Presolar Al 2O 3 grains as probes of stellar nucleosynthesis and galactic chemical evolution

Abstract

Ninety-two presolar oxide grains have been isolated from primitive meteorites and have been analyzed for their O-isotopic ratios. Fifty-two of these have been analyzed for their MgAl isotopic compositions as well. An origin around red giant stars is likely for the majority of grains, which have 17O excesses and moderate 18O depletions, relative to solar. However, many individual stars with different masses and initial compositions are required to explain the range of O-isotopic ratios and inferred 26Al 27Al ratios observed in the grains. Grains with 17O and 18O depletions probably originated in O-rich red giants of very low mass (M≲1.4M ⊙) and low metallicity; these grains' compositions may reflect the chemical evolution of the Galaxy. Another group of grains has large 18O depletions, suggestive of extra mixing in low-mass asymptotic giant branch (AGB) stars. A few grains have enrichments in 18O. Possible explanations for these grains include dredge-up of this isotope in early thermal pulses in AGB stars or an origin in red giants of unusually high metallicity. One grain is very highly enriched in 17O and depleted in 18O, and may have formed in a AGB star undergoing hot-bottom-burning or in a massive star in the Of-WN phase.