Emission and absorption in circumstellar dust shells: an isothermal approach

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Abstract
An isothermal approach has been used to construct an extensive grid of circumstellar dust shell models. The effect on the output spectrum of the star and shell temperature and the optical depth of the shell is investigated for power opacity laws and silicate opacity laws. It is found that, for silicate shells, there exists a large domain in which the 10 and 20-µm features are in absorption for all values of the star temperature and that the principal factor influencing whether the features are in emission or absorption is the shell temperature. In general low shell temperatures give rise to absorption features whilst high shell temperatures give rise to emission features. The model is compared with observations using a variety of colour–colour diagrams and a semi-empirical silicate opacity law. It is found that the model accounts remarkably well for observations of cool stars with silicate emission features.

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