LTE radiative transfer models for the three main CO isotopologues. The four rows show the best match between the observations and the models for, from top to bottom, the full field of view studied here, clusters 1 to 6, and the 7th and 8th clusters. The control parameters of the family of models are written on top of each associated row. The left and middle columns show the joint histogram of the (1–0) lines of ¹³CO vs. ¹²CO, and of ¹³CO vs. C¹⁸O. The right column shows the joint histogram of the ¹³CO/ C¹⁸O vs. ¹²CO /¹³CO intensity ratios. The colour look-up tables show the number of lines of sight that fall within a given bin of the histogram. The white curves present the LTE intensity variations as a function of the ¹³CO kinetic temperature for different fixed ¹³CO opacities (0.03, 0.1, 0.3, 0.5, 0.65, 1.2, 1.75, 2.5).