Halo Star Abundances and r-Process Synthesis

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] ≤ -2.5, stellar abundance data (for both halo field stars and globular cluster stars) provides strong confirmation of the occurrence of a robust r-process mechanism for the production of the main r-process component, at mass numbers A ∼ 130-140. The identification of an environment provided by massive stars and associated Type II supernovae as an r-process site seems compelling. Scatter in the ratio [r-process/Fe] provides a measure of the level of inhomogeneity characteristic of the halo gas at that early epoch. 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. For the mass region A ∼ 130, the abundance patterns in even the most metal deficient stars are not consistent with the solar system r-process abundance distribution, providing evidence for the fact that the r-process isotopes identified in solar system matter are in fact the products of two distinct r-processes nucleosynthesis events. We review recent observational studies of heavy element abundances in low metallicity stars and explore some implications of these results for nucleosynthesis and early Galactic chemical evolution.