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Line emission from clumpy photodissociation regions
Burton, Michael G.; Hollenbach, D. J.; Tielens, A. G. G. M.
AA(NASA, Ames Research Center, Moffett Field; California, University, Irvine), AB(NASA, Ames Research Center, Moffett Field, CA), AC(NASA, Ames Research Center, Moffett Field, CA)
Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 365, Dec. 20, 1990, p. 620-639. (ApJ Homepage)
Publication Date:
NASA/STI Keywords:
H Ii Regions, Infrared Spectra, Interstellar Gas, Molecular Clouds, Photodissociation, Carbon Monoxide, Emission Spectra, Fine Structure, Hydrogen, Line Spectra, Ultraviolet Radiation
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A theoretical parameter study of dense photodissociation regions is presented. It is found that when the gas density is sufficiently high relative to the FUV flux, self-shielding of the molecules can move the C(+)/CO and H/H2 atomic-molecular transitions close to the surfaces of the molecular cloud, where they can feel the full effect of heating by the FUV radiation field. For n of 100,000/cu cm, collisional deexcitation of the FUV-pumped H2 can move the lower levels toward the LTE, producing line ratio resembling those of shocked regions for these low-v levels, while the high-v level line ratios retain a 'fluorescent' value. Appreciable emission in high-J transitions of CO originates in this warm molecular gas. Comparison with observations suggests that a small volume filling factor of high density clumps embedded within a moderate density interclump medium are a common phenomenon in photodissociation regions.

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