For refining cosmochemical studies, we are investigating the statistical equilibrium (non-LTE) of neutral and singly-ionized calcium (CaI/II) in the atmospheres of solar-type stars. The envisioned applications are a) detailed abundance studies beyond classical models, b) using CaI/II as a gravity indicator for ultra-metal-poor (UMP) stars and c) detailed modelling of the CaII IR triplet lines for the astrophysical parametrization of the Gaia mission. We present first results for some well-studied reference cases, both at solar and low metallicity.

We employ a new Infrared Flux Method (IRFM) temperature scale (Ramírez & Meléndez 2005a, b) in order to determine Li, O, and Fe NLTE abundances in a sample of relatively unevolved (dwarfs, turn-off, subgiants) metal-poor stars. We show that the analysis of the permitted OI triplet and FeII lines leads to a plateau in [OI/FeII] over the broad metallicity range $-3.2 <$ [Fe/H] $<-$0.7, independent of temperature and metallicity, and with a star-to-star scatter of only 0.1 dex. The Li abundance in halo stars is also found to be independent of temperature and metallicity (Spite plateau), with a star-to-star scatter of just 0.06 dex over the metallicity range $-3.4 <$ [Fe/H] $<-1$. Our Li abundance (Meléndez & Ramírez 2004) is higher than previously reported values, but still lower than the primordial abundance suggested by WMAP data and BBN.

We have determined the chemical composition of the carbon dwarf G77-61, from Keck IR and optical spectra. We present here a new analysis with the oxygen abundance measured for the first time. We show that G77-61 is extremely metal-poor ([Fe/H]$\,{=}\,{-}$4), with large overabundances of C, N and O ([C/Fe] = 3.2, [N/Fe] = 2.2, [O/Fe] = 2.2). It also shows moderate enhancements of Ca and Mg, Na, and Cr of typically 0.5 dex relative to Fe. We discuss the possible origin of these peculiarities.

The Sun has typical abundances of both oxygen and sulphur for its metallicity, age and galactic orbit, when compared to the nearby solar-type stars. This result favors a solution of the old riddle of the solar overabundance in oxygen with respect to the local interstellar medium as caused by the recent infall of metal-poor gas over the disk, rather than the competing explanations of an outward migration of the Sun from a inner, and metal richer, birthplace in the disk, or a last-minute supernova which might have enriched the proto-solar nebula.

Oxygen abundances for a large sample of dwarf and giant stars kinematically selected to be part of the Galactic thin and thick disks have been determined from a non-LTE analysis of the O I triplet lines at 777 nm. The abundance analysis was performed using the infrared flux method temperature scale, trigonometric surface gravities, and accurate atomic data. Within this framework, the ionization balance of iron lines could not be satisfied and so we adopted the iron abundances from Fe II lines only given that they are relatively less sensitive to changes in the atmospheric parameters. We show the resulting [O/Fe] vs. [Fe/H] relationship and briefly discuss its implications.

We develop and test a method for the estimation of carbon abundance ratios, [C/Fe], and metallicities, [Fe/H], in metal-deficient stars, based on application of artificial neural networks and synthesis models to medium-resolution (1–2 Å) spectra in the region of the CH G-band feature at $\lambda$ 4300 Å. We calibrate this method by comparison to modern carbon abundance determinations for N $\simeq 125$ stars reported in the recent literature. As a first application, we estimate the abundance of carbon and [Fe/H] for the sample of 56 stars identified as carbon-rich, relative to stars of similar metal abundance, in the sample of “strong G-band” stars discussed by Beers, Preston, & Shectman.

We present new carbon-enhanced stars identified from the third public release of the Sloan Digital Sky Survey, SDSS-DR3. We have generated synthetic spectra with varying carbon abundances, and compare them with the SDSS spectra. We have also performed a preliminary analysis of s-process enhancement in several SDSS carbon-enhanced stars. Spectral features that are sensitve to stellar luminosity and temperature have also been explored. These methods will be applied to the large set of public SDSS data, as well as to the forthcoming data from SEGUE, the Sloan Extension for Galactic Understanding and Evolution, in order to study carbon enhancement at different metallicities, the fraction of s-process enhancement that occurs in carbon-enhanced stars, and possibly isotopic carbon abundances and nitrogen abundances.

The LTE abundances of sulfur (S) were explored in the sample of 15 metal-poor stars with the metallicity range of −4<[Fe/H]<−1.5, based on the equivalent widths of the S I(1) 9212 and 9237 Å lines measured on high-resolution spectra, which were observed by the Keck I HIRES. Combining our results and those of Takada-Hidai et al. (2005), we found that the behavior of [S/Fe] against [Fe/H] shows a nearly flat trend in the range of metallicity down to [Fe/H]∼−4.

To determine ages of individual (field) stars, composition, mass and distance should be known accurately, which is usually not the case. An alternative way is to use turn-off colours of field star populations. This method led in the past to ages significantly higher than those of globular clusters of the same metallicity. We show that colour-based relative ages between the field and cluster population indicate that they have similar if not equal ages. First steps using large samples of stars from SDSS are presented.

The preliminary results of abundance analysis are presented for extremely metal-poor carbon star HD 112869 = TT CVn = CGCS 3319. The radial velocity was found to be −137.7 km s−1. Our LTE abundance analysis supports an extremely low metallicity for TT CVn, [Fe/H] = −3.2, and a significant overabundance of carbon and neutron-capture elements. The 12C/13C ratio in the atmosphere of HD 112869 is high.

The abundance patterns of extremely metal-poor (EMP) stars provide us with important information on nucleosynthesis in supernovae (SNe) formed in a Pop III or EMP environment, and thus on the nature of the first stars in the Universe. We review nucleosynthesis yields of various types of those SNe, focusing on core-collapse (black-hole-forming) SNe with various progenitor masses, explosion energies (including Hypernovae), and asphericity. We discuss the implications of the observed trends in the abundance ratios among iron-peak elements, and the large C/Fe ratio observed in certain EMP stars with particular attention to recently discovered hyper metal-poor (HMP) stars. We show that the abundance pattern of the HMP stars with [Fe/H] <−5 and other EMP stars are in good accord with those of black-hole-forming supernovae, but not pair-instability supernovae. This suggests that black-hole-forming supernovae made important contributions to the early Galactic (and cosmic) chemical evolution. Finally we discuss the nature of First (Pop III) Stars.

The first generation of stars to form in the universe may have been very massive, and, due to the absence of initial metals, they could have retained most of their mass until their death and thus explode as pair instability supernovae. These supernovae encounter the late burning phases beyond carbon burning in an implosive/explosive way, leading to very powerful thermonuclear-powered explosions, up to a hundred times more powerful than ordinary supernovae. For primordial stars, these explosions also produce a peculiar abundance pattern, showing a strong odd-even pattern in the elemental abundances, a sharp drop-off of nucleosynthetic production beyond the iron group, and no r-process contribution. These results are greatly altered if only a small mass of 14N is dredged down into the helium burning core before the star becomes unstable. Such mixing could be a consequence of differential rotation or convective overshooting.

We present a new analysis of the abundances observed in extremely metal poor stars based on both a new generation of theoretical presupernova models and explosions of zero metallicity massive stars and a new abundance analysis of an homogeneous sample of stars having $\rm [Fe/H]\leq -2.5$ (Cayrel et al. 2004).

Core collapse supernovae are responsible for at least half of the galactic inventory of Fe-group elements and probably for most of the Fe-group abundances seen in metal poor stars. Recent simulations show the emergence of a proton-rich ($Y_e>0.5$) region in the innermost ejected mass zones due to the neutrino interaction with matter. We explore the nucleosynthesis implications of these findings that result in enhanced abundances of 45Sc, 49Ti, and 64Zn, which is consistent with chemical evolution studies and observations of low metallicity stars.

We provide new nucleosynthesis yields depending on metallicity and energy (i.e., (normal supernovae and hypernovae), and show the evolution of heavy element abundances from C to Zn in the solar neighborhood. We then show the chemodynamical simulation of the Milky Way Galaxy and discuss the G-dwarf problem. We finally show the cosmological simulation and discuss the galaxy formation and chemical enrichment.

PN G 135.9 +55.9 is an extraordinary nebula discovered recently in the Galactic halo (Tovmassian et al. 2001). The first studies estimated its oxygen abundance to be 1/100 of the solar value or even less.

Being extremely metal-poor, PNG 135.9+55.9 offers an unprecedented opportunity to check our understanding of the evolution of intermediate-mass stars at very low metallicity, by complementing the data obtained from metal-poor giants (Spite et al 2005). Indeed, PNG 135.9+55.9 and some of those stars are snapshots of the evolution of very similar stars at different times.

We present our most recent abundance analysis for this object, providing stringent limits on the abundances of C, N, O, and Ne, to be confronted with the predictions for the yields of low metallicity intermediate mass stars.

A detailed abundance analysis of four giants in the metal-rich bulge globular cluster NGC 6553 is carried out, based on optical high resolution échelle spectra obtained with UVES at the ESO VLT-UT2 Kueyen telescope. A mean radial heliocentric velocity of −1.86 km s−1 is found. Stellar parameters are derived from spectroscopic data based on Fe I and Fe II lines. Enhanced abundance ratios for the α-elements Mg and Si with respect to Ca and Ti are obtained. The odd-Z elements are typically solar. A solar value for the r-process element Eu ([Eu/Fe] = +0.05 ±0.06) was also found.

Our understanding on the nitrogen origin has recently greatly changed. New data on nitrogen abundances in very metal-poor stars ($-4<[Fe/H]<-3$) show a quite surprising result: a high N/O ratio suggestive of high levels of production of primary nitrogen in massive stars. Moreover, none of the stars measured so far has N/O ratios as low as the ones observed in Damped Lyman-α systems (DLAs) (currently the systems showing the lowest N/O ratios in the universe). We studied the implications of the above new data set for our understanding on the nitrogen enrichment in the Milky Way. We find that, to explain the new observations, we need to adopt stellar yields computed with stellar models in which rotation is taken into account and assume that stars born at Z <10−5 contribute a lot more N than the most recent calculations available in the literature for Z= 10−5. The implications of our findings for our understanding of the nature of Damped Lyman-α systems (DLAs) are also briefly discussed.

Nine 20$\,M_\odot$ models were computed with metallicities ranging from solar, through $Z=10^{-5}$ ([Fe/H]∼−3.1) down to $Z=10^{-8}$ ([Fe/H]∼−6.1) and with initial rotational velocities between 0 and 600 km s−1 to study the impact of initial metallicity and rotational velocity (Hirschi (2005)). The very large amounts of 14N observed (∼0.03 M[odot ]) are only produced at $Z=10^{-8}$ (PopII 1/2). The strong dependence of the 14N yields on rotation and other parameters like the initial mass and metallicity may explain the large scatter in the observations of 14N abundance. The metallicity trends are best reproduced by the models with $\upsilon_{ini}/\upsilon_c \sim 0.75$, which is slightly above the mean observed value for OB solar metallicity stars. Indeed, in the model with $\upsilon_{ini}$ = 600 km s−1 at $Z=10^{-8}$, the 16O yield is reduced due to strong mixing. This allows in particular to reproduce the upturn for C/O and a slightly decreasing [C/Fe], which are observed below [Fe/H]∼−3.

The abundance distributions of cluster and field halo populations will be considered. Abundance data for most elements of the Periodic Table are much sparser for globular clusters than for individual halo field stars, mainly due to the relative lack of high resolution spectra in the blue for the former group. But the available data do suggest that, in spite of remaining uncertainties in the observational data, that chemical composition differences in the heavier elements (Fe-peak and beyond) are small, indicating common nucleosynthetic origins for all halo populations.