OXYGEN ISOTOPES IN CIRCUMSTELLAR Al20 3 GRAINS FROM METEORITES AND STELLAR NUCLEOSYNTHESIS

We have found a ~311m Al20 3 grain (B39) in the Bishunpur LL3.1 chondrite that is enriched in 170 by a factor of ~6.8 e60/170 = 385 ± 9) and depleted in 180 by almost 60% e60/180 = 853 ± 30) relative to solar system oxygen and has an initial 26Al/27Al = 1.7 ± 0.2 x 103, ~34 times greater than the initial solar system value. The isotopic compositions of B39 and two other Al20 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 Al20 3 grains provide very precise isotopic data for stellar ejecta that complement spectroscopic observations of oxygen-rich stars. Isotopic differences indicate that the Al20 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. Subject headings: circumstellar matter dust, extinction nuclear reactions, nucleosynthesis, abundances stars: AGB and post-AGBsolar system: formation