Panel 1): CH₃CN spectrum towards the HC in black, LTE model fit in dashed purple, and fitting with optical depth correction in filled red. Panels 2)−6): rotation diagram of CH₃CN derived for each substructure, with dots for p-CH₃CN and triangles for o-CH₃CN. Black fittings are only for the p-lines when assuming they are optically thin, and red fittings are for both o- and p-lines by optical depth correction. Panel 7): spectrum towards the HC in black and the LTE model fit in red. Panel 8): rotation diagram of in HC with black dots (p-) and triangles (o-) from observation, and fitting is o-lines excluded by assuming lines are optically thin. For lines without detection, an upper limit equal to the 3σ rms is shown as arrow. The estimated temperature and total column density from each fitting is indicated in each panel. Errors are derived from scatter in the data points.