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.

Recent systematic surveys providing massive datasets of white dwarf stars show that there is still a deficit of a factor of 2.5 in the ratio of white dwarfs showing helium absorption lines to those with hydrogen absorption lines in the temperature range 30 000 K < Teff < 45 000 K. Since all white dwarfs have to evolve through this temperature range along similar sequences in the color-magnitude diagram, this implies that most of the helium atmosphere white dwarfs evolve into hydrogen atmosphere hot DA stars in that temperature range and then back again to helium atmosphere white dwarfs. I describe possible scenarios for this chameleon-like disguises of white dwarfs with helium dominant atmospheres and a new kind of pulsating white dwarfs predicted to appear near the lower end of the temperature range 45 000–30 000 K from these scenarios. Recently searches have discovered a pulsating DA white dwarf, WD1017−138, in the predicted temperature range. This discovery opens the possibility of a direct test of the explanation for the deficit of helium atmosphere white dwarfs at these temperatures by asteroseismic probing of the atmospheric layers of the hot DAV stars.

We present an investigation of the dust formation process in the atmospheres of the oxygen-rich AGB stars RR Aql, S Ori, GX Mon and R Cnc using spatially and spectrally resolved mid-infrared interferometric observations. We successfully compared multi epoch observations to a radiative transfer model of the dust shells, where the central stellar intensity profile is described by a series of dust-free dynamic model atmospheres based on self-excited pulsation models. We show that the Al2O3 shells have inner radii between 1.9 and 2.2 stellar photospheric radii, and that the silicate shells have inner radii between 4.1 and 4.6 stellar photospheric radii. The best-fit photospheric angular diameters are consistent with independent estimates. The model dust temperatures at the inner radii of 1.9–2.2 stellar radii and 4.1–4.6 stellar radii are consistent with dust condensation temperatures of Al2O3 and silicates, respectively.

Stellar models including atomic diffusion processes (gravitational settling, radiative accelerations, thermal diffusion, in addition to the purely diffusive term) have now been evolved throughout most of stellar evolution. We review some of the major roles of diffusion processes in stellar evolution. The emphasis is on interior properties. Examples include Populations I and II stars. It is emphasized that competing advective processes such as mass loss or meridional circulation modify internal concentrations differently from turbulent diffusion even when they lead to the same surface abundances. Radiative accelerations play a major role in horizontal branch and sdB stars.

Horizontal-branch stars with effective temperatures larger than approximately 11 500 K show abundance anomalies as well as other peculiar observational properties believed to be due to atomic diffusion in their atmosphere. These stars possess low rotational velocities that makes it possible for atomic diffusion to come into play and are therefore of great interest with respect to diffusion theory. Observational anomalies of blue horizontal-branch stars found in globular clusters such as photometric jumps and gaps are reviewed. Recent detections of vertical stratification of elements are also discussed. These results are compared to predictions of atmospheric modeling while including vertical stratification of the elements. The atmospheric structure of these models is calculated self-consistently while taking into account vertical stratification of the elements.

The atmospheres of upper mainsequence chemically peculiar stars (ApBp stars) are known to be strongly affected by atomic diffusion. In recent years, both observations and models have shown that, as expected, the classical picture of abundance stratification is far too simple. A new generation of observational and modelling tools is appearing to take into account the complexity of the situation. In this talk, I will give an overview on recent results concerning the models for ApBp stars atmospheres. I will evaluate the two main approaches to estimate quantitatively the abundance stratifications: those using the so-called equilibrium hypothesis vs. those calculating time-dependent diffusion.

We present time-dependent, self-consistent diffusion calculations for Cr, Mn and Fe in a Teff = 12 000 K and logg = 4.0 stellar atmosphere. The temporal evolution of the chemical stratifications is followed for the zero field case and for magnetic fields of varying strength and direction relative to the stellar surface. Preliminary conclusions are preceded by a comparison of the present results to equilibrium solutions.

The atomic (microscopic) diffusion of individual elements in stellar interiors and atmospheres lead to their accumulation or depletion in specific layers. The selective radiative accelerations on individual elements, which work in opposition to the effects of the pressure and thermal gradients, have important consequences. When acting on elements like iron or nickel, which are important contributors to the opacity in some stellar layers, the resulting overabundance can lead to global macroscopic effects, like extra convective zones or wave excitation by kappa mechanism. The competition between atomic diffusion and independent hydrodynamical processes including turbulent (macroscopic) diffusion has been extensively studied in previous years using parametrized computations. An important macroscopic consequence of atomic diffusion was however ignored: the double-diffusive mixing, also referred to as fingering or thermohaline convection, directly induced by μ-gradient inversion created by local heavy element accumulation. Taking this process into account will modify the studies of the element abundance variations inside stars and in their atmospheres.

The standard model of stellar structure is unable to account for various observational facts, such as anomalies in the surface composition, and there is now a broad consensus that some extra mixing must occur in the radiation zones, in addition to the always present convective overshoot or penetration. The search for the causes of this extra mixing started in the late seventies, and it was quickly realized – in particular by Sylvie Vauclair and her co-workers – that some mild turbulence must be present to counteract the effect of gravitational settling and radiative levitation. What could be responsible for this turbulence? One suggestion was the internal gravity waves emitted at the boundary of convection zones, but it is still not established whether these waves will lead to true mixing. However they transport angular momentum, and therefore they generate differential rotation, which may be shear-unstable and thus lead to turbulence. Another way to transport angular momentum and produce an unstable rotation profile is through the large-scale circulation which is induced by the structural adjustments as the star evolves, or by the torques applied to it (due to stellar wind, accretion, tides). These processes participate in what is called the “rotational mixing”; their implementation in stellar evolution codes – again under Sylvie's impulse – has given birth to a new generation of stellar models, which agree much better with the observational constraints, although there is still room for improvement.

A simple mixing-length discussion of vertical diffusive transport of a scalar by small-scale turbulent convection is presented, likening it to the microscopic transport in a classical gas. If the scalar is passive, the transport is governed by the well known telegraph equation. Temperature, on the other hand, influences the dynamics of the small-scale motion by modifying the buoyancy that drives the turbulent eddies; it leads to a rather more complicated equation, which, for relatively rapid variation reduces to the same wave equation as for a passive scalar, with maximal propagation speed comparable with the rms vertical eddy velocity; but in the slow diffusive limit it reduces, for good reason, to the classical diffusion equation with a diffusivity enhanced by a factor 3/2 over that pertaining to a passive scalar.

One of the key issues of stellar evolution theory is the influence of the transport processes related to rotationally driven macroscopic motions on the internal structure and the evolution of stars. Turbulent mixing of chemical elements due to differential rotation in stellar radiative zones is currently taken into account in many stellar evolution codes through transport coefficients firstly derived by Zahn (1992, A&A, 265, 115). The purpose of our work is to constrain one of these coefficients, the radial diffusion coefficient driven by radial differential rotation through local direct numerical simulations of steady homogeneous stably stratified sheared turbulence, and to compare the results with phenomenological models. In particular, we have determined the dependence of the turbulent diffusion coefficient on thermal diffusivity and chemical stratification.

Internal gravity waves are excited in stellar interiors thanks to turbulent convective regions, to opacity bumps and to tides when there is a close stellar or planetary companion. Then, they propagate in stably stratified stellar radiation zones where they deposit/extract angular momentum by damping processes and corotation resonances. This modifies the rotational evolution as well as the internal mixing in stars. In this review, we will present the state of the art for modelling such transport mechanisms with a peculiar focus on the most recent advances taking into account the influence of rotation, shear, and magnetic fields on internal gravity waves. Next, we will discuss consequences for stars dynamical evolution and what can be learnt from stellar seismology.

Much progress has recently been made in understanding and quantifying vertical mixing induced by double-diffusive instabilities such as fingering convection (usually called thermohaline convection) and oscillatory double-diffusive convection (a process closely related to semiconvection). This was prompted in parts by advances in supercomputing, which allow us to run Direct Numerical Simulations of these processes at parameter values approaching those relevant in stellar interiors, and in parts by recent theoretical developments in oceanography where such instabilities also occur. In this paper I summarize these recent findings, and propose new mixing parametrizations for both processes that can easily be implemented in stellar evolution codes.

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.

Stellar modelling challenges the physicist to provide accurate data for a lot of processes in a wide range of conditions. This ends in the use of patchy results from microscopic models that aimed initially at peculiar physical domains. Very often various quantities such as thermodynamics quantities, opacities, diffusion coefficients or conductivities, are obtained from different sources. It is thus of importance to construct first principles models of general application. We focus this talk on warm dense matter, as encountered in low-mass or compact stars, and where pressure ionization has dramatic effects. We present equation of state computations from a model we have developed that treat quantum mechanically bound as well as delocalized electrons. It goes beyond the usual Average-Atom models in treating separately ionic charges, and extends from low to high densities. On the experimental side recent years have seen significant advances in the creation and the diagnostics of plasmas under these extreme conditions. A brief account will be given on what might be expected as far as quantities of astrophysical interest are concerned.

The direct measurements of nuclear reaction cross sections at astrophysical energies are usually difficult to be performed in terrestrial laboratories, mainly because of the Coulomb barrier. An alternative way of studying these processes is via the indirect methods among which the Trojan Horse (THM) represents a powerful tool for studying charged-particles induced reactions at astrophysically energies without the need of extrapolation procedures. The recent THM studies about the lithium and boron destroying reactions will be here shown and discussed.

Several years ago, M. Asplund and coauthors (2004) proposed a revision of the Solar composition. The use of this new prescription for Solar abundances in standard stellar models generated a strong disagreement between the predictions and the observations of Solar observables. Many claimed that the Standard Solar Model (SSM) was faulty, and more specifically the opacities used in such models. As a result, activities around the stellar opacities were boosted. New experiments (J. Bailey at Sandia on Z-Pinch, The OPAC consortium at LULI200) to measure directly absorbtion coefficients have been realized or are underway. Several theoretical groups (CEA-OPAS, Los Alamos Nat. Lab., CEA-SCORCG, The Opacity Project – The Iron Project (IPOPv2)) have started new sets of calculations using different approaches and codes. While the new results seem to confirm the good quality of the opacities used in SSM, it remains important to improve and complement the data currently available. We present recent results in the case of the photoionization cross sections for Ni XIV (Ni13+) from IPOPv2 and possible implications on stellar modelling.

We aim to improve the calculation of the Mg I line profiles in benchmark stars and to assess the impact of the new quantum mechanical calculations of inelastic collisions with hydrogen. The method is based on the non-Local Thermodynamic Equilibrium (NLTE) line formation using updated atomic data for a simple model atom of Mg I. Focus on the formation of the Mg I b triplet lines at 5167, 5172 and 5183 Å is presented for the Sun, Arcturus, HD 84937 (metal-poor dwarf) and HD 122563 (metal-poor giant). This study is preliminary and essentially tests the new atomic data. In this context, we show that NLTE effects are smaller for this triplet when quantum mechanical H collisions are included compared with the use of the semi-classical Drawin's formula.

Seismology of Sun and stars associated to precise solar neutrino detections reveals microscopic and macroscopic behaviors of radiative zones that were not anticipated by stellar modeling predictions. These results allow us to check the hypotheses of the equations used in stellar evolution for long. In this review, I list some of the problems which emerge from these confrontations both on the description of the Sun or solar-like stars and on the envelopes of intermediate-type stars, then I summarize the cases at least partly solved in using 3D MHD simulations, new opacity calculations and laboratory experiments. This review shows also how we hope to progress on the others inside our OPAC consortium or in using the coming detection of neutrino fluxes.

It is clear that in order to explain many observed stellar phenomenae, atomic diffusion must be included in stellar models. Observed surface abundances allow us to constrain other transport pro- cesses which compete with atomic diffusion. We will focus on the effect of the flow of matter induced by mass-loss in stellar models.