ATOMIC CARBON IN INTERSTELLAR CLOUDS

We have computed models of interstellar clouds in order to determine t he transition from the warm, photon-dominated region at the edge to th e cold, dark interior. We find that the equilibrium value of the C/CO ratio in the interior is crucially dependent on the fractional ionizat ion of the gas, which varies with the cosmic ray flux, the metallicity , and the kinetic temperature. When the degree of ionization in the cl oud centre exceeds a critical value, the ratio n(C)/n(CO) increases by about two orders of magnitude, to n(C)/n(CO) approximate to 0.1. Howe ver, this increase is not reflected in the intensities of the CI fine structure emission lines, when they arise predominantly in the photon- dominated region. Instead, the rotational transitions of carbon-bearin g species, such as CH and CN, are better indicators of the atomic carb on content of the cloud interior.