Upper part: sum of all line fluxes emitted by the different molecules between 9.7 and 38 μm. The vertical dashed line indicates the main model as plotted in Figs. 4 and 5, which is broadly consistent with the observations. Lower part: colour of the water line emission spectrum in the form of a ratio of two o-H₂ O lines at about 12 and 27 μm. In the series of 4 models on the left, the gas/dust mass ratio in the disc is varied, the calculated gas temperatures are used, and R_in = 0.07 au is assumed. In the series of 4 models in the centre, the gas/dust mass ratio in the disc is varied while T_gas =T_dust and R_in = 0.07 au are assumed. In the series of models on the right, the disc inner radius R_in is varied while assuming g/d = 100 and using the calculated gas temperatures. “H₂O” is the combined emission from o-H₂O and p-H₂O, and “OH” isthe combined emission from rotational OH lines and ro-vibrational OH_H lines; see Table 3.