AB initio determination of the ratio of H2 column density to CO(J=1 -->0) integrated antenna temperature.
Observations of the CO(J=1 → 0) line are commonly used as a tracer for molecular material in interstellar clouds. The ratio of the H2 column density to the integrated intensity of this line is often taken to be constant, despite theoretical and observational uncertainty on this point. Cloud models are presented which attempt to identify how this ratio depends on cloud parameters, testing a simple theoretical argument that suggests its invariance with respect to density. The apparent constancy of the ratio can be understood if clouds are clumpy on a scale of AV≲2 mag.