¹³CO (horizontal axis) and C¹⁸O (verticalaxis) line luminosities from the physical–chemical models of protoplanetary disks (color dots) by Miotello et al. (2016) 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, 10⁻², 5 × 10⁻³, 10⁻³, 5 × 10⁻⁴, and 10⁻⁵, 10⁻⁴ M_⊙.