Rotation diagrams for W51 (top) and Orion KL (bottom). The rotation temperature, T_rot, is related to the inverse of the slope of the best‐fit line, and the column density is obtained from the intercept at E_upper = 0 (cf. Johansson et al. 1984; Blake et al. 1987). Both plots are presented on the same scale to point out the difference in the rotation temperatures between the two sources.