A model for multiple isothermal circumstellar dust shells

Abstract

We have extended our previously described single and double-shell isothermal circumstellar dust shell models to the case of an arbitrary number of independent isothermal shells. We envisage that this model, the code for which is available on-line, may be useful in three ways: (i) as a tool for the quick representation of the spectrum of dust enshrouded objects, (ii) as an initial guide to the choice of parameters to be used in more sophisticated models and (iii) as a teaching aid for a simple application of radiative transfer theory. For illustrative purposes this model has been used to represent the spectral energy distribution of two evolved objects with optically thick circumstellar dust shells, the post asymptotic giant branch (post-AGB) star IRAS 22036+5306 and the proto-planetary nebula (PPN) M1-92. Both of these objects have very broad spectral energy distributions, their flat distributions having proven difficult to model in the past. The satisfactory fit of the models supports the considerable evidence that some evolved objects in particular, may have multiple discrete circumstellar dust shells, possibly ejected from the central star at different epochs and with different grain compositions. In addition, the isothermal nature of the models suggests the possibility that these shells may be quite limited in their temperature range and physical extent.