2000ApJ...530..783W

New high-resolution, very high signal-to-noise spectra of ultra-metal-poor (UMP) giant stars HD 115444 and HD 122563 have been gathered with the High-Resolution Echelle Spectrometer of the McDonald Observatory 2.7 m telescope. With these spectra, line identification and model atmosphere analyses have been conducted, emphasizing the neutron-capture elements. Twenty elements with Z>30 have been identified in the spectrum of HD 115444. This star is known to have overabundances of the neutron-capture elements, but it has lacked a detailed analysis necessary to compare with nucleosynthesis predictions. The new study features a line-by-line differential abundance comparison of HD 115444 with the bright, well-studied halo giant HD 122563. For HD 115444, the overall metallicity is [Fe/H]≃-3.0. The abundances of the light and iron-peak elements generally show the same pattern as other UMP stars (e.g., overdeficiencies of manganese and chromium, overabundances of cobalt), but the differential analysis indicates several nucleosynthesis signatures that are unique to each star.

Synthetic spectrum analyses reveal substantial overabundances of the heavier neutron-capture elements (Z≥56; elements barium and beyond) in HD 115444. Thus with [Eu/Fe]≃+0.9, for example, HD 115444 is a moderate version of the extremely neutron-capture-rich UMP giant CS 22892-052 ([Fe/H]≃-3.1, [Eu/Fe]≃+1.7). The abundance pattern of the heavier neutron-capture elements is consistent with scaled solar system r-process-only abundances (with little contribution from the s-process). In HD 115444, [Ba/Eu]=-0.73, while in CS 22892-052 this ratio is -0.79. Thus HD 115444 becomes the second UMP r-process-rich halo giant unambiguously identified from a very detailed abundance analysis. Abundances of the lighter neutron-capture elements strontium, yttrium, and zirconium are, however, nearly identical in HD 115444 and HD 122563.

Along with the heavier neutron-capture elements, the 4019 Å line of Th II has been detected in HD 115444, yielding log {epsiv}(Th) = -2.23±0.07. Comparing the observed thorium abundance in HD 115444, along with CS 22892-052, with other theoretical estimates of the time-zero abundance suggests an age for both of these UMP stars of 15.6±4 Gyr, consistent with previous radioactive age estimates for CS 22892-052 and other Galactic and cosmological age determinations.