Line luminosity of C¹⁸O vs. ¹³CO (J = 2−1) obtained implementing isotope-selective processes. Different colors represent models with different disk gas masses. In the upper panels models with different values for R_c are presented separately. In the lower panels models with different values for γ are shown separately. Models displayed in panel a) and d) are also reported in gray in panels b), c) and e), f) respectively in order to facilitate the comparison.