NASA IRTF/TEXES spectra (in black) of IK Tau (left) and VY CMa (right) near the wavelengths (wavenumbers) of 10.7 μm (930 cm −1 ; top), 10.5 μm (950 cm −1 ; middle), and10.3 μm (966 cm −1 ; bottom). The wavenumber axes are in the topocentric frame of reference. The target spectra are normalised and zoomed. The normalised calibrator spectra are plotted in magenta and the fractional atmospheric transmissions are shown in grey. Only the target data with atmospheric transmission greater than 0.8 are plotted. The dark blue dotted lines in each spectrum indicate the positions of the labelled NH₃ rovibrational transitions in the stellar rest frame and the green solid lines indicatethe telluric lines (in the observatory frame) that blend with the NH₃ lines of VY CMa. We assume the systemic velocities of IK Tau and VY CMa to be 34 km s −1 and 22 km s−1, respectively.