Probing the Galactic Chemical Evolution of Si and Ti with Presolar Sic Grains

Presolar SiC grains of the types mainstream, Y, and Z are believed to have formed in thermally pulsing asymptotic giant branch stars with a range of metallicity: mainstream grains in stars of close-to-solar metallicity, Y grains in stars of around half-solar metallicty, and Z grains in stars of around one-third solar metallicity. From their Si and Ti isotopic ratios, it is possible to obtain information on both neutron capture processes that take place in the He intershell and initial compositions of the parent stars of the grains. Since Z grains formed in stars with the lowest metallicity, their study will likely provide insight into the Galactic chemical evolution of these elements as well as nuclear processes in low-metallicity stars. A preliminary comparison of data on Z grains with models of AGB stars confirms that Z grains formed in low-metallicity stars (Z≤0.006). The 1 C/ C ratios of the Z grains indicate that in these stars cool bottom processing operates during the third dredge-up.