Comparison between the rotational diagrams of the ¹²CO v=1-0 and v=2-1 transitions to test the assumption of LTE. The left panel shows the ¹²CO v=1-0 rotational diagrams (see also Figure 8). The right panel shows the non-LTE test, where the ¹²CO v=2-1 integrated fluxes are shown together with expected the models based on the results of the ¹²CO v=1-0 rotational diagrams. As the models overproduce the observed ¹²CO v=2-1 fluxes, invalidating the assumption that the levels can be characterised by a single temperature.