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 CS 31062-050 and LP 625-44. CS 31062-050 is known to be a so-called CEMP-r/s star, which exhibits large excesses of s-process elements such as barium (Ba) and lead (Pb), as well as a significant enhancement of europium (Eu) that cannot be explained by conventional s-process production in asymptotic giant branch star models. Our analysis of the high-resolution spectrum for this object has determined, for the first time, the abundances of Ir and Os, elements in the third peak of the r-process nucleosynthesis. They also exhibit significant excesses relative to the predictions of standard s-process calculations. These two elements are not detected in a similar quality spectrum of LP 625-44; the derived upper limits on their abundances are lower than the abundances in CS 31062-050. We compare the observed abundance patterns of neutron-capture elements, including Os and Ir, in these two stars with recent model calculations of the s-process, and we discuss possible interpretations.