Effect of the chemical conversion of CO on the CO abundance after 1, 3 and 10 Myr. The example model shown here has M_* = 0.32 M_⊙ and α_visc = 10⁻³. Top panels: CO abundances have been calculated using the DALI chemical network (see Sect. 2.3.1).Bottom panels: we have included the chemical conversion of CO into other species, as described in Sect. 2.3.2. Colors show the CO abundance with respect to H₂, where white indicates CO/H₂ ≤ 10⁻⁸. The dashed black line shows where ≤5% of the total amount of oxygen is in O(g)+OH(g), the gas temperature T_gas ≤ 35 K and outwardCO column is ≥10¹⁶ cm⁻². Grain-surface chemistry is calculated below this black dashed line. Red contours show the ¹³ CO J = 3− 2 emitting region, enclosing 25 and 75% of the total ¹³CO flux. Similarly, the orange contours show the C¹⁸O J = 3− 2 emitting region.