Abstract
We present a global balance "Strömgren sphere" approach for the case of dusty HII regions. From this model, we obtain prescriptions for the outer radius of the nebulae as a function of the Strömgren radius R_S (of the corresponding dust-free nebula) and the dust optical depth. We also obtain a new, approximate analytic solution for the radiative transfer problem, giving analytic forms for the ionization fraction as a function of radius. These solutions are compared with the results obtained from the Strömgren sphere approach. Our results can be used to evaluate under what conditions the presence of dust can have an important effect on the structures of HII regions.
