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

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.