Astronomy and Astrophysics – Astronomy
Scientific paper
Jul 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999apj...519..222a&link_type=abstract
The Astrophysical Journal, Volume 519, Issue 1, pp. 222-235.
Astronomy and Astrophysics
Astronomy
39
Ism: Dust, Extinction, Galaxy: Evolution, Ism: Abundances, Nuclear Reactions, Nucleosynthesis, Abundances, Stars: Agb And Post-Agb
Scientific paper
Most meteoritic presolar grains of SiC and Al_2O_3 condensed around red giant and asymptotic giant branch (AGB) stars prior to the formation of the solar system. Here we use new and previously published isotopic data of presolar SiC and Al_2O_3 grains to constrain the Galactic chemical evolution (GCE) of Si, Ti, and O isotopic ratios. The ^12Csolar^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 reflect 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 chi^2 fit to the SiC isotopic data has allowed us to accurately estimate the relative GCE paths for the Si and Ti isotopes. The fit estimates the nucleosynthetic components of the grains' compositions, 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 fit increase significantly more slowly with metallicity, relative to the other isotopes, than predicted by the corrected GCE model. Conversely, ^49Ti increases more rapidly in the fit than predicted. These discrepancies probably reflect errors in the supernova models used to calculate the GCE paths. Our fit also suggests that the typical ISM at solar metallicity is enriched in ^29Si and ^30Si (by ~8% and ~5%, respectively) relative to solar. The GCE of the O isotopes cannot yet be so well constrained, but the Al_2O_3 data are most consistent with a 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 addition or removal of a small amount of supernova material. However, we can probably rule out a supernova explanation for the apparent enrichment of solar O relative to young stars and the local ISM and ^18O relative to molecular clouds.
Nittler Larry R.
O'D. Alexander Conel M.
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