Left: C¹⁸O fractional abundance map with different labeled masks which correspond to the legend of the right panel. Right: C¹⁸O fractional abundance as a function of H₂ column density. The gray error bars indicate the measured uncertainties in C¹⁸O fractional abundance and H₂ column density. The colors of the markers correspond to different regions indicated in the legend. The Pearson correlation coefficient isshown in the lower left. The black dashed line represents the linear fit to the observed relation.