High Temperature Dust Condensation Around an AGB Star: Evidence from a Highly Pristine Presolar Corundum
Journal Article - Open Access
NAVAL RESEARCH LAB WASHINGTON DC WASHINGTON United States
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Corundum alpha-Al2O3 and amorphous or metastable Al2O3 are common components of circumstellar dust observed around O-rich asymptotic giant branch AGB stars and found in primitive meteorites. We report a detailed isotopic and microstructural investigation of a unique presolar corundum grain, QUE060, identified in an acid residue of the Queen Alexandra Range 97008 LL3.05 meteorite. Based on its O and Mg isotopic compositions, this 1.4 mu m diameter grain formed in a low- or intermediate-mass AGB star. It has four developed rhombohedral 011 faces of corundum and a rough, rounded face with cavities. High Mg contents MgAl 0.004 are due to the decay of radioactive Al-26. No spinel MgAl2O4 inclusions that might have exsolved from the corundum are observed, but there are several high-Mg domains with modulated structures. The subhedral shape of grain QUE060 is the first clear evidence that corundum condenses and grows to micrometer sizes in the extended atmospheres around AGB stars. The flat faces indicate that grain QUE060 experienced little modification by gas-grain and grain-grain collisions in the interstellar medium ISM and solar nebula. The Mg distribution in its structure indicates that grain QUE060 has not experienced any severe heating events since the exhaustion of Al-26. However, it underwent at least one very transient heating event to form the high-Mg domains. A possible mechanism for producing this transient event, as well as the one rough surface and cavity, is a single grain-grain collision in the ISM. These results indicate that grain QUE060 is the most pristine circumstellar corundum studied to date.