Deep SDSS Optical Spectroscopy of Distant Halo Stars. III. Chemical
  analysis of extremely metal-poor stars
  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 SDSS. We follow a methodology 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. 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 {\alpha}-elements, ~ +0.4, although
a number of stars for which only [Fe/H] upper limits could be estimated point
to higher [{\alpha}/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.
