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

774 J. R. Goicoechea et al.: The ionization fraction gradient across the Horsehead edge: an archetype for molecular cloudsδx ≃ 23 ′′ in the maps). The resulting density gradient used in thephotochemical and radiative transfer models is shown in Fig. 4.In the next sections we constrain the ionization fraction gradientin the cloud by comparing synthetic and observed H 13 CO + andDCO + spectra along the same cut.3.2. Photochemical modelsWe have updated the Meudon PDR code to model our observationsof the Horsehead. The code has been described in detailelsewhere (e.g., Le Bourlot et al. 1993; Le Petit et al. 2006;Goicoechea & Le Bourlot 2007) and benchmarked against otherPDR codes by Röllig et al. (2007). In this section we summarizethe most relevant upgrades and model features for this work.3.2.1. UV radiative transfer and dust propertiestel-00726959, version 1 - 31 Aug 2012Fig. 2. HOC + and H 13 CO + J = 1–0 lines towards the Horsehead PDR(upper and middle panels) observed with the IRAM-30 m telescope.Solid lines are radiative transfer models with T k = 60 K, n(H 2 ) =5 × 10 4 cm −3 , n(H) = 500 cm −3 and [e − ] = 5 × 10 −5 . Three differentabundances are shown, thick-grey line: [HOC + ] = 4.0 × 10 −12 and[H 13 CO + ]= 1.5 × 10 −11 ; red dashed line: abundances ×2; blue thin line:abundances ÷2. For completeness, the HCO + J = 1–0 line towards thePDR is also shown (lower panel). This transition is very opaque, asshown by the low H 12 CO + /H 13 CO + J = 1–0 line intensity ratio (∼7).The resulting line profile is thus broadened and it suffers from scatteringby low-density foreground gas that we do not model here.maps of the Horsehead edge (Goicoechea et al. 2006). It enablesus to observationally benchmark the abundance gradients predictedby chemical models, even if it does not produce perfectfits to line profiles in all cloud positions. In this paper, we analyzea horizontal cut of the H 13 CO + and DCO + line emission alongthe direction of the illuminating star (δy = 15 ′′ ). Figure 1 showsthat this cut (blue dashed line) goes across the DCO + emissionpeak (δx ∼ 45 ′′ ), which we identify as the “shielded core”, andacross the HCO emission peak, the “PDR” (δx ∼ 15 ′′ ).3.1. Geometry and density gradientThe Horsehead edge has an almost edge-on geometry with aline-of-sight depth of l depth ≃ 0.1 pc(e.g.,Habart et al. 2005)and a spatial scale in the plane of the sky of ≃0.002 pc arcsec −1 .We determine the density profile from observations by fittingthe 1.2 mm dust continuum emission (IRAM-30 m/MAMBO)along the δy = 15 ′′ direction (Hily-Blant et al. 2005). In this fit,we adopt a dust opacity per unit (gas+dust) mass column densityof κ 1.2 = 0.003 cm 2 g −1 at 1.2 mm (computed for “MRNgrains”: Mathis et al. 1977, see below), our best knowledge ofthe dust grains temperature (from ∼15 K in the core to ∼30 Kin the PDR; e.g., Ward-Thompson et al. 2006) andapower-lawdensity profile n H (r) = n(H)+2n(H 2 ) ∝ r −p ,wherer is the distancefrom the shielded core towards the illuminated edge of thecloud. Best fits are obtained for a steep density gradient in thecloud edge (p ≃ 3) and a flatter one towards the core (p ≃ 0.5).The turnover point occurs at a core radius of r ≃ 0.04 pc (orThe code solves the UV radiative transfer problem takinginto account dust scattering and gas absorption. Anisotropicscattering of UV photons by dust grains is included by explicitycalculating the wavelength-dependent grain albedo andg-asymmetry parameters (Goicoechea & Le Bourlot 2007). Thisenables the specific computation of the UV radiation field (continuum+lines)and thus, the direct integration of consistentphotoionization and photodissociation rates. We use two typesof dust populations: (i) a mixture of graphite+silicate grains;and (ii) PAHs (see next paragraph). More precisely, we adopta power-law size distribution (n(a) ∝ a −3.5 ) with minimumand maximum radius of ∼5 and∼250 nm respectively (forgraphite+silicate grains). Wavelength-dependent optical properties(Q efficiencies and g factors) are interpolated from Laor &Draine (1993) tabulations. With a standard gas-to-dust mass ratio(∼100), this grain mixture (“MRN grains”) reproduces themain characteristics of the standard interstellar extinction curvewith N H /A V = 1.9 × 10 21 cm −2 and R V = 3.1.In order to complete our description of the dust populations,in this work we have also added smaller aromaticgrains. Observationally, the AIB emission towards theHorsehead (produced by free PAHs according to the mostaccepted theory; Léger & Puget 1984; Allamandola et al. 1985)clearly separates the H ii region and PDR (where the emissionis bright) from the regions shielded from UV radiation,where no AIB emission is detected (Abergel et al. 2003;Habart et al. 2005; Compiègne et al. 2007; 2008). However, thesize distribution and PAH abundance in dense regions shieldedfrom UV radiation are uncertain. It may vary from “negligible”,if PAHs coagulate into larger PAH aggregates(Boulanger et al. 1990; Rapacioli et al. 2006) to “high” abundances(though they will not be detected in the mid–IR due tothe lack of UV photons to excite them). We used the followingPAH properties: a size distribution with ∼0.4 and ∼1.2 nmradii limits (Desert et al. 1990) and optical parameters from Li& Draine (2001). This size distribution is compatible with PAHshaving a mean radius of ∼0.6 nm and N C ∼ 100 carbonatoms assuming N C ≃ 500 a 3 (Bakes & Tielens 1994). Theextinction curve and the efficiency of the photoelectric heatingmechanism depend on the mass fraction put into PAHs(Bakes & Tielens 1994). Depending on the PAH abundance,they contribute to the total dust mass by ∼1% for [PAH] = 10 −7and ∼10% for [PAH] = 10 −6 .