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2019 Cool Sci @ STScI

Alumina Polymorphism in the Circumstellar Dust Shells of Asymptotic Giant Branch STars

Presented by: Ben Sargent (STScI)
Category: Science Colloquia   Duration: 3 minutes   Broadcast date: January 16, 2019
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Abstract: Many emission features remain unidentified in infrared spectra of asymptotic giant branch (AGB) stars. In particular, features at ~ 11, 20, 28, and 32 microns wavelength have been noted in mid-infrared spectra of oxygen-rich AGB stars. Here, I present models of dust excess emission in 36 spectra from the Short Wavelength Spectrometer (SWS) on the Infrared Space Observatory (ISO) and the Infrared Spectrograph (IRS) on the Spitzer Space Telescope of 24 AGB stars. The models include opacities of grains composed of mixtures of various polymorphs of alumina obtained by preparing bayerite and boehmite at high temperatures, and these dust components provide satisfactory fits to the 11, 20, 28, and 32 micron features. Though not a direct conclusion from this study, the presence of grains of the various polymorphs of alumina in circumstellar dust shells around AGB stars suggests corundum may have a role in giving rise to the 13 micron feature seen in AGB spectra.