ar X iv : a st ro - p h / 02 10 12 5 v 1 4 O ct 2 00 2 1 Observations of Neutron - Capture Elements in the Early Galaxy

Neutron-capture elements in low metallicity Galactic halo stars vary widely both in overall contents and detailed abundance patterns. This review discusses recent observational results on the n-capture elements, discussing the implications for early Galactic nucleosynthesis of: (a) the star-to-star " bulk " variations in the n-capture/Fe abundance ratios; (b) the distinct signature of rapid n-capture synthesis events in many (most?) of the lowest metallicity stars; (c) the existence of metal-poor stars heavily enriched in the products of slow n-capture synthesis reactions; and (d) the now-routine detection of radioactive thorium (and even uranium in one and possibly two cases) in the spectra of metal-poor stars. Neutron-capture elements are those heavy elements whose isotopes are primarily synthesized in neutron bombardment reactions; these include all elements with Z > 30. The relative rates of neutron ingestion by nuclei compete with β-decay rates to determine the final abundance distribution. If the neutron flux is small enough that all unstable nuclei may undergo β-decay between successive neutron captures, element synthesis proceeds up the valley of β stability. This slow neutron addition is called the s-process, and occurs principally in the He-burning zones of low and intermediate mass AGB stars. If however the neutron flux is very large compared to β-decay rates, extremely neutron-rich nuclei are created which then rapidly undergo β-decay back toward stability. This rapid neutron-capture (n-capture) synthesis, called the r-process, can occur in several situations during supernova deaths of high-mass stars and their creation of neutron-star remnants. Isotopes of the n-capture elements may be created by the s-process, the r-process, or both. The solar-system abundances of these isotopes have been built in roughly equal measure of products of the r-and s-process. The manner in which isotopic (and hence * The authors all thank our colleagues who have contributed to the results presented here.