We present FLAMES/GIRAFFE spectroscopy obtained at the Very Large Telescope (VLT). Using these observations we have been able for the first time to observe the Li I doublet in the Main Sequence stars ...of a Globular Cluster. We also 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 which makes use of both 1D and 3D model atmospheres. We find that SG stars show, on average, a Li abundance higher 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 an unambigous evidence that the Li abundance changes with evolutionary status. 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 mechanism can explain the cosmological lithium problem, is still an open question.
We present a detailed analysis of the carbon and nitrogen abundances of two dwarf carbon-enhanced metal-poor (CEMP) stars: SDSS J1349-0229 and SDSS J0912+0216. We also report the oxygen abundance of ...SDSS J1349-0229. These stars are metal-poor, with Fe/H < -2.5, and were selected from our ongoing survey of extremely metal-poor dwarf candidates from the Sloan Digital SkySurvey (SDSS). The carbon, nitrogen and oxygen abundances rely on molecular lines which form in the outer layers of the stellar atmosphere. It is known that convection in metal-poor stars induces very low temperatures which are not predicted by `classical' 1D stellar atmospheres. To obtain the correct temperature structure, one needs full 3D hydrodynamical models. Using CO5BOLD 3D hydrodynamical model atmospheres and the Linfor3D line formation code, molecular lines of CH, NH, OH and C2 were computed, and 3D carbon, nitrogen and oxygen abundances were determined. The resulting carbon abundances were compared to abundances derived using atomic CI lines in 1D LTE and NLTE. There is not a good agreement between the carbon abundances determined from C2 bands and from the CH band, and molecular lines do not agree with the atomic CI lines. Although this may be partly due to uncertainties in the transition probabilities of the molecular bands it certainly has to do with the temperature structure of the outer layers of the adopted model atmosphere. We explore the influence of the 3D model properties on the molecular abundance determination. In particular, the choice of the number of opacity bins used in the model calculations and its subsequent effects on the temperature structure and molecular line formation is discussed. (Abridged)
Astron.Astrophys. 425 (2004) 615-626 We examine the large sample of lambda Boo candidates collected in Table 1 of
Gerbaldi et al. (2003) to see how many of them show composite spectra. Of the
132 ...lambda Boo candidates we identify 22 which definitely show composite
spectra and 15 more for which there are good reasons to suspect a composite
spectrum. The percentage of lambda Boo candidates with composite spectra is
therefore > 17 and possibly considerably higher. For such stars the lambda Boo
classification should be reconsidered taking into account the fact that their
spectra are composite. We argue that some of the underabundances reported in
the literature may simply be the result of the failure to consider the
composite nature of the spectra. This leads to the legitimate suspicion that
some, if not all, the lambda Boo candidates are not chemically peculiar at all.
A thorough analysis of even a single one of the lambda Boo candidates with
composite spectra, in which the composite nature of the spectrum is duly
considered, which would demonstrate that the chemical peculiarities persist,
would clear the doubt we presently have that the stars with composite spectra
may not be lambda Boo at all.
Approximately 20% of very metal-poor stars (Fe/H < -2.0) are strongly enhanced in carbon (C/Fe > +1.0). Such stars are referred to as carbon-enhanced metal-poor (CEMP) stars. We present a chemical ...abundance analysis based on high resolution spectra acquired with UVES at the VLT of three dwarf CEMP stars: SDSS J1349-0229, SDSS J0912+0216 and SDSS J1036+1212. These very metal-poor stars, with Fe/H < -2.5, were selected from our ongoing survey of extremely metal-poor dwarf candidates from the SDSS. Among these CEMPs, SDSS J1349-0229 has been identified as a carbon star (C/O > +1.0). First and second peak s-process elements, as well as second peak r-process elements have been detected in all stars. In addition, elements from the third r-process peak were detected in one of the stars, SDSS J1036+1212. We present the abundance results of these stars in the context of neutron-capture nucleosynthesis theories.
We give a progress report about the activities within the CIFIST Team related to the search for extremely metal-poor stars in the Sloan Digital Sky Survey's spectroscopic catalog. So far the search ...has provided 25 candidates with metallicities around or smaller -3. For 15 candidates high resolution spectroscopy with UVES at the VLT has confirmed their extremely metal-poor status. Work is under way to extend the search to the SDSS's photometric catalog by augmenting the SDSS photometry, and by gauging the capabilities of X-shooter when going to significantly fainter targets.
Most Globular Clusters are believed to host a single stellar populations. They can thus be considered a good place to study the Spite plateau and probe for possible evolutionary modifications of the ...Li content. We want to determine the Li content of subgiant (SG) and Main Sequence (MS) stars of the old, metal-poor globular cluster NGC 6397. This work was aimed not only at studying possible Li abundance variations but to investigate the cosmological Li discrepancy. Here, we present FLAMES/GIRAFFE observations of a sample of 84 SG and 79 MS stars in NGC 6397 selected in a narrow range of B-V colour and, therefore, effective temperatures. We determine both Teff and A(Li) using 3D hydrodynamical model atmospheres for all the MS and SG stars of the sample. We find a significant difference in the Li abundance between SG stars and MS stars, the SG stars having an A(Li) higher by almost 0.1 dex on average. We also find a decrease in the A(Li) with decreasing Teff, both in MS and SG stars, albeit with a significantly different slope for the two classes of stars. This suggests that the lithium abundance in these stars is, indeed, altered by some process, which is Teff-dependent. The Li abundance pattern observed in NGC 6397 is different from what is found among field stars, casting some doubt on the use of Globular Cluster stars as representative of Population II with respect to the Li abundance. None of the available theories of Li depletion appears to satisfactorily describe our observations.
Solar-like oscillations have been observed in numerous red giants from ground and from space. An important question arises: could we expect to detect non-radial modes probing the internal structure ...of these stars? We investigate under what physical circumstances non-radial modes could be observable in red giants; what would be their amplitudes, lifetimes and heights in the power spectrum (PS)? Using a non-radial non-adiabatic pulsation code including a non-local time-dependent treatment of convection, we compute the theoretical lifetimes of radial and non-radial modes in several red giant models. Next, using a stochastic excitation model, we compute the amplitudes of these modes and their heights in the PS. Very distinct results are found depending on the evolutionary status of the star.
We investigate sulphur abundance in 74 Galactic stars by using high
resolution spectra obtained at ESO VLT and NTT telescopes. For the first time
the abundances are derived, where possible, from ...three optical multiplets:
Mult. 1, 6, and 8. By combining our own measurements with data in the
literature we assemble a sample of 253 stars in the metallicity range -3.2 <
Fe/H < +0.5. Two important features, which could hardly be detected in
smaller samples, are obvious from this large sample: 1) a sizeable scatter in
S/Fe ratios around Fe/H ~ -1 ; 2) at low metallicities we observe stars
with S/Fe ~ 0.4, as well as stars with higher S/Fe ratios. The latter do
not seem to be kinematically different from the former ones. Whether the latter
finding stems from a distinct population of metal-poor stars or simply from an
increased scatter in sulphur abundances remains an open question.
We present VLT-UVES Li abundances for 28 halo dwarf stars between Fe/H=-2.5 and -3.5, 10 of which have Fe/H<-3. Four different T_eff scales have been used. Direct Infrared Flux Method (IRFM) has been ...used on the basis of 2MASS infrared photometry. H_alpha wings have been fitted against synthetic grids computed by means of 1D LTE atmosphere models, assuming different self-broadening theories. Finally, a grid of H_alpha profiles has been computed by means of 3D hydrodynamical atmosphere models. The Li I doublet at 670.8 nm has been used to measure A(Li) by means of 3D hydrodynamical NLTE spectral syntheses. An analytical fit of A(Li)(3D, NLTE) as a function of equivalent width, T_eff, log g, and Fe/H has been derived and is made available. A(Li) does not exhibit a plateau below Fe/H=-3. A strong positive correlation with Fe/H appears, not influenced by the choice of the T_eff estimator. From a linear fit, we obtain a strong slope of about 0.30 dex in A(Li) per dex in Fe/H, significant to 2-3 sigma, and consistent among all the four T_eff estimators. A significant slope is also detected in the A(Li)--T_eff plane, driven mainly by the coolest stars in the sample which appear Li-poor. Removing such stars does not alter the behavior in the A(Li)-Fe/H plane. The scatter in A(Li) increases by a factor of 2 towards lower metallicities, while the plateau appears very thin above Fe/H=-2.8. The meltdown of the Spite plateau below Fe/H\sim-3 is established, but its cause is unclear. If the primordial A(Li) is the one derived from WMAP, 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 fact that no star below Fe/H=-3 lies above the plateau suggests that they formed at plateau level and underwent subsequent depletion.
CONTEXT:The detailed chemical abundances of extremely metal-poor (EMP) stars are key guides to understanding the early chemical evolution of the Galaxy. Most existing data are, however, for giant ...stars which may have experienced internal mixing later. AIMS: We aim to compare the results for giants with new, accurate abundances for all observable elements in 18 EMP turnoff stars. METHODS:VLT/UVES spectra at R ~45,000 and S/N~ 130 per pixel (330-1000 nm) are analysed with OSMARCS model atmospheres and the TURBOSPECTRUM code to derive abundances for C, Mg, Si, Ca, Sc, Ti, Cr, Mn, Co, Ni, Zn, Sr, and Ba. RESULTS: For Ca, Ni, Sr, and Ba, we find excellent consistency with our earlier sample of EMP giants, at all metallicities. However, our abundances of C, Sc, Ti, Cr, Mn and Co are ~0.2 dex larger than in giants of similar metallicity. Mg and Si abundances are ~0.2 dex lower (the giant Mg/Fe values are slightly revised), while Zn is again ~0.4 dex higher than in giants of similar Fe/H (6 stars only). CONCLUSIONS:For C, the dwarf/giant discrepancy could possibly have an astrophysical cause, but for the other elements it must arise from shortcomings in the analysis. Approximate computations of granulation (3D) effects yield smaller corrections for giants than for dwarfs, but suggest that this is an unlikely explanation, except perhaps for C, Cr, and Mn. NLTE computations for Na and Al provide consistent abundances between dwarfs and giants, unlike the LTE results, and would be highly desirable for the other discrepant elements as well. Meanwhile, we recommend using the giant abundances as reference data for Galactic chemical evolution models.
