Modelling the constancy of X

Abstract

Observations of the CO J=1-0 line are commonly used as a tracer for molecular material in clouds. The ratio of the H₂ column density to the integrated intensity of this line,X, is often taken to be constant, despite theoretical and observational uncertainty. We have tried to identify how this ratio depends on cloud parameters, testing a simple theoretical argument suggesting its invariance with respect to density. The apparent constancy can be understood if clouds are clumpy on scales of Av ≈ 1-2 mag.