Selected doublets of HC₉N ν₁₉ = 1 in the 31–50 GHz domain. Left panels: lines of HC₉N in the ground vibrational state. The rotational quantum numbers are indicated in the top-right side of each panel. The same transitions for the ν₁₉ = 1 state are shown in the middle (e component) and right (f component) panels. The intensity scale is antenna temperature in mK. The abscissa corresponds to local standard of rest (LSR) velocities in km s⁻¹. The vertical violet dotted line at −26.5 km s⁻¹ indicates the systemic velocity of the envelope (Cernicharo et al. 2000, 2018). Two additional ν₁₉ doublets are shown in Fig. 1.