Silicon and carbon isotopic ratios in AGB stars: SiC grain data, models, and the galactic evolution of the Si Isotopes

Abstract

Presolar SiCgrains of the mainstream,Y, and Z type are believed to come fromcarbon stars.We compared theirCand Si isotopic ratios with theoretical models for the envelope compositions of AGB stars. Two sets of models (FRANEC and Monash) use a range of stellar masses (1.5–5M ) and metallicities, different prescriptions for mass loss, and two sets of neutron-capture cross sections for the Si isotopes. They predict that the shifts in Si isotopic ratios and the increase of 12C/13C in the envelope during third dredge-up are higher for higher stellar mass, lower metallicity, and lower mass-loss rate. Because the 22Ne neutron source dominates Si nucleosynthesis, the effect of the 13C source is negligible. Comparison of the model predictions with grain data confirms an AGB origin for these grains, with Yand Z grains having originated in stars with lower than solar metallicity. The Si isotopic ratios of the Z grains favor the Si cross sections by Guber et al. over those by Bao et al. The 12C/13C ratios of low-metallicity models are much higher than those found in Z grains, and cool bottom processing must be invoked to explain the grains’ C isotopic ratios. By combining Z grain Si data with the models, we determined the evolution of the 29Si/ 28Si ratio in the Galaxy as function of metallicity Z. At Z