The Metal Star Chemical Abundances

The main content of this thesis is to search for specific metal-poor stars using low-resolution spectra and to study the chemical enrichment history of metal-poor stars based on high/middle resolution spectra.The first work made use of the database of Sloan Digital Sky Survey (SDSS) to search for metal-poor stars enriched by Pair Instability Supernovae (PISNe) explo-sion. Very massive, first generation (Population Ⅲ) stars (140M(?)≤M≤260M(?)) end their lives as PISNe, which have been predicted by theories, but no relics of PISNe have been observed yet. Among the distinct characteristics of the yields of PISN, as predicted by theoretical calculations, are a strong odd-even effect, and a strong overabundance of Ca with respect to iron and the Solar ratio. We use the latter characteristic to identify metal-poor stars in the Galactic halo that have been pre-enriched by PISN, by comparing metallicites derived from strong, co-added Fe lines detected in low-resolution (i.e., R～2000) spectra of the Sloan Digital Sky Survey (SDSS), with metallicities determined by the SDSS Stellar Parameters Pipeline (SSPP). The latter are based on the strength of the Ca II K line and as-sumptions on the Ca/Fe abundance ratio. Stars are selected as candidates if their metallicity derived from Fe lines is significantly lower than the SSPP metallicities. In a sample of12,300stars for which SDSS spectroscopy is available, we have identified18candidate stars. Higher resolution and signal-to-noise ratio spectra of these candidates are being obtained with the Very Large Telescope of the European Southern Observatory and the XSHOOTER spectrograph, to determine their abun-dance patterns, and to verify our selection method. We plan to apply our method to the data base of several million stellar spectra to be acquired with the LAMOST telescope in the next five years.The second work analyzed thorium abundances for77metal-poor stars in the metallicity range of-3.5<[Fe/H]<-1.0, based on "snapshot" spectra obtained with VLT-UT2/UVES during the HERES Survey. We were able to determine the thorium abundances with better thanσ confidence for17stars, while for60stars we derived upper limits. For five stars common with previous studies, our results were in good agreement with the literature results. The thorium abundances span a wide range of about4.0dex, and scatter exists in the distribution of log(Th/Eu) ratios for lower metallicity stars, supporting previous studies suggesting the r-process is not universal. We derived ages from the log(Th/Eu) ratios for12stars, resulting in large scattered ages, and two stars with significant enhancement of Th relative to Eu are found, indicating the "actinide boost" does not seem to be a rare phenomenon and thus highlighting the risk in using log(Th/Eu) to derive stellar ages.CEMP-no giant HE0557-4840was found and reported by Norris et al.(2007) for the first time. With [Fe/H]=-4.8, it is one of the four most metal-poor stars found so far. We use the ultraviolett spectrum obtained in2007, and together with the spectra observed in2006, to make a detailed chemical abundances analysis of this object. We confirmed most of the element abundances of Norris et al.(2007), and also improved the upper limits for heavy elements; in addtion, we also measured the Si abundance and an upper limit for Zn. NLTE and3D corrections were calcu-lated for this star for the first time, and with the much more accurate abundances, the abundance pattern of HE0557-4840was compared with several nucleosynthesis models.