AAS 207th Meeting, 8-12 January 2006
Session 182 Stellar Winds, Circumstellar Matter, and Activity
Poster, Thursday, 9:20am-4:00pm, January 12, 2006, Exhibit Hall

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[182.16] SiO2 Around O-rich AGB Stars - Mineralogy & The "13\mum" Feature

K. D. DePew, A. K. Speck, C. Dijkstra (University of Missouri-Columbia), A. M. Hofmeister (Washington University, Saint Louis)

Asymptotic Giant Branch (AGB) Stars have circumstellar shells which are either carbon or oxygen rich (C-rich or O-rich). The nature of the dust grains can be identified using IR spectroscopy. AGB stars have several interesting infrared spectral features. Approximately half the oxygen-rich AGB stars to be investigated spectroscopically exhibit a feature at ~13 \mum. The feature has several enigmatic correlations. It has been shown that the 13 \mum feature is more often found in semi-regular variable stars than in Miras. The presence of the feature in supergiant spectra is rare (Speck et al. 2000). Furthermore, the 13\mum feature has been correlated with various other spectral features, although these correlations are not universally accepted.The carrier of this feature has not yet been unequivocally identified, but has been attributed to various dust species, including corundum (\alpha-Al2O3), spinel (MgAl2O3) and silica (SiO2). Recently it has been shown that spinel is unlikely to be the carrier of the 13 \mum feature, but both corundum and silica remain contenders. In order to constrain the carrier of the 13 \mum feature, we use the one-dimensional radiative transfer code DUSTY to model the effects of composition and optical depth on the shape and strength of the emerging 13 \mum feature from various silica-silicate and silica-oxide mixtures, as well as explore the effects of different grain morphologies on scattering cross-section. These spectra are then compared to the observed spectra which exhibit the 13 \mum feature. We present the pros and cons for silica as the carrier species and how it would fit into the bigger picture of stardust formation, including its relation to evidence from theoretical and laboratory studies.

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