ABUNDANCES OF NEUTRON-CAPTURE ELEMENTS IN METAL-POOR STARS

We use a large set of high S/N, high resolution spectra of 19 stars wi th -2.8 <[Fe/H]< 0 to study the abundances of neutron-rich elements in metal-poor stars. Basic data (atmospheric parameters, iron abundances , abundance indices, atomic and line parameters) are carefully examine d both for the Sun and for the program stars, and extensive use is mad e of comparisons with synthetic spectra. New analyses of solar abundan ces of Sr, La, and Ce are presented; deduced abundances agree well wit h meteoritic results. Our stellar abundances are briefly compared with nucleosynthesis predictions. The main results are: 1. Those elements whose solar abundances are mainly attributed to the s-process (e.g. Ba and La) are overdeficient in extremely metal-poor stars ([Fe/H]< -2) with respect to those elements whose solar abundances are mainly attri buted to the r-process (e.g. Eu). We did not find any clear evidence f or a plateau in abundance ratios like [Ba/Eu] at these low values of [ Fe/H]. 2. Eu itself begins to decline (with respect either to Fe or Mg ) in the most metal-poor stars, with perhaps a sharp drop in stars wit h [Fe/H]< -2.5. If the r-process mainly occurs in SN explosions of mas sive stars, then the abundances of its products in the ejecta are a fu nction of initial stellar mass and/or metallicity. 3. The abundance pa ttern of neutron-capture elements in metal-poor stars show clear diffe rences with respect to scaled solar-system r-process nucleosynthesis p redictions; e.g. there is a relative excess of Ba (attributed mainly t o the s-process). This pattern may be explained if the contribution of the s-process to the solar abundances of heavy elements is overestima ted, or the production of heavy elements through the r - process was d ifferent when [Fe/H] was much lower than the present value, or there w as an early onset of the contribution by the main component of the s-p rocess in the Galactic chemical evolution.