Modelling infrared spectra of a few compact HII regions

Abstract

The observed near infrared to sub-mm spectra of five compact HII regions, viz., IRAS 18116-1646, 18162-2048, 19422+2427, 22308+5812 and 18434-0242, consisting of the continuum as well as several features, have been explained well by radiative transfer through a spherical dust cloud. In addition to the equilibrium thermal emission from the normal dust grains (BG), the non-equilibrium processes, e.g. transient heating of the very small grains (VSG) as well as the Polycyclic Aromatic Hydrogen (PAH)are also taken into account by a novel scheme of segmenting the main cloud into individual "onion skin" cloud shells. The following have been concluded about these compact HII regions : (i)the uniform radial density distribution gives an excellent fit to the entire body of the data (other cases e.g. r to the power -1, r to the power -1.5 or r to the power -2 etc can be ruled out); (ii)the PAH features are predicted very well only if these molecules exist exclusively in the central region (a few % of radius) of the cloud; and (iii)the silicate grains constitute only 5-25% of the BG component (rest being graphite).