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Science 3 September 2004:
Vol. 305. no. 5689, pp. 1455 - 1457
DOI: 10.1126/science.1101099
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Reports

Polymorphism in Presolar Al2O3 Grains from Asymptotic Giant Branch Stars

Rhonda M. Stroud,1* Larry R. Nittler,2 Conel M. O'D. Alexander2

We report microstructural and isotopic analyses of two presolar Al2O3 grains. Aluminum oxide is important cosmically, because its presence has been detected in the infrared spectra of the circumstellar envelopes of O-rich asymptotic giant branch stars and because it is predicted to be the first solid to condense in these stellar environments. The two grain structures, one corundum and the other amorphous, confirm that asymptotic giant branch stars produce both phases. The variation in structure and Ti content demonstrates that Al2O3 can condense in the absence of TiO2 seed clusters but that Ti may be important in determining the crystal structure.

1 Code 6360, Naval Research Laboratory, Washington, DC 20375, USA.
2 Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015, USA.

* To whom correspondence should be addressed. E-mail: stroud{at}nrl.navy.mil

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Science. ISSN 0036-8075 (print), 1095-9203 (online)