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

Liu, N. and Savina, M. R. and Davis, A. M. and Gallino, R. and Straniero, O. and Gyngard, F. and Pellin, M. J. and Willingham, D. G. and Dauphas, N. and Pignatari, Marco and Bisterzo, S. and Cristallo, S. and Herwig, F. (2014) Barium Isotopic Composition of Mainstream Silicon Carbides from Murchison: Constraints for s-Process Nucleosynthesis in AGB Stars. ASTROPHYSICAL JOURNAL, 786 (1). ISSN 1538-4357

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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 δ(138Ba/136Ba) values are found, down to −400 permil, which can only be modeled with a flatter 13C profile within the 13C pocket than is normally used. The dependence of δ(138Ba/136Ba) predictions on the distribution of 13C within the pocket in AGB models allows us to probe the 13C profile within the 13C pocket and the pocket mass in asymptotic giant branch (AGB) stars. In addition, we provide constraints on the 22Ne(α,n)25Mg rate in the stellar temperature regime relevant to AGB stars, based on δ(134Ba/136Ba) values of mainstream grains. We found two nominally mainstream grains with strongly negative δ(134Ba/136Ba) 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, 130Ba and 132Ba, in single SiC grains. These isotopes are destroyed in the s-process in AGB stars. By comparing their abundances with respect to that of 135Ba, we conclude that there is no measurable decay of 135Cs (t1/2= 2.3 Ma) to 135Ba in individual SiC grains, indicating condensation of barium, but not cesium into SiC grains before 135Cs decayed.

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