Range of parameters compatible with the distances measured in the observations. Top: distance between the ionization front (IF) and the H/H₂ transition (dissociation front). The blue zone represents the upper limit that has been derived for the distance between the IF and the dissociation front. The blue dashed lines indicates the distance found by the Meudon PDR code for two radiation fields (G₀ = 50 and 186) and different thermal pressures (P_th). The models compatible with the observations are those in the blue zone. Bottom: same but for the distance between the IF and C/CO transition. The domain of parameters that reproduce the distances is represented by the black dashed squares.