Time-Dependent Photodissociation Regions

We present theoretical models of the time-dependent thermal and chemical structure of molecular gas suddenly exposed to far-ultraviolet (FUV) (6 eV 0/n> 10-2 cm3, the emergent H₂vibrational line intensities are initially larger than the final equilibrium values. The H₂lines are excited by FUV fluorescence and by collisional excitation in warm gas. Most of the H₂intensity is generated at a characteristic hydrogen column density of N 1021 cm 2, which corresponds to an FUV optical depth of unity caused by dust opacity. The time dependence of the H₂intensities arises because the initial abundances of H₂at these depths is much higher than the equilibrium values, so that H2 initially competes more effectively with dust in absorbing FUV photons. Considerable column densities of warm (T 1000) K H₂gas can be produced by the FUV pumping of H₂vibrational levels followed by collisional de-excitation, which transfers the energy to heat. In dense (n 10 cm -3) gas exposed to high (G₀ 10 ) fluxes, this warm gas produces a 2-1 S(1)/1-0 5(1) H₂line ratio of 0.1, which mimics the ratio found in shocked gas. In lower density regions, the FUV pumping produces a pure-fluorescent ratio of 0.5. We also present calculations of the time dependence of the atomic hydrogen column densities and of the intensities of O I 6300 A, S II 6730 A, Fe 111.64 mum, and rotational OH and H2O emission. Potential applications include star4orming regions, clouds near active galactic nuclei, and planetary nebulae. We apply our models to five planetary nebulae and conclude that only BD + 30 3639 shows evidence of enhanced H₂emission due to (high) nonequilibrium H₂abundances.