A SUBARU/HIGH DISPERSION SPECTROGRAPH 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, metal-poor ( 2:7 1⁄2Fe=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 and 3683 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 On-line material: machine-readable table