r-Process Abundance Signatures in Metal-Poor Halo Stars

John J. Cowan
University of Oklahoma

ABSTRACT

Abundance observations indicate the presence of rapid-neutron capture (i.e., r-process) elements in old Galactic halo and globular cluster stars. Recent observations of the r-process enriched star BD +17 3248 include new abundance determinations for the neutron-capture elements Cd I (Z=48), Lu II (Z = 71) and Os II (Z = 76), the first detections of these elements in metal-poor r-process enriched halo stars. Combining these and previous observations, we have now detected 32 n-capture elements in BD +17 3248 - this is the most of any metal-poor halo star to date. For the r-process rich stars, such as CS 22892-052 and BD +17 3248, abundance comparisons among the r-process-rich halo stars show that the heaviest neutron-capture elements (i.e., Ba and above) are consistent with a scaled solar system r-process abundance distribution. The lighter neutron-capture element abundances in these stars, however, do not conform to the solar pattern. These comparisons suggest the possibility of multiple synthesis mechanisms for the neutron-capture elements. The heavy element abundance patterns in most metal-poor halo stars do not resemble that of CS 22892-052, but the presence of heavy elements such as barium (Ba, Z =56) in nearly all metal-poor stars without s-process enrichment indicates that the r-process in the early Galaxy is a common phenomenon.