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

Abstract

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.