Photochemical model of the Horsehead PDR. Upper panel: PDR density profile (n_H = n(H) + 2n(H₂) in    cm^-3). Middle panel: predicted abundance (relative to n_H) of H₂CO (blue) and HCO (red). Lower panel: predicted HCO/H₂CO abundance ratio. In the two bottom panels, models shown as solid lines include pure gas-phase chemistry and models shown as dashed lines include gas-phase as well as grain surface chemistry. The horizontal bars show the measured H₂CO abundances and abundance ratios.