N₂H⁺ distribution (left panel), column density (middle panel) and radial J = 4−3 integrated line intensity profile (right panel) when photodesorption is included in the chemistry (model CH-PD with CO and N₂ abundances of 3 × 10^-6; dashed black lines). The dotted blue lines show a model with photodesorption rates increased by a factor 100 (model CH-PD 100x), and the red solid lines show the fiducial model without photodesorption (model CH). The simulated emission is convolved with a beam. Observations by Q13 are shown in gray in the right panel with the 3σ-error depicted in the lower right corner.