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Hubble Space Telescope Observations of Neutron-Capture Elements in Very Metal Poor Stars

Using the Goddard High-Resolution Spectrograph (GHRS) of the Hubble Space Telescope (HST) we have detected the neutron-capture elements osmium, platinum, and lead in the very metal poor ([Fe/H] ≃ -2.8) Galactic halo star HD 115444. This star joins the metal-poor giant HD 126238 ([Fe/H] ≃ -1.7) as the second Galactic halo star for which "third neutron-capture peak" elements have been detected. We have also determined upper limits on these same elements for the neutron-capture deficient star HD 122563 ([Fe/H] ≃ - 2.7). We have identified zirconium and germanium features in the HST spectra of all three stars; this marks the initial identification of the latter element in halo stars. We compare and contrast the spectra and abundances of the neutron-capture elements in these three stars. Combining the new HST observations of the third neutron-capture peak elements of HD 115444 with ground-based data, we find the stellar abundance distribution, over a wider atomic number range than ever before possible, is consistent with the solar system r-process abundance curve. Comparison of this star to the more metal-rich HD 126238 finds that the ratio of the third-peak element abundances to that of the pure r-process element europium is the same in both stars. Earlier ground-based studies have found that abundances of the lighter elements, such as barium and europium, in HD 115444 exceed those of HD 122563 by ~0.7 dex. Our osmium, platinum, and lead upper limits show a similar difference and are not inconsistent with either the scaled solar r-process or the solar total distribution. Thus, the second and third neutron-capture peak element (Z ≥ 56) abundances in all three stars are consistent with scaled solar system r-process abundances. However, the zirconium abundance is approximately the same in HD 122563 and HD 115444, so this element (near the first neutron-capture peak) is overabundant (somewhat) in HD 122563 and (slightly) in HD 115444 with respect to the solar r-process abundances. Germanium (synthesized in approximately equal amounts by the r- and the s-process in solar material) is underabundant in all three stars, but does seem to scale with metallicity—it is identical in HD 115444 and HD 126238, but significantly greater in the higher metallicity HD 126238. These new results support previous observations that demonstrate the operation of the r-process, including the synthesis of the heaviest such elements, early in the history of the Galaxy. Implications of these results for early Galactic chemical evolution are discussed.