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

The surface magnetic field configuration at the Ap star 78 Vir is reconstructed in the framework of the magnetic charge description (MCD) method from the analysis of Stokes IQUV spectra obtained using the MuSiCoS spectropolarimeter at Pic du Midi Observatory. Only the magnetically-sensitive Fe ii lines were employed in the analysis, supposing that iron is evenly distributed over the surface of 78 Vir. A stellar atmosphere model with Teff = 9250 K, log(g) = 4.5 provides the best fit to the majority of lines analysed at the level of χ2 = 1.68÷6.30. We show that the global magnetic field structure of 78 Vir can be well-described by a slightly decentered magnetic dipole. The derived inclinations of the stellar rotational axis to the line of sight i = 24° as well as to the magnetic dipole axis β = 125° agree reasonably well with previous estimations by other authors, whereas the sky-projected position angle of the stellar rotation axis Ω ~ 110°, increasing clockwise toward the north celestial pole, is determined here for the first time.

A new solution is emerging for ground-based asteroseismology with the potential opening of Dome C in Antarctica as an astronomical site. At a latitude of 75° South, many sources becomes observable without interruption for several weeks, as a space instrument is doing. However, bringing a high resolution grating spectrometer in such a site would be a heavy operation. Hence, a study has been conducted for a different type of velocity seismometer, based on an interferometer (Mosser et al. 2003), leading to the concept of a compact, monolithic instrument, without moving parts, called SIAMOIS. A single instrument of this type behind a 2-m class telescope could reach the required sensitivity to observe solar-type stars of magnitude <5, with a velocity precision of few cm s-1 after 5 days. The implementation of a robotic 80-cm telescope (IRAIT) as a first equipment could make possible the preliminary tests of this new seismometer.

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 obtained for 31 field Am stars with good Hipparcos parallaxes, as well as for 36 normal A and F stars. Radial and projected rotational velocities were determined as well. We examine the Li abundance as a function of the stellar parameters: for normal stars, it is clearly bimodal for Teff < 7500 K, while Am-Fm stars are all somewhat Li-deficient in this range. The most Li-deficient stars – either Am or normal – tend to be at least slightly evolved, but the reverse is not true.

Red giant (RGB) stars in the field and in globular clusters present abundance anomalies that can not be explained by standard stellar evolution models. Some of these peculiarities, in particular those concerning lithium, carbon and nitrogen for stars more luminous than the bump, attest the presence of extra-mixing processes at play inside the stars. Although their nature remains unclear, rotation has often been invoked as a possible source for mixing inside RGB stars (Sweigart & Mengel 1979; Charbonnel 1995; Denissenkov & Tout 2000). Much work has been done during the last decade on the description of rotation-induced mixing and on the transport of angular momentum and chemicals by meridional circulation and shear turbulence (Zahn 1992, etc.). Within this framework, we present the first fully consistent computations of rotating low mass and low metallicity stars from the Zero Age Main Sequence (ZAMS) to the upper RGB. When self-consistent evolution of the internal distribution of angular momentum is taken into account, it is found that meridional circulation and shear instability by themselves are not able to produce the required amount of mixing to account for the observed abundance patterns.

Main theoretical formulae and some computational results of evolutionary segregation of Hg isotopes due to light-induced drift in the atmospheres of chemically peculiar stars have been represented.

The spectra of a number of reportedly sharp-line B, A and F stars have been observed in a 60-Å window with a resolving power of 120 000. As a first step in analysis of these data, the spectra have been synthesized to see how well simple models fit. The information obtained from both successful modelling and from discrepant fits is discussed.

This poster describes the first results of a study of magnetic Ap stars in nearby northern open clusters. Using the MuSiCoS spectropolarimeter on the 2 m Bernard Lyot telescope at Pic du Midi observatory, high-resolution circular polarisation spectra have been obtained for 9 Ap stars proposed as members of 4 open clusters (M 39, IC 4756, the Pleiades and the Hyades). The aim of this study is to explore the evolution of chemical peculiarities and magnetic fields of Ap 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 the atmospheres of the extremely metal poor stars varies when the star evolves. This could be explained by a mixing with deep layers but such a mixing is not predicted by the theory.

Diffusion in cool B stars of the main sequence has been shown to strongly affect opacities and convection in cool B stars of the main sequence. We show here that diffusion in B stars maintains or enhances the excitation of pulsations in these stars. This result conflicts with observations as cool B stars that show evidence of diffusion, the HgMn stars, are stable to the current detection level. We discuss possible implications of this discrepancy for the models.