Panels a and b: C¹⁷O line intensity radial profiles (solid lines) obtained with six 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 = 10⁻⁴M_⊙ (left panel), 10⁻²M_⊙ (right panel) and γ = 0.8, 1, 1.5 shown in dark blue, light blue, and purple, respectively (top, middle, and bottom panels). Panels c and d: column densities of gas-phase (solid lines) and ice-phase (dashed lines) of C¹⁷ O calculated from the surface to the midplane shown as function of the disk radius for six models with simple power-law surface density (R_out = 200 au, M_disk = 10⁻⁴, 10⁻²M_⊙). Top, middle, and bottom panels: models with γ = 0.8, 1, 1.5, respectively. The dotted black lines indicate the column density at which CO self-shielding becomes inefficient (N = 10¹⁵cm⁻²).