Abstract

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 (Tdust ≠20-30 K). We complemented these observations with a map of the p-H2CO 303-202 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 H 2CO abundances (2-3 × 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 H 2CO abundance in the core. In contrast, the formation of H 2CO 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 (∼3) while the PDR displays an out-of-equilibrium value (∼2). Conclusions. Photo-desorption of H 2CO ices is an efficient mechanism to release a significant amount of gas-phase H2CO into the Horsehead PDR. © 2011 ESO.