AAS 196th Meeting, June 2000
Session 35. High Resolution Spectroscopy at Visible and Ultraviolet Wavelengths
Topical Session Oral, Wednesday, June 7, 2000, 8:30-10:00am, 10:45am-12:30pm, 2:30-4:00pm, 4:15-6:00pm, Highland A/K

## [35.08] Prospecting for Precious Metals in Ultra-Metal-Poor Stars

R. S. French (University of Texas at Austin)

The chemical compositions of the most metal-poor halo stars are living records of the very early nucleosynthetic history of the Galaxy. Only a few prior generations, if not a single one, of element-donating supernovae could have been responsible for the heavy elements observed in ultra-metal-poor (UMP; [Fe/H] < --2.5) stars. Abundances of the heavy neutron-capture elements (Z > 30) can yield direct information about the supernova progenitors to UMP stars, and abundances of unstable thorium and uranium (Z = 90, 92) can potentially provide age estimates for the Galactic halo. Already, many studies have demonstrated that abundances of rare-earth elements (56 \leq Z \leq 72) in UMP stars are completely consistent with their production in rapid neutron-capture synthesis (r-process) events, usually believed to occur during supernovae explosions. Therefore, mapping the entire abundance pattern of UMP stars is of significant interest. In particular, abundances of the most massive stable elements (Os arrow Pb or 76 \leq Z \leq 82) could provide crucial information about the so-called third r-process peak,'' and are critical to the radioactive-dating technique that uses unstable thorium as a chronometer. Until recently, abundance determinations for these elements have been virtually non-existent, as the strongest relevant transitions lay in the vacuum UV, inaccessible to ground-based observation. The availability of high-resolution space-based spectrometers has opened up new regions of spectral coverage, including precisely the range in wavelength needed to make these sensitive measurements. We have undertaken a study of about 10 metal-poor halo giants to determine the abundances of several of the heaviest neutron-capture elements including platinum, osmium, lead, and gold. Preliminary results indicate that the abundance pattern of heavy neutron-capture elements (56 \leq Z \leq 82) in UMP stars does mimic a scaled solar system r-process. Thus, the ability to estimate the initial abundances of thorium and uranium is greatly reinforced.