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The abundance of atomic carbon near the ionization fronts in M17 and S140
Keene, J.; Blake, G. A.; Phillips, T. G.; Huggins, P. J.; Beichman, C. A.
AA(California Institute of Technology, Pasadena), AB(California Institute of Technology, Pasadena), AC(California Institute of Technology, Pasadena), AD(New York University, NY), AE(California Institute of Technology, Jet Propulsion Laboratory, Pasadena)
Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 299, Dec. 15, 1985, p. 967-980. (ApJ Homepage)
Publication Date:
NASA/STI Keywords:
Carbon, Gas Ionization, Interstellar Chemistry, Interstellar Matter, Molecular Clouds, Carbon Monoxide, Optical Thickness, Photodissociation, Photoproduction, Time Dependence
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The 492 GHz ground-state line of atomic carbon in the edge-on ionization fronts in M17 and S140 were observed. It was found that, contrary to expectation, the C I emission peaks farther into the molecular cloud from the ionization front than does the CO. In fact the peak C I abundance in M17 occurs more than 60 mag of visual extinction into the cloud from the ionization front. Calculations of the ratio of C I to CO column densities yield values of 0.1-0.2. These observations do not support chemical models which predict that neutral atomic carbon should be found only near the edges of molelcular clouds. Other models are discussed which may explain the observations.

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