Sylvie Vauclair

CoRoT measures solar-like oscillations and granulation in stars hotter than the Sun.

Oscillations of the Sun have been used to understand its interior structure. The extension of similar studies to more distant stars has raised many difficulties despite the strong efforts of the international community over the past decades. The CoRoT (Convection Rotation and Planetary Transits) satellite, launched in December 2006, has now measured oscillations and the stellar granulation signature in three main sequence stars that are noticeably hotter than the sun. The oscillation amplitudes are about 1.5 times as large as those in the Sun; the stellar granulation is up to three times as high. The stellar amplitudes are about 25% below the theoretic values, providing a measurement of the nonadiabaticity of the process ruling the oscillations in the outer layers of the stars.

The Seismic Structure of the Sun

Global Oscillation Network Group data reveal that the internal structure of the sun can be well represented by a calibrated standard model. However, immediately beneath the convection zone and at the edge of the energy-generating core, the sound-speed variation is somewhat smoother in the sun than it is in the model. This could be a consequence of chemical inhomogeneity that is too severe in the model, perhaps owing to inaccurate modeling of gravitational settling or to neglected macroscopic motion that may be present in the sun. Accurate knowledge of the suns structure enables inferences to be made about the physics that controls the sun; for example, through the opacity, the equation of state, or wave motion. Those inferences can then be used elsewhere in astrophysics.

METALLIC FINGERS AND METALLICITY EXCESS IN EXOPLANETS' HOST STARS: THE ACCRETION HYPOTHESIS REVISITED

While the metallicity excess observed in the central stars of planetary systems is confirmed by all recent observations, the reason for this excess is still a subject of debate: is it primordial, or the result of accretion, or both? The basic argument against an accretion origin is related to the mass of the outer convective zones, which varies by more than 1 order of magnitude among the considered stars, while the observed overabundances of metals are similar. We show here that in previous discussions a fundamental process was forgotten: thermohaline convection induced by the inverse μ-gradient. Metallic fingers might be created that dilute the accreted matter inside the star. Introducing this effect may reconcile the overabundances expected in cases of accretion with the observations of stars of different masses.

Diffusion in main-sequence stars: Radiation forces, time scales, anomalies

The abundance anomalies generated by diffusion in the envelope of main-sequence stars are studied. It is shown that in low-mass stars (Mapproximately-less-than1.2M/sub sun/) diffusion leads to underabundances while in more massive stars (Mapproximately-greater-than1.3M/sub sun/) diffusion leads to overabundances of at least some elements. In general the overabundance and underabundance factors generated (up to 10/sup 7/) are larger than the observed anomalies in stars of the main sequence (rarely up to 10/sup 6/). It is established that diffusion can lead to the largest anomalies observed. For particular elements (Sr, Eu,...), it is shown where more accurate calculations are needed. Approximate formulae are developed for radiative accelerations. They allow the reader to carry out calculations for cases of special interest to him and also to evaluate the uncertainty of the calculations. (AIP)

Asteroseismology of the planet-hosting star mu Arae. I. The acoustic spectrum

We present HARPS spectroscopy of μ Arae (HD 160691) performed for studying the origin of the metallicity excess in this planet-hosting stars. The asteroseismologic campaign led to the previously reported discovery of a 14 earth mass planetary companion (Santos et al. 2004b, A&A, 426, L19). The present analysis reinforces this interpretation by excluding other possible processes for explaining the observed Doppler variation and leads to the identification of up to 43 p-mode oscillations with l = 0-3, frequencies in the range 1.3-2.5 mHz and amplitudes in the range 10-40 cm s -1 .

Accurate p-mode measurements of the G0V metal-rich CoRoT target HD 52265

Context. The star HD 52265 is a G0V metal-rich exoplanet-host star observed in the seismology field of the CoRoT space telescope from November 2008 to March 2009. The satellite collected 117 days of high-precision photometric data on this star, showing that it presents solar-like oscillations. HD 52265 was also observed in spectroscopy with the Narval spectrograph at the same epoch. Aims. We characterise HD 52265 using both spectroscopic and seismic data. Methods. The fundamental stellar parameters of HD 52265 were derived with the semi-automatic software VWA, and the projected rotational velocity was estimated by fitting synthetic profiles to isolated lines in the observed spectrum. The parameters of the observed p modes were determined with a maximum-likelihood estimation. We performed a global fit of the oscillation spectrum, over about ten radial orders, for degrees l = 0 to 2. We also derived the properties of the granulation, and analysed a signature of the rotation induced by the photospheric magnetic activity. Results. Precise determinations of fundamental parameters have been obtained: Teff = 6100 ± 60 K, log g = 4.35 ± 0.09, [M/H] = 0.19 ± 0.05, as well as v sini = 3.6 +0.3 −1.0 km s −1 . We have measured a mean rotation period P rot = 12.3 ± 0.15 days, and find a signature of differential rotation. The frequencies of 31 modes are reported in the range 1500–2550 μHz. The large separation exhibits a clear modulation around the mean value Δν = 98.3 ± 0.1 μHz. Mode widths vary with frequency along an S-shape with a clear local maximum around 1800 μHz. We deduce lifetimes ranging between 0.5 and 3 days for these modes. Finally, we find a maximal bolometric amplitude of about 3.96 ± 0.24 ppm for radial modes.

The radius and mass of the close solar twin 18 Scorpii derived from asteroseismology and interferometry

The growing interest in solar twins is motivated by the possibility of comparing them directly to the Sun. To carry on this kind of analysis, we need to know their physical characteristics with precision. Our first objective is to use asteroseismology and interferometry on the brightest of them: 18 Sco. We observed the star during 12 nights with HARPS for seismology and used the PAVO beam-combiner at CHARA for interferometry. An average large frequency separation 134.4 ± 0.3 μHz and angular and linear radiuses

Element segregation in low metallicity stars and the primordial lithium abundance

Observational constraints on the primordial lithium abundance are important for the evaluation of the baryonic density of the universe. Its precise determination, however, suffers from uncertainties arising from the possible depletion of this element inside stars. Here we present and discuss new results for the lithium abundances in Population II stars obtained with the most recent stellar models including the best available physics. We show that it is possible to account for the general behavior of lithium observed in Population II stars without any free parameters. Macroscopic motions inside the stars are needed, but it is not necessary to specify their exact nature in order to interpret the observational data. This fact allows us to derive a parameter-free value for the primordial lithium abundance: log Lip = 2.35 ± 0.10 in the log H = 12 scale.

INFLUENCE OF THERMOHALINE CONVECTION ON DIFFUSION-INDUCED IRON ACCUMULATION IN A STARS

Atomic diffusion may lead to heavy-element accumulation inside stars in certain specific layers. Iron accumulation in the Z-bump opacity region has been invoked by several authors to quantitatively account for abundance anomalies observed in some stars, or to account for stellar oscillations through the induced κ-mechanism. These authors, however, never took into account the fact that such an accumulation creates an inverse μ-gradient, unstable for thermohaline convection. Here, we present results for A-F stars, where abundance variations are computed with and without this process. We show that iron accumulation is still present when thermohaline convection is taken into account, but much reduced compared to when this physical process is neglected. The consequences of thermohaline convection for A-type stars as well as for other types of stars are presented.

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 , which implies that it is a planet, not a brown dwarf.