Model calculations for different ¹²CO/¹³CO ratios compared to the observed ¹³CO lines adopting the dissociation radius by Mamon et al. (1988). The solid red line represents the observations. The long-dashed green, short-dashed blue, dotted purple, and dot-dashed light blue lines are for intrinsic isotope ratios 10, 15, 20, and 30, respectively. Transitions J = 10–9 and 9–8 have been smoothed for a better visualization.