title: Polymorphism in presolar Al2O3 grains from asymptotic giant branch stars.
abstract: 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.
authors.affiliation: Code 6360, Naval Research Laboratory, Washington, DC 20375, USA. stroud@nrl.navy.mil
authors.forename: Rhonda M
authors.initials: RM
authors.lastname: Stroud
authors.forename: Larry R
authors.initials: LR
authors.lastname: Nittler
authors.forename: Conel M O'D
authors.initials: CM
authors.lastname: Alexander
mesh_terms: D000537:Aluminum Oxide; D055580:Astronomical Phenomena; D001256:Astronomy; D019443:Cosmic Dust; D014025:Titanium
chemical_list: D019443:Cosmic Dust; C009495:titanium dioxide; D014025:Titanium; D000537:Aluminum Oxide