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L. R. Nittler, C. M. O'D. Alexander (Carnegie Institution of Washington)
An important question in the theory of galactic chemical evolution is the effect of inhomogeneous mixing of stellar ejecta in the interstellar medium. Although elemental abundances and isotopic ratios in the Galaxy should evolve monotonically on average, heterogeneous mixing of material from individual stars can in principle lead to large variations about the mean in localized regions. The Si isotopic composition of presolar SiC grains from meteorites is believedto largely reflect the initial variability of the grains' parent AGB stars, but it is unknown how much of this variability is due to mean chemical evolution and how much to incomplete mixing of material from distinct supernovae. 18O/16O ratios in presolar oxide grains in meteorites also reflect chemical evolution. Since these grains probably originated in a similar population of low mass stars to those that produced the SiC grains, they should provide some insight into the problem. We are modeling the effect of inhomogeneous mixing of supernova ejecta with the interstellar medium on Si and O isotopic ratios, as well as elemental abundances. Consideration of recent supernova yields, calculated with updated reaction rates (Hoffman et al, 2001,Ap. J., 549, 1085), suggests that inhomogeneous interstellar medium enrichment by supernovae should lead to a comparable level of variability of Si and O isotopic ratios. In contrast, the meteoritic measurements indicate a significantly higher level of variability for O compared to Si. This suggests that either the circumstellar oxide grains and SiC grains formed in very different stellar populations or that local enrichment of molecular clouds is not the correct explanation for isotopic variability in presolar grains. Monte Carlo modeling of inhomogeneous chemical evolution is in progress and should help quantify the level of allowed isotopic heterogeneity in the interstellar medium and aid in the interpretation of the presolar grain isotopic data.