BARIUM ISOTOPIC COMPOSITION OF MAINSTREAM SILICON CARBIDES FROM MURCHISON: CONSTRAINTS FORs-PROCESS NUCLEOSYNTHESIS IN ASYMPTOTIC GIANT BRANCH STARS

Abstract

Short title: Barium isotopic composition of mainstream SiCs 10 NuGrid collaboration, http://www.nugridstars.org. ABSTRACTWe 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( 138 Ba/ 136 Ba) values are found, down to −400 ‰, which can only be modeled with a flatter 13 C profile within the 13 C pocket than is normally used. The dependence of δ( 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(α,n) 25 Mg rate in the stellar temperature regime relevant to AGB stars, based on δ( 134 Ba/ 136 Ba) values of mainstream grains. We found two nominally mainstream grains with strongly negative δ( 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 ½ = 2.3 Ma) to 135 Ba in individual SiC grains, indicating condensation of barium, but not cesium into SiC grains before 135 Cs decayed.

s-Processing from MHD-induced mixing and isotopic abundances in presolar SiC grains

“…Anyhow, an important, independent confirmation that the 13 C reservoir had to be rather different from what had been often adopted in the past came recently from the analysis of s-elements in presolar SiC grains made by Liu et al (2014a), Liu et al (2014b) and Liu et al (2015). These authors compared isotopic and elemental ratios of s-elements in individual presolar SiC grains, as measured either by themselves or by Nicolussi et al (1997) and Barzyk et al (2007), with s-process computations.…”

Partial mixing and the formation of 13C pockets in AGB stars: effects on the s-process elements

“…Because such compositions can be measured to very high precision, particularly when using the Resonant Ionisation Mass Spectromety (RIMS) technique (e.g. Liu et al 2014aLiu et al ,b, 2015, stardust SiC grains provide excellent constraints for stellar models of the s process. On the other hand, we do not know a priori the initial mass and metallicity of the parent star of each grain.…”

The 13C Neutron Source and s-Processing in AGB Stars

“…Recent spectroscopic observations in young open clusters and experimental constraints from presolar grains [8,9] suggested that s-processing should occur in 13 C reservoirs both larger and differently-shaped than previously assumed. We found in magnetic buoyancy a physical mechanism suitable to produce an extended 13 C-pocket (5 × 10 −3 M ⊙ ) with a rather flat 13 C distribution.…”