Population diagrams of HNCO, HNCS, HC₃N, and NO including observational data points (filled symbols) and points from representative non-LTE models (empty symbols; Sect. 4.2). The main input parameters (n_H2, T_kin, and N_tot) of the non-LTE models are indicated within the boxes, along with the linear fit to the data points (dashed line; same as in Fig. 9). For HNCO, squares and triangles represent the K_a = 0 and K_a = 1 transitions, respectively (as in Fig. 9).