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 particula r interest are the heavy element compositions of extremely metal-deficient stars. At metallicities [Fe/H] less than or equal to -2.5, stellar abundanc e data (for both halo field stars and globular cluster stars) provides stro ng confirmation of the occurrence of a robust r-process mechanism for the p roduction of the main r-process component, at mass numbers A greater than o r similar to 130-140. The identification of an environment provided by mass ive stars and associated Type II supernovae as an r-process site seems comp elling. Scatter in the ratio [r-process/Fe] provides a measure of the level of inhomogeneity characteristic of the halo gas at that early epoch. Incre asing levels of s-process enrichment with increasing metallicity reflect th e delayed contributions from the intermediate mass stars that provide the s ite for s-process nucleosynthesis during the AGE phase of their evolution. For the mass region A less than or similar to 130, the abundance patterns i n even the most metal deficient stars are not consistent with the solar sys tem r-process abundance distribution, providing evidence for the fact that the r-process isotopes identified in solar system matter are in fact the pr oducts 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 ear ly Galactic chemical evolution.