Line intensity as a function of column density for non-LTE radiative transfer models at the median gas volume density and temperature for each CN group. The full and dotted lines assume a ionisation fraction of 0 and 1.4 × 10^-4, respectively. The horizontal dashed lines represent the median observed intensity computed for each group. The symbols show the best column density for each group according to the RADEX models, with full circles for models with x_e = 0 and open squares for x_e = 1.4 × 10^-4.