Panel a: C¹⁸O column densities as a function of H₂ column densities. Panel b: C¹⁸O fractional abundances as a function of H₂ column densities. Panel c: similar to Fig. 5a but for N₂H⁺ column densities. Panel d: similar to Fig. 5b, but for N₂H⁺ fractional abundances. Panel e: C¹⁸O fractional abundances as a function of N₂H⁺ fractional abundances. Panel f: N₂H⁺/C¹⁸O abundance ratios as a function of H₂ column densities. In all panels, the values of NW, SE, and EX are shown with red, green, and blue symbols, respectively. In panels c–f, only data from NW and SE are presented.