Median of the ¹³CO (upper panel) and C¹⁸O (lower panel) J = 2−1 line luminosities in different mass bins for compact disk models are presented with filled circles. Red solid lines show the fit functions presented in Eqs. (A.1) and (A.2). The translucent symbols show all model results, while the gray solid lines show the fit to the maximum and minimum luminosities, which can be used to estimate uncertainties on the mass measurements.