(a) C¹⁸O fractional abundances, , as a function of time, as calculated with different H₂ column densities and an assumed thickness of 0.68 pc. The shaded region corresponds to the observed C¹⁸O fractional abundance range. (b) Similar to Fig. 13, but overlaid with the isochronic lines which are based on the results in a. The markers represent the observed relationship between C¹⁸O fractional abundance and H₂ column density with different regions labeled in different colors. c and e are similar to a, but for an assumed thickness of 0.34 pc and 1 pc, respectively. Panels d and f are similar to panel b, but for an assumed thickness of 0.34 pc and 1 pc, respectively.