Relative abundance of CO in the gas phase in equilibrium between depletion and desorption as a function of density. The depletion is caused by the loss of CO from the gas phase as the molecules freeze onto dust grains (eq. [12] in text). The depletion is a function of density because the freezeout is proportional to the dust‐gas collision rate and inversely proportional to the cosmic‐ray‐induced desorption rate, which is itself independent of density.