NH₃ rovibrational spectra of VY CMa in the transitions aR(0, 0) (top) and aQ(2, 2) (bottom). The black line in the top-left panel shows the observed spectrum from the NASA IRTF while those in other panels show the old spectra from the McMath Solar Telescope, as reproduced from Fig. 1 of McLaren & Betz (1980, © AAS. Reproduced with permission). Red lines show the modelled spectra in all panels. The modelled spectrum in the top-left panel was divided by a smoothed version of itself as in the real data (see Sect. 3.4) in order to induce the similar distortion in the line wings.