Analysis of neutron capture elements in metal-poor stars

We derived model atmosphere parameters (T-eff, log g, [Fe/H], V-t) for 90 m etal-deficient stars (-0.5 < [Fe/H] < -3), using echelle spectra from the E LODIE library (Soubiran et al. 1998). These parameters were analyzed and co mpared with current determinations by other authors. The study of the follo wing elements was carried out: Mg. Si. Ga, Sr, Y. Ba, La, Ce, Nd, and Eu. T he relative contributions of s- and r-processes were evaluated and interpre ted through theoretical computations of the chemical evolution of the Galax y. The chemical evolution models (Pagel & Tautvaisiene 1995, Timmes et al. 1995) depict quite well the behaviour of [Si/Fe], [Ca/Fe] with [Fe/H]. The trend of [Mg/Fe] compares more favourably with the computations of Pagel & Tautvaisiene: (1995) than those of Timmes et al. (1995). The runs of n-capt ure elements vs, metallicity are described well both by the model of Pagel & Tautvaisiene (1995, 1997) and by the model of Travaglio ct al. (1999) at (Fe/H] > -1.5, when the matter of the Galaxy is sufficiently homogeneous. T he analysis of n-capture element abundances confirms the jump in [Ba/Fe] at [Fe/H]= -2.5. Some stars front our sample at [Fe/H] < -2.0 show a large sc atter of Sr, Ba, Y, Ce. This scatter is not caused by the errors in the mea surements, and may reflect the inhomogeneous nature of the prestellar mediu m at early stages of galactic evolution. The matching of [Ba/Fe], [Eu/Fe] v s. [Fe/H] with the inhomogeneous model by Travaglio et al. (2001a) suggests that at [Fe/H] < -2.5, the essential contribution to the n-rich element ab undances derives from the r-process. The main sources of these processes ma y be low mass SN II. The larger dispersion of s-process element abundances with respect to a-rich elements may arise both from the birth of metal-poor stars in globular clusters with following different evolutionary paths and (or) from differences in s-element enrichment in Galaxy populations.