FIRST DETECTION OF PLATINUM, OSMIUM, AND LEAD IN A METAL-POOR HALO STAR-HD-126238

Using the Goddard High Resolution Spectrograph (GHRS) of the Hubble Sp ace Telescope (HST), we have detected the very heavy elements osmium, platinum, and lead in the metal-poor ([Fe/H] = -1.7) giant HD 126238, the first such detections of these elements in a Galactic halo star. O s and Pt are synthesized predominantly via rapid neutron captures (i.e ., the r-process), and this nuclear region defines one of the three ma jor r-process peaks identified in solar system material. Within the er ror limits, the elemental abundances of Os and Pt are consistent with solar system abundances scaled to the metallicity of HD 126238. Elemen t-by-element comparisons of the data from HST and from ground-based da ta demonstrate, for the first time, the operation of the r-process for all of the elements from Ba to Pt in the progenitor (or progenitors) of this halo star. The newly determined Pb abundance, predominantly pr oduced in slow neutron captures (i.e., the s-process) for solar materi al, can best be explained as resulting from a combination of s- and r- process nucleosynthesis contributions, prior to the formation of HD 12 6238. The s-process fraction is likely to have been generated only fro m the most massive stellar sites of the s-process.