Effect of chemical CO depletion through grain-surface chemistry on the ¹²CO 2–1 intensityprofile (left panels) and ¹³CO 2–1 intensity profile (right panels) after 1 Myr (orange), 3 Myr (light blue/dark red), and 10 Myr (blue/black). Top and bottom rows:models with M_* = 0.32 M_⊙ and M_* = 1.0 M_⊙, respectively. The profile without chemical CO depletion is shown as a dashed line. The gas outer radii (R_CO, 90%) are shown as a cross at arbitrary height below the profile. After 1 Myr the chemical CO depletion is not significant enough to change the intensity profile and R_CO, 90%. After 10 Myr chemical CO depletion causes R_CO, 90% to increase (for details, see Sect. 4.3). Figures C.1 and G.1 give a full overview of the ¹³CO 2–1 intensity profile of the models without and with chemical CO depletion.