Simulated ¹²CO (J = 2−1) versus ¹³CO (J = 2−1) line luminosity of T Tauri disk models are presented: disk masses are color-coded and different values of the critical radius R_c are shown bydifferent symbol sizes. DALI model results from this work (R_c= 0.5, 1, 2, 5, 15 au) are shown as smaller circles, while results from Miotello et al. (2016) (R_c = 30, 60, 200 au) are shown using larger symbols. Lupus ¹²CO and ¹³CO (J = 2−1) line luminosity are presented with black squares (empty squares if the ¹²CO line is partially absorbed by the cloud), where the ¹³CO nondetections are shown as upper limits by the black arrows and the ¹²CO (and ¹³CO) nondetections are shown as upper limits by the gray arrows. The dotted gray line shows the average of the ¹³CO Lupus upper limits.