Evolution of the theoretical power spectrum of solar-like oscillations along the ascending phase on the red giant branch.

CoRoT and Kepler observations of red giants reveal rich spectra of non-radial solar-like oscillations allowing to probe their internal structure. An important question comes from the observation of mixed modes : When during the star’s ascension on the RGB are mixed-modes more likely to be detectable ? We follow the evolution of a star on the RGB and investigate the effect of its ascension on theoretical power spectrum. Equilibrium models (computed with the code ATON) represent four different stages of a star on the RGB. The mass of the star (1.5M") is in the typical mass range of stars observed by CoRoT and Kepler. We used a non-radial non-adiabatic code to compute the theoretical solar-like oscillations of these models. An important output of these calculations is the theoretical lifetimes of the modes. Then we computed the oscillation amplitudes through a stochastic excitation model. These computations allow us to draw theoretical power spectrum and discuss the possibility to observe mixed-modes at different evolutionary stages on the RGB. We found that structure modifications in a star ascending the RGB have an important impact on theoretical power spectrum of solar-like oscillations. Efficiencies of trapping and lifetimes of mixed modes are indeed strongly affected by this evolution

Comparisons for Esta-Task3: Cles and Cesam

We present the results of comparing three different implementations of the microscopic diffusion process in the stellar evolution codes CESAM and CLES. For each of these implementations we computed models of 1.0, 1.2 and 1.3 M$_{\odot}$. We analyse the differences in their internal structure at three selected evolutionary stages, as well as the variations of helium abundance and depth of the stellar convective envelope. The origin of these differences and their effects on the seismic properties of the models are also considered.Comment: 10 pages, 8 figures, Joint HELAS and CoRoT/ESTA Workshop on Solar/Stellar Models and Seismic Analysis Tools, Novembre, Porto 2007 To be published in EAS Publications Serie

Hybrid gamma Doradus/delta Scuti Stars: Comparison Between Observations and Theory

Gamma Doradus are F-type stars pulsating with high order g-modes. Their instability strip (IS) overlaps the red edge of the delta Scuti one. This observation has led to search for objects in this region of the HR diagram showing p and g-modes simultaneously. The existence of such hybrid pulsators has recently been confirmed (Handler 2009) and the number of candidates is increasing (Matthews 2007). From a theoretical point of view, non-adiabatic computations including a time-dependent treatment of convection (TDC) predict the existence of gamma Dor/delta Sct hybrid pulsators (Dupret et al. 2004; Grigahcene et al. 2006). Our aim is to confront the properties of the observed hybrid candidates with the theoretical predictions from non-adiabatic computations of non-radial pulsations including the convection-pulsation interaction.Comment: 3 pages, 3 figures, Poster at "Stellar Pulsation: challenges for theory and observation", Santa Fe, June 200

Microscopic Diffusion in Stellar Evolution Codes: First Comparison results of ESTA-Task~3

We present recent work undertaken by the Evolution and Seismic Tools Activity (ESTA) team of the CoRoT Seismology Working Group. The new ESTA-Task 3 aims at testing, comparing and optimising stellar evolution codes which include microscopic diffusion of the chemical elements resulting from pressure, temperature and concentration gradients. The results already obtained are globally satisfactory, but some differences between the different numerical tools appear that require further investigations.Comment: to appear in "Stellar Evolution and Seismic Tools for Asteroseismology", (Eds) C. W. Straka, Y. Lebreton and M. J. P. F. G. Monteiro, EAS Publications Series, 200

Microscopic Diffusion in Stellar Plasmas

We review the basic equations describing the microscopic diffusion of elements in stars. We describe the two formalisms commonly used to describe a stellar plasma, i.e., the Chapman-Enskog method and the Burgers equations. We point out the underlying assumptions about the physical state of the plasma itself. We briefly describe the different approximations which are often used to solve these equations, and discuss their validity. One of the major problems lies in the calculation of the collision integrals, and we discuss the different approaches to solving these integrals, emphasizing on their domain of validity

Input from opacity data in computation of pulsation instability

Several types of variable stars are found along the HR diagram whose pulsations are driven by the κ-mechanism. Given their nature, the precise (Teff - L) domain where these pulsators are located is highly dependent on the value of opacity and on its variation inside the star. We analyze the sensitivity of opacity driven pulsators of spectral-type A and B (δ Scuti, β Cephei and SPB stars) to the opacity tables (OP/OPAL) and to the chemical composition of the stellar matter. We also briefly discuss the effect of opacity on pulsators whose oscillations are not driven by the κ-mechanism, such as γ Doradus and solar-like stars

Evolution and seismology of alpha Centauri.

Solar-like oscillations detected in both components of the binary system alpha Centauri provide strong constraints on the fundamental parameters of the stellar system. We model alpha Centauri by means of a Levenberg-Marquardt minimization algorithm including seismic and classical constraints. Computations, that were perfomed decreasing significanly the weight of alpha Cen B seismic data in the calibration procedure, predict small separations in good agreement with new observations of solar-like oscillations in alpha Cen B by Bedding (these proceedings)

New light on the old problem of lithium pre-main sequence depletion: models with 2D radiative-hydrodynamical convection

peer reviewedThe T[SUB]eff[/SUB] location of pre-main sequence (PMS) evolutionary tracks depends on the treatment of overadiabaticity. We present here the PMS evolutionary tracks computed by using the mixing length theory (MLT) of convection in which the α[SUB]MLT[/SUB] = l/H[SUB]p[/SUB] parameter calibration is based on 2D hydrodynamical models by Ludwig et al. These MLT-α[SUP]2D[/SUP] stellar models and tracks are very similar to those computed with non-grey ATLAS9 atmospheric boundary conditions and full spectrum of turbulence (FST) convection model both in the atmosphere and in the interior. The comparison of the new tracks with the location on the Hertzsprung-Russell (HR) diagram of PMS binaries is not completely satisfactory, as some binary components are located at too low T[SUB]eff[/SUB]. Besides, the PMS lithium depletion in the MLT-α[SUP]2D[/SUP] tracks is still much larger than that expected from the observations of lithium in young open clusters. This result is similar to that of FST models. Thus, in spite of the fact that 2D radiative-hydrodynamical models should provide a better convection description than any local model, their introduction is not sufficient to reconcile theory and observations. Lithium depletion in young clusters points towards a convection efficiency which, in PMS, should be smaller than in the MS. The PMS lithium depletion decreases significantly in FST models if we reduce the solar metallicity down to the value suggested by Asplund et al., but the corresponding solar model does not reproduce the depth of the convective zone as determined by helioseismology

How asteroseismology can help to precisely constrain properties of planet-host stars

Nowadays more than 500 exoplanets have been discovered, mainly studied by radial velocity and transit measurements. Precise knowledge on their characteristics is crucial to develop theories of planetary formation and evolution. In that aim, not only star and planet(s) masses but also the evolutionary stage of systems are needed. From radial velocity measurements one has to assume the inclination and the stellar mass of the system to disentangle the mass of the planet. When transit is observable, one can measure the ratio of planetary and stellar radii. Finally, the degree of evolution of the system is determined by the one of the star. Thus the host star must be well known to obtain a full set of system properties. However, determination of stellar parameters such as the mass, radius and its evolution from classical observables (Teff, log g, [Fe/H]) suffers of large uncertainties. This is particularly true for dwarf stars on the Main Sequence. Fortunately we can obtain better constrains with the help of asteroseismology. That latter approach probes the stellar properties through observation of oscillations present in stars. With the launches of high-precision photometry space missions, CoRoT and Kepler, we are now able to detect oscillations in a huge number of stars. In particular Kepler photometry, primarily intended to detect transits of planet, can give accurate stellar parameters of planetary systems as it also affords to mak

Testing the forward approach in modelling β Cephei pulsators: setting the stage

peer reviewedThe information on stellar parameters and on the stellar interior we can get by studying pulsating stars depends crucially on the available observational constraints: both seismic constraints (precision and number of detected modes, identification, nature of the modes) and ``classical'' observations (photospheric abundances, effective temperature, luminosity, surface gravity). We consider the case of β Cephei pulsators and, with the aim of estimating quantitatively how the available observational constraints determine the type and precision of our inferences, we set the stage for Hare&Hound exercises. In this contribution we present preliminary results for one simple case, where we assume as ``observed'' frequencies a subset of frequencies of a model and then evaluate a seismic merit function on a dense and extensive grid of models of B-type stars. We also compare the behaviour of χ^2 surfaces obtained with and without mode identification