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Photodissociation regions. I - Basic model. II - A model for the Orion photodissociation region
Tielens, A. G. G. M.; Hollenbach, D.
AA(NASA, Ames Research Center, Moffett Field, CA), AB(NASA, Ames Research Center, Moffett Field, CA)
Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 291, April 15, 1985, p. 722-754. NASA-supported research. (ApJ Homepage)
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NASA/STI Keywords:
Astronomical Models, Gas Ionization, Molecular Clouds, Nebulae, Photodissociation, Abundance, Brightness Temperature, Emission Spectra, Far Ultraviolet Radiation, Fine Structure, Gas Density, Gas Temperature, H Ii Regions, Infrared Spectra
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A theoretical parameter study of the temperature and chemical structure of dense photodissociation regions and their resultant spectrum is presented. Models are discussed which are relevant not only to the dust and gas between molecular clouds and H II regions, but also apply to any neutral cloud illuminated by intense FUV fluxes. The models relate observed line and continuum emission from these regions to physical parameters such as the gas density and temperature, the elemental and chemical abundances, the local radiation field, and the grain properties. The results are applied to observational data from the OMC-1 region. The model shows that the observed high brightness temperature of the C I 609 microns line can be explained by emission from the C(+)/C/CO transition region. This difference with previous chemical models is due to a higher gas phase elemental abundance of carbon, to the charge exchange reactions of C(+) with S and SiO, and to carbon self-shielding.

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