Radial profile of the disk layers where the disk becomes optically thick (τ = 1 surface), for different CO isotopologues of the VSI-unstable disk radiative transfer model. We illustrate the τ = 1 surface for the J:2–1 and J:3–2 transitions of three CO isotopologues, ¹²CO, ¹³CO and C¹⁸O. We assume a face-on disk orientation for the calculation. The gray dashed lines show where Z = AR for A =0.1;0.2;0.3;0.4 and 0.5. The solid black line shows the grid boundary in colatitude of the simulated disk, located approximately at Z = 0.45 R.