Nonequilibrium Photodissociation Regions: Ionization-Dissociation Fronts

We discuss the theory of coupled ionization-dissociation fronts produced when molecular clouds are exposed to λ < 1110 Å radiation from hot stars. A steady, composite structure is developed, which generally includes an ionized outflow away from the cloud, an ionization front, a layer of photodissociated gas, a photodissociation front, and a shock wave preceding the photodissociation front. We show that the properties of the structure are determined by two dimensionless parameters ψ and σ, and by the Alfvén speed in the preshock gas. For a broad range of parameters of interest, the ionization front and the hydrogen photodissociation front do not separate, the H₂ photodissociation and photoionization take place together, and a classical hydrogen "photodissociation region" (PDR) does not exist. We also show that even when a distinct PDR exists, in many cases the dissociation front propagates too rapidly for the usual stationary models of PDRs to be applicable. We discuss several famous PDRs, e.g., in M 17 and Orion, and conclude that they cannot be described by equilibrium PDR models.