Predicted behaviour of the ¹²CO, ¹³CO and C¹⁸O J = 2 → 1 isotopologue lines around 1.3 mm. The left plot shows the impact of model parameters on the predicted ¹²CO line flux, see Fig. 10 for more explanations. The two figures on the right show the impacts of model parameters on the ¹²CO /¹³CO and ¹³CO/C¹⁸O line ratios. Note that the direction of effects is sometimes inversed on the r.h.s., for example the dependency on disk flaring parameter β. Less flaring leeds to weaker CO lines in general, but to larger line ratios. On the right, both an increase and a decrease of the amorphous carbon dust volume fraction Vol(AC) lead to higher line ratios, here the larger arrows correspond to Vol(AC) = 0%.