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Hot bottom burning in asymptotic giant branch stars and its effect on oxygen isotopic abundances

Boothroyd, Arnold I. and Sackmann, I.-Juliana and Wasserburg, G. J. (1995) Hot bottom burning in asymptotic giant branch stars and its effect on oxygen isotopic abundances. Astrophysical Journal Letters, 442 (1). L21-L24. ISSN 2041-8205. doi:10.1086/187806.

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A self-consistent calculation of asymptotic giant branch (AGB) evolution was carried out, including nucleosynthesis at the base of the convective envelope (hot bottom burning). Hot bottom burning was found to occur for stars between ~4.5 and ~7 M_☉, producing envelopes with ^(18)O/^(16)O ≾ 10^(-6) and 10^(-3) ≾ ^(17)O/^(l6)O ≾ 10^(-1). The ^(17)O abundance depends sensitively on the nuclear ^(17)O-destruction rate; this rate is only loosely constrained by the requirement that first and second dredge-up models match O-isotope observations of red giant branch (RGB) stars (Boothroyd, Sackmann, & Wasserburg 1994). In some cases, high mass-loss rates can terminate hot bottom burning before further ^(17)O enrichment takes place or even before all ^(18)O is destroyed. These predictions are in accord with the very limited stellar observations of J type carbon stars on the AGB and with some of the circumstellar Al_2O_3 grains from meteorites. In contrast, precise data from a number of grains and data from most low-mass Sand C AGB stars (≾ 1.7 M_☉) lie in a region of the ^(18)O/^(l6)O versus ^(17)O/^(16)O diagram that is not accessible by first and second dredge-up or by hot bottom burning. We conclude that for AGB stars, the standard models of stellar evolution are not in accord with these observations. We surmise that an additional mixing mechanism must exist that transports material from the cool bottom of the stellar convective envelope to a depth at which ^(18)O is destroyed. This "cool bottom processing" mechanism on the AGB is similar to extra mixing mechanisms proposed to explain the excess ^(13)C (and depleted ^(12)C) observed in the earlier RGB stage of evolution and the large ^7Li depletion observed in low-mass main-sequence stars.

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Wasserburg, G. J.0000-0002-7957-8029
Additional Information:© 1995 American Astronomical Society. Received 1994 October 21; accepted 1995 January 6. We appreciate the support supplied by the Kellogg Radiation Laboratory. A. I. B. thanks S. D. Tremaine and P. G. Martin for the support provided by the Canadian Institute for Theoretical Astrophysics. I.-J. S. wishes to thank R. F. Christy for challenging discussions and gentlemanly help during the many tasks of daily life. This work was supported by grants from the National Science Foundation PHY-8817296, the Natural Sciences and Engineering Research Council of Canada, and NASA grants NAGW-3040, NAGW-3337, and NAGW-3297. Caltech Division Contribution 5464(858).
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Natural Sciences and Engineering Research Council of Canada (NSERC)UNSPECIFIED
Subject Keywords:dust, extinction; nuclear reactions, nucleosynthesis, abundances; stars: abundances stars: AGB and post-AGB
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Caltech Division of Geological and Planetary Sciences5464
Lunatic Asylum Lab858
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Official Citation:Hot bottom burning in asymptotic giant branch stars and its effect on oxygen isotopic abundances Boothroyd, Arnold I.; Sackmann, I.-Juliana; Wasserburg, G. J. Astrophysical Journal, Part 2 - Letters (ISSN 0004-637X), vol. 442, no. 1, p. L21-L24 1994
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:43054
Deposited By: Ruth Sustaita
Deposited On:17 Dec 2013 23:55
Last Modified:10 Nov 2021 16:32

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