Time-Dependent Photodissociation Regions

Abstract

view Abstract Citations (57) References (27) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Time-dependent Photodissociation Regions Hollenbach, David ; Natta, Antonella Abstract We present theoretical models of the time-dependent thermal and chemical structure of molecular gas suddenly exposed to far-ultraviolet (FUV) (6 eV <hv < 13.6 eV) radiation fields and the consequent time- dependent infrared emission of the gas. We focus on the response of molecular hydrogen for cloud densities ranging from n = 103 to 106 cm -3 and FUV fluxes G_{0 }= 103 106 times the local FUV interstellar flux. For G0/n> 10-2 cm3, the emergent H_{2 }vibrational line intensities are initially larger than the final equilibrium values. The H_{2 }lines are excited by FUV fluorescence and by collisional excitation in warm gas. Most of the H_{2 }intensity 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 H_{2 }intensities arises because the initial abundances of H_{2 }at 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 H_{2 }gas can be produced by the FUV pumping of H_{2 }vibrational levels followed by collisional de-excitation, which transfers the energy to heat. In dense (n 10 cm -3) gas exposed to high (G_{0 } 10 ) fluxes, this warm gas produces a 2-1 S(1)/1-0 5(1) H_{2 }line 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 H_{2 }emission due to (high) nonequilibrium H_{2 }abundances. Publication: The Astrophysical Journal Pub Date: December 1995 DOI: 10.1086/176562 Bibcode: 1995ApJ...455..133H Keywords: INFRARED: ISM: LINES AND BANDS; ISM: CLOUDS; ISM: MOLECULES; ISM: PLANETARY NEBULAE: GENERAL; ULTRAVIOLET: ISM; Photodissociation; Chemical Composition; Temperature Distribution; Radiation Distribution; Fluorescence; Molecular Gases; Planetary Nebulae; Hydrogen Clouds; Hydroxyl Emission; Active Galactic Nuclei; Excitation; Gas Density; Time Dependence; Photons; Astrophysics full text sources ADS | data products SIMBAD (7)