CO abundance as a function of A_V for selected models. In all the diagrams, the C/CO transition zone is shifted towards the surface as a function of increasing Γ_mech. For M1 and M2, the abundance of CO is below the displayed range for Γ_mech > 10^-20 erg cm^-3 s^-1 (these curves are not shown). The other curves almost overlap except for Γ_mech = 10^-20 erg cm^-3 s^-1. In models M3 and M4 the abundance of CO drops slightly beyond the transition zone compared to the case of no mechanical heating. The curves overlap except for Γ_mech = 10^-16 erg cm^-3 s^-1 where the C⁺/C/CO transition does not fully take place.