The effects of dust on compact and ultracompact HII regions

We calculate numerical models of dusty H II regions using the Cloudy photoionization code with a grain size distribution. Dust sublimation causes a depletion of grain sizes and types within the ionized region, with large graphite grains being able to exist closer to the star than smaller graphite grains or silicate grains. We investigate the time-dependent hydrodynamic expansion of dusty H II regions and find that the fraction of ionizing photons absorbed by dust decreases with time. Furthermore, dusty H II regions stall earlier and at smaller radii than their dust-free counterparts. Comparison is made between our models and observable parameters, such as the electron density. We find that the electron density in dusty ionized regions estimated from radio continuum observations is likely to be an overestimate, and we quantify the discrepancy. Finally, we calculate the infrared emission from dusty H II regions and their surrounding circumnebular dust shells using the DUSTY code. We find that the far-infrared emission depends strongly on the parameters assumed for the circumnebular dust shell.