A Subaru/High Dispersion Spectrograph Study of Lead (Pb) Abundances in Eights‐Process Element–rich, Metal‐poor Stars

We report the abundances of neutron-capture elements in eight carbon-rich, metal-poor (-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 λ4057 and λ3683 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.