Effect of chemical depletion of CO through grain-surface chemistry on the CO abundanceafter 1 Myr (left panels) and 10 Myr (right panels). The example model shown has M_* = 0.32 M_⊙ and α_visc = 10⁻³. The colors show the CO abundance with respect to H₂, where white indicates CO/H₂ ≤ 10⁻⁸. The black contours show the ¹²CO 2–1 emittingregion, enclosing 25 and 75% of the total ¹²CO flux. Similarly, the red contours show the ¹³CO 2–1 emitting region.