Temperature and emission-line structure at the edges of H II regions

Abstract

Models of ionization fronts located at the edges of expanding H II regions are presented. These fronts are of the weak D-type and are preceded by shocks in the H I clouds. Since the energy input time is smaller than the cooling time, the gas is found to heat up to a high temperature immediately following ionization. At the trailing edge of the front, the temperature decreases and the ionized gas merges with the main bulk of the nebula where the physical processes are in equilibrium. The emission in [0 ii] and [N ii] lines is greatly enhanced because of the high temperature at the front. The emission in these and other important lines is calculated and compared with H . Effects of different velocities of flow, of different exciting stars, and of different gas densities on the structure of the fronts are also investigated. Observations of bright rims of H ii regions where enhanced emission of [N II] lines has been reported are discussed on the basis of these models.