The galactic evolution of Si, Ti, and O isotopic ratios

Most meteoritic presolar grains of SiC and Al2O3 condensed around red giant and asymptotic giant branch (AGB) stars prier to the formation of the solar system. Here we use new and previously published isotopic data of presolar SiC and Al2O3 grains to constrain the Galactic chemical evolution (GCE) of Si, Ti, and O isotopic ratios. The C-12/C-13 ratios of the SiC grains limit the amount of He-shell material mixed to the surface of the parent AGE stars during third dredge-up to a few percent. Thus, the linear correlations observed between the SiC Si and Ti isotopic ratios probably reflect the average GCE trends of these elements. Moreover, the close proximity of these trends to the solar isotopic composition indicates that the latter cannot be very unusual for these elements. A chi(2) fit to the SiC isotopic data has allowed us to accurately estimate the relative GCE paths for the Si and Ti isotopes. The fit estimates the nucleosynthetic components of the grains' compositions, the metallicities of their parent stars, and the mean GCE paths of the isotopes. For most of the isotopes, our results agree remarkably well with the GCE calculations of Timmes, Woosley, & Weaver and Timmes & Clayton after they are corrected to pass through solar. However, the abundances of both Si-30 and Ti-47 in the fit increase significantly more slowly with metallicity, relative to the other isotopes, than predicted by the corrected GCE model. Conversely, Ti-49 increases more rapidly in the fit than predicted. These discrepancies probably reflect errors in the supernova models used to calculate the GCE paths. Our fit also suggests that the typical ISM at solar metallicity is enriched in Si-29 and Si-30 (by similar to 8% and similar to 5%, respectively) relative to solar. The GCE of the O isotopes cannot yet be so well constrained, but the Al2O3 data are most consistent with a GCE path that passes close to solar rather than one that is O-18 depleted as suggested by measurements of molecular clouds. The inferred depletion of Si-29 and Si-30 in the Sun could be explained by the addition or removal of a small amount of supernova material. However, we can probably rule out a supernova explanation for the apparent enrichment of solar O relative to young stars and the local ISM and O-18 relative to molecular clouds.