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

Presolar SiC grains of the mainstream, Y, and Z type are believed to come from carbon stars. We compared their C and 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 (1.5-5 M_☉) 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 Y and 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 < 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.