Oxygen isotopes in circumstellar Al_2O_3 grains from meteorites and stellar nucleosynthesis
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.
© 1994 American Astronomical Society. Received 1994 March 16; accepted 1994 April 29. We thank David Dearborn, Mounib El Eid, Daniel Schaerer, Michael Wischer, and Peter Wannier for useful discussions. The enthusiasm of I.-J. Sackmann and Arnold Boothroyd for these grains has been particularly rewarding. Work was supported by NASA grants NAGW-3040, NAGW- 3337 and NAGW-3297. This is Division Contribution 5389(847).
Published - 1994ApJ___430L__81H.pdf