8 research outputs found
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 () 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
), but the outer planet Kepler-107c is more than twice as
dense (about ) as the innermost Kepler-107b (about
). 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 starâplanet 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 starâplanet 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 starâplanet 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 starâplanet 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
International audienc
Spectroscopic and seismic analysis of red giants in eclipsing binaries discovered by Kepler
International audienceEclipsing binaries (EBs) are unique targets for measuring accurate stellar properties and constraining stellar evolution models. In particular, it is possible to measure masses and radii at the few percent level for both components of a double-lined spectroscopic EB (SB2-EB). On the one hand, detached EBs hosting at least one star with detectable solar-like oscillations constitute ideal test objects to verify the ability of ensemble asteroseismology to derive stellar properties. On the other hand, the oscillations and surface activity of stars that belong to EBs offer unique information about the evolution of binary systems. This paper builds upon previous works dedicated to red giant stars (RG) in EBs; so far 20 known systems have been discovered by the NASA Kepler mission. We report the discovery of 16 RGs in EBs, which are also from the Kepler data, leading to a total of 36 confirmed RG stars in EBs from the original Kepler mission. This new sample includes three SB2-EBs with oscillations, resulting in a total of 14 known SB2-EBs with an oscillating RG component. This sample also includes six close systems in which the RG display a clear surface activity and complete oscillation suppression. Based on dedicated high-resolution spectroscopic observations (Apache Point Observatory, Observatoire de Haute Provence), we focus on three main aspects. Firstly, from the extended sample of 14 SB2-EBs, we confirm that the simple application of the asteroseismic scaling relations to RGs overestimates masses and radii of RGs by about 15% and 5%. This bias can be reduced by employing either new asteroseismic reference values for RGs or model-based corrections of the asteroseismic parameters. Secondly, we confirm that close binarity leads to a high level of photometric modulation (up to 10%) and a suppression of solar-like oscillations. In particular, we show that it reduces the lifetime of radial modes by a factor of up to 10. Thirdly, we use our 16 new systems to complement previous observational studies that aimed to constrain tidal dissipation in interacting binaries. We confirm the important role of the equilibrium tide in binary evolution, but we also identify systems with circular orbits despite relatively young ages, which suggests the need to explore complementary tidal dissipation mechanisms in the future. Finally, as a by-product, we report the measurements of mass, radius, and age of three M-dwarf companion stars
The dynamical evolution of close-in binary systems formed by a super-Earth and its host star
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 (Râ) range from low-density sub-Neptunes containing volatile elements1 to higher-density rocky planets with Earth-like2 or iron-rich3 (Mercury-like) compositions. Such astonishing diversity in observed small exoplanet compositions may be the product ofdifferent initial conditions of the planet-formation process or different evolutionary paths that altered the planetary properties after formation4. Planet evolution may be especially affected by either photoevaporative mass loss induced by high stellar X-ray and extreme ultraviolet (XUV) flux5 or giant impacts6. Although there is some evidence for the former7,8, there are no unambiguous findings so far about the occurrence of giant impacts in an exoplanet system. Here, we characterize the two innermost planets ofthe compact and near-resonant system Kepler-107 (ref. 9). We show that they have nearly identical radii (about 1.5-1.6Râ), but the outer planet Kepler-107 c is more than twice as dense (about 12.6 g cm-3) as the innermost Kepler-107 b (about 5.3 g cm-3). In consequence, Kepler-107 c must have a larger iron core fraction than Kepler-107 b. This imbalance cannot be explained by the stellar XUV irradiation, which would conversely make the more-irradiated and less-massive planet Kepler-107 b denser than Kepler-107 c. Instead, the dissimilar densities are consistent with a giant impact event on Kepler-107 c that would have stripped off part of its silicate mantle. This hypothesis is supported by theoretical predictions from collisional mantle stripping10, which match the mass and radius of Kepler-107 c