Probing the Neutron‐Capture Nucleosynthesis History of Galactic Matter

The heavy elements formed by neutron capture processes have an interestinghistory from which we can extract useful clues to and constraints upon both thecharacteristics of the processes themselves and the star formation andnucleosynthesis history of Galactic matter. Of particular interest in thisregard are the heavy element compositions of extremely metal-deficient stars.At metallicities [Fe/H] <= -2.5, the elements in the mass region past barium (A>= 130-140 have been found (in non carbon-rich stars) to be pure r-processproducts. The identification of an environment provided by massive stars andassociated Type II supernovae as an r-process site seems compelling. Increasinglevels of heavy s-process (e.g., barium) enrichment with increasingmetallicity, evident in the abundances of more metal-rich halo stars and diskstars, reflect the delayed contributions from the low- and intermediate-mass (M\~ 1-3 Msol) stars that provide the site for the main s-process nucleosynthesiscomponent during the AGB phase of their evolution. New abundance data in themass region 60 <~ A <~ 130 is providing insight into the identity of possiblealternative r-process sites. We review recent observational studies of heavyelement abundances both in low metallicity halo stars and in disk stars,discuss the observed trends in light of nucleosynthesis theory, and exploresome implications of these results for Galactic chemical evolution,nucleosynthesis, and nucleocosmochronology.Comment: 47 pages, 2 tables, 11 figures; To appear in PAS