New ultra metal-poor stars from SDSS: follow-up GTC medium-resolution
  spectroscopy
  Context. The first generation of stars formed in the Galaxy left behind the
chemical signatures of their nucleosynthesis in the interstellar medium,
visible today in the atmospheres of low-mass stars formed afterwards. Sampling
the chemistry of low-mass provides insight into the first stars. Aims. We aim
to increase the samples of stars with extremely low metal abundances,
identifying ultra metal-poor stars from spectra with modest spectral resolution
and signal-to-noise ratio (S/N). Achieving this goal involves deriving reliable
metallicities and carbon abundances from such spectra. Methods. We carry-out
follow-up observations of faint, V>19, metal-poor candidates selected from SDSS
spectroscopy and observed with OSIRIS at GTC. The SDSS and follow-up OSIRIS
spectra have been analyzed using the FERRE code to derive effective
temperatures, surface gravities, metallicities and carbon abundances. In
addition, a well-known extremely metal-poor star has been included in our
sample to calibrate the analysis methodology. Results. We have observed and
analyzed five metal-poor candidates from modest-quality SDSS spectra. All stars
in our sample have been confirmed as extremely metal-poor stars, in the
[Fe/H]<-3.3 regime. We report the recognition of J173403+644632, a
carbon-enhanced ultra metal-poor dwarf star with [Fe/H]=-4.3 and [C/Fe]=+3.1.
