The Galactic Evolution of Si, Ti, and O Isotopic Ratios

Abstract

Most meteoritic presolar grains of SiC and condensed around red giant and asymptotic giant Al 2 O 3 branch (AGB) stars prior to the formation of the solar system. Here we use new and previously published isotopic data of presolar SiC and grains to constrain the Galactic chemical evolution Al 2 O 3 (GCE) of Si, Ti, and O isotopic ratios. The 12C/13C ratios of the SiC grains limit the amount of He-shell material mixed to the surface of the parent AGB stars during third dredge-up to a few percent. Thus, the linear correlations observed between the SiC Si and Ti isotopic ratios probably re—ect 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 s2 —t to the SiC isotopic data has allowed us to accurately estimate the relative GCE paths for the Si and Ti isotopes. The —t estimates the nucleosynthetic components of the grainscompositions, 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 30Si and 47Ti in the —t increase signi—cantly more slowly with metallicity, relative to the other isotopes, than predicted by the corrected GCE model. Con- versely, 49Ti increases more rapidly in the —t than predicted. These discrepancies probably re—ect errors in the supernova models used to calculate the GCE paths. Our —t also suggests that the typical ISM at solar metallicity is enriched in 29Si and 30Si (by D8% and D5%, respectively) relative to solar. The GCE of the O isotopes cannot yet be so well constrained, but the data are most consistent with a Al 2 O 3 GCE path that passes close to solar rather than one that is 18O depleted as suggested by measurements of molecular clouds. The inferred depletion of 29Si and 30Si in the Sun could be explained by the addi- tion or removal of a small amount of supernova material. However, we can probably rule out a super- nova explanation for the apparent enrichment of solar O relative to young stars and the local ISM and 18O relative to molecular clouds. Subject headings: dust, extinctionGalaxy: evolutionISM: abundances ¨ nuclear reactions, nucleosynthesis, abundances ¨ stars: AGB and post-AGB