OXYGEN ISOTOPES IN CIRCUMSTELLAR AL2O3 GRAINS FROM METEORITES AND STELLAR NUCLEOSYNTHESIS

We have found a approximately 3 mum Al2O3 grain (B39) in the Bishunpur LL3.1 chondrite that is enriched in O-17 by a factor of approximately 6.8 (O-16/O-17 = 385 +/- 9) and depleted in O-18 by almost 60% (O-16/ O-18 = 853 +/- 30) relative to solar system oxygen and has an initial Al-26/Al-27 = 1.7 +/- 0.2 x 10(3), approximately 34 times greater than the initial solar system value. The isotopic compositions of B39 and two other Al2O3 grains previously reported from the Orgueil and Murchi son meteorites show that these grains formed directly from the ejecta of low-mass AGB stars with C/O < 1. A simple theoretical analysis is p resented showing that the oxygen systematics of the grains are a natur al consequence of main-sequence evolution followed by first dredge-up. Al-26 is the result of third dredge-up. Circumstellar Al2O3 grains pr ovide very precise isotopic data for stellar ejecta that complement sp ectroscopic observations of oxygen-rich stars. Isotopic differences in dicate that the Al2O3 grains come from separate stars of different mas s and initial oxygen composition that originated in molecular clouds d ifferent from the one in which the solar system formed.