Observations of tides and circularization in red-giant binaries from Kepler photometry

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.Comment: Proceedings paper for the J-P Zahn Symposion, Paris, 6 Pages, 2 Figure

A giant impact as the likely origin of different twins in the Kepler-107 exoplanet system

Measures of exoplanet bulk densities indicate that small exoplanets with radius less than 3 Earth radii ($R_\oplus$) range from low-density sub-Neptunes containing volatile elements to higher density rocky planets with Earth-like or iron-rich (Mercury-like) compositions. Such astonishing diversity in observed small exoplanet compositions may be the product of different initial conditions of the planet-formation process and/or different evolutionary paths that altered the planetary properties after formation. Planet evolution may be especially affected by either photoevaporative mass loss induced by high stellar X-ray and extreme ultraviolet (XUV) flux or giant impacts. Although there is some evidence for the former, there are no unambiguous findings so far about the occurrence of giant impacts in an exoplanet system. Here, we characterize the two innermost planets of the compact and near-resonant system Kepler-107. We show that they have nearly identical radii (about $1.5-1.6~R_\oplus$), but the outer planet Kepler-107c is more than twice as dense (about $12.6~\rm g\,cm^{-3}$) as the innermost Kepler-107b (about $5.3~\rm g\,cm^{-3}$). In consequence, Kepler-107c must have a larger iron core fraction than Kepler-107b. This imbalance cannot be explained by the stellar XUV irradiation, which would conversely make the more-irradiated and less-massive planet Kepler-107b denser than Kepler-107c. Instead, the dissimilar densities are consistent with a giant impact event on Kepler-107c that would have stripped off part of its silicate mantle. This hypothesis is supported by theoretical predictions from collisional mantle stripping, which match the mass and radius of Kepler-107c.Comment: Published in Nature Astronomy on 4 February 2019, 35 pages including Supplementary Information materia

Evolution of interacting low-mass stars : multitechnical observations and modeling of multiple systems

Cette thèse est consacrée à l'étude des étoiles de faible masse ayant dans leur environnement proche d'autres étoiles ou des planètes. Nous nous sommes concentrés sur l'influence des interactions avec ces compagnons sur l'évolution stellaire ainsi que leurs conséquences observables.Dans la première partie, nous présentons le modèle d'évolution des systèmes étoile–planète que nous avons développé au cours de cette thèse, nommé ESPEM (Évolution des Systèmes Planétaires Et Magnétisme). Ce modèle prend en compte de façon ab-initio des effets du vent stellaire magnétisé et de la dissipation de marée sur la rotation stellaire et l'orbite planétaire, simultanément avec l'évolution structurelle de l'étoile. Premièrement, nous l'utilisons pour étudier l'évolution séculaire de la rotation des étoiles hôtes de systèmes planétaires et montrons notamment que cette évolution peut être significativement différente de celle des étoiles isolées. Ensuite, nous examinons les prédictions de ce modèle concernant l'architecture orbitale des systèmes étoile–planète. Nos résultats suggèrent une interprétation aux distributions de périodes orbitales et de de rotation stellaire observées.Dans la deuxième partie, nous montrons en quoi l'observation d'étoiles binaires évoluées permet de tester les théories astrophysiques, notamment l'astérosismologie et l'interaction de marée. Dans un premier temps, nous présentons les résultats d'un programme d'observations que nous avons mené pendant plus de deux ans et qui nous a permis de caractériser 16 systèmes binaires à éclipses. Ensuite, nous comparons ces résultats avec ceux que nous avons obtenus en analysant cet échantillon à l'aide d'outils astérosismiques dans le but de vérifier l'exactitude de ces derniers. Enfin, en élargissant l'échantillon étudié à 30 autres étoiles binaires évoluées, nous testons la théorie de l'évolution de marée. Ceci nous permet à la fois de valider la théorie et de comprendre l'évolution des systèmes observés dans ce travail.Ce travail met en avant deux aspects de la spécificité des systèmes multiples. Premièrement, il montre en quoi l'évolution des étoiles est impactée par la présence d'un compagnon stellaire ou planétaire. Deuxièmement, il met en avant l'intérêt des étoiles binaires pour tester les théories astrophysiques et renforce la compréhension actuelle de l'évolution stellaire.This thesis is devoted to the study of low-mass stars having other stars or planets in their immediate environment. We focused on the influence of interactions with these companions on stellar evolution and their observable consequences.In the first part, we present the model of evolution of star–planet systems that we developed during this thesis, called ESPEM (French acronym for Evolution of Planetary Systems and Magnetism). This model incorporates ab-initio prescriptions to quantify the effects of magnetized stellar wind and tidal dissipation on stellar rotation and planetary orbit, simultaneously with the star's structural evolution. First, we use it to study the secular evolution of the rotation of planet-host stars and show that this evolution can be significantly different from that of isolated stars. Next, we examine the predictions of this model regarding the orbital architecture of star–planet systems. Our results suggest an interpretation to the observed distributions of orbital and stellar rotation periods.In the second part of the manuscript, we show how the observation of advanced binary stars allows us to test astrophysical theories, in particular asteroseismology and tidal interaction. First, we present the results of an observation program that we conducted for more than two years and that allowed us to characterize 16 eclipsing binary systems. Then, we compare these results with those obtained by analyzing this sample using asteroseismic tools to verify the accuracy of the latter. Finally, by extending the studied sample to 30 other advanced binary stars including an evolved primary, we test the theory of tidal evolution. This allows us both to validate the theory and to understand the evolution of the systems observed in this work.This work highlights two aspects of the specificity of multiple systems. First, it shows how the evolution of stars is affected by the presence of a stellar or planetary companion. Second, it emphasizes the interest of binary stars in testing astrophysical theories and reinforces the current understanding of stellar evolution

Evolution of star–planet systems under magnetic braking and tidal interaction

Context. With the discovery over the last two decades of a large diversity of exoplanetary systems, it is now of prime importance to characterize star–planet interactions and how such systems evolve. Aims. We address this question by studying systems formed by a solar-like star and a close-in planet. We focus on the stellar wind spinning down the star along its main-sequence phase and tidal interaction causing orbital evolution of the systems. Despite recent significant advances in these fields, all current models use parametric descriptions to study at least one of these effects. Our objective is to introduce ab initio prescriptions of the tidal and braking torques simultaneously, so as to improve our understanding of the underlying physics. Methods. We develop a one-dimensional (1D) numerical model of coplanar circular star–planet systems taking into account stellar structural changes, wind braking, and tidal interaction and implement it in a code called ESPEM. We follow the secular evolution of the stellar rotation and of the semi-major axis of the orbit, assuming a bilayer internal structure for the former. After comparing our predictions to recent observations and models, we perform tests to emphasize the contribution of ab initio prescriptions. Finally, we isolate four significant characteristics of star–planet systems: stellar mass, initial stellar rotation period, planetary mass and initial semi-major axis; and browse the parameter space to investigate the influence of each of them on the fate of the system. Results. Our secular model of stellar wind braking accurately reproduces the recent observations of stellar rotation in open clusters. Our results show that a planet can affect the rotation of its host star and that the resulting spin-up or spin-down depends on the orbital semi-major axis and on the joint influence of magnetic and tidal effects. The ab initio prescription for tidal dissipation that we used predicts fast outward migration of massive planets orbiting fast-rotating young stars. Finally, we provide the reader with a criterion based on the characteristics of the system that allows us to assess whether or not the planet will undergo orbital decay due to tidal interaction

Testing tidal theory for evolved stars by using red-giant binaries observed by Kepler

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