Photochemical model of the Horsehead PDR. The left column shows the full-resolution model and the right column shows the model convolved with a Gaussian of 6′′ FWHM. Upper panel: PDR density profile (n_H = n(H)+ 2n(H₂) in  cm^-3). Middle panel: predicted abundance (relative to n_H) of HCO (red), H₂CO (blue) and CH₃OH (green). Lower panel: predicted H₂CO/CH₃OH abundance ratio. Models shown as dashed lines include pure gas-phase chemistry and models shown as solid lines include both gas-phase and grain surface chemistry. The CH₃OH abundance predicted by the pure gas-phase model has been multiplied by 10³ in the middle panel so that it could fit in the figure.