H2CO in the Horsehead PDR: photo-desorption of dust grain ice mantles
LERMA – LRA, UMR 8112, Observatoire de Paris and École Normale Supérieure, 24 rue Lhomond, 75231 Paris, France
e-mail: firstname.lastname@example.org; email@example.com
2 IRAM, 300 rue de la Piscine, 38406 Grenoble Cedex, France
3 Departamento de Astrofísica, Centro de Astrobiología, CSIC-INTA, Carretera de Ajalvir, Km 4, Torrejón de Ardoz, 28850 Madrid, Spain
4 LUTH UMR 8102, CNRS and Observatoire de Paris, Place J. Janssen, 92195 Meudon Cedex, France
Received: 13 May 2011
Accepted: 22 August 2011
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.
Key words: astrochemistry / ISM: clouds / ISM: molecules / ISM: individual objects: Horsehead nebula / radiative transfer / radio lines: ISM
© ESO, 2011