CH₃CN disk around IRAS 20126+4104. The observer lies at y = −∞ and sees the disk edge on. The curved arrow indicates the sense of rotation. The points mark the estimated positions of the same peaks as in Fig. 7, and colour-coded in the same way. The grey area outlines a tentative trailing spiral arm. The starred symbol marks the position of the protostar and the two dashed lines are two lines of sight crossing the regions where the low- and high-energy lines peak. The two solid arrows indicate the two regions where the observed lines are emitted.