Efficient radiative transfer in dust grain mixtures

The influence of a dust grain mixture consisting of spherical dust grains with different radii and/or chemical composition on the resulting temperature structure and spectral energy distribution of a circumstellar shell is investigated. The comparison with the results based on an approximation of dust grain parameters representing the mean optical properties of the corresponding dust grain mixture reveal that (1) the temperature dispersion of a real dust grain mixture decreases substantially with increasing optical depth, converging towards the temperature distribution resulting from the approximation of mean dust grain parameters, and (2) the resulting spectral energy distributions do not differ by more than 10% if ≥ 2 grain sizes are considered which justifies the mean parameter approximation and the many results obtained under its assumption so far. Nevertheless, the dust grain temperature dispersion at the inner boundary of a dust shell may amount to ≫100K and has therefore to be considered in the correct simulation of, e.g., chemical networks. In order to study the additional influence of geometrical effects, a twodimensional configuration – the HH30 circumstellar disk – was considered, using model parameters from Cotera et al. (2001) and Wood et al. (2002). A drastic inversion of the large to small grain temperature distribution was found within the inner ∼ 1AU of the disk. Subject headings: radiative transfer — scattering — methods: numerical — circumstellar matter