Evaporation and condensation kinetics of Corundum: The origin of the 13 μm feature of oxygen-rich AGB stars
- Division of Earth and Planetary Sciences, Graduate School of Science, Kyoto University, Kitashirakawa Oiwake-cho, Kyoto 606-8502 (Japan)
- Department of Natural History Sciences, Hokkaido University, N10 W8, Sapporo 060-0810 (Japan)
- Department of Earth and Planetary Science, The University of Tokyo, 7-3-1 Hongo, Tokyo 113-0033 (Japan)
Corundum is predicted to be the most abundant refractory circumstellar dust and has been suggested as a candidate emitting the 13 μm feature in infrared spectra of oxygen-rich asymptotic giant branch stars. Crystalline dust has morphologies reflecting the anisotropic crystal structure and formation conditions or processes and may emit characteristic infrared features. In this study, evaporation and condensation experiments of corundum along different crystallographic orientations were performed in vacuum to determine key kinetic parameters controlling morphological anisotropy: evaporation and condensation coefficients (α{sub e} and α{sub c}), the degrees of kinetic hindrance on evaporation and condensation. Plausible shapes of corundum condensates were estimated from the experimental results to evaluate the infrared features of corundum condensates. The evaporation coefficients of corundum are 0.02–0.2 at 1600–1785°C, which increase with temperature and show notable anisotropy. The order of α{sub e} along the crystallographic c, a, and m axes is α {sub e}{sup m} ≫ α {sub e}{sup a} > α {sub e}{sup c} irrespective of temperature. The obtained condensation coefficients along the c, a, and m axes at 1575°C and a supersaturation ratio of ∼4 are 0.04–0.06, 0.06–0.08, and 0.1–0.2, respectively. The morphology of condensed circumstellar corundum expected from our experiments is oblate and slightly flattened to the c axis and is consistent with the fact that no presolar corundum with eccentric shapes has been found. The mass absorption coefficient of oblate corundum slightly flattened to the c axis shows a peak at 13 μm without any accompanying strong peaks. These results strongly indicate that corundum condensed anisotropically in circumstellar environments is a carrier of the unidentified 13 μm feature around oxygen-rich evolved stars.
- OSTI ID:
- 22872452
- Journal Information:
- Astrophysical Journal, Supplement Series, Vol. 218, Issue 1; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 0067-0049
- Country of Publication:
- United States
- Language:
- English
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