Probing the Neutron‐Capture Nucleosynthesis History of Galactic Matter

The heavy elements formed by neutron‐capture processes have an interesting history from which we can extract useful clues to and constraints upon both the characteristics of the processes themselves and the star formation and nucleosynthesis history of Galactic matter. Of particular interest in this regard 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‐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 heavy s‐process (e.g., barium) enrichment with increasing metallicity, evident in the abundances of more metal rich halo stars and disk stars, reflect the delayed contributions from the low‐ and intermediate‐mass (M∼1–3 M⊙) stars that provide the site for the main s‐process nucleosynthesis component during the asymptotic giant branch phase of their evolution. New abundance data in the mass region 60≲A≲130 are providing insight into the identity of possible alternative r‐process sites. We review recent observational studies of heavy‐element abundances in both low‐metallicity halo stars and disk stars, discuss the observed trends in light of nucleosynthesis theory, and explore some implications of these results for Galactic chemical evolution, nucleosynthesis, and nucleocosmochronology.