Model HII regions and ionization fronts

Previous HII region models have all been constructed assuming a uniform density throughout the nebula. In this investigation uniform pressure model HII regions were constructed for three different pressures, four different heavy element abundances, two stellar temperatures, and two different assumptions about the far ultraviolet opacity of dust grains. These models are used to investigate the dependence of observable emission line strengths upon heavy element abundances. The ionization structures of the trace elements C, N, O, Ne, Si, and S are also computed. The frequency dependence of the line profiles of radio recombination lines were studied and a test of the expanding nebula hypothesis is proposed. It is found that ionization fronts are closely related to phase boundaries. Model ionization fronts for several different velocities were constructed by methods similar to those of Hjellming (1966). These models were tested for thermal stability. Only very low velocity fronts show instability. In these cases, thermal conduction may stabilize the front. The formation of molecular hydrogen in ionization fronts by the reaction H- + H+ → H2 + e- was studied and shown to be capable of explaining Copernicus satellite observations of low column density, high rotational temperature H2 in unreddened stars. The influence of the exciting star on the region outside the nebula is discussed. The dissociation of H2, ionization of CI and HI and formation of CI and HI radio recombination lines is studied