NanoSIMS studies of Ba isotopic compositions in single presolar silicon carbide grains from AGB stars and supernovae

Abstract

We have studied 74 single presolar silicon carbide grains with sizes between 0.2 and 2.6 ?m from the Murchison and Murray meteorites for Ba isotopic compositions using NanoSIMS. We also analyzed 7 SiC particles either consisting of sub-micron-size SiC grains or representing a morphologically and isotopically distinct subgroup. Of the 55 (likely) mainstream grains, originating from asymptotic giant branch (AGB) stars, 32 had high enough Ba contents for isotopic analysis. For 26 of them, CsHx interferences were either negligible or could be corrected with confidence. They exhibit typical s-process Ba isotopic patterns with slightly higher than solar 134Ba/136Ba and lower than solar 135,137,138Ba/136Ba ratios. Results are generally well explained in the context of neutron capture nucleosynthesis in low mass (13 Mu) AGB stars and provide constraints on AGB models, by reducing the needed 13C spread from factor of ~20 down to 2. Out of the 19 supernova X grains, three had sufficient concentrations for isotopic analysis. They tend to exhibit higher than solar 134Ba/136Ba and 138Ba/136Ba ratios, close to solar 137Ba/136Ba, and 135Ba/136Ba lower than solar but higher than in mainstream grains. This signature could indicate a mixture of n-burst type Ba with either normal Ba more s-process-rich than solar, or normal Ba plus weak s-process Ba. In the n-burst component Cs may have to be separated from Ba at ~10 years after the SN explosion. Depending on predictions for its composition, another possibility is early separation (at ~1 year) coupled with addition of some unfractionated n-burst matter. Abundances of trace elements (Sr, Zr, Cs, La, and Ce) analyzed along with Ba signify that implantation may have been an important process for their introduction.