Spectra of the CH₃CN (12_K–11_K) lines computed within the 5σ contours of the dust continuum map (see Sect. 3.1) up to the higher component detectable (K = 8). The gray profile shows the integrated spectrum and compares the measured profile and the XCLASS synthetic spectra obtained in LTE approximation for: 1) the CH₃CN K-ladder (red); 2) the CHCN K-ladder (blue); 3) and the opacity profile (green; same scale as the intensity one). The position of the K-ladders for the CH₃CN and its isotopomer is marked. Details in Sect. 3.1 and Table 4.