Emission, absorption and self-absorption in circumstellar dust shells

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Abstract
An extended spherical circumstellar dust shell model, in which the radiative transfer problem is treated accurately, is used to construct an extensive grid of model results in order to investigate quantitatively the dependence of the 10 and 20 μm silicate features on model parameters. A wide range of results is presented graphically, and these results demonstrate that compact shells characterized by inward peaking density distributions exhibit stronger emission features and weaker absorption features than those of the reverse characteristics. In the transition of the 10 μm feature from emission to absorption a self-absorption domain is identified where the wings of the feature appear in emission while the centre is in absorption. The underlying factor producing the reversal from emission to absorption is the existence of increasing quantities of dust at low temperatures. However, the presence of such low temperature material may be brought about by a number of independent parametric alterations to the shell structure. Reversal temperatures for the 10 and 20 μm features are defined according to the temperature of the ‘effective radiating surface’ (the imaginary surface corresponding to unity optical depth at a specified wavelength) at which reversal from emission to absorption takes place. It is shown that these temperatures are not constant but show a marked dependence on the shell parameters. The effects of varying the strength of the silicate opacity features are discussed in relation to circumstellar dust opacities.

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