Observational constraints on  the critical metallicity for low mass star formation

The Universe emerged from the Big Bang with a very simple chemical
composition: hydrogen, helium and traces of lithium.  The first stars
that formed had this chemical composition, however, from the
theoretical point of view only very massive stars should have formed,
due to the lack of efficient cooling mechanisms that allow the
formation of low mass stars.  Thus there should exist a critical
metallicity, below which no low-mass stars, that are long-lived and
may still be observed at the present time, can be formed.  To put
observational constraints on this critical metallicity we have to
search for extremely metal-poor stars and derive the metal-weak tail
of the metallicity distribution function.  The critical metallicity
should manifest itself as a sharp drop in the MDF.  In this search one
finds many extremely metal-poor stars (EMP) that hold in their
atmospheres the fossil record of the chemical composition of the early
phases of the Galactic evolution.  The chemical analysis of such
objects provides important constraints on these early phases and
especially on the masses of their progenitors that produced the metals
observed in their atmospheres.  To pursue this objective it is
necessary to treat large amounts of data.  With an automatic
procedure, we analysed objects with colours of Turn-Off stars from the
Sloan Digital Sky Survey to select a sample of candidate EMP
stars. During the French-Italian X-Shooter GTO, used as a pilot
programme, we observed a small sample of these candidates.  We could
confirm the low metallicity of our sample of stars, and we succeeded
in finding the present record metal-poor star: SDSS J102915+172927
with [M/H]=-5, Z<= 6.9x10^{-7}.  This star shows no enhancement of CNO
elements, as do the previously found extremely iron-poor stars and,
surprisingly, shows no lithium.