(a): CO depletion factor (f_d) versus number density of hydrogen nuclei (n_H). The data for each position are shown, along with their uncertainties. Astrochemical models are shown for ages ranging from 10⁵ yr to 10⁷ yr and for ζ = 10⁻¹⁸ s⁻¹ (dotted lines) and 10⁻¹⁷ s⁻¹ (solid lines) and 10⁻¹⁶ s⁻¹ (dashed lines) with other parameters set to be T = 15 K and A_v = 20 mag. (b): CO depletion factor (f_d) versus visual extinction (A_v). The data for each position are shown, along with their uncertainties. Astrochemical models are shown here for n_H = 10⁵ cm⁻³, T = 15 K and ζ = 10⁻¹⁷ s⁻¹ with solid lines (colors represent different times, as in legend of (a)). An equivalent model for n_H = 10⁴ cm⁻³ is shown with dotted lines.