[tel-00726959, v1] Caractériser le milieu interstellaire ... - HAL - INRIA

A&A 534, A49 (2011)tel-00726959, version 1 - 31 Aug 2012Table 5. Column densities and abundances.Column densities[cm −2 ]Abundances[X] =N(X)(N(H)+2 N(H 2 ))Molecule PDR Dense-coreN H 3.8 × 10 22 6.4 × 10 22N(o-H 2 CO) 7.2 × 10 12 9.6 × 10 12N(p-H 2 CO) 3.6 × 10 12 3.2 × 10 12N(HCO) a 3.2 × 10 13 50 K), so the deuterium chemistry is drivenmainly by CH 2 D + , as opposed to colder regions (20 K) like theHorsehead dense-core, where H 2 D + is the main actor. Owing tothe low temperature in the core it is likely that a non-negligiblefraction of CO is frozen on the dust grains, enhancing the deuteriumfractionation.Another way to form deuterated molecules in cold environmentsis trough D addition or H-D substitution reactions on thesurface of dust grains (Hidaka et al. 2009). In the Horsehead corethough, desorption from the grain mantles is not efficient in releasingproducts into the gas-phase (see Sect. 4). It is then morelikely that the gas-phase HDCO and D 2 CO molecules detectedhere are formed in the gas-phase. Nevertheless, there can still bea considerable amount of deuterated H 2 CO trapped in the icesaround dust grains.4. H 2 CO chemistryWe used a one-dimensional, steady-state photochemical model(Le Bourlot et al. 1993; Le Petit et al. 2006) to study the H 2 COchemistry in the Horsehead. The physical conditions have alreadybeen constrained by our previous observational studiesand we keep the same assumptions for the density profile (displayedin the upper panel of Fig. 6), radiation field (χ = 60in Draine units), elemental gas-phase abundances (see Table 6in Goicoechea et al. 2009b) and cosmic ray ionization rate(ζ = 5 × 10 −17 s −1 ).Unlike other organic molecules like methanol, which canonly be efficiently formed on the surface of grains (Tielens &Whittet 1997; Woon 2002; Cuppen et al. 2009), formaldehydecan be formed in both the gas-phase and on the surface of grains.Next, we investigate these two different scenarios.4.1. Pure gas-phase chemistry modelsWe used the Ohio State University (osu) pure gas-phase chemicalnetwork upgraded to photochemical studies. We included the