Spectral fits of the CH₃CN 6₀–5₀, 6₁–5₁, 6₂–5₂, and 6₃–5₃ transitions extracted in the four regions R1, R2, B1, and B2. In the upper panels of each region, the transitions producing the emission lines (see Table 1) are indicated, and their corresponding frequencies are marked by small vertical purple lines. The spectra are centred at the frequency of the 6₀–5₀ transition (110.3835 GHz). In regions R1 and R2, each transition shows a main peak at +9–10 km s⁻¹ (orange curves) and a secondary component redshifted by ∼3–4 km s⁻¹ (blue curves). In regions B1 and B2, each transition shows a main peak at +6 km s⁻¹ (orange curves) and a secondary component blueshifted by ∼5 km s⁻¹ (blue curves). In all panels, the red curves show the total multi-component fits. The dashed vertical line marks the systemic velocity, +8.5 km s⁻¹ (Lee et al. 2014).