Panel a: ¹³CO line intensity radial profiles (solid lines) obtained with three representative disk models with input surface density distribution Σ_gas (dashed lines) given by the self-similar disk model (see Eq. (1)). The model parameters are R_c = 200 au, M_disk = 10⁻³ M_⊙, and γ = 0.8, 1, 1.5 shown in dark blue, light blue, and purple, respectively (top, middle, and bottom panels). Panel b: column densities of gas-phase (solid lines) and ice-phase (dashed lines) ¹³ CO calculated from the surface to the midplane shown as function of the disk radius for the three representative self-similar disk models. The dotted black line indicates the column density at which CO self-shielding becomes inefficient (N = 10¹⁵ cm⁻²).