Formation of SiC grains in circumstellar envelops of carbon stars
| Autor: | Dorschner, Johann, Henning, Thomas, Stognienko, Ralf, Kozasa, Takashi |
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| Jazyk: | angličtina |
| Rok vydání: | 1995 |
| Předmět: | |
| Zdroj: | 第28回月・惑星シンポジウム = Proceedings of the 28th ISAS Lunar and Planetary Symposium. :177-177 |
| Popis: | The emission feature around 11.3 micrometer observed towards carbon stars is considered to be due to SiC grains condensed in the circumstellar envelopes. The isotopic anomalies of SiC discovered in carbonaceous meteorites strongly support the formation of SiC grains in carbon-rich AGB (Asymptotic Giant Branch) stars. The statistical analysis of IRAS (Infrared Astronomical Satellite) carbon stars revealed a systematic variation of the emission features; the peak wavelength shifts towards longer ones, the strength decreases, and the width becomes wider as the strength at 11.3 micrometer decreases. The absorption spectrum measured for the powder SiC samples varies from a sample to others, depending on the preparation procedure of the samples. The optical constants derived from the measurements of the thin film of crystalline alpha-SiC being used, the absorption spectrum of small alpha-SiC grains are completely different from the absorption spectrum of the powder samples; the very strong and sharp resonance feature appears at 10.75 micrometer for a small spherical particle, which is the Frohlich band arising from the surface modes. Thus it is expected that the appearance of the resonance feature of small SiC grains heavily depends on the morphology and the chemical composition of impurities as well as the surrounding medium. Paying attention to the behavior of the resonance feature, we investigated what kind of dust grains shows the observed emission feature, and then demonstrated the possibility of the formation of such a grains based on a theory of homogeneous nucleation and grain growth. We used the optical constants of alpha-SiC tabulated by Choyke and Palik (1985 in Hand Book of Optical Constants of Solids), and of glassy carbon by Edo (1983). The result of calculations are as the followings: (A) Small SiC grains coated by carbon can show the single-peaked and smooth resonance feature similar to the emission feature observed towards carbon stars; as the volume fraction of carbon mantle increases, the peak wavelength shifts towards longer ones, the strength decreases and the width becomes wider. the resonance feature peaks at 11.3 micrometer when the volume fraction of carbon mantle is 0.4. (B) Chemical equilibrium calculations generally predict that condensation of carbon precedes SiC condensation. However, based on a theory of nucleation and grain growth taking into account the non-L.T.E. (Local Thermodynamic Equilibrium) effect, i.e. the difference between temperatures of gas and small clusters, the condensation of SiC grains occurs before carbon condensation in circumstellar envelopes of carbon stars. Thus we can expect the formation of core-mantle type grains consisting of a SiC core and a carbon mantle. (C) The formation process of dust grains in circumstellar envelopes of carbon stars is divided into the following two cases according to the mass loss rate: In the case of mass loss rates smaller than 1.5 x 10(exp -5) solar mass/yr, SiC grains condense first and then are coated by carbon mantle gradually; the radius of SiC core is very small, at most 0.05 micrometer. Otherwise, after an avalanche of nucleation of SiC, SiC and carbon simultaneously condense on the nuclei to form composite grains; carbon grains containing SiC as an impurity. The full paper will be published in Astronomy and Astrophysics and is available by requesting to the author. 資料番号: AA0063352047 |
| Databáze: | OpenAIRE |
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