Constraints on Heterogeneous Galactic Chemical Evolution from Meteoritic Stardust

Abstract

Meteorites contain presolar grains that formed in stars prior to solar system formation and preserve a record of stellar and galactic evolutionary processes. We consider a stochastic model of heterogeneous mixing of supernova ejecta into a homogeneous interstellar medium (ISM), first presented by Lugaro et al. The predictions of the model for Si, Ti, and O isotopic ratios are compared with the compositions of presolar SiC, Al2O3, and MgAl2O4 grains. Model parameters that reproduce the range of Si isotopic ratios in SiC grains fail to reproduce the strong observed correlation between SiC Si and Ti isotopic ratios and the range of 18O/16O ratios in the oxide phases. The Si-Ti correlation in SiC implies that heterogeneous mixing of SNe ejecta can account for at most 40% of the range of grain isotopic compositions. This result further implies that the ejecta from diverse SNe are well mixed to the percent level in the ISM, consistent with recent results based on stellar and interstellar abundance measurements. However, the presolar grain data provide much more stringent limits on the range of relative element ratios (e.g., Mg/Fe) than can be derived from astronomical observations. The large scatter in metallicity observed for disk stars of a given age cannot be explained by heterogeneous mixing of stellar ejecta. The range of 18O/16O ratios measured in presolar oxide grains implies either that they formed in stars with a wider range of metallicities than the SiC progenitor stars or that their parent stars experienced moderate amounts of cool bottom processing.