Silicon and Carbon Isotopic Ratios in AGB Stars: SiC Grain Data, Models, and the Galactic Evolution of the Si Isotopes

Abstract

PresolarSiCgrainsofthemainstream,Y,andZtypearebelievedtocomefromcarbonstars.WecomparedtheirCand Si isotopicratios withtheoretical modelsfor theenvelopecompositions of AGB stars.Two setsof 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 12 C/ 13 C in the envelope during third dredge-up are higher for higher stellar mass, lower metallicity, and lower mass-loss rate. Because the 22 Ne neutron source dominates Si nucleosynthesis, the effect of the 13 C 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 12 C/ 13 C 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 29 Si/ 28 Si ratio in the Galaxy as function of metallicity Z .A tZ <0:01 this ratio rises much faster than current Galactic evolution models predict and suggests an early source of the heavy Si isotopes not considered in these models. Subject headingg dust, extinction — Galaxy: evolution — nuclear reactions, nucleosynthesis, abundances — stars: AGB and post-AGB — stars: carbon