Large-Scale C I Emission from Molecular Clouds with Associated Ultraviolet Sources

Details Large-Scale C I Emission from Molecular Clouds with Associated Ultraviolet Sources Large-Scale C I Emission from Molecular Clouds with Associated Ultraviolet Sources

Feb 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999apj...512..768p&link_type=abstract

The Astrophysical Journal, Volume 512, Issue 2, pp. 768-783.

Astronomy and Astrophysics

Astronomy

32

Ism: Individual: Name: Cepheus A, Ism: Individual: Ngc Number: Ngc 2024, Ism: Individual: Alphanumeric: S140, Ism: Individual: Alphanumeric: W3, Ism: Molecules, Radio Lines: Ism

Scientific paper

We present large-scale observations of the ^3P_1-->^3P_0 transition of neutral carbon, as well as observations of ^12CO, ^13CO, and C^18O, in four molecular clouds with nearby ultraviolet sources: W3, L1630/NGC 2024, S140, and Cep A. The observations cover approximately 30^'x30^' regions with a 3' beam. A typical C^0 column density is about 1.6x10^17 cm^-2. The overall extent and morphology of the C I emission is similar to that of the ^12CO and ^13CO J=2-->1 emission. There is a strong correlation of C I and ^13CO line intensities. The column densities and line strengths of the C I lines imply that these lines arise in gas at the edge of the molecular cloud that is dissociated by ultraviolet radiation. The correlation of C I and ^13CO intensity can arise as a result of a combination of column density variations, volume density variations, and unresolved cloud structure. In the latter case, however, any unresolved structures containing both C I and CO need not be more than a few times smaller than the telescope beam. Taken together with C^+ observations, the C I data imply that only about half of the gas-phase carbon in molecular clouds is in CO. The observations indicate that the large ^13CO/C^18O abundance ratio (~20) seen at the edges of molecular clouds results from isotope-selective photodissociation of C^18O.

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