Some insight on the first generation of stars can be obtained from the
chemical composition of their direct descendants, extremely metal-poor stars
(EMP), with metallicity less than or equal to 1/1000 of the solar metalllicity.
Such stars are exceedingly rare, the most successful surveys, for this purpose,
have so far provided only about 100 stars with 1/1000 the solar metallicity and
4 stars with about 1/10000 of the solar metallicity. The Sloan Digital Sky
Survey has the potential to provide a large number of candidates of extremely
low metallicity. X-Shooter has the unique capability of performing the
necessary follow-up spectroscopy providing accurate metallicities and abundance
ratios for several elements (Mg, Al, Ca, Ti, Cr, Sr,...) for EMP candidates. We
here report on the results for the first two stars observed in the course of
our franco-italian X-Shooter GTO. The two stars were targeted to be of
metallicity around -3.0, the analysis of the X-Shooter spectra showed them to
be of metallicity around -2.0, but with a low alpha to iron ratio, which
explains the underestimate of the metallicity from the SDSS spectra. The
efficiency of X-Shooter allows an in situ study of the outer Halo, for the two
stars studied here we estimate distances of 3.9 and 9.1 Kpc, these are likely
the most distant dwarf stars studied in detail to date.Comment: Invited review at the Conference: X-shooter 2010: in memory of R.
Pallavicini, To be published on Astronomische Nachrichten, 1 reference
changed, tables 2 and 3 sorted by atomic numbe
