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

982 M. Gerin et al.: HCO mapping of the Horsehead: tracing the illuminated dense molecular cloud surfacestel-00726959, version 1 - 31 Aug 2012Fig. 4. Photochemical models of a unidimensional PDR. Upper panels show the density gradient (n H = n(H) + 2n(H 2 )incm −3 )usedinthecalculation. Middle panels show the predicted HCO and H 13 CO + abundances (relative to n H ). The H 13 CO + abundance inferred from observationsin the cold core (“the DCO + peak”, see the offsets in Table 2) is shown with blue lines. The HCO abundance inferred from observations in thePDR (“the HCO peak”, see the offsets in Table 2) is shown with red lines. Lower panels show the HCO/H 13 CO + abundance ratio predicted bythe models whereas the HCO/H 13 CO + column density ratio inferred from observations is shown as blue arrows and red lines (for the cold coreand PDR respectively). Each panel compares two different models: left-side models show a standard chemistry (dashed curves) versus the samenetwork upgraded with the addition of the H 2 CO + photon → HCO + H photodissociation (solid curves). Right-side models show the previousupgraded standard model (solid curves) versus a chemistry that adds the O + CH 2 → HCO + H reaction with a rate of 5.01 × 10 −11 cm 3 s −1 (dottedcurves). The inclusion of the O + CH 2 reaction has almost no effect on H 13 CO + for the physical conditions prevailing in the Horsehead, but triggersan increases of the HCO abundance in the PDR by two orders of magnitude.emission towards the “the DCO + peak” arises from the quiescent,cold and dense core, whereas HCO, in the same line ofsight, arises predominantly from the warmer and more turbulentouter cloud layers. We note that the presence of a foregroundlayerofmorediffuse material (A V ∼ 2 mag) was already introducedby Goicoechea et al. (2006) to fit the CS J = 2–1 scatteredline emission. The analysis of CO J = 4–3 and CI 3 P 1 − 3 P 0 mapsled Philipp et al. (2006) to propose the presence of a diffuse envelope,with A V ∼ 2 mag, and which contributes to about halfthe mass of the dense filament traced by C 18 O and the dust continuumemission. The hypothesis of a surface layer of HCO istherefore consistent with previous modeling of molecular emissionof the Horsehead.We conclude 1) that HCO and HCO + have similar abundancesin the PDR; and 2) that the HCO abundance drops by atleast one order of magnitude between the dense and warm PDRregion and the cold and shielded DCO + core.4. HCO chemistry4.1. Gas-phase formation: PDR modelsIn order to understand the HCO and H 13 CO + abundances andHCO/H 13 CO + column density ratio inferred from observations,we have modeled the steady state gas phase chemistry at theHorsehead edge. The density distribution in the PDR is wellrepresented by a density gradient n H (δx) ∝ δx 4 ,whereδxis the distance from the edge towards the cloud interior andn H = n(H) + 2n(H 2 ) (see the top panels of Fig. 4). The densityreaches a constant n H value of 2 × 10 5 cm −3 in an equivalentlength of ∼10 ′′ Habart et al. (2005); Goicoechea et al. (2006).The cloud edge is illuminated by a FUV field 60 times the meaninterstellar radiation field (G 0 = 60 in Draine units). We usedthe Meudon PDR code 2 , a photochemical model of a unidimensionalPDR (see Le Bourlot et al. 1993; Le Petit et al. 2006;Goicoechea & Le Bourlot 2007, for a detailed description). Ourstandard chemical network is based on a modified version of theOhio State University (osu) gas-phase network, updated for photochemicalstudies (see Goicoechea et al. 2006). It also includes13 C fractionation reactions Graedel et al. (1982) and specificcomputation of the 13 CO photodissociation rate as a function ofdepth. The ionization rate due to cosmic rays in the models isζ = 5 × 10 −17 s −1 . Following our previous work, we chose thefollowing elemental gas phase abundances: He/H = 0.1, O/H =3 × 10 −4 ,C/H = 1.4 × 10 −4 ,N/H = 8 × 10 −5 ,S/H = 3.5 × 10 −6 ,13 C/H = 2.3 × 10 −6 ,Si/H = 1.7 × 10 −8 and Fe/H = 1.0 × 10 −9 .In Fig. 4, we investigate the main gas-phase formation routesfor HCO in a series of models “testing” different pathways leadingto the formation of HCO. HCO and H 13 CO + predictions areshown in Fig. 4 (middle panels). In all models the HCO abundancepeaks near the cloud surface at A V ≃ 1.5 (δx ≃ 14 ′′ )where the ionization fraction is high (e − /H ∼ 5 × 10 −5 ). Dueto the low abundance of metals in the model (as represented bythe low abundance of Fe), the ionization fraction in the shielded2 Publicly available at http://aristote.obspm.fr/MIS/