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

submitted to Astronomy and Astrophysics, 11 pages, abridged abstract.Far-UV photons strongly affect the physical and chemical state of molecular gas in the vicinity of young massive stars. We have obtained maps of the HCO and H13CO+ ground state lines towards the Horsehead edge at 5'' angular resolution with a combination of IRAM PdBI and 30m observations. These maps have been complemented with IRAM-30m observations of several excited transitions at two different positions. 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 than HCO+ in the PDR (~1-2 x10^{-9} with respect to H2). Pure gas-phase chemistry models fail to reproduce the observed HCO abundance by ~2 orders of magnitude, except if reactions of OI 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. The measured HCO/H13CO+ abundance ratio is large towards the PDR (~50), and much lower toward the gas shielded from FUV radiation (10^{-10}) together with large HCO/H13CO+ abundance ratios (>1) are sensitive diagnostics of the presence of active photochemistry induced by FUV radiation