Deep SDSS optical spectroscopy of distant halo stars

Abstract

Aims. We present the results of an analysis for 107 extremely metal-poor (EMP) stars with metallicities less than [Fe/H] = −3.0, identified from 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 follow the same methodology as in previous papers of this series, based on comparisons of the observed spectra with synthetic spectra. The abundances of Fe, Ca, and Mg are determined by fitting spectral regions 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 analyse our sample with the SEGUE Stellar Parameter Pipeline, in order to obtain additional determinations of the atmospheric parameters, iron and alpha-element abundances, to compare with ours, and to infer [C/Fe] ratios. Results. We find that, in these moderate to low signal-to-noise and medium-resolution spectra in this metallicity regime, Ca is usually the only element that exhibits 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 measure [Ca/Fe] and [Mg/Fe] for EMP stars in the SDSS spectra, and conclude that most of the stars exhibit the usual level of enhancement for α-elements, ∼+0.4, although a number of stars for which only [Fe/H] upper limits could be estimated point to 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 identity 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.