Chemical analysis of very metal-poor turn-off stars from SDSS-DR12
  The most metal-poor stars are the relics of the early chemical evolution of
the Galaxy. Their chemical composition is an important tool to constrain the
nucleosynthesis in the first generation of stars. The aim is to observe a
sample of extremely metal-poor star (EMP stars) candidates selected from the
Sloan Digital Sky Survey Data Release 12 (SDSS DR12) and determine their
chemical composition. We obtain medium resolution spectra of a sample of six
stars using the X-Shooter spectrograph at the Very Large Telescope (VLT) and we
used ATLAS models to compute the abundances. Five stars of the sample have a
metallicity [Fe/H] between -2.5 dex and -3.2 dex. We confirm the recent
discovery of SDSS J002314.00+030758.0. As a star with an extremely low [Fe/H]
ratio. Assuming the alpha-enhancement [Ca/Fe]=+0.4 dex, we obtain [Fe/H]=-6.1
dex. We could also determine its magnesium abundance and found that this star
exhibits a very high ratio [Mg/Fe] >= +3.60~dex assuming [Fe/H] = -6.13 dex. We
determined the carbon abundance and found A(C)=6.4 dex. From this carbon
abundance, this stars belongs to the lower band of the A(C)-[Fe/H] diagram.
