Column density ratio of HCO and H¹³CO⁺ molecules as a functionof N(H₂) (upper panel) and T_dust (lower panel). The symbols are the same as explained in Fig. 10. The bigger dots indicate sources with their X(HCO) higher than 10⁻¹⁰ (Gerin et al. 2009). The purple and blue stars (upper panel only) represent N(HCO)/N(H¹³CO⁺) ratios from the PDR and cold gas region in the Horsehead nebula (Gerin et al. 2009). The red and blue dashed line ellipses are confidence ellipses showing distributions of these two populations (HII and non-HII regions) with 1 and 2σ levels. In the upper plot, the red and blue lines indicate the best-linear regression fits data points of HII and non-HII regions (IR bright and IR dark non-HIIs). Upper panel: the black horizontal dashed line presents that N(HCO)/N(H¹³CO⁺) ratios is 1.