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 <= [Fe/H] <= -1.6) Galactic halo giant stars, based on Hubble Space Telescope UV and 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 degrees 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 alpha-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 degrees 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.