Hubble Space Telescope Observations of Heavy Elements in Metal-poor Galactic Halo Stars

We present new abundance determinations of neutron-capture elements Ge, Zr, Os, Ir, and Pt in a sample of 11 metal-poor ( 3:1 1⁄2Fe/H 1:6) Galactic halo giant stars, based on Hubble Space Telescope UVand Keck I optical high-resolution spectroscopy. The stellar sample is dominated by r-process–rich stars such as the well-studied CS 22892 052 and BD +17 3248 but also includes the r-process–poor, bright giant HD 122563. Our results demonstrate that abundances of the third r-process peak elements Os, Ir, and Pt in these metal-poor halo stars are very well correlated among themselves and with the abundances of the canonical r-process element Eu (determined in other studies), thus arguing for a common origin or site for r-process nucleosynthesis of heavier (Z > 56) elements. However, the large (and correlated) scatters of [Eu, Os, Ir, Pt /Fe] suggest that the heaviest neutron-capture r-process elements are not formed in all supernovae. In contrast, the Ge abundances of all program stars track their Fe abundances, very well. An explosive process on iron peak nuclei (e.g., the -rich freezeout in supernovae), rather than neutron capture, appears to have been the dominant synthesis mechanism for this element at low metallicities: Ge abundances seem completely uncorrelated with Eu. The correlation (with very small scatter) of Ge and Fe abundances suggests that Ge must have been produced rather commonly in stars, even at early times in the Galaxy, over a wide range of metallicity. The Zr abundances show much the same behavior as Ge with (perhaps) somewhat more scatter, suggesting some variations in abundance with respect to Fe. The Zr abundances also do not vary cleanly with Eu abundances, indicating a synthesis origin different than that of heavier neutron-capture elements. Detailed abundance distributions for CS 22892 052 and BD +17 3248, combining the new elemental determinations for Os–Pt and recently published Nd and Ho measurements, show excellent agreement with the solar system r-process curve from the elements Ba to Pb. The lighter n-capture elements, including Ge, in general fall below the same solar system r-process curve that matches the heavier elements. Subject headinggs: Galaxy: abundances — Galaxy: evolution — Galaxy: halo — nuclear reactions, nucleosynthesis, abundances — stars: abundances — stars: Population II