Observations of neutral carbon in the NGC 1977 bright rim

Strong neutral carbon emission at 610 μm (429 GHz) has been detected from a bright-rimmed cloud abutting and H II region NGC 1977. The similarity of velocity and width between 13CO and C I lines suggests both lines originate in the same region. A model for the density and temperature structure of the cloud, based on 13CO and 12CO observations, has been used to estimate the carbon abundance. the abundances of both C I and 13CO increase with depth into the cloud away from the rim. The carbon abundance reaches its peak value nearer the rim than does the 13CO abundance. This variation in the relative abundance distributions of CO and C I confirms the importance of photodissociation in the chemistry of molecular clouds, and the C I line to studies of the interaction of hot stars with clouds