Tribute to Georges Michaud.


I salute Georges Michaud on the occasion of his 65th birthday for his seminal contributions to astrophysics.

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 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.

Radiative accelerations are crucial quantities to study element diffusion instars. They strongly depend on atomic properties of ions in a way more or lesssimilar to opacities or synthetic spectra. Computations of radiativeaccelerations however have also some specific requirements which make them verysensitive to the quality of atomic data. We shall present various aspects of theradiative accelerations computations and discuss, through examples, how they areso closely related to the knowledge of atomic data.

Atomic diffusion has been suggested to play a significant role in a large variety of CP stars to reproduce observations. In this paper I will briefly present the physics included in the self-consistent models that include atomic diffusion and radiative accelerations. I will discuss their effect on the evolution and on the internal structure of Pop I and Pop II stars, as well as the effect on the predicted surface abundances. I will also present the effect on cluster age determination.

The accuracy with which it is possible to calculate theoretically the positions ofenergy levels and the low density (in frequency) of the grid on which opacity sampling is calculated bothlimit the accuracy with which the effect ofblends can be determined. For species such as Fe for which a large number of transitionsare involved in calculations, this leads to a small effect since, statistically, cancellations occur. However the effect can be large for an element such as Li with only a few transitions contributing to its radiative acceleration, grad(Li), at any given stellar radius. In particular, the Fe lines are shown to modify grad(Li) in and below the Fe convection zone but the modification depends on Fe level positions.This is analyzed quantitatively here using the OPAL data base. The grad(Li) are found to be uncertain by a factor of 2 in Pop I stars but of 1.03 in Pop II stars.

Whereas transition probabilities needed for the computation of radiativeforces due to bound-bound absorptions are generally availablephotoionisation and bremsstrahlung require going one step further thanthe dipolar approximation in treating the radiation matter interaction.I will try to give an overview of the current status on both theoreticaland practical points.
Autoionization resonances deserve further investigation: though they candominate photoionization cross sections very little is known on how theymight impact radiative forces. I will show through recent literaturethat specific atomic calculations have to be performed.


I present a discussion of the atomic input data required for state-of-the-art radiative acceleration calculations in magnetic stellar atmospheres, and of the numerical methods appropriate to this problem.

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.

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.

We have begun a general high resolution spectroscopic survey of the entire class of roAp stars to study systematics of the pulsations for the class, and to make detailed studies of individual stars. We have high spectral resolution (R ~ 110 000), high time resolution VLT UVES data for 16 of the 35 known members of the class, two of which were discovered during this survey. We show here some examples of studies of pulsation amplitude and phase as a function of line depth in Hα and in the 6145 Å line of Nd iii. The two of these lines together give a nearly continuous map of pulsation amplitude and phase as a function of optical depth from about -5 ≤ log τ5000 ≤ -2, or even higher. Our results for these lines and those of Fe support studies showing settling of Fe and levitation of Nd. The pulsation behaviour provides an independent constraint on atmospheric structure and abundance distribution in roAp stars, hence is important in tests of radiative diffusion calculations.


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 show that in rapidly oscillating Ap stars with resolvedmagnetically split lines, different pulsation behaviours can beexpected for the π and σ line components. We outline theprospects that this opens for diagnosis of the pulsation ofthese stars. We present a first application to the strongly magneticstar HD 166473. While the predicted effect could not be observed(possibly due to the low pulsation amplitude), marginal detection ofvariation of the mean magnetic field modulus with the pulsationfrequency is achieved.

Contrary to what is still often admitted when studying the chemical evolution of galaxies, the element abundances observed in stellar atmospheres are often not representative of the initial composition of the protostellar nebula. Element abundances may be altered during stellar evolution by atomic diffusion and/or accretion, if not by nuclear reactions. Atomic diffusion occurs in any type of stars, with time scales extremely different according to the macroscopic motions with which they have to compete. Accretion may occur in solar type stars during planetary formation, which seems to be a common process. Following the success of helioseismology, asteroseismology is now becoming a fundamental tool for penetrating the secrets of the internal structure of stars. In particular it may be used to derive their internal abundances compared to the observed ones. In the present paper, I focus on main-sequence stars. I first recall the situations where abundance stratification can act for exciting or stabilizing stellar oscillations, second I describe how seismic observations can be used to derive abundance gradients inside stars.

A brief description of the various types of variable stars occuring in the advanced stages of stellar evolution is given.This includes the Planetary Nebulae Nuclei variables (PNNV), the PG 1159 variables, the sdB pulsators and the DB and DA white dwarf pulsators. In this review, the emphasis is put on the importance of diffusion for eitherthe excitation mechanism or the properties of the oscillations.

The ratio of DA to DB white dwarfs is temperature sensitive. No DBs are found between 45 000 K and 30 000 K, and this exclusion zone is known as the “DB gap”. A working hypothesis to explain the DB gap is proposed. Since the temperatures of the upper and the lower bounds of the DB gap coincide with the effective temperatures where the He II/III and He I/II convection zones show up respectively, convective mixing is suspected to be the cause of the appearance of DBs outside the DB gap. Inversely, chemical separation due to gravitational settling in the convectively stable atmosphere is suspected to be the cause of the presence of the DB gap. According to this scenario, the white dwarfs which will turn back to DBs after the DB gap phase are expected to have a superadiabatic layer which is nonetheless convectively stable due to chemical separation, when they are located near the lower bound of the DB gap. Such white dwarfs are expected to be pulsationally unstable.


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.

Due to radiative acceleration and gravitational settling, stars would display vast differences in their surface composition, if these were not smoothed out, at least partly, by some competing processes. Possible mechanisms have been suggested and examined by Georges Michaud and his collaborators: mass loss, turbulent mixing and meridional circulation. They will be discussed here in the light of more recent developments.