Luminosity diagrams for the C¹⁸O and ¹³CO lines. Left: translucent points are results of WB14 assuming CO/C¹⁸O = 550 and traced points are results of this work using the assumptions from WB14. Traced stars are results of M16 and contours are results of this work with the grain-surface chemistry network. The points and contours are colored according to the disk mass of the model. Right: points are values of individual test models. Black point is the grain-surface chemistry model, white points are modified versions of it. Color of the arrow shows the added modification to the model: grain chemistry turned off (black), parametric vertical structure (no HE, red), isotope-selective photodissociation (ISO on, blue), and the assumptions of WB14 (as WB14, gray). All models have the same parameters: M_d = 0.01 M_⊙, R_c = 60 au, M_⋆ = 1.0 M_⊙, L_⋆ = 1.0 L_⊙, and T_⋆ = 4000 K. Models with parametric vertical structure have h_c = 0.1, ψ = 0.1.