Difference between the velocity of the peak of the emission in the PV plots along the direction with PA = −45° toward core A and the systemic velocity for several transitions of CH₃CN and CH₃CN v₈ = 1 as a function of upper level excitation energy. The systemic velocity V_sys is ~49.5 km s^-1. The open circles correspond to the v₈ = 1 transitions. The different K and (K,l) transitions are indicated.