Results of photo-ionisation models. For each hydrogen density the calculation is stopped when the observed N(H₂) is reached. The blue shaded regions give the 1 and 2σ range on the total column density (top) or column density ratios (bottom) from the observations. The solid, dotted, dashed and dashed-dotted lines correspond to an in situ radiation field of respectively χ = 0,1,10,30 times the local Galactic field. The cold-gas-only model (T = 80 K, log N(H₂) = 20.13) is represented by black lines while the orange lines represent the model with an additional warm component (T = 300 K and log N(H₂) = 19).