H2CO in the Horsehead PDR: photo-desorption of dust grain ice mantles

Aims. For the first time we investigate the role of the grain surface chemistry in the Horsehead photo-dissociation region (PDR). Methods. We performed deep observations of several H2CO rotational lines toward the PDR and its associated dense-core in the Horsehead nebula, where the dust is cold (T-dust similar or equal to 20-30 K). We complemented these observations with a map of the p-H2CO 3(03)-2(02) line at 218.2 GHz (with 12 angular resolution). We determine the H2CO abundances using a detailed radiative transfer analysis and compare these results with PDR models that include either pure gas-phase chemistry or both gas-phase and grain surface chemistry. Results. The H2CO abundances (similar or equal to 2-3 x 10(-10)) with respect to H-nuclei are similar in the PDR and dense-core. In the dense-core the pure gas-phase chemistry model reproduces the observed H2CO abundance. Thus, surface processes do not contribute significantly to the gas-phase H2CO abundance in the core. In contrast, the formation of H2CO on the surface of dust grains and subsequent photo-desorption into the gas-phase are needed in the PDR to explain the observed gas- phase H2CO abundance, because the gas-phase chemistry alone does not produce enough H2CO. The assignments of different formation routes are strengthen by the different measured ortho-to-para ratio of H2CO: the dense-core displays the equilibrium value (similar to 3) while the PDR displays an out-of-equilibrium value (similar to 2). Conclusions. Photo-desorption of H2CO ices is an efficient mechanism to release a significant amount of gas-phase H2CO into the Horsehead PDR.