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

Context. Far-UV photons 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 (PAHs for instance) are not easily accessible towards high extinction line-of-sights. Furthermore, gas phase diagnostics provide information on the velocity fields. Methods. We have obtained maps of the HCO and H13CO+ ground state lines towards the Horsehead edge at 5 angular resolution with a combination of Plateau de Bure Interferometer (PdBI) and the IRAM-30m telescope observations. These maps have been complemented with IRAM-30m 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 than HCO in the PDR (≃1-2×10−9 with respect to H2). To our knowledge, this is the highest HCO abundance ever measured. Pure gas-phase chemistry models fail to reproduce the observed HCO abundance by ∼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/H13CO+ abundance ratio is large towards the PDR (≃50), and much lower toward the gas shielded from FUV radiation (.1). We propose that high HCO abundances (&10−10) together with large HCO/H13CO+ abundance ratios (&1) are sensitive diagnostics of the presence of active photochemistry induced by FUV radiation.