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

Presolar grains of the mainstream, Y and Z type are believed to have an origin in carbon stars. We compared the C and Si isotopic ratios of these grains [1] with the results of theoretical models for the envelope compositions of AGB stars. Two sets of models (FRANEC, Monash) use a range of stellar masses (1.5 to 5 MÖ), metallicities, different prescriptions for mass loss, and two sets of neutron-capture cross-sections for the Si isotopes [2, 3]. 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. The Guber et al. [3] cross-sections result in larger shift in the 30Si/28Si ratios and smaller shifts in the 29Si/28Si ratios than the Bao et al. [2] cross-sections. Because the 22Ne neutron source dominates Si nucleosynthesis, the effect of the 13C source is negligible.