Panel a: ¹³CO line intensity radial profiles (solid lines) obtained with three representative disk models with input surface density distribution Σ_gas (dashed lines) chosen to be a simple power-law (see Eq. (2)). The model parameters are R_out = 200 au, M_disk = 10⁻⁴ M_⊙, and γ = 0.8, 1, 1.5 shown in dark blue, light blue, and purple, respectively (top, middle, and bottom panels). No beam convolution is applied to the simulated images and a distance of 150 pc is assumed. Panel b: ¹³ CO line intensity radial profiles compared with input surface density profiles as in panel a, but convolved with a 0.2′′ beam. Panel c: column densities of gas-phase ¹³CO calculated from the surface to the midplane shown as function of the disk radius for the three representative models with simple power-law surface density. The dotted black line indicates the column density at which CO self-shielding becomes inefficient (N = 10¹⁵ cm⁻²). Ice-phase CO column densities are not shown in the plot as they are below the values shown here.