Efficient Radiative Transfer in Dust Grain Mixtures

The influence of a dust grain mixture consisting of spherical dust grainswith different radii and/or chemical composition on the resulting temperaturestructure and spectral energy distribution of a circumstellar shell isinvestigated. The comparison with the results based on an approximation of dustgrain parameters representing the mean optical properties of the correspondingdust grain mixture reveal that (1) the temperature dispersion of a real dustgrain mixture decreases substantially with increasing optical depth, convergingtowards the temperature distribution resulting from the approximation of meandust grain parameters, and (2) the resulting spectral energy distributions donot differ by more than 10% if >= 2^5 grain sizes are considered whichjustifies the mean parameter approximation and the many results obtained underits assumption so far. Nevertheless, the dust grain temperature dispersion atthe inner boundary of a dust shell may amount to >>100K and has therefore to beconsidered in the correct simulation of, e.g., chemical networks. In order to study the additional influence of geometrical effects, atwo-dimensional configuration -- the HH30 circumstellar disk -- was considered,using model parameters from Cotera et al. (2001) and Wood et al. (2002). Adrastic inversion of the large to small grain temperature distribution wasfound within the inner approx. 1AU of the disk.Comment: ApJ, accepte