In the last two decades, considerable efforts have been devoted to the study of white dwarfs pulsators. Owing to ground-based multi-sites observational campaigns, and more recently to the long time-based high-precision photometric observations with the Kepler satellite, a large number of pulsating white dwarfs have been studied through the asteroseismology method. I emphasize here the accuracy that this method achieves on the determination of the total mass, one of the fundamental stellar parameter that can be derived from asteroseismology, among others. Then I discuss the issue of the age determination, emphasizing the contribution of the neutrinos cooling and the importance of the outer layer mass fraction. I show that some observations throw some doubts on the assumption that the cooling is the only cause of the observed frequency variations. Finally I summarize the results demonstrating that the entire stellar angular momentum is lost during the evolutionary phases prior to the white dwarf stage. While the asteroseismology of white dwarf stars has given a rich harvest of results on their internal structure and evolution, there are still some challenges to face. Determining accurate mass of the outer layers, specifically for the DA white dwarfs, is one of them. An increasing number of white dwarf pulsators are now known to exhibit frequency and amplitude temporal variations. The physical mechanism producing such variabilities has to be identified.

Heavy elements are observed in the atmospheres of many DA and DB white dwarfs, and their presence is attributed to the accretion of matter coming from debris disks. Several authors have deduced accretion rates from the observed abundances, taking into account the mixing induced by the convective zones and the gravitational settling. The obtained values are different for DA and DB white dwarfs. Here we show that an important process was forgotten in all these computations: thermohaline mixing, induced by the inverse μ-gradient built during the accretion process. Taking this mixing into account leads to an increase of the derived accretion rates, specially for DA white dwarfs, and modifies the conclusions.

The pre-white dwarf pulsators of PG 1159 type, or GW Virginis variable stars, are in a phase of rapid evolution towards the white dwarf cooling sequence. The rate of change of their nonradial g-mode frequencies can be measured on a reasonably short time scale. From a theoretical point of view, it was expected that one could derive the rate of cooling of the stellar core from such measurements. At the cool end of the GW Virginis instability strip, it is predicted that the neutrinos flux dominates the cooling. PG 0122+200 which defines the red edge of the instability strip is in principle a good candidate to check this prediction. It has been followed-up through multisite photometric campaigns for about fifteen years. We report here the first determination of the rate of change of its 7 largest amplitude frequencies. We find that the amplitudes of the frequency variations are one to two orders of magnitude larger than predicted by theoretical models based on the assumption that these variations are uniquely caused by cooling. The time scales of the variations are much shorter than the ones expected from a neutrino dominated core cooling. These results point to the existence of other mechanisms responsible for the frequency variability. We discuss the role of nonlinearities as one possible mechanism.

Four probable Ap members of the open cluster NGC 6475, were investigated through detailed spectrum synthesis. Cluster membership was confirmed for all four stars, however chemical abundance analysis shows that only three are truly Ap. Derived $v \sin i$  values and chemical abundances for 20 elements ranging form C to Eu are presented.

We present our current knowledge of the solar chemical composition based on the recent significant downward revision of the solar photospheric abundances of the most abundant metals very recently reviewed in detail by Asplund et al.  (2005a). These new solar abundances result from the use of a 3D hydrodynamical model of the solar atmosphere instead of the classical 1D hydrostatic models,accounting for departures from LTE, and improved atomic and molecular data. With these abundances, the new solar metallicity, Z, and Z/X, decrease to Z = 0.012 and Z/X = 0.0165 respectively, almost a factor of 2 lower than earlier widely used values. While resolving a number of longstanding problems, the new 3D-based solar photospheric composition also poses serious challenges for the standard solar model.

Main theoretical formulae and some computational results ofevolutionary segregation of Hg isotopes due to light-induced driftin the atmospheres of chemically peculiar stars have beenrepresented.


Time-resolved spectroscopic observations of non-radial oscillations in magnetic Apstars reveal unexpected and complex pulsation behaviour in spectral linesof individual chemical elements. This diverse rapid spectroscopic variability isclosely related to the presence of strong photospheric vertical abundance gradientsbuilt up by chemical diffusion. Consideration of chemical stratification and simultaneous interpretation of the pulsationalvariability of different chemical species have made it possible a tomographicmapping of the radial dependence of pulsationproperties of the atmospheres of roAp stars. Combination of the informationobtained using pulsation tomography with horizontal mapping using Doppler imagingopens a unique possibility to construct unprecedented three-dimensional maps of pulsationwaves, chemical spots and magnetic field in the atmospheres of peculiar A stars.


We present briefly some results of the ESO Large Program “First Stars"where it is shown that the abundance ratios of some light elements in theatmospheres of the extremely metal poor stars varies when the starevolves. This could be explained by a mixing with deep layers butsuch a mixing is not predicted by the theory.

A review of some of the most important studies of the past several decades pertaining to the study of atomic diffusion in stellar atmospheres is presented. A brief description of various studies of radiative accelerations and diffusion calculations in stellar atmospheres is given. Several physical phenomena such as ambipolar diffusion, light-induced drift and the effect of magnetic fields on diffusion are discussed. Results from recent self-consistent model atmospheres including elemental stratification are also reviewed. Particularities of radiative acceleration calculations in stellar atmospheres as well as difficulties and future challenges related to diffusion calculations in stellar atmospheres are also presented.

In this paper I give an overview of the numerous ways inwhich rotation and magnetic fields can interact under stellarinterior conditions.I first provide “tutorial”examples of how magnetic fields can (1) alter existing stellarinternal flows, (2) generate internal flows, and of how rotationcan (3) amplify or (4) destroy magnetic fields. The upshot of allthis is that treating rotation or magnetic fields in isolationof one another, as intermediate steps towards the “full picture”,may yield a situation that can only be applied meaningfully undervery limited and specific astrophysical circumstances, if any.

o Leo was a binary of two Am dwarfs. One is now evolving and is nearly as coolas the Sun, but still bears spectrum characteristics of an Am dwarf. How candiffusion theory explain such an enigma?

We briefly report on an ongoing numerical project that aimsat simulating, with the ASH code, the intricate magnetohydrodynamic processespresent in the inner region of A-type stars. We mainly focus our attention, onthe dynamics of the convective core, its associated differential rotation and meridionalciculation and the dynamo action than can occur in suchtubulent MHD system. We indeed find that magnetic fields with amplitude greater than 10 kG are likely to be present in the core of A-type stars.

We report the discovery, using FORS1 at the ESO-VLT and ESPaDOnS at the Canada-France-Hawaii Telescope, of magnetic fields and chemical peculiarities in the Herbig Ae/Be stars HD 72106 and HD 101412. These stars may well represent pre-main sequence progenitors of the magnetic Ap/Bp stars. At the same time, we fail to confirm claims by Hubrig et al. (2004) of the presence of magnetic fields in the Herbig Ae star HD 139614.

Observational data on the early galactic abundances of the light elements lithium, beryllium and boron are combined with data related to the reionization of the intergalactic medium (IGM) in a search of processes happening in the early universe. Early massive metal-free stars (Pop III), largely held responsible for the reionization of the IGM, are proposed to have been also the injectors of the lithium-6 plateau, through their winds. In this sense the evolution of the 6Li/9Be ratio appears to be a key parameter for the history of nucleosynthesis as a monitor of the early formation of metals and their subsequent injection in high energy particles. 


We have used the CFHT and Gemini South telescopes to obtain high-resolution, high signal-to-noise optical and near-infrared spectra of several lines of neutral helium in a number of helium-peculiar, HgMn and related stars. Since the helium lines observed have 3He isotope shifts of up to 1 Å and sample a large range of continuous opacities in the program stars, these spectra are being used to search for the presence of 3He in their photospheres and, where 3He is present, examine the possible stratification of both isotopes of helium in their atmospheres. Analysis of a number of previously identified 3He stars provides strong evidence for different vertical stratification profiles of 3He and 4He in these stars. 4He is generally enhanced deep in the photospheres but sharply depleted high in the atmospheres while 3He appears to be confined to a narrow layer above the 4He.

Wide Binary Millisecond Pulsars are composed of a rapidly spinning neutron star and a low-mass white dwarf. An important key to understanding the spin-down evolution of the pulsar is the determination of its age. Under the assumption that the pulsar and white dwarf formed coevally, the age of the pulsar can be inferred by comparing the observed temperature of the cooling white dwarf with evolutionary calculations. The rate of cooling is affected by the composition profile and thickness of the hydrogen-rich envelope. The latter property is largely governed by the evolution of the white-dwarf progenitor during the mass-loss phase of the evolution but is also affected to some degree by stellar winds and chemical diffusion. Based on an extensive analysis of parameter space, we find that self-induced stellar winds are not likely to have a significant effect on the cooling but that diffusion in the envelope can shorten the cooling times in some systems by a significant factor. This effect generally leads to better agreement between cooling and spin-down times.


Preliminary abundances of lithium and a few other elements have been obtainedfor 31 field Am stars with good Hipparcos parallaxes, as well as for36 normal A and F stars. Radial and projected rotational velocities were determined as well. We examine the Li abundance as a function ofthe stellar parameters: for normal stars, it is clearly bimodal forTeff < 7500 K, while Am-Fm stars are all somewhat Li-deficientin this range. The most Li-deficient stars – either Am or normal – tendto be at least slightly evolved, but the reverse is not true.


The solar interior has been scrutinized by two different and independent probes during the last twenty years with important revisions of the solar model, including a recent heavy element abundancerevision. Today, we get a quantitatively coherent picture (even incomplete) of the solar (stellar) radiative zones. In this review, we recallthe clues for solar gravitational settling definitively established by the seismic determination of the photospheric helium content. We comment also on the need for mixing in the transition region between radiation and convectionin the case of the Sun and of population II stars. We finally list the open questions and the importance to continue more precise investigations of the solar (stellar) radiative zone in detecting gravity modes with the project DynaMICS.

Abundances anomalies and helioseismology provideconstraints on the turbulent transport of chemical species in stellar radiativezone. We use these constraints to derive upper bounds on thevelocity and lengthscale of the vertical turbulent motions.We also present numerical simulations showingthat a strong anisotropy between horizontal and vertical motionscould help understand the observed transport.