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

We present large-scale observations of the 3P1→3P0 transition of neutral carbon, as well as observations of 12CO, 13CO, and C18O, in four molecular clouds with nearby ultraviolet sources: W3, L1630/NGC 2024, S140, and Cep A. The observations cover approximately 30'×30' regions with a 3' beam. A typical C0 column density is about 1.6×1017 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/C18O abundance ratio (~20) seen at the edges of molecular clouds results from isotope-selective photodissociation of C18O.