Oxygen isotopes in circumstellar Al_2O_3 grains from meteorites and stellar nucleosynthesis

Abstract

We have found a ~3 µm Al_2O_3 grain (B39) in the Bishunpur LL3.1 chondrite that is enriched in ^(17)O by a factor of ~ 6.8 (^(16)O/^(17)O = 385 ± 9) and depleted in 180 by almost 60% (^(16)O/^(18)O = 853 ± 30) relative to solar system oxygen and has an initial ^(26)Al/^(27)Al = 1.7 ± 0.2 x 10^3, ~34 times greater than the initial solar system value. The isotopic compositions of B39 and two other Al_2O_3 grains previously reported from the Orgueil and Murchison meteorites show that these grains formed directly from the ejecta of low-mass AGB stars with C/0 < 1. A simple theoretical analysis is presented showing that the oxygen systematics of the grains are a natural consequence of main-sequence evolution followed by first dredge-up. ^(26)AI is the result of third dredge-up. Circumstellar Al_2O_3 grains provide very precise isotopic data for stellar ejecta that complement spectroscopic observations of oxygen-rich stars. Isotopic differences indicate that the Al_2O_3 grains come from separate stars of different mass and initial oxygen composition that originated in molecular clouds different from the one in which the solar system formed.

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