Nonequilibrium Photodissociation Regions with Advancing Ionization Fronts

We have modeled the thermal and chemical structure of photodissociation regions (PDRs) where the interface between the H II region and the neutral cloud (the ionization front IF) is moving with a velocity v_IF into the PDR. This situation applies, for example, to PDRs associated with photoevaporating clumps, blister H II regions, and expanding H II regions. Although the chemical and temperature structure of the PDR achieves a steady state value in the frame of the IF, the chemical and thermal structure of these nonequilibrium models differ from static equilibrium values because of the advection of molecular gas through the PDR toward the IF. We have studied PDRs with hydrogen nucleus densities n ranging from 10⁴ to 10⁶ cm^-3 and FUV fluxes χ = 10⁴-10⁵ times the local average FUV flux, such as is appropriate for many PDRs associated with dense H II regions near O stars. The velocity of the ionization front v_IF is varied between 0 and 1 km s^-1, the range predicted for the advance of a D-type ionization front into a photoevaporating PDR. We predict intensities of the [O I] 63 μm and the [C II] 158 μm fine-structure lines, the pure rotational H₂ 0-0 S(0) 28.22 μm, S(1) 17.03 μm, S(2) 12.28 μm, S(3) 9.67 μm, S(4) 8.03 μm, and S(5) 6.91 μm lines; the H₂ v = 1-0 S(1) and H₂ v = 2-1 S(1) vibrational lines; and the CO 1-0 and 2-1 rotational lines.

We find that there is no H/H₂ photodissociation front for models with χ/n <= 0.2v_IF, where n is in cm^-3 and v_IF is in km s^-1. The H₂ is rapidly advected to the IF before it can photodissociate. For χ/n > 0.2v_IF an H/H₂ photodissociation front exists, but the front moves progressively nearer to the cloud surface as χ/n declines.

We also find that the nonequilibrium models always have a well-defined C⁺/CO transition layer, because the CO does not self-shield as effectively as H₂ and the CO photodissociation timescale is shorter than the flow time across the PDR. This layer is, however, shifted slightly nearer to the cloud surface compared to the equilibrium models. The [C II] and [O I] fine-structure line intensities are relatively insensitive to v_IF in the range studied, because of the relative rapid photodissociation of CO and photoionization of C I. We conclude that nonequilibrium effects have a relatively minor effect on previous analysis of the physical conditions of PDRs based on [O I], [C II], and IR continuum intensities. The CO low-J lines can be a factor of 2 stronger compared to the static equilibrium models. The H₂ rotational and vibrational lines can be enhanced by a factor of 3. We consider the Orion Bar PDR and conclude that nonequilibrium effects are probably minor.