Analysis of neutron capture elements in metal-poor stars

We derived model atmosphere parameters (T-eff, log g, [Fe/H], V-t) for 90 metal-deficient stars (-0.5 < [Fe/H] < -3), using echelle spectra from the ELODIE library (Soubiran et al. 1998). These parameters were analyzed and compared with current determinations by other authors. The study of the following elements was carried out: Mg. Si. Ga, Sr, Y. Ba, La, Ce, Nd, and Eu. The relative contributions of s- and r-processes were evaluated and interpreted through theoretical computations of the chemical evolution of the Galaxy. 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-capture 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. The 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 scatter of Sr, Ba, Y, Ce. This scatter is not caused by the errors in the measurements, and may reflect the inhomogeneous nature of the prestellar medium at early stages of galactic evolution. The matching of [Ba/Fe], [Eu/Fe] vs. [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 abundances derives from the r-process. The main sources of these processes may 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.