The development of H I dissociation zones around new H II regions

Abstract

Results of computations for a model of the time development of H I photodissociation zones in the molecular gas around O and early B main-sequence stars and their associated H II regions are presented. The computations are for a grid of values of T(eff) from 20 to 45 kK and of ambient gas particle densities from 30 to 3000 atoms/cu cm. The model calculations show that in the case of atomic zones around H II regions, in low-density gas, the unshocked H I zone will persist for up to half the main-sequence lifetime of the star, whereas in high-density gas the zone will last only a few percent of the stellar lifetime. A star with T(eff) at the lower end of the range will dissociate to a radius several times the radius of the H II region, whereas the maximum width of the dissociation zone around the star at the early end of the range will be less than the ionized radius. The sizes and masses of both H II regions and their H I zones increase with decreasing density of the surrounding gas.