Definition of the different transition zones. Left: zoom in version of the normalized integrated intensity profiles shown in the left panel of Fig. 3 at the vertical position of the PDR, but only including the HCO⁺ J = 4–3 (solid blue curve), the CO J = 3–2 (solid red curve), the Hα (solid black curve), and the H₂ lines (solid green curve). The location of the ionization front, the dissociation front, and the C⁺/C/CO transition zone are shown by the black, green, and red vertical dashed lines, respectively. The dotted red and blue lines display the ±1σ significance levels for CO and HCO⁺, respectively, corresponding to the standard deviation of the average in the δy direction. Right: CO J = 3–2 integrated intensity map, showing the horizontal cut used to extract the intensity profiles at the vertical position of the PDR. The yellow rectangle shows the area where the emission of the different tracers was averaged, but in the zoom in version shown in the left panel this area was restricted between δx = −3″ to 15″ for the purpose of defining the transition zones. The two crosses, the red vertical line, and the different contours are the same as in Fig. 2. The angular resolution is plotted in the bottom left corner.