Based on 3D hydrodynamical model atmospheres computed with the CO5BOLD code and 3D non-LTE (NLTE) line formation calculations, we study the effect of the convection-induced line asymmetry on the derived 6Li abundance for a range in effective temperature, gravity, and metallicity covering the stars of the Asplund et al. (2006) sample. When the asymmetry effect is taken into account for this sample of stars, the resulting 6Li/7Li ratios are reduced by about 1.5% on average with respect to the isotopic ratios determined by Asplund et al. (2006). This purely theoretical correction diminishes the number of significant 6Li detections from 9 to 4 (2σ criterion), or from 5 to 2 (3σ criterion). In view of this result the existence of a 6Li plateau appears questionable. A careful reanalysis of individual objects by fitting the observed lithium 6707 Å doublet both with 3D NLTE and 1D LTE synthetic line profiles confirms that the inferred 6Li abundance is systematically lower when using 3D NLTE instead of 1D LTE line fitting. Nevertheless, halo stars with unquestionable 6Li detection do exist even if analyzed in 3D-NLTE, the most prominent example being HD 84937.

Thanks to the high multiplex and efficiency of Giraffe at the VLT we have been able for the first time to observe the Li I doublet in the Main Sequence stars of a globular cluster. At the same time we observed Li in a sample of Sub-Giant stars of the same B-V colour.

Our final sample is composed of 84 SG stars and 79 MS stars. In spite of the fact that SG and MS span the same temperature range we find that the equivalent widths of the Li I doublet in SG stars are systematically larger than those in MS stars, suggesting a higher Li content among SG stars. This is confirmed by our quantitative analysis carried out making use of 1D hydrostatic plane-parallel models and 3D hydrodynamical simulations of the stellar atmospheres.

We derived the effective temperatures of stars in our the sample from Hα fitting. Theoretical profiles were computed using 3D hydrodynamical simulations and 1D ATLAS models. Therefore, we are able to determined 1D and 3D-based effective temperatures. We then infer Li abundances taking into account non-local thermodynamical equilibrium effects when using both 1D and 3D models.

We find that SG stars have a mean Li abundance higher by 0.1 dex than MS stars. This result is obtained using both 1D and 3D models. We also detect a positive slope of Li abundance with effective temperature, the higher the temperature the higher the Li abundance, both for SG and MS stars, although the slope is slightly steeper for MS stars. These results provide an unambiguous evidence that the Li abundance changes with evolutionary status.

The physical mechanisms responsible for this behaviour are not yet clear, and none of the existing models seems to describe accurately these observations. Based on these conclusions, we believe that the cosmological lithium problem still remains an open question.

We recently presented (Sbordone et al., 2009a) the largest sample to date of lithium abundances in extremely metal-poor (EMP) Halo dwarf and Turn-Off (TO) stars. One of the most crucial aspects in estimating Li abundances is the Teff determination, since the Li I 670.8 nm doublet is highly temperature sensitive. In this short contribution we concentrate on the Teff determination based on Hα wings fitting, and on its sensitivity to the chosen stellar gravity.

We present FLAMES/GIRAFFE spectroscopy obtained with the Very Large Telescope (VLT). Using these observations, we have been able (for the first time) to observe the Lii doublet in the main-sequence (MS) stars of a globular cluster. We also observed Li in a sample of subgiant (SG) stars of the same B − V colour. Our final sample is composed of 84 SG and 79 MS stars. In spite of the fact that SG and MS stars span the same temperature range, we find that the equivalent widths of the Lii doublet in SG stars are systematically greater than in MS stars, suggesting a higher Li content among SG stars. This is confirmed by our quantitative analysis, which makes use of both 1D and 3D model atmospheres. We find that SG stars show, on average, a higher Li abundance, by 0.1 dex, than MS stars. We also detect a positive slope of Li abundance with effective temperature: the higher the temperature the higher the Li abundance, both for SG and MS stars, although the slope is slightly steeper for MS stars. These results provide unambiguous evidence that the Li abundance changes with evolutionary state. The physical mechanisms that contribute to this are not yet clear, since none of the proposed models seem to describe accurately the observations. Whether such a mechanism can explain the cosmological lithium problem is still an open question.

The abundance derivation of heavy r-elements may provide a better understanding of the r-process, and the determination of several reference r-elements should allow a better determination of the star's age. The spatial ultraviolet (UV) region presents a large number of lines of heavy elements, and in some cases such as Bi, Pt, Au, detectable lines are only available in the UV. The extreme “r-process star” CS 31082-001 ([Fe/H] = −2.9) was observed in the spatial UV in order to determine abundances of the heavy elements, using STIS on board HST.

In the framework of he ESO Large Program “First Stars”, high resolution (R=45000) high S/N ratio spectra have been obtained for a sample of Extremely Metal Poor Stars (EMP stars), 35 giants and 18 turnoff stars. Among them 37 have a very low metallicity: [Fe/H]< −2.9.

We present the largest sample available to date of lithium abundances in extremely metal poor (EMP) Halo dwarfs. Four Teff estimators are used, including IRFM and Hα wings fitting against 3D hydrodynamical synthetic profiles. Lithium abundances are computed by means of 1D and 3D-hydrodynamical NLTE computations. Below [Fe/H]~−3, a strong positive correlation of A(Li) with [Fe/H] appears, not influenced by the choice of the Teff estimator. A linear fit finds a slope of about 0.30 dex in A(Li) per dex in [Fe/H], significant to 2–3 σ, and consistent within 1 σ among all the Teff estimators. The scatter in A(Li) increases significantly below [Fe/H]~−3. Above, the plateau lies at 〈A(Li)3D, NLTE〉 = 2.199 ± 0.086. If the primordial A(Li) is the one derived from standard Big Bang Nucleosynthesis (BBN), it appears difficult to envision a single depletion phenomenon producing a thin, metallicity independent plateau above [Fe/H] = −2.8, and a highly scattered, metallicity dependent distribution below.

The presence of convective motions in the atmospheres of metal-poor halo stars leads to systematic asymmetries of the emergent spectral line profiles. Since such line asymmetries are very small, they can be safely ignored for standard spectroscopic abundance analysis. However, when it comes to the determination of the 6Li/7Li isotopic ratio, q(Li)=n(6Li)/n(7Li), the intrinsic asymmetry of the 7Li line must be taken into account, because its signature is essentially indistinguishable from the presence of a weak 6Li blend in the red wing of the 7Li line. In this contribution we quantity the error of the inferred 6Li/7Li isotopic ratio that arises if the convective line asymmetry is ignored in the fitting of the λ6707 Å lithium blend. Our conclusion is that 6Li/7Li ratios derived by Asplund et al. (2006), using symmetric line profiles, must be reduced by typically Δq(Li) ≈ 0.015. This diminishes the number of certain 6Li detections from 9 to 4 stars or less, casting some doubt on the existence of a 6Li plateau.

From NLTE computations of the magnesium abundance in a sample of extremely metal-poor giants we derive [Mg/Fe]=+0.7, leading to [Al/Mg]=−0.80 and [Na/Mg]=−0.85 in the early Galaxy. The ratio [O/Mg] should be near to the solar value. Measurements of nitrogen abundances derived from the analysis of the NH band in eight more stars confirm the large scatter of the ratios [N/Fe] and [N/O] in the early Galaxy.

We have studied the lithium abundance in 18 extremely metal-poor main-sequence turnoff stars as a function of [Fe/H] and $T_{\rm eff}$, using high-quality VLT/UVES spectra. The sample covers the range $-3.3\le [{\rm Fe}/{\rm H}]\le -2.5$, with half of the stars below [Fe/H] = −3.0. $T_{\rm eff}$ is determined from H$\alpha$ line profiles as well as from B-V, V-K, J-H and J-K colours. The behaviour of A(Li) as a function of metallicity is markedly different when different temperature scales are adopted. However, even when applying standard depletion corrections, it is a robust result that the Li abundance in extremely metal poor dwarfs is far below the prediction of standard big bang nucleosynthesis using a baryonic density consistent with the WMAP data.

This contribution retraces the scientific careers of Monique and François. It highlights the impressive contributions that they have brought to astrophysics, from the discovery of the lithium plateau in subdwarfs the second year of operation of the Canada-France-Hawaii telescope, to the exceptional contribution of Monique to the ESO VLT Large Programme “First Stars”, passing by several other findings which have marked our knowledge of the nuclear evolution of our Galaxy and of the Magellanic Clouds.

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.

High dose helium implantation followed by a suitable thermal treatment induces defects such as cavities and dislocations. Gettering efficiency of this technique for metallic impurities has been widely proved. Nevertheless, dopants, as well as point defects, interact with this defect layer. Due to the presence of vacancy type defects after helium implantation, boron diffusion can be largely influenced by such a buried layer. In this paper, we study the influence of helium induced defects on boron diffusion. The boron diffusion in presence of these defects has been analyzed as a function of different parameters such as distance between boron profile and defect layer and defect density. Our results demonstrate that the major impact known as boron enhanced diffusion can be partially or completely suppressed depending on parameters of experiments. Moreover, these results clarify the interaction of boron with extended He-induced defects.

We present a determination of the lithium abundance from high quality spectra in an extremely metal poor star where the lithium line had not been detected.

We have built a new grid of synthetic spectra in the wavelength range λλ 4600-5600 Å for [α-elements/Fe] = 0.0, +0.2, +0.4 and +0.6. The Lick indices Mg2, Mgb, Fe5270 and Fe5335 are measured in all grid spectra, and their behaviour with stellar parameters is studied.

For the calculation of this new grid, the set of atomic and molecular constants was revised and model atmospheres with overshooting and mixing length parameters appropriate to reproduce the wings of the Hβ line (suitable to indicate temperatures) were computed. We also present a grid in the wavelength range λλ 6000 – 10200 Å. A computation of a high-resolution grid of spectra in the range λλ 3800–7000 Å, as well as a full grid in the range λλ 3000–10200 Å, to be further used in conjunction with evolutionary population synthesis models, are also underway.

The origin of the elements in the early Galaxy has been thought to be fairly well understood as primordial matter polluted by ejectae of type II supernovae. Some recent observational results have led to reconsider in more detail this statement. As a result, it is confirmed in some respects, but curiously, when the same metallicity can be found both in the halo and in the thick disk, the stars contaminated by SN la ejectae are more the halo stars than the thick disk stars.

After having studied in great detail the observational HR diagram (log Teff, Mbol) composed by 40 main sequence stars of the Hyades (Perryman et al.,1997, A&A., in press), we have tried to apply the same method to the observational main sequences of the three next nearest open clusters: Coma Berenices, the Pleiades, and Praesepe. This method consists in comparing the observational main sequence of the clusters with a grid of theoretical ZAMSs. The stars composing the observational main sequences had to have reliable absolute bolometric magnitudes, coming all from individual Hipparcos parallaxes, precise bolometric corrections, effective temperatures and metal abundances from high resolution detailed spectroscopic analyses. If we assume, following the work by Fernandez et al. (1996, A&A,311,127), that the mixing-lenth parameter is solar, the position of a theoretical ZAMS, in the (log Teff, Mbol) plane, computed with given input physics, only depends on two free parameters: the He content Y by mass, and the metallicity Z by mass. If effective temperature and metallicity of the constituting stars of the 4 clusters are previously known by means of detailed analyses, one can deduce their helium abundances by means of an appropriate grid of theoretical ZAMS’s. The comparison between the empirical (log Teff, Mbol) main sequence of the Hyades and the computed ZAMS corresponding to the observed metallicity Z of the Hyades (Z= 0.0240 ± 0.0085) gives a He abundance for the Hyades, Y= 0.26 ± 0.02. Our interpretation, concerning the observational position of the main sequence of the three nearest clusters after the Hyades, is still under way and appears to be greatly more difficult than for the Hyades. For the moment we can say that:

1. ‒ The 15 dwarfs analysed in detailed in Coma have a solar metallicity: [Fe/H] = -0.05 ± 0.06. However, their observational main sequence fit better with the Hyades ZAMS.

2. ‒ The mean metallicity of 13 Pleiades dwarfs analysed in detail is solar. A metal deficient and He normal ZAMS would fit better. But, a warning for absorption in the Pleiades has to be recalled.

3. ‒ The upper main sequence of Praesepe, (the more distant cluster: 180 pc) composed by 11 stars, analysed in detail, is the one which has the best fit with the Hyades ZAMS. The deduced ‘turnoff age’ of the cluster is slightly higher than that of the Hyades: 0.8 Gyr instead of 0.63 Gyr.

The aim of our stellar population study is to investigate the kinematical and chemical characteristics of the thin disk, thick disk and halo of the Galaxy. We have selected 51 stars in 2 astrometric and photometric surveys at l = 42°, b = +79° (Soubiran 1992) and l = 167°, b = +47° (Ojha et al. 1994), on the basis of the Reduced Proper Motion Diagram. They were observed with the 193cm telescope at Observatoire de Haute-Provence, with the CARELEC spectrograph (dispersion of 66Å/mm, FWHM of 3.0Å, range λλ4600 – 5500Å), together with 43 comparison stars with known fundamental parameters. The derivation of Teff, logg and [Fe/H] was done differentially using a grid of synthetic spectra and the comparison stars, as described in Cayrel et al. (1991). Twenty of the target stars were found to be more deficient than −0.5. In the (V, [Fe/H]) distribution, the halo stars are clearly separated from the other stars with a mean of (V, [Fe/H]) ≃ (−210km/s, – 1.4dex). Because of the small size of the sample, it was not possible to discriminate the thick disk from the thin disk. We have taken 200 more spectra, and with these new observations, we hope to be able to deconvolve the 3 populations in the (U, V, W, [Fe/H]) space as we did previously with the (U, V) velocity (Soubiran 1993).

From models of the Sun atmosphere obtained with ATLAS 9 (Kurucz 1992) it appears that the diffusion limit of the transfer equation becomes valid only at depths larger than TR ~ 10. Even with physics as consistent as possible one restores the atmosphere with an accuracy not better than ~ 40%, but with small consequences on solar calibrated models. Despite the eigenmodes have turning points located in the atmosphere, the study of the oscillations reveals differences less than 2μHz when the atmosphere is restored with various assumptions.

A grid of 200 synthetic spectra was built in the wavelength region λλ 478-530 nm, and a method was developed for deriving the stellar parameters temperature, gravity and metallicity from low-resolution stellar spectra. The parameters of the observed spectra are derived by dividing the observed spectrum by that of a reference star. The resulting δf(λ) = log F*/Fref is then used, in conjunction with the grid of synthetic spectra, to derive the final parameters, through a perturbation method. The method was applied to a sample of 41 stars in the Basel field SA 141 (1 = 245°, b = −85.8°). Since this direction points closely to the south galactic pole, the binning of stars into appropriate distance intervals provides a coarse view of the metallicity distributions associated with the major galactic populations: we find the thin disk, thick disk and halo components near the current best estimates of their scale heights above the galactic plane. The first results were presented in Cayrel et al. (1991a, A&A, 247, 108; 1991b, A&A, 247, 122). Further observations are in progress, with the aim of having a sample of about 1000 stars.