Theoretical study of dust-filled H II regions

Text includes handwritten formulasMotivated by recent observational data in the infrared indicating strong continuum emission from diffuse nebulae and by some considerably improved calculations of atomic parameters governing nebular processes, a comprehensive, theoretical model for HII regions was devised from which the ionization and electron structures were derived. Spherically symmetric models including both gas and dust grain processes were calculated for a variety of exciting stellar spectrums and for various gas and dust densities and dust parameters. The results of our dust-free models using older values of atomic parameters agreed quite well with previously published models. The inclusion of the new rates for atomic processes produced model HII regions with electron temperatures which averaged several hundred degrees below those of past calculations. The presence of dust was found to decrease both the size and temperature of the model regions, with the dust dominating the structural determination once the dust-to-gas ratio exceeded 1 In some models dominated by dust, the temperature profiles were lowered by more than one thousand degrees compared to those of the dust-free models, indicating that HII regions may be much cooler than previously anticipated. Calculations of the infrared emission from the dust within the region gave rather good agreement with observational studies, lending strong support to models of HII regions that include both gas and dust