THE SPECTRAL ENERGY-DISTRIBUTION OF STAR-FORMING REGIONS

We present exact solutions of the radiative transfer problem for cloud s containing quantum-heated dust particles, with attention to dust des truction as appropriate for modeling star-forming regions. The dust mo del by Siebenmorgen & Krugel is compared with models containing only l arge grains heated under steady state conditions. We demonstrate that both models can be distinguished by studying the surface brightness mo rphology of objects at mid-infrared wavelengths. The emission bands of polycyclic aromatic hydrocarbons in media with high optical depth is studied and the multicomponent grain emission and temperature structur es inside opaque clouds is discussed. We present excellent fits of a s ample of well-known star-forming regions by calculating beam-matched s pectral energy distributions. The current problems of modeling the inf rared emission from star-forming regions are summarized. We suggest th at a scenario of disklike structures embedded in the circumstellar dus t envelope might better match the observations. The disk grains should be large and of inhomogeneous structure. The grain absorption cross s ections of fluffy grains are calculated by using the Maxwell-Garnett m ixing rule. We find that compositional inhomogeneities of dust grains will affect the silicate bands and increase the absorptivity at far-in frared and submillimeter wavelengths.