CO ladders for a PDR model with n = 10³ cm^-3 and G₀ = 10³ for different values of mechanical heating. The numbers in the legend correspond to Γ_mech in terms of the surface heating. Left: CO line intensities as a function of the rotational level J. Notice the boost in the intensities is much larger at high-J compared to low-J. Right: optical depth at the line centres (τ₀) for the same lines. The optical depth generally decreases as a function of increasing Γ_mech, but the most significant effect is that the low-J CO lines become optically thinner with τ₀ ~ 1.