Column density fractionation ratios plotted against emission line ratios for all models from our parameter space. (blue: ER, red: ER  ± 10   %, dashed: ER  ±    20%). Models with line intensities  < 0.01    K   km   s^-1 have been omitted. The symbols follow the coding from Fig. 9. Left panel: N(CO)/N(¹³CO) vs. T_mb(¹³CO(2 − 1))/T_mb( [CI] 610   μm), middle panel: N(C)/N(¹³C) vs. T_mb( [CII] )/T_mb(H¹³CO ⁺ (1 − 0)), right panel: vs. T_mb(¹³CO(3 − 2))/T_mb( [CII] ).