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?

The surface magnetic field configuration at the Ap star 78 Viris reconstructed in the framework of the magnetic charge description (MCD)method from the analysis of Stokes IQUV spectra obtained using the MuSiCoSspectropolarimeter at Pic du Midi Observatory. Only the magnetically-sensitiveFe ii lines were employed in the analysis, supposing that iron is evenlydistributed over the surface of 78 Vir. A stellar atmosphere model withTeff = 9250 K, log(g) = 4.5 provides the best fit to the majority oflines analysed at the level of χ2 = 1.68÷6.30. We show that theglobal magnetic field structure of 78 Vir can be well-described by a slightlydecentered magnetic dipole. The derived inclinations of the stellar rotationalaxis to the line of sight i = 24° as well as to the magnetic dipole axisβ = 125° agree reasonably well with previous estimations by otherauthors, whereas the sky-projected position angle of the stellar rotation axisΩ ~ 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 thepotential opening of Dome C in Antarctica as an astronomical site. At alatitude of 75° South, many sources becomes observable withoutinterruption for several weeks, as a space instrument is doing. However,bringing a high resolution grating spectrometer in such a site would be aheavy operation. Hence, a study has been conducted for a different type ofvelocity seismometer, based on an interferometer (Mosser et al. 2003), leading to the concept of a compact, monolithicinstrument, without moving parts, called SIAMOIS. A singleinstrument of this type behind a 2-m class telescope could reach therequired sensitivity to observe solar-type stars of magnitude <5, with avelocity precision of few cm s-1 after 5 days. The implementation ofa robotic 80-cm telescope (IRAIT) as a first equipment could make possiblethe 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 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.

Red giant (RGB) stars in the field and in globular clusters presentabundance anomalies that can not be explained by standard stellar evolutionmodels. Some of these peculiarities, in particular those concerninglithium, carbon and nitrogen for stars more luminous than the bump, attestthe presence of extra-mixing processes at play inside the stars. Althoughtheir nature remains unclear, rotation has often been invoked as a possiblesource for mixing inside RGB stars (Sweigart & Mengel 1979;Charbonnel 1995; Denissenkov & Tout 2000). Much work hasbeen done during the last decade on the description of rotation-inducedmixing and on the transport of angular momentum and chemicals bymeridional circulation and shear turbulence (Zahn 1992, etc.). Within this framework, wepresent the first fully consistent computations of rotating low mass andlow metallicity stars from the Zero Age Main Sequence (ZAMS) to the upperRGB. When self-consistent evolution of the internal distribution of angularmomentum is taken into account, it is found that meridional circulation andshear 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 ofevolutionary segregation of Hg isotopes due to light-induced driftin the atmospheres of chemically peculiar stars have beenrepresented.

The spectra of a number of reportedly sharp-line B, A and F stars havebeen 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 beensynthesized to see how well simple models fit. The informationobtained from both successful modelling and from discrepant fits isdiscussed.

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 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.

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 ofpulsations in these stars. This result conflicts with observations as cool B starsthat show evidence of diffusion, the HgMn stars, are stable to the currentdetection level. We discuss possible implications of this discrepancy for the models.