Barium Isotopic Composition of Mainstream Silicon Carbides from Murchison: Constraints for s-Process Nucleosynthesis in AGB Stars

Abstract

We present barium, carbon, and silicon isotopic compositions of 38 acid-cleaned presolar SiC grains from Murchison. Comparison with previous data shows that acid washing is highly effective in removing barium contamination. Strong depletions in $\delta$(${}^{138}$Ba/${}^{136}$Ba) values are found, down to $-$400 permil, which can only be modeled with a flatter ${}^{13}$C profile within the ${}^{13}$C pocket than is normally used. The dependence of $\delta$(${}^{138}$Ba/${}^{136}$Ba) predictions on the distribution of ${}^{13}$C within the pocket in AGB models allows us to probe the ${}^{13}$C profile within the ${}^{13}$C pocket and the pocket mass in asymptotic giant branch (AGB) stars. In addition, we provide constraints on the ${}^{22}$Ne$(\alpha ,n{)}^{25}$Mg rate in the stellar temperature regime relevant to AGB stars, based on $\delta$(${}^{134}$Ba/${}^{136}$Ba) values of mainstream grains. We found two nominally mainstream grains with strongly negative $\delta$(${}^{134}$Ba/${}^{136}$Ba) values that cannot be explained by any of the current AGB model calculations. Instead, such negative values are consistent with the intermediate neutron capture process ($i$-process), which is activated by the Very Late Thermal Pulse (VLTP) during the post-AGB phase and characterized by a neutron density much higher than the $s$-process. These two grains may have condensed around post-AGB stars. Finally, we report abundances of two $p$-process isotopes, ${}^{130}$Ba and ${}^{132}$Ba, in single SiC grains. These isotopes are destroyed in the $s$-process in AGB stars. By comparing their abundances with respect to that of ${}^{135}$Ba, we conclude that there is no measurable decay of ${}^{135}$Cs ($t_{1/2}$= 2.3 Ma) to ${}^{135}$Ba in individual SiC grains, indicating condensation of barium, but not cesium into SiC grains before ${}^{135}$Cs decayed.

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