Ratio of the ¹²CO₂ Q-branch flux and the fluxes of several H₂O line complexes, plotted as a function of cavity size for the models with input spectra with L_* = 0.2, 0.4, 0.8 L_⊙. Note the different vertical scales of these panels. Properties of these lines can be found in Table 2. Data points are connected by dashed lines if the line flux drops below 10⁻¹⁵ erg s⁻¹ cm⁻², indicating that the lines will become more difficult to observe.