Sample of observed HCN lines in the mid-IR (black histogram) with their best fits (spherically symmetric models; red curves) grouped in bands. The blue solid lines in the range 13–14 μm were calculated with the spherically symmetric envelope model that better fits the lines at 7.5 μm and vice versa (see Table 2 and text). The green solid lines were calculated with the asymmetric envelope model (see Sect. 4.3.2). They are vertically shifted to improve the visibility. The gray curves are the atmospheric transmission. The black arrows point at possible red-shifted wings. The vertical dashed-dotted and dashed lines are the systemic velocity (0 km s⁻¹) and the terminal Doppler velocities derived in this work (≃8 km s⁻¹), respectively. All the fits are reasonably good except those to the lines of band 2ν₂ (δ) − ν₂(π), Q branch (see text).