Line-wing over line-core main-beam temperature ratio of the observed CO lines in the well studied sources CRL 618, CRL 2688, and OH 231.8+4.2 (J = 2–1 data are taken from previous works); see Table 2. Note the different behavior of the relative line-wing intensity between CRL 618, which shows particularly hot high-velocity outflows, and OH 231.8+4.2, in which the flows remain quite cold. CRL 2688 is an intermediate case. 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.