Astronomy and Astrophysics – Astrophysics
Scientific paper
Dec 1990
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1990apj...365..620b&link_type=abstract
Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 365, Dec. 20, 1990, p. 620-639.
Astronomy and Astrophysics
Astrophysics
284
H Ii Regions, Infrared Spectra, Interstellar Gas, Molecular Clouds, Photodissociation, Carbon Monoxide, Emission Spectra, Fine Structure, Hydrogen, Line Spectra, Ultraviolet Radiation
Scientific paper
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.
Burton Michael G.
Hollenbach David J.
Tielens Alexander G. G. M.
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