2011A&A...534A..49G

For the first time we investigate the role of the grain surface chemistry in the Horsehead photo-dissociation region (PDR). 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≃20-30K). We complemented these observations with a map of the p-H₂CO 3₀₃-2₀₂ line at 218.2GHz (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. The H₂CO abundances (≃2-3x10^–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 (∼3) while the PDR displays an out-of-equilibrium value (∼2). Photo-desorption of H₂CO ices is an efficient mechanism to release a significant amount of gas-phase H₂CO into the Horsehead PDR.