Can Stellar Dynamics Explain the Metallicity Distributions of Presolar Grains?

Abstract

The majority of presolar circumstellar SiC and Al 2 O 3 grains found in meteorites are inferred to have originated in asymptotic giant branch (AGB) stars that were more metal-rich than the Sun despite being much older than the Sun. We test the suggestion of Clayton that this reflects the overwhelming diffusion of AGB stars from the more metal-rich inner Galaxy to the protosolar cloud. Weighting the orbital diffusion model of Wielen, Fuchs, & Dettbarn to take into account the higher density of AGB stars and molecular clouds in the inner Galaxy, we find that most parent stars of presolar grains should have diffused to the presolar cloud from their birthplaces at smaller Galactocentric radii. However, the majority of these stars are expected to have had subsolar metallicities, in conflict with the grain data. The self-consistency of the diffusion model is discussed in the context of stellar abundance data in the solar neighborhood. The disagreement between the model predictions and the observations might indicate that the inferred metallicities of presolar grain parent stars are wrong, that the adopted diffusion parameters are incorrect, and/or that stellar orbital diffusion is not the correct explanation for the grain data.