¹³CO (horizontal axis) and C¹⁸O (vertical axis) line luminosities from the physical–chemical models of protoplanetary disks (color dots) by Williams & Best (2014) overlaid on the measured line luminosities from the velocity-aligned stacking method (data points with error bars). The C¹⁸O non-detected disks are presented with open circles. Red, orange, yellow, green, blue, purple, and gray dots denote the models with disk masses of 10⁻¹, 3 × 10⁻², 10⁻², 3 × 10⁻³, 10⁻³, 3 × 10⁻⁴, and 10⁻⁴ M_⊙. The left panel is for the typical ISM abundance of C¹⁸O, while the right panel is for the C¹⁸O abundance that is three times lower the typical value.