A Subaru/HDS study of Lead (Pb) abundances in eight s-process-element-rich, metal-poor stars

We report the abundances of neutron-capture elements in eight carbon-rich, metalpoor (−2.7 ≤[Fe/H]≤ −1.9) stars observed with the Subaru Telescope High Dispersion Spectrograph. The derived abundance patterns indicate that the neutron-capture elements in these objects primarily originated from s-process nucleosynthesis, although the [Ba/Eu] abundance ratios in some objects are lower than that of the solar-system s-process component. The present analysis has yielded the Pb abundances for seven objects, as well as an upper limit for one object, from use of the Pb I 4057 Å and 3683 Å lines. The values of [Pb/Ba] in these objects cover a wide range, between −0.3 and +1.2. Theoretical studies of s-process nucleosynthesis at low metallicity are required to explain this large dispersion of the [Pb/Ba] values. Variations in radial velocity have been found for two of the eight objects, suggesting that, at least in these instances, the observed excess of s-process elements is due to the transfer of material across a binary system including an AGB star. Comparisons with predictions of AGB nucleosynthesis models are discussed. Subject headings: nuclear reactions, nucleosynthesis, abundances – stars: abundances – stars: AGB and post-AGB – stars: carbon – stars: Population II National Astronomical Observatory, Mitaka, Tokyo, 181-8588 Japan; email: [email protected], [email protected] Department of Physics and Astronomy, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK; email: [email protected], [email protected] Research School of Astronomy & Astrophysics, The Australian National University, Mount Stromlo Observatory, Cotter Road, Weston, ACT 2611, Australia; [email protected] Department of Physics and Astronomy, Michigan State University, East Lansing, MI 48824-1116; email: [email protected] Based on data collected at the Subaru Telescope, which is operated by the National Astronomical Observatory of Japan.