Carbon-enhanced Metal-poor Stars: Osmium and Iridium Abundances in the Neutron-Capture-enhanced Subgiants CS 31062-050 and LP 625-44

We have investigated the abundances of heavy neutron-capture elements,including osmium (Os) and iridium (Ir), in the two Carbon-Enhanced Metal-Poor(CEMP) subgiants CS31062-050 and LP625-44. CS31062-050 is known to be aso-called CEMP-r/s star, which exhibits large excesses of s-process elementssuch as barium (Ba) and lead (Pb), as well as a significant enhancement ofeuropium (Eu) that cannot be explained by conventional s-process production inAsymptotic Giant Branch star models. Our analysis of the high-resolutionspectrum for this object has determined, for the first time, the abundances ofIr and Os, elements in the third peak of the r-process nucleosynthesis. Theyalso exhibit significant excesses relative to the predictions of standards-process calculations. These two elements are not detected in asimilar-quality spectrum of LP625-44; the derived upper limits on theirabundances are lower than the abundances in CS31062-050. We compare theobserved abundance patterns of neutron-capture elements, including Os and Ir,in these two stars with recent model calculations of the s-process, and discusspossible interpretations.Comment: 12 pages, 2 figures, 1 table, ApJ Letter, in pres