Rotational diagrams and optical depths of the slab model (N_H = 5 × 10²¹ cm^-2, T_gas = 450 K) with the lowest combining all the ¹²CO, ¹³CO, and C¹⁸O data (cross in Fig. 6). In the left panels observations are shown in black and models in red. In the right panels the optical depths of the observed transitions are shown in black, other transitions are plotted in red. A single-temperature and single-density slab does not correctly describe the rotational diagrams of the three CO isotopologs simultaneously. Error bars on the rotational diagram are 1σ.