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

International audienceContext: 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 H13CO+ 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), ≃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. Based on observations obtained with the IRAM Plateau de Bure interferometer and 30 m telescope. IRAM is supported by INSU/CNRS (France), MPG (Germany), and IGN (Spain). 30m maps of the ground state lines are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/494/97