a) PV plot of the ¹³CH₃CN(13–12) K = 2 line emission (excitation energy of 96 K) in G31.41 along a direction with PA = 68° passing through the HMC (dotted line in Fig. 9). The vertical and horizontal dashed lines indicate, respectively, the position of the continuum peak and the systemic velocity. The cross in the bottom right corner denotes the angular and spectral resolutions. b) Same as top panel, but for the CH₃CN(12–11) ν₈ = 1K,l = 6, 1 line (excitation energy of 778 K).