The abundances of atomic carbon and carbon monoxide compared with visual extinction in the Ophiuchus molecular cloud complex

We have observed emission from the 492 GHz lines of C I toward six positions in the Ophiuchus molecular cloud complex for which accurate visual extinctions are available. We find that the column density of C I increases with A_v to greater than 2 x 10^(17) cm^(-2) at 100 mag, the column-averaged fractional abundance reaches a peak of about 2.2 x 10^(-5) for A_v in the range 4-11 mag and the column-averaged abundance ratio of C I to CO decreases with A_v from about 1 at 2 mag to greater than ~0.03 at 100 mag. These results imply that, while C I is not the primary reservoir of gaseous carbon even at cloud edges, its fractional abundance remains high for at least 10 mag into the cloud and may be significant at even greater depths