Wing intensity ratio between the observed lines and the CO J = 2–1 transition (from previous works) in the well-studied sources CRL 618, CRL 2688, and OH 231.8+4.2; see Table 2. A comparison between these ratios and estimates from our simple, general models reveals values of the characteristic temperature compatible with estimates from detailed models (see text). Squares (continuous line) represent ¹²CO data and triangles (dashed lines) show ¹³CO data. Each rotational transition is represented by the energy (in K) of its upper level.