Photochemical model of the Horsehead PDR displayed at the full resolution of the model (left column) and convolved with a Gaussian of 6′′-FWHM (right column). Top: Horsehead density profile n_H = n(H) + 2n(H₂). Middle: predicted abundance of selected small hydrocarbons molecules and cations. Bottom: abundances of the same hydrocarbons relative to the abundance of C₃H⁺. The illuminating star is positioned at the right of the plots. The symbols present the measured range of possible abundances for C₂H (blue vertical segment), C₃H₂ (green vertical segment), and C₃H⁺ (red filled rectangle), inferred at a typical resolution of 6′′. The legend at the bottom of the figure presents the line coding for the curves: C₂H in plain blue, C₂H⁺ in dashed blue, C₃H in plain red, C₃H⁺ in dashed red, and C₃H₂ in green.