Top: abundance distribution of C⁺ and CO and the corresponding ¹³C isotopologues for a model with n_surf = 10⁴ cm^-3, 1000 χ₀ and 1000 M_⊙. The blue curve shows the corresponding gas temperature structure determining the emissivity. Bottom: resulting integrated optically thin line emissivity ∫ϵdv in K km s^-1/(cm^-3 pc) for the [C ii] and [¹³C ii] lines (black curves) as a function of the clump radius. The red line shows the intensity ratio provided by the two curves.