HCO mapping of the Horsehead: tracing the illuminated dense molecular cloud surfaces

Context. Far-UV photons (FUV) strongly affect the physical and chemical state of molecular gas in the vicinity of young massive stars. Aims. Finding molecular tracers of the presence of FUV radiation fields in the millimeter wavelength domain is desirable because IR diagnostics (for instance PAHs) are not easily accessible along high extinction line-of-sights. Furthermore, gas phase diagnostics provide information on the velocity fields. Methods. We have obtained maps of the HCO and (HCO+)-C-13 ground state lines towards the Horsehead edge at 5 '' angular resolution with a combination of Plateau de Bure Interferometer (PdBI) and the IRAM-30 m telescope observations. These maps have been complemented with IRAM-30 m observations of several excited transitions at two different positions. Results. Bright formyl radical emission delineates the illuminated edge of the nebula, with a faint emission remaining towards the shielded molecular core. Viewed from the illuminated star, the HCO emission almost coincides with the PAH and CCH emission. HCO reaches a similar abundance to HCO+ in the photon dissociation region (PDR), similar or equal to 1-2 x 10(-9) with respect to H-2. To our knowledge, this is the highest HCO abundance ever measured. Pure gas-phase chemistry models fail to reproduce the observed HCO abundance by similar to 2 orders of magnitude, except if reactions of atomic oxygen with carbon radicals abundant in the PDR (i.e., CH2) play a significant role in the HCO formation. Alternatively, HCO could be produced in the PDR by non-thermal processes such as photo-processing of ice mantles and subsequent photo-desorption of either HCO or H2CO, and further gas phase photodissociation. Conclusions. The measured HCO/(HCO+)-C-13 abundance ratio is large towards the PDR (similar or equal to 50), and much lower toward the gas shielded from FUV radiation (less than or similar to 1). We propose that high HCO abundances (greater than or similar to 10(-10)) together with large HCO/(HCO+)-C-13 abundance ratios (greater than or similar to 1) are sensitive diagnostics of the presence of active photochemistry induced by FUV radiation.