Multigrain dust cloud models of compact HII regions

We present a generalization of the method of Efstathiou & Rowan-Robinson for the solution of the radiative transfer problem in dust clouds that takes into account a distribution of grain species and sizes. The method includes treatment of multiple scattering from grains and can be applied to axisymmetric density distributions. In this paper, however, we restrict our attention to the special case of spherical symmetry. We find that, in the parameter space we have explored, composite grains with 'average' absorption and scattering efficiencies approximate very well the emission from the grain mixture of Rowan-Robinson. We also re-examine the sample of compact H n regions of Crawford & Rowan-Robinson, and find that the combined effect of the multigrain calculations and a new model for interstellar grains by Rowan-Robinson is to improve considerably the agreement of models that assume high-optical-depth (Av ≈ 20-200) homogeneous clouds around hot stars with the IRAS data. For four objects we are able to compare our models with submillimetre data (350-1300 (xm). For W3(OH) the fit is good, but for the other three objects the models predict too much flux at long wavelengths. Possible reasons for this are discussed.