S-PROCESS BA ISOTOPIC COMPOSITIONS IN PRESOLAR SIC FROM THE MURCHISONMETEORITE

We have measured, by thermal ionization mass spectrometry, the isotopi c composition of Ba in interstellar SiC-enriched samples (the KJ serie s) prepared from the Murchison meteorite. Ba in these samples is stron gly enriched in components plausibly understood as s-process nucleosyn thetic products. Inferred s-process compositions are different from th e average s-process contribution to solar system materials in the sens e of indicating lower effective neutron exposure. There are variations in s-process compositions which correlate with grain size, indicating lower neutron exposure for the larger grains. The inferred s-process compositions differ significantly from previous theoretical prediction s, indicating the need for revision of relevant neutron capture cross sections and providing a target for more detailed modeling of s-proces s nucleosynthesis in specific stellar environments.