Deep SDSS optical spectroscopy of distant halo stars - III. Chemical analysis of extremely metal-poor stars

摘要

Aims. We present the results of an analysis of 107 extremely metal-poor (EMP) stars with metallicities lower than [Fe/H] = ? 3.0 , identified in medium-resolution spectra in the Sloan Digital Sky Survey (SDSS). Our analysis provides estimates of the stellar effective temperatures and surface gravities, as well as iron, calcium, and magnesium abundances. Methods. We followed the same method as in previous papers of this series. The method is based on comparisons of the observed spectra with synthetic spectra. The abundances of Fe, Ca, and Mg were determined by fitting spectral regions that are dominated by lines of each element. In addition, we present a technique to determine upper limits for elements whose features are not detected in a given spectrum. We also analyzed our sample with the SEGUE stellar parameter pipeline to obtain additional determinations of the atmospheric parameters and iron and alpha-element abundances, which we thend compare with ours. In addition, we used these parameters to infer [C/Fe] ratios. Results. Ca is typically the only element in these spectra with a moderate to low signal-to-noise ratio and medium resolution in this metallicity regime with lines that are sufficiently strong to reliably measure its abundance. Fe and Mg exhibit weaker features that in most cases only provide upper limits. We measured [Ca/Fe] and [Mg/Fe] for EMP stars in the SDSS spectra and conclude that most of the stars exhibit the typical enhancement level for α -elements, ~ +0.4, although some stars for which only [Fe/H] upper limits could be estimated indicate higher [ α /Fe] ratios. We also find that 26% of the stars in our sample can be classified as carbon-enhanced metal-poor (CEMP) stars and that the frequency of CEMP stars also increases with decreasing metallicity, as has been reported for previous samples. We identify a rare, bright ( g = 11.90 ) EMP star, SDSS J134144.61 + 474128.6, with [Fe/H] = ? 3.27 , [C/Fe] = + 0.95 , and elevated magnesium ([Mg/Fe] = + 0.62 ), an abundance pattern typical of CEMP-no stars.