Plots of the peak positions in the PV plots in Fig. 6 versus the energy of the lower level of the corresponding transition. The black points correspond, from left to right, to the K = 2,3,4,5,7,8 and v₈ = 1 K,l = 1,1 lines, while the white point denotes the CH₃¹³CN K = 2 line. Top: absolute value of the relative velocity of the peak with respect to the systemic velocity, V_sys ≃ −3.9 km s^-1. The bars correspond to half the velocity channel. Bottom: absolute value of the distance of the peak from the position of the continuum peak, measured along the plane of the disk (i.e. along the direction with PA = +37°).