Neutron Capture Nucleosynthesis in Early Galactic Environments

Abstract.

The heavy elements formed by neutron capture processes have an interesting history from which we can extract useful clues to and constraints upon the star formation and nucleosynthesis history of Galactic matter. Of particular interest are the heavy element compositions of extremely metal-deficient stars. At metallicities [Fe/H] $\le -2.5$, the elements in the mass region past barium (A $\approx$ 140) have been found to be pure r-process products. The identification of an environment provided by massive stars and associated Type II supernovae as an r-process site seems compelling. Increasing levels of s-process enrichment with increasing metallicity reflect the delayed contributions from the intermediate mass stars that provide the site for s-process nucleosynthesis during the AGB phase of their evolution. We review recent observational studies of heavy element abundances in low metallicity stars and explore same implications of these results for nucleosynthesis and early Galactic chemical evolution.