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Published July 20, 1994 | Published
Journal Article Open

Predictions of oxygen isotope ratios in stars and of oxygen-rich interstellar grains in meteorites


We carried out detailed, self-consistent calculations for stars from 1 to 9 M_☉ over a wide range of metallicities, following the evolution and nucleosynthesis from the pre-main sequence to the asymptotic giant branch (AGB), in order to provide a self-consistent grid for evaluating stellar oxygen isotopic variations. These were calculated for first and second dredge-up, and for some masses also for third dredge-up and "hot bottom" convective envelope burning on the AGB. We demonstrate that ^(16)O/^(17)O in red giant envelopes is primarily a function of the star's mass, while ^(16)O/^(18)O is primarily a function of the initial composition. Uncertainties in the ^(17)O-destruction rate have no effect on the ^(16)O/^(17)O ratio for stars from 1 to 2.5 M_☉, but do affect the ratios for higher masses: the stellar ^(16)O/^(17)O observations are consistent with the Landré et al. (1990) rates using ƒ = 0.2 for ^(17)O(p, y)^(18)F and ^(17)O(p, ɑ)^(14)N, and with the Caughlan & Fowler (1988) rates using ƒ ~ 1. The stellar ^(16)O/^(18)O observations require ƒ ~ 0 in the Caughlan & Fowler ^(18)O(p, ɑ)^(15)N rate. First dredge-up has the largest effect on the oxygen isotope ratios, decreasing ^(16)O/^(17)O significantly from the initial value and increasing ^(16)O/^(18)O slightly. Second and third dredge-up have only minor effects for solar metallicity stars. The absence of very low observed ^(16)O/^(18)O ratios is consistent with a major increase in the ^(18)O(ɑ, y)^(22)Ne rate over the Caughlan & Fowler (1988) value. Hot bottom burning in stars above about 5 M_☉ can cause a huge increase in ^(16)O/^(18)O (to ≳10^6), and possibly a significant decrease in ^(16)O/^(17)O; these are accompanied by a huge increase in ^7Li and a value of ^(12)C/^(13)C ≈ 3. The oxygen isotope ratios in the Al_2O_3 grains (Orgueil grain B, the Murchison 83-5 grain, and the new Bishunpur B39 grain) can be accounted for if they originated in stars that did NOT have the same initial ^(16)O/^(18)O ratio. Thus one cannot assume uniform isotope ratios, even for stars of nearly solar composition. The grains' ^(16)O/^(17)O ratios, together with the ^(26)Mg excesses that indicate grain formation in a ^(26)Al-rich environment, indicate that the Orgueil grain B and Murchison 83-5 grain originated in stars of roughly 1.5 M_☉, during third dredge-up on the AGB. The new Bishunpur B39 grain originated in a star of either 2 or of 4-7 M_☉.

Additional Information

© 1994 American Astronomical Society. Received 1994 March 16; accepted 1994 April 29. We wish to thank Gary Russ and especially Roberto Gallino for illuminating and animated discussions, and for helpful comments on the Letter. We wish to thank Steven E. Koonin for the support supplied by the Kellogg Radiation Laboratory. One of us (A. I. B.) wishes in addition to thank Scott D. Tremaine and Peter G. Martin for the support provided by the Canadian Institute for Theoretical Astrophysics. This work was supported in part by a grant from the National Science Foundation PHY-8817296, a grant from the Natural Sciences and Engineering Research Council of Canada, and a Caltech Division Contribution 5390(848)

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Published - 1994ApJ___430L__77B.pdf


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August 20, 2023
August 20, 2023