Field halo stars: the globular cluster connection

High resolution and high signal-to-noise spectra of about 20 metal-poor stars have been analysed. The correlations between the relative abundances of 16 elements have been studied, with a special emphasis on the neutron-capture ones. This analysis reveals the existence of two subpopulations of field halo stars, namely Pop IIa and Pop IIb. They differ by the behaviour of the s-process elements versus the alpha and r-process elements. A scenario for the formation of these stars is presented, which closely relates the origin of field halo stars to the evolution of globular clusters. According to this scenario, the two sub-populations originate from two different stages in the globular cluster's chemical evolution. Based on observations carried out at the European Southern Observatory (La Silla, Chile)