Neutron-Capture Element Abundances in Halo Stars

Abundance comparisons of neutron-capture elements – those formed in slow (sprocess) and rapid (r-process) neutron-capture nucleosynthesis – are providing important new information about the nature of the earliest Galactic nucleosynthesis and the first stellar generations (see recent reviews by Truran et al. 2002, Sneden & Cowan 2003 and Cowan & Thielemann 2004). Most previous studies have been restricted to elements detectable with ground-based observations. We have recently completed a long-term study of the n-capture elements Ge, Zr, Os and Pt – all of these elements with dominant atomic transitions in the UV – in a sample of 11 metal-poor Galactic halo stars using the Hubble Space Telescope, HST (Cowan et al. 2004). These observations were supplemented with Keck I High Resolution Echelle Spectrograph (HIRES) observations of the n-capture element Ir. Our focus has been to examine correlations among these elements in an attempt to help to identify the sites of nucleosynthesis for these elements and also to trace their evolution with Galactic metallicity.