Correlated Strontium and Barium Isotopic Compositions of Acid-Cleaned Single Mainstream Silicon Carbides From Murchison

Abstract

We present strontium, barium, carbon, and silicon isotopic compositions of 61 acidcleaned presolar SiC grains from Murchison. Comparison with previous data shows that acid washing is highly effective in removing both strontium and barium contamination. For the first time, by using correlated 88 Sr/ 86 Sr and 138 Ba/ 136 Ba ratios in mainstream SiC grains, we are able to resolve the effect of 13 C concentration from that of 13 C-pocket mass on s-process nucleosynthesis, which points towards the existence of large 13 C-pockets with low 13 C concentrations in AGB stars. The presence of such large 13 C-pockets with a variety of relatively low 13 C concentrations seems to require multiple mixing processes in parent AGB stars of mainstream SiC grains.

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

“…Also the 13 C abundance inside the reservoir must be different than commonly inferred previously, remaining rather flat and at a low concentration over most of the pocket mass. These indications become now general constraints that must be taken into account by any mixing model aiming at producing an adequate reservoir of 13 C. In fact, as we shall see in the next Sections, our suggestions for the mixing of protons into the He-rich zone at TDU yield a partially-mixed layer that has the general characteristics suggested by Liu et al (2015). Basically, the papers quoted so far, although not adopting a specific physical model, derived from their exploratory approach convincing constraints on their parameters, which must now be repoduced by any further modelling.…”

“…For the rest, they found that purely stellar and Galactic observational data can be fitted, with different combinations of the parameters, by rather different choices of the 13 C reservoir. If confirmed, this indication would emphasize even more the relevance of presolar SiC data and of the Liu et al (2014a), Liu et al (2014b) and Liu et al (2015) analysis in informing us on otherwise inaccessible details of the mixing required for driving s-processing.…”

“…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.…”

“…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. In particular, Liu et al (2015) performed extensive sensitivity studies by letting the main parameters of the 13 C reservoir (extension, abundance by mass of 13 C and 14 N, etc.) vary in an exploratory way.…”

Neutron-captures in Low Mass Stars and the Early Solar System Record of Short-lived Radioactivities

“…In this contribution we shall in particular consider the approach by [22] that was already shown to be able to produce extended 13 C reservoirs and to induce neutron captures capable of reproducing not only the solar-system distribution of s-elements, but also the more stringent constraints [27][28][29] that are posed by the isotopic composition of presolar SiC grains: see e.g. [30].…”