CH, CO, and E(B-V) as molecular gas tracers in a translucent cloud

The effectiveness of three molecular hydrogen gas tracers-the CH(2)Pi(1/2), J = 1/2, F = 1-1 transition, the CO(1-0) transition, and the color excess, E(B-V)-is evaluated for the translucent molecular cloud MBM 16. The CH emission line and E(B-V) are well correlated with each other but not with the CO(1-0) emission line. It is likely that in translucent molecular clouds CO is not a linear tracer of the molecular hydrogen column density because, in these objects, the CO/H-2 ratio is undergoing rapid fluctuations as a function of the molecular hydrogen column density. For translucent and diffuse molecular clouds, and possibly for the translucent envelopes of giant molecular clouds, CH observations should be used to calibrate the CO to H-2 conversion factor.