Barium isotopic composition of mainstream silicon carbides from Murchison: Constraints for s-process nucleosynthesis in asymptotic giant branch stars

• Published in: The Astrophysical journal 2014-05, Vol.786 (1)
• Main Authors: Liu, Nan, Davis, Andrew M., Pellin, Michael J., Dauphas, Nicolas, Savina, Michael R., Gallino, Roberto, Bisterzo, Sara, Straniero, Oscar, Cristallo, Sergio, Gyngard, Frank, Willingham, David G., Pignatari, Marco, Herwig, Falk
• Format: Article
• Language: English
• Subjects: ABUNDANCE; ALPHA REACTIONS; ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; BARIUM ISOTOPES; CAPTURE; CARBON 13; CESIUM 135; DISTRIBUTION; DUSTS; GIANT STARS; INTERMEDIATE NEUTRONS; ISOTOPE RATIO; MAGNESIUM 25; NEON 22 TARGET; NEUTRON DENSITY; NEUTRON EMISSION; NUCLEAR PHYSICS AND RADIATION PHYSICS; NUCLEOSYNTHESIS; S PROCESS; SILICON CARBIDES; STARS
• ISSN: 0004-637X; 1538-4357
• DOI: 10.1088/0004-637X/786/1/66

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({sup 138}Ba/{sup 136}Ba) values are found, down to –400‰, which can only be modeled with a flatter {sup 13}C profile within the {sup 13}C pocket than is normally used. The dependence of δ({sup 138}Ba/{sup 136}Ba) predictions on the distribution of {sup 13}C within the pocket in asymptotic giant branch (AGB) models allows us to probe the {sup 13}C profile within the {sup 13}C pocket and the pocket mass in AGB stars. In addition, we provide constraints on the {sup 22}Ne(α, n){sup 25}Mg rate in the stellar temperature regime relevant to AGB stars, based on δ({sup 134}Ba/{sup 136}Ba) values of mainstream grains. We found two nominally mainstream grains with strongly negative δ({sup 134}Ba/{sup 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 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, {sup 130}Ba and {sup 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 {sup 135}Ba, we conclude that there is no measurable decay of {sup 135}Cs (t {sub 1/2} = 2.3 Ma) to {sup 135}Ba in individual SiC grains, indicating condensation of barium, but not cesium into SiC grains before {sup 135}Cs decayed.