Si and C Isotopes in Presolar Silicon Carbide Grains From AGB Stars

We compare the Si and C isotopic data of presolar SiC grains with predictions of AGB star models computed with a wide range of input parameters. The models confirm an AGB star origin for SiC grains of type mainstream, Y and Z. Cool bottom processing during the AGB phase is required to explain simultaneously the Si and C data for Z grains. The Z grain data together with the new models indicate that the interstellar 29Si/28Si ratio was high at low metallicity, relative to models of Galactic Chemical Evolution.