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Circumstellar Hibonite and Corundum and Nucleosynthesis in Asymptotic Giant Branch Stars

Choi, Byeon-Gak and Wasserburg, G. J. and Huss, Gary R. (1999) Circumstellar Hibonite and Corundum and Nucleosynthesis in Asymptotic Giant Branch Stars. Astrophysical Journal Letters, 522 (2). L133-L136. ISSN 2041-8205. doi:10.1086/312239.

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We report the discovery of two hibonite grains (CaAl_(12)O_(19)) whose isotopic compositions show that they formed in the winds of red giant and asymptotic giant branch (AGB) stars. While hibonite is the second major phase (after corundum, Al_2O_3) expected to condense from stellar ejecta with C/O < 1, it has not previously been found. One circumstellar hibonite grain is highly enriched in ^(17)O and slightly depleted in ^(18)O relative to the solar composition and has large excesses in ^(26)Mg and ^(41)K, decay products of ^(26)Al and ^(41)Ca. The inferred initial values (^(26)Al/^(27)Al)0 ≈ 5 × 10^(-3) and (^(41)Ca/^(40)Ca)0 ≈ 1.5 × 10^(-4) of this grain are consistent with models of nucleosynthesis in an AGB star. The other hibonite is enriched in ^(17)O, strongly depleted in ^(18)O, shows no evidence of ^(41)Ca and formed with (^(26)Al/^(27)Al)0 ≈ 2 × 10^(-2). The low ^(18)O/^(16)O and very high (^(26)Al/^(27)Al)_0 may indicate substantial proton exposure during cool bottom processing in a low-mass parent star. The low upper limit on ^(41)Ca/^(40)Ca (≤ 3.2 × 10^(-5)) implies that little or no He-shell material had been dredged into the envelope when this grain formed. We also report isotopic compositions for 12 new circumstellar corundum grains. The compositions of 11 of these grains are consistent with current models for red giant and AGB stars. One corundum grain has extremely high ^(17)O/^(16)O and near-solar ^(18)O/^(16)O and may have formed in a star that was initially enriched in ^(17)O and ^(18)O.

Item Type:Article
Related URLs:
URLURL TypeDescription
Wasserburg, G. J.0000-0002-7957-8029
Huss, Gary R.0000-0003-4281-7839
Additional Information:© 1999 American Astronomical Society. Received 1999 June 25; accepted 1999 July 12; published 1999 August 12. We thank R. Gallino for valuable comments and suggestions. This work was supported by NASA, NAG5-4083, and NAG5-8158. Contribution 8644(1035), Division of Geological and Planetary Sciences of Caltech.
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Subject Keywords: nuclear reactions, nucleosynthesis, abundances; stars: AGB and post-AGB; stars: winds, outflows
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Other Numbering System NameOther Numbering System ID
Caltech Division of Geological and Planetary Sciences8644
Lunatic Asylum Lab1035
Issue or Number:2
Record Number:CaltechAUTHORS:20131212-080207314
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Official Citation: Circumstellar Hibonite and Corundum and Nucleosynthesis in Asymptotic Giant Branch Stars Byeon-Gak Choi et al. 1999 ApJ 522 L133
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:42968
Deposited By: Ruth Sustaita
Deposited On:12 Dec 2013 16:45
Last Modified:10 Nov 2021 16:31

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