Gas mass of the models versus the retrieved gas mass from the C¹⁸O flux using Eq. (4), and the mass-weighted average of the temperature in the C¹⁸O emitting region. For the blue shaded region a temperature of 40 K is used, the gray shaded region uses the temperature of each model shown in the bottom panel. The shaded areas are the minimum and maximum values, with the mean value indicated by the solid line. The dashed line indicates the one-to-one correspondence between the two masses. The typical range in temperatures is indicated by the gray shaded region in the bottom panel.