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.