Fig. 17.

Rotational population diagrams for p-H₂CO computed with a non-LTE excitation code. All models adopt the same gas temperature (T_k = 200 K) and column density (N_tot = 1.8 × 10¹³ cm⁻²), but three different n(H₂) values. For simplicity, only rotational transitions in the K_a = 0 (filled symbols) and 2 (empty symbols) ladders are shown. We note that at densities <10⁸ cm⁻³, the set of transitions with the K_a = 2 number are also split into two components: (i) transitions with K_c = J − 1 (empty symbols connected with solid lines) and (ii) transitions with K_c = J − 2 (empty symbols connected with dashed lines).