We present preliminary results on the chemical abundance patterns of extremely metal-poor stars obtained during an ongoing observing program with Subaru/HDS. High-resolution, high signal-to-noise spectra have been obtained for 14 stars with [Fe/H]$\lesssim -3$. Five of them exhibit clear overabundances of carbon, a remarkable characteristic found only in the most metal-poor range. One of the carbon-rich stars, HE 1327–2326, has [Fe/H]$_{\rm NLTE}=-5.4$, the lowest Fe abundance known. No stars with ${-}5\,{<}\,$[Fe/H]$\,{<}\,{-}4$ have yet been found in our program, suggesting that quite different enrichment processes were responsible for stars with [Fe/H]$\,{<}\,{-}5$ and [Fe/H]$\,{>}\,{-}4$. While neutron-capture elements are deficient in most of our stars, one star (BS 16550–087) exhibits large enhancements of its light neutron-capture elements (Sr, Y and Zr), providing a strong constraint on models for the production of such elements in the very early Galaxy.

We present the results of analysis of “snapshot” spectra (i.e., R=20,000 and S/N=50 per pixel) of 253 metal-poor halo stars $-3.8<$[Fe/H]$<-1.5$ obtained in the HERES survey. The spectra are analysed using an automated line profile analysis method based on the Spectroscopy Made Easy (SME) codes of Valenti & Piskunov (1996). Elemental abundances of moderate precision (absolute r.m.s. errors of order 0.25 dex) have been obtained for 22 elements, C, Mg, Al, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Sr, Y, Zr, Ba, La, Ce, Nd, Sm, and Eu, where detectable. The results are presented and discussed, particularly trends and scatter in the abundance distributions.

We describe the discovery of HE 1327–2326, a dwarf or subgiant with $\mbox{[Fe/H]}=-5.4$. The star was found in a sample of bright metal-poor stars selected from the Hamburg/ESO survey. Its abundance pattern is characterized by very high C and N abundances. The detection of Sr which is overabundant by a factor of 10 as compared to iron and the Sun, suggests that neutron-capture elements had already been produced in the very early Galaxy. A puzzling Li depletion is observed in this unevolved star which contradicts the value of the primordial Li derived from WMAP and other Li studies. Possible scenarios for the origin of the abundance pattern (Pop. II or Pop. III) are presented as well as an outlook on future observations.

We discuss the fraction of carbon stars (C-stars) and of C-enhanced stars among samples of candidate extremely metal poor (EMP: [Fe/H] $\le -3.0$ dex) stars selected from the Hamburg/ESO Survey (HES), obtaining a total for C-rich stars with [C/Fe] $\ge$ +1.0 dex of 14.4$\pm4$%. We also present the key results of detailed abundance analyses of a sample of 14 C-stars selected in this way.

A new class of carbon-rich stars was revealed by large surveys of very metal-poor objects, the carbon-enhanced metal-poor stars (CEMPs). This carbon enhancement is reminiscent of that found in classical CH stars, which despite being halo stars, are not as metal-poor as CEMPs. Although a mass-transfer scenario similar to that formerly at work in CH stars could account for the abundance pattern of CEMPs, differences arise for some key heavy elements. Moreover, statistical studies find 14%; of metal-poor C-rich stars among very metal-poor stars. Thus, this important population of stars represents a precious testimony for nucleosynthesis and chemical evolution at early stages of the Galaxy. We have started a detailed analysis of a large sample of both CH and metal-poor C-rich stars. Here we present results concerning the chemical composition obtained via high resolution and high signal-to-noise VLT-UVES spectra, with special emphasis on the challenges encountered during the abundance analysis. The discussion also includes preliminary results of our ongoing radial velocity monitoring programme which aims at evaluating the relevance of the binary scenario.

The earliest phases of the chemical evolution of our Galaxy are analysed in the light of the recent VLT results (concerning abundance patterns in the most metal-poor stars of the Galactic halo) and of stellar nucleosynthesis calculations. It is argued that, among the various suggestions made in order to explain the observed abundance patterns, nucleosynthesis in asymmetric supernova explosions appears most promising. The data suggest a correlation between asymmetry and metallicity, which is hard to justify theoretically. The data also confirm the absence of dispersion in abundance ratios, at least up to the Fe peak, in the early Galaxy; this may be related to the (presently poorly known) timescales of homogeneisation of the interstellar medium, but also on small yield variations among massive stars. Finally, the metallicity distribution of halo stars may provide important constraints on the formation of the Milky Way; at present, it is not clear whether observations support the hierarchical formation scenario.

A stochastic model of the chemical enrichment of metal-poor systems by core-collapse supernovae is used to study the scatter in relative elemental abundances in extremely metal-poor stars. The resulting scatter in abundance ratios is demonstrated to be crucially dependent on the as yet uncertain supernovae yields. The relatively small star-to-star scatter observed in many of these abundance ratios, e.g. by Cayrel et al. (2004), is tentatively explained by the averaging of a large number of contributing supernovae and by the cosmic selection effects favoring contributions from supernovae in a certain mass range for the most metal-poor stars. “Spurs”, very narrow sequences in abundance-ratio diagrams, may disclose a single-supernova origin of the elements of the stars on the sequence and would thus be an indication of an unmixed interstellar medium (ISM). Verification of the existence of such features, called single supernova sequences (SSSs), is challenging. This will require samples of several hundred stars with abundance ratios observed to accuracies of 0.05 dex or better.

We have determined the CNO abundances from OH, CH and NH lines in the VLT/UVES UV spectra of HE0107-5240 and CD-38 245 together with the abundances of several metal species, including FeII, whose lines were not detected in blue spectra. The UV abundance analyses support those determined from the blue spectra, in particular the abundance from the FeII lines is in good agreement with the Fe abundance derived from the Fe I lines. It seems unlikely that the large overabundances of CNO have been produced in a medium-mass AGB star which now evolved to a white dwarf. The oxygen abundance is consistent with the predictions of Umeda & Nomoto (2003, 2005), who proposed that HE0107-5240 has formed from a gas cloud which has been enriched by the yields of a $\approx 25$ solar mass Population III star exploding as a supernova of low explosion energy E${=} 3.10^{50}$ erg with mixing and fallback. The odd-even effect predicted in the original models are not evident in the Sc and Co abundances of HE0107-5240, which are better fitted by the low density variant progenitor model.

We present the main results of our study of photometric solar-analog stars.

We present a compiled catalogue of effective temperatures, surface gravities iron and magnesium abundances, along with distances, velocity components, and orbital elements for stars in the solar vicinity. Abundances were found from 1412 determinations in 31 publications for 876 dwarfs and subgiants by means of a three-step iteration averaging procedure, with weights assigned to each source of data as well as to each individual determination. The assumed coverage completeness for data sources containing more than 5 stars, as of late 2003, exceeds 90%. For the vast majority of stars in the catalogue, the spatial-velocity components were derived from modern high-precision astrometric observations, and their galactic-orbit elements were computed using a three-component model of the Galaxy, consisting of a disk, a bulge, and a massive extended halo.

We have observed with UVES@VLT 15 field Blue Straggler Stars for which kinematical attribution to the Halo or Thick Disk is available. We present here the first results of our detailed chemical analysis.

We have derived [Fe/H] and [α/Fe] abudnances for Cepheids with Galactocentric distances of 11 to 17 kpc. The stars are as metal-poor as much older open clusters at comparable distances. Like the clusters, and despite their youth, the Cepheids also show enhanced [α/Fe] abundances indicating enhanced contributions from SNe II relative to the Solar neighborhood.

We investigate the main differences between static 1D and 3D time-dependent model stellar atmospheres of red giants at very low metallicities. We focus in particular on the impact of 3D LTE spectral line formation on the derivation of elemental abundances for the extremely metal-poor ([Fe/H] $\approx-$5.3) red giant HE 0107-5240.

We present preliminary oxygen abundances in a sample of four red-giants belonging to the bulge population. The abundances were derived from OH molecular transitions in the infra-red high-resolution spectra obtained with the Phoenix spectrograph on Gemini South. The target stars were taken from the previous study by McWilliam & Rich (1994) and selected in order to span a range in [Fe/H] from $-$1.0 to +0.5. Our oxygen results are found to be enhanced and fall above the Milky Way disk trend; in agreement with the high abundances obtained previously for other $\alpha-elements$. It is important to note, however, that the enhanced OH abundances obtained here are based upon stellar parameters taken directly from McWilliam & Rich (1994). Significant revisions to these published values are found more recently in McWilliam & Rich (2003) Such revisions would result in derived oxygen abundances that are systematically lower and perhaps in better agreement with the Milky Way trend. The conclusion is that further efforts are needed in order to better define the stellar parameters for the target stars, as they are crucial in order to decide the important issue of whether the oxygen abundances are enhanced in the bulge K-giants, as we find here. Or, on the other hand, if they follow the trend defined for the Milky Way disk.

In da Silva et al. (2005) is presented the detailed spectroscopic analysis of 60 evolved stars, which were previously studied for accurate radial velocity variations (Setiawan et al. 2003). The observations were made with FEROS and the 1.52m ESO telescope of La Silla, partially in the ESO-Observatório Nacional (Brazil) agreement. The main goal of this study is to determine precise atmospheric fundamental data (Teff, log g, [Fe/H] and micro-turbulence velocity) to derive ages and masses of the stars, trying to link them to the detected radial velocity variabilities. Here, due the short space at our disposal, we discuss only the effective temperature determination, comparing the values obtained from the excitation equilibrium of FeI and from (V-K) index with those from (B-V).

We present a detailed and uniform study of oxygen abundances in a large set of 155 metal-rich dwarfs. EW measurements were carried out for the [OI] 6300 Å line and the OI triplet, while spectral synthesis was performed for several OH lines. NLTE corrections were calculated and applied to the LTE abundance results derived from the triplet. Abundances from [OI], the OI triplet and near-UV OH were obtained in 103, 87 and 77 dwarfs, respectively. A good agreement between [O/H] ratios from forbidden and OH lines is found, while the NLTE triplet shows a systematically lower abundance. Nevertheless, the consistency with other indicators improves if we consider LTE triplet results. In any case, discrepancies between OH, [OI] and the OI triplet hardly exceed 0.2dex. All three indicators show that, on average, [O/Fe] decreases with [Fe/H] in the metallicity range −0.8<[Fe/H]<0.5.

We present non–Local Thermodynamic Equilibrium (non–LTE) calculations for neutral carbon spectral line formation, carried out for a grid of model atmospheres covering the range of late-type stars. The results of our detailed calculations suggest that the carbon non–LTE corrections in these stars are higher than usually adopted, remaining substantial even at low metallicity. For the most metal-poor stars in the sample of Akerman et al (2004), the effects are of the order of $\Delta\log\epsilon_{\rm C} \simeq -0.35\ldots-0.45$ (when neglecting H collisions). Applying our results to those observations, the apparent [C/O] upturn seen in their LTE analysis is no longer present, thus revealing no need to invoke contributions from Pop. III stars to the carbon nucleosynthesis.

There seems to be no consensus on the [O/Fe] values found in metal-poor stars nor their trend with metallicity: different indicators give different results. We present here [O/Fe] derived from three different oxygen abundance indicators ([OI], OI and OH UV lines) for a sample of thirteen subgiant stars with metallicities in the range $-3\le\mathrm{[Fe/H]\le-1.5$. Oxygen and iron abundances were determined from the analysis of high $S/N$ spectra acquired with the UVES spectrograph at VLT. We found good agreement between [O/Fe] estimates based on OH and the estimates based on [Oi] ($\mathrm{mean\ difference}\sim-0.09\pm0.25(\mathrm{s.d.})$), although the scatter is not insignificant. Unfortunately, good agreement is not reached for the third indicator ($\mathrm{mean\difference}\sim0.19\pm0.22(\mathrm{s.d.})$). Our [OI] and OH-based estimates do not show a well defined linear trend with metallicity. Another interesting result is that the abundances based on molecular lines ($\mathrm{[O/Fe]}\sim0.45$) are in general lower than previous published results for turn-off stars even though a lower solar oxygen abundance ($A(\mathrm{O})=8.74$) was assumed.

We have derived abundances of O, Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Fe, Ni and Ba for 43 metal-poor field stars, mostly stars at the turn-off point and on the subgiant branch, in the interval ${-}3.0{<}$[Fe/H]${<}{-}0.4$. The analysis is differential relative to the Sun. Oxygen abundances, with consideration of NLTE effects, were derived from the OI 777.4 nm triplet lines. We find [O/Fe] to gradually increase with decreasing [Fe/H], though considerably slower than has earlier been obtained from OH lines in the UV. A scatter in [O,Mg,Ca,Ti/Fe] at a given [Fe/H] is found and we argue that this scatter is partly real. The deviations from the mean trends of abundance ratios vs [Fe/H] are found to correlate in non-trivial ways for different abundances. Similar trends are found from results of accurate studies by other groups. This seems to give further evidence for the hypothesis that the stars once formed in different subsystems, with different star-formation rates.

The paper is in press in A&A, may be obtained as astro-ph/0505118.

The UVES Paranal Observatory Project (POP), is an ESO public database of about 400 stars whose high quality spectra were obtained with UVES, the high resolution spectrometer of the VLT. All stars were observed with two instrument modes, in order to cover almost completely the optical region (300–1000 nm). The resolving power is about 80000, and for most of the spectra, the typical S/N ratio is 300–500 in the V band. Program stars fall into two groups, stars belonging to open clusters IC2391 and NGC6475, and bright field stars. For field stars, the only selection criterion applied was to cover the largest possible variety of spectral types in the HR diagram, including peculiar objects, e.g., Ap and Bp stars, Wolf-Rayet stars, Be stars, carbon stars and metal poor stars. The spectra have been reduced, coadded and merged and various products can be downloaded from a public area. For each star the final spectrum may be displayed through a dedicated user-friendly Spectral Preview Interface. The database is accessible at http://www.eso.org/uvespop