CO vibrational ratio versus the inferred radius of emission for observational data (gray points) and analytic (left) and RADEX (right) model results (colored lines). For the RADEX models, two different assumption for the radiation field are shown weakly irradiated (W = 0.01, dashed) and strongly irradiated (W = 0.3, dotted) cases. For the highest column onlythe LTE model and the weakly irradiated RADEX model are shown. The density for the RADEX models is 10¹² cm⁻³.