be converted into a true mass of 1.85 (+0.52,-0.42) M_Jupiter, which implies that it is a planet, not a brown dwarf.Comment: Published in Proceedings of the National Academy of Sciences (5 pages, 5 figures, 3 tables). Available at http://www.pnas.org/cgi/doi/10.1073/pnas.130329111

Excitation stochastique des oscillations stellaires. <br />Application à la mission spatiale COROT.

Ces travaux ont fait l'objet de deux publications:- Samadi R. & Goupil M.J., "Excitation of stellar p-modes by turbulent convection. I. Theoretical formulation", 2001, Astronomy & Astrophysics, vol. 370, p. 136 (http://fr.arxiv.org/abs/astro-ph/0101109)- Samadi R., Goupil M.J., Lebreton Y. , "Excitation of stellar p-modes by turbulent convection. II. The Sun", 2001, Astronomy & Astrophysics, vol. 370, p 147 (http://fr.arxiv.org/abs/astro-ph/0101111)The observed solar p-mode oscillations are damped due to several damping mechanisms and in the other hand they are believed to be excited stochastically by turbulent convection.Turbulent motions, which occur in the convection zone, generate acoustic power which in turn is injected into the p-mode oscillations and thus ensures the modes excitation. Solar-like oscillations are therefore meant as stochastically excited oscillations and concern low massive stars with an outer convective zone.Providing that accurate measurements of the oscillation amplitudes and damping rates are available it is possible to evaluate the power injected into the modes and thus- by comparisonwith the observations- to constrain current theories.In the present work we review the basic theory : inconsistencies are identified and removed and controversy between previous authors are solved. As a result I propose a new formulation which generalizes previous ones. This formulation enable investigations of various possible stellar turbulent models. The entropy contribution to excitation is found to originate from the advection of the Eulerian entropy fluctuations by the turbulent velocity field and dominates the contribution of the Reynolds stress.Numerical computations performed in the solar case and in the case of Procyon reveal the sensitivity to the free parameters - involved in the theory- and to the model of stellar turbulence. Application to several low intermediate stars (1 Mo The forthcoming space mission COROT will provide high-quality data of solar oscillations. In order to optimize the scientific return of the mission we have developed a numerical simulation of the whole photometric chain. This simulation allows use to assess the instrument response which has been convoluted with the oscillations power. We conclude that the high performances of COROT will provide accurate constraints for the theory of the stochastic excitation developed in the present work.On a tout lieu de croire que les oscillations acoustiques du Soleil, qui sont sujettes à différents mécanismes complexes d'amortissement, sont excitées stochastiquement par les mouvements incohérents de matière dans la zone convective.Ces mouvements turbulents sont à l'origine d'une puissance acoustique injectée dans les oscillations et en assurent la survie. On désigne alors par oscillations de type solaire des oscillations acoustiques excitées par ce mécanisme, elles concernent les étoiles de masse intermédiaire dotées d'une zone convective supérieure.Pour peu que des mesures précises d'amplitude et d'amortissement soient disponibles il est possible d'évaluer cette puissance acoustique et de contraindre la physique du phénomène.Dans ce travail les bases de cette théorie ont été reconsidérées : les inconsistances ont été identifiée et supprimées et les controverses entre les auteurs précédents résolues. Il en résulte une nouvelle formulation qui généralise le traitement de la turbulence. Le terme d'advection des fluctuations turbulentes de l'entropie par le champ de vitesse est identifiée comme la source d'excitation dominant largement la source liée au stress de Reynolds. Les études quantitatives effectuées pour un modèle du Soleil et de Procyon ont révélé la grande sensibilité aux paramètres libres, introduits dans la théorie, ainsi qu'à la modélisation de la turbulence. L'application de cette formulation à des étoiles de masse intermédiaire ( 1 M oLa future mission spatiale COROT devrait collecter des données de haute qualité sur ce type d'étoile. Dans cette perspective et pour optimiser le retour scientifique nous avons développé un modèle numérique de la chaîne photométrique de l'instrument. Ce modèle nous a permis d'apprécier la réponse de l'instrument que nous avons alors convolué avec nos estimations des puissances. On établit que les hautes performances de COROT devraient être capables de nous fournir les contraintes observationnelles sur la théorie développée dans le présent travail

Etude des mécanismes d'excitation stochastique des oscillations stellaires par la convection turbulente

Alors que les oscillations solaires n'ont pas fini de nous révéler tous leurs secrets, des oscillations acoustiques analogues sont détectées dans un nombre croissant d'étoiles. Comme sur le Soleil, ces oscillations (dite de type solaire) sont amorties par des mécanismes complexes et encore mal connus et excitées par la turbulence dans l'enveloppe convective supérieure des étoiles. Grâce à la qualité photométrique exceptionnelle des missions spatiales CoRoT (CNES) et Kepler(NASA) ainsi qu'à la continuité long terme des observations qu'elles fournissent, on mesure maintenant précisément fréquences, amplitudes et durées de vie de ces oscillations dans une variété d'étoiles dotées de caractéristiques diverses concernant leur stade évolutif, paramètres fondamentaux, composition chimique, champ magnétique, rotation ... etc. Plus que ne le fait la mesure de leurs fréquences, la mesure des amplitudes et durées vies des modes de type solaire nous fournit des contraintes sur les propriétés statiques et dynamiques de la convection, sur la physique des modes et enfin sur la stratification en surface des étoiles. Le jeux conséquent d'étoiles pulsantes détectées par CoRoT et Kepler nous révéle aussi que les amplitudes et durées de ces oscillations varient d'une étoile à l'autre selon des lois d'échelles caractéristiques qui dépendent d'un nombre restreint de paramètres stellaires (masse, luminosité, température effective ... etc). Ce mémoire de thèse résume les travaux que j'ai menés dans ce contexte depuis plus de dix ans en collaboration avec mes collègues et avec les étudiants que j'ai encadré. Ces travaux ont cherché à comprendre et mieux modéliser les amplitudes des oscillations excitées par la convection turbulente, notamment les lois d'échelles observées. Ce faisant, ils ont visé à établir des diagnostics sur les propriétés statiques et dynamiques des régions convectives, avec pour objectif à plus long terme d'améliorer la modélisation des processus de transport convectif dans les intérieurs stellaires

Stellar acoustic radii and ages from seismic inversion techniques

Context: Determining stellar characteristics such as the radius, mass or age is crucial for the study of stellar evolution, exoplanetary systems or the characterisation of stellar populations in the Galaxy. Asteroseismology is currently the most promising tool to accurately determine these characteristics. However, a key question is how to reduce the model dependence of asteroseismic methods. Method: We extend the SOLA inversion technique to new global characteristics in addition to the mean density (see Reese et al. 2012). We apply our methodology to the acoustic radius and an age indicator based on the sound speed derivative. The results from SOLA inversions are compared with estimates based on the small and large frequency separations for several test cases, including differing mixing-lengths, and the presence or absence of non-adiabatic effects or turbulent pressure. Results: We show that SOLA inversions yield accurate results in all test cases, unlike the other techniques which are more sensitive to surface effects. We observe that the acoustic radius and mean density inversions are more robust than the age indicator inversions, which are limited to relatively young stars with radiative cores

Transit least-squares survey. IV. Earth-like transiting planets expected from the PLATO mission

International audienceIn its long-duration observation phase, the PLATO satellite (scheduled for launch in 2026) will observe two independent, non-overlapping fields, nominally one in the northern hemisphere and one in the southern hemisphere, for a total of four years. The exact duration of each pointing will be determined two years before launch. Previous estimates of PLATO's yield of Earth-sized planets in the habitable zones (HZs) around solar-type stars ranged between 6 and 280. We use the PLATO Solar-like Light curve Simulator (PSLS) to simulate light curves with transiting planets around bright (mV ≤ 11) Sun-like stars at a cadence of 25 s, roughly representative of the >15 000 targets in PLATO's high-priority P1 sample (mostly F5-K7 dwarfs and subdwarfs). Our study includes light curves generated from synchronous observations of 6, 12, 18, and 24 of PLATO's 12 cm aperture cameras over both 2 and 3yr of continuous observations. Automated detrending is done with the Wotan software, and post-detrending transit detection is performed with the transit least-squares (TLS) algorithm. Light curves combined from 24 cameras yield true positive rates (TPRs) near unity for planets ≥1.2 R⊕ with two transits. If a third transit is in the light curve, planets as small as 1 R⊕ are recovered with TPR ~ 100%. We scale the TPRs with the expected number of stars in the P1 sample and with modern estimates of the exoplanet occurrence rates and predict the detection of planets with 0.5 R⊕ ≤ Rp ≤ 1.5 R⊕ in the HZs around F5-K7 dwarf stars. For the long-duration observation phase (2yr + 2yr) strategy we predict 11-34 detections, and for the (3 yr + 1 yr) strategy we predict 8-25 discoveries. These estimates neglect exoplanets with monotransits, serendipitous detections in stellar samples P2-P5, a dedicated removal of systematic effects, and a possible bias of the P1 sample toward brighter stars and high camera coverage due to noise requirements. As an opposite effect, Earth-sized planets might typically exhibit transits around P1 sample stars shallower than we have assumed since the P1 sample will be skewed toward spectral types earlier than the Sun-like stars assumed in our simulations. Moreover, our study of the effects of stellar variability on shallow transits of Earth-like planets illustrates that our estimates of PLATO's planet yield, which we derive using a photometrically quiet star similar to the Sun, must be seen as upper limits. In conclusion, PLATO's detection of about a dozen Earth-sized planets in the HZs around solar-type stars will mean a major contribution to this as yet poorly sampled part of the exoplanet parameter space with Earth-like planets

Extraction of seismic indices and stellar granulation parameters for CoRoT and Kepler red giants using the MLEUP method. Main results and perspectives

In the framework of the SPACEInn project, a Stellar Seismic Indices (SSI - http://ssi.lesia.obspm.fr/) database was developed in order to provide the scientific community with oscillations and granulation signatures for a large set of red-giant type stars. For this purpose, we have developed the method MLEUP able to extract simultaneously the seismic indices (the equidistance Δv, the frequency vmax and the height Henv of the maximum oscillation power) and granulation parameters (the e-folding time τeff and the variance of the integrated brightness fluctuations σ2). This method has been tested in terms of precision and accuracy, using Monte Carlo simulations. Then we applied it to all stars observed by CoRoT and all long-cadence Kepler lightcurves. In total, we yield seismic indices and granulation parameters for about 5,000 stars for CoRoT and more than 13,000 for Kepler. In this paper, we focus on the main results for both seismic indices Δv and vmax as well as for the stellar parameters (mass, radius and luminosity) seismically inferred. Then, in the perspective of Gaia, we discuss about the possibility to derive other seismic quantities like e.g. a seismic effective temperature

Current updated grids of 1D stellar models: Cestam mockgrid.

International audienc

Extraction of seismic indices and stellar granulation parameters for CoRoT and Kepler red giants using the MLEUP method. Main results and perspectives

In the framework of the SPACEInn project, a Stellar Seismic Indices (SSI - http://ssi.lesia.obspm.fr/) database was developed in order to provide the scientific community with oscillations and granulation signatures for a large set of red-giant type stars. For this purpose, we have developed the method MLEUP able to extract simultaneously the seismic indices (the equidistance Δv, the frequency vmax and the height Henv of the maximum oscillation power) and granulation parameters (the e-folding time τeff and the variance of the integrated brightness fluctuations σ2). This method has been tested in terms of precision and accuracy, using Monte Carlo simulations. Then we applied it to all stars observed by CoRoT and all long-cadence Kepler lightcurves. In total, we yield seismic indices and granulation parameters for about 5,000 stars for CoRoT and more than 13,000 for Kepler. In this paper, we focus on the main results for both seismic indices Δv and vmax as well as for the stellar parameters (mass, radius and luminosity) seismically inferred. Then, in the perspective of Gaia, we discuss about the possibility to derive other seismic quantities like e.g. a seismic effective temperature