Statistics – Applications
Scientific paper
Dec 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995apj...455..133h&link_type=abstract
The Astrophysical Journal, vol. 455, p. 133
Statistics
Applications
42
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
Scientific paper
We present theoretical models of the time-dependent thermal and chemical structure of molecular gas suddenly exposed to far-ultraviolet (FUV) (6 eV less than hv less than 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/cu cm and FUV fluxes G0 = 103-106 times the local FUV interstellar flux. For G0/n greater than 10-2 cu cm, the emergent H2 vibrational line intensities are initially larger than the final equilibrium values. The H2 lines are excited by FUV fluorescence and by collisional excitation in warm gas. Most of the H2 intensity is generated at a characteristic hydrogen column density of N approximately 1021/sq cm, which corresponds to an FUV optical depth of unity caused by dust opacity. The time dependence of the H2 intensities arises because the initial abundances of H2 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 approximately 1000) K H2 gas can be produced by the FUV pumping of H2 vibrational levels followed by collisional de-excitation, which transfers the energy to heat. In dense (n greater than or approximately 105/cu cm) gas exposed to high (G0 greater than or approximately 104) fluxes, this warm gas produces a 2-1 S(1)/1-0 S(l) H2 line ratio of approximately 0.1, which mimics the ratio found in shocked gas. In lower density regions, the FUV pumping produces a pure-fluorescent ratio of approximately 0.5. We also present calculations of the time dependence of the atomic hydrogen column densities and of the intensities of 0 I 6300 Å, S II 6730 Å, Fe II 1.64 microns, and rotational OH and H20 emission. Potential applications include star-forming regions, clouds near active galactic nuclei, and planetary nebulae. We apply our models to five planetary nebulae and conclude that only BD +30deg3639 shows evidence of enhanced H2 emission due to (high) nonequilibrium H2 abundances.
Hollenbach David
Natta Antonella
No associations
LandOfFree
Time-Dependent Photodissociation Regions does not yet have a rating. At this time, there are no reviews or comments for this scientific paper.
If you have personal experience with Time-Dependent Photodissociation Regions, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Time-Dependent Photodissociation Regions will most certainly appreciate the feedback.
Profile ID: LFWR-SCP-O-1258103