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 sola r system. Here we use new and previously published isotopic data of presola r SiC and Al2O3 grains to constrain the Galactic chemical evolution (GCE) o f Si, Ti, and O isotopic ratios. The C-12/C-13 ratios of the SiC grains lim it the amount of He-shell material mixed to the surface of the parent AGE s tars during third dredge-up to a few percent. Thus, the linear correlations observed between the SiC Si and Ti isotopic ratios probably reflect the av erage GCE trends of these elements. Moreover, the close proximity of these trends to the solar isotopic composition indicates that the latter cannot b e very unusual for these elements. A chi(2) fit to the SiC isotopic data ha s allowed us to accurately estimate the relative GCE paths for the Si and T i isotopes. The fit estimates the nucleosynthetic components of the grains' compositions, the metallicities of their parent stars, and the mean GCE pa ths of the isotopes. For most of the isotopes, our results agree remarkably well with the GCE calculations of Timmes, Woosley, & Weaver and Timmes & C layton after they are corrected to pass through solar. However, the abundan ces of both Si-30 and Ti-47 in the fit increase significantly more slowly w ith metallicity, relative to the other isotopes, than predicted by the corr ected GCE model. Conversely, Ti-49 increases more rapidly in the fit than p redicted. These discrepancies probably reflect errors in the supernova mode ls 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% a nd similar to 5%, respectively) relative to solar. The GCE of the O isotope s 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 de pleted as suggested by measurements of molecular clouds. The inferred deple tion of Si-29 and Si-30 in the Sun could be explained by the addition or re moval of a small amount of supernova material. However, we can probably rul e out a supernova explanation for the apparent enrichment of solar O relati ve to young stars and the local ISM and O-18 relative to molecular clouds.