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Article Dans Une Revue Astronomy and Astrophysics - A&A Année : 2011

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

Résumé

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 H2CO 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 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). Conclusions: Photo-desorption of H2CO ices is an efficient mechanism to release a significant amount of gas-phase H2CO into the Horsehead PDR.

Dates et versions

hal-00992495 , version 1 (18-05-2014)

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V. Guzmán, J. Pety, J. R. Goicoechea, M. Gerin, E. Roueff. H2CO in the Horsehead PDR: photo-desorption of dust grain ice mantles. Astronomy and Astrophysics - A&A, 2011, 534, pp.49. ⟨10.1051/0004-6361/201117257⟩. ⟨hal-00992495⟩
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