On s-process abundance evolution in the galactic disk

The s-process abundances in C, S, and Ba II stars are examined to investigate whether such stars may have detectably influenced the galactic s-process abundance curve over the lifetime of the galactic disk. These peculiar red giants apparently exhibit two characteristic types of abundance curve, one resulting from neutron exposure distributions concentrated at low exposure levels, and the other from exposure distributions restricted to large levels. There is some evidence that both high- and low-mass stars can produce each type of abundance curve, thus probably ruling out the s-process as a probe of the stellar initial mass function at early epochs. Although stellar s-process abundances do not closely mimic solar/galactic abundances, it remains plausible that abundance evolution due to these stars may be too small to detect. The possible presence of a predisk component to the synthesis pool amplifies this conclusion.The solar system s-process buildup path branches at 93Zr such that tau/sub n/approx.0.3tau/sub β/. In the Ba II and S stars this branching is characterized by tau/sub n/<tau/sub β/, and probably by tau/sub n/very-much-less-thantau/sub β/, leading to speculation that solar system abundances may be composite in nature. Unfortunately, stellar observations appear to require too high an accuracy to be helpful in studying this question. Analyses of the principal branching points in the solar system buildup path indicate that solar abundances are consistent with a single component s-process occurring under more or less unique temperature and neutron flux conditions, but that uncertainties in the basic nuclear data prevent any certainty in this conclusion