  Accurate relative abundances have been obtained for a sample of 21 mildly
metal-poor stars from the analysis of high resolution and high signal-to-noise
spectra. With these accurate results, correlations between relative abundances
have been searched for, with a special emphasis on the neutron capture
elements. This analysis shows that the r elements are closely correlated to the
alpha elements, which is in agreement with the generally accepted idea that the
r-process takes place during the explosion of massive stars. The situation is
more complex as far as the s elements are concerned. Their relation with the
alpha elements is not linear. In a first group of stars, the relative abundance
of the s elements increases only slightly with the alpha elements overabundance
until the latter reaches a maximum value. For the second group, the s elements
show a rather large range of enhancement and a constant (and maximum) value of
the alpha elements overabundance. This peculiar behaviour leads us to
distinguish between two sub-populations of metal-poor stars, namely PopIIa
(first group) and PopIIb (second group). We suggest a scenario of formation of
metal-poor stars based on two distinct phases of chemical enrichment, a first
phase essentially consisting in supernova explosions of massive stars, and a
second phase where the enrichment is provided by stellar winds from
intermediate mass stars. More specifically, we assume that all thick disk and
field halo stars were born in globular clusters, from which they escaped,
either during an early disruption of the cluster (PopIIa) or, later, through an
evaporation process (PopIIb).
