Observed emission (left) and inferred optical depth (middle) of the ¹³CO J = 3-2 line and corresponding calculated column density of ¹³CO (right) assuming optically thin emission and the same excitation temperature distribution derived for SO₂. The dashed white circle in the middle panel marks the 60 mas radius inside of which the derived column densities are not reliable because of high optical depths. The white and magenta contours in the left and right panels, respectively, mark emission equal to three times the RMS noise level.