Abundances Of Neutron-Capture Elements In Metal-Poor Stars
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We use a large set of high S/N, high resolution spectra of 19 stars with -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 examined both for the Sun and for the program stars, and extensive use is made of comparisons with synthetic spectra. New analyses of solar abundances of Sr, La, and Ce are presented; deduced abundances agree well with 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 attributed to the r-process (e.g. Eu). We did not find any clear evidence for 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 with [Fe/H]< -2.5. If the r-process mainly occurs in SN explosions of massive stars, then the abundances of its products in the ejecta are a function of initial stellar mass and/or metallicity. 3. The abundance pattern of neutron-capture elements in metal-poor stars show clear differences with respect to scaled solar-system r-process nucleosynthesis predictions; e.g. there is a relative excess of Ba (attributed mainly to the s-process). This pattern may be explained if the contribution of the s-process to the solar abundances of heavy elements is overestimated, or the production of heavy elements through the r - process was different when [Fe/H] was much lower than the present value, or there was an early onset of the contribution by the main component of the s-process in the Galactic chemical evolution.