Photochemical model of the Horsehead PDR. A_V increases from right to left; the PDR edge, delineated by the red vertical line in Fig. 2, corresponds to A_V = 0. a) Horsehead density profile n_H = n(H) + 2n(H₂). b) Predicted abundance of CF⁺ in red, HF in blue and C⁺ in green. The red horizontal bars show the measured CF⁺ abundances, and their length represents the beam size. c) Predicted intensity profiles.