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C. Sneden (U. Texas, Austin)
All of the elements of the Periodic Table beyond the iron peak are synthesized principally in neutron bombardment reactions. The abundance distributions of these elements in metal-poor halo Population II stars offer unique ways to gain insight into very early Galactic nucleosynthesis. This review highlights recent high resolution spectroscopic studies of the lowest metallicity stars, which have revealed two main features of their neutron-capture element contents. First, extremely large star-to-star scatter, reaching over two orders of magnitude, is observed in the overall bulk levels of the neutron-capture elements compared to the iron-peak elements. This is the most direct evidence of the effects of individual nucleosynthesis events that must have occurred in a poorly mixed halo of the newly formed Galaxy. Second, the detailed abundance ratios among the neutron-capture elements are very different than their solar-system values, and these ratios clearly point to the dominance of so-called rapid neutron-capture nucleosynthesis that accompanied the deaths of the short-lived high mass stars that were the first element donors of the Galaxy. The onset of contributions from slow neutron-capture events came later, as indicated by the generally higher metallicity levels of stars that have neutron-capture element abundance ratios that are more nearly like those of the Sun.
The long-lived radioactive element thorium is now being routinely detected in low metallicity stars, raising the possibility of direct age estimates for the Galactic halo. In at least one star, uranium also has been discovered. The current state of the thorium and uranium observational evidence is reviewed, estimates are given of the Galactic chronometric age from comparisons of thorium and uranium contents to those of lighter stable neutron-capture elements, and suggestions are made for improving the reliability of this age estimate.
This work has been supported by several NSF and NASA STScI grants, most recently by grants AST-9987162 and GO-8342 and GO-8111.