Photodissociation-region models of interstellar hydroxyl masers

Dense warm photodissociation regions are possible sites for the formation of interstellar hydroxyl masers, because, in addition to containing fractional abundances of OH which are large compared to those in dense cool gas in dark clouds, they do not possess high material velocities relative to the central stars, a property which they share with some observed hydroxyl masers. We have calculated the column densities of OH in dense equilibrium photodissociation regions which form in the vicinity of early-type stars. They lie in the range of several times 10¹⁵ cm^-2. The largest OH column densities are obtained in photodissociation regions with densities and sizes comparable to those thought to be typical of the hydroxyl-maser regions. The column densities of OH in hydroxyl masers in star-forming regions are probably several times higher than the maximum theoretical values that we have obtained. Hence, photo-dissociation regions are possible sites for the hydroxyl masers, only if the observational lines-of-sight to them are at fairly small angles to their surfaces, a conclusion which is consistent with the genrrally accepted view that maser spots are longer than they are wide.