Total CO abundance (gas and ice) from the chemical models, at 1, 3 and 10 Myr, mapped onto the temperature and density structure of a T Tauri and Herbig disk model (see Fig. A.1 and Appendix A). A cosmic-ray ionisation rate of 10⁻¹⁷ s⁻¹ is assumedthroughout the disk. The orange area shows the disk region that has a temperature above 40 K and is thus not included in our chemical models. See Fig. A.1 for the CO abundance in that region using a simple CO chemistry. The white contours shows the area within which 25% and 75% of the C¹⁸O flux is emitted. The black and grey dotted lines show the 20 and 30 K contours, respectively. The Herbig disk is always warmer than 20 K. The molecules in red denote the dominant carbon carrier in the regions CO is depleted. The orange dashed line encompasses the region that is completely shielded from UV radiation (see Sect. 2.1).