Fraction of CO in gas phase in our fiducial model, as a function of total hydrogen density. CO is able to form efficiently, reaching the canonical abundance in gas above n_H, tot = 3 − 4 × 10³ cm⁻³, before freeze-out on dust grains starts to deplete it, as expected. We also find that the abundance does not significantly vary with time, similarly to the OPR, as long as depletion remains almost negligible.