Panel a: gas density and temperature along the CN high abundance layer (dotted-green line in Fig. 2a). Panel c: column densities of CN molecules in the (N = 1, J = 3/2) (dotted), (N = 3, J = 7/2) (solid) and (N = 5, J = 11/2) (dashed) levels. Panel b: intensities of the N = 5−4, N = 3−2, and N = 2−1 transitions, for comparison with panel c. The vertical lines mark the location of the column density peaks of CN molecules in the three levels.