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

J. R. Goicoechea et al.: Low sulfur depletion in the Horsehead PDR 573Fig. 8. IRAM-30 m HCS + (2–1) single-dish observations (histograms) at different positions of the Horsehead. Offsets in arcsec refer to the (0,0) positionof the C 18 O(2–1) map (see Fig. 4). Radiative transfer models for HCS + at selected positions are also shown (curves). Predicted line profileshave been convolved with the telescope angular resolution at each frequency. Intensity scale is in main beam temperature.tel-00726959, version 1 - 31 Aug 2012Table 7. Single-component radiative transfer model parameters.ParameterT kn(H 2 )v turbχ(C 34 S)χ(HCS + )Value20–25 K(7–12) × 10 4 cm −30.3–0.5 km s −1(3 ± 1) × 10 −10(4± 2) × 10 −11medium is inhomogeneous and dense clumps and a more diffuseinterclump medium coexist. The same argumentation hasbeen used to interpret HCN and H 13 CN observations in theOrion Bar PDR (Lis & Schilke 2003). In addition, it is wellknown that low-J CS lines may not be a good column densitytracer if their emission is scattered by a low density halo(González-Alfonso & Cernicharo 1993). This process can bea common effect in optically thick lines of high-dipole momentmolecules such as CS or HCO + (Cernicharo & Guélin 1987).In this scenario, the CS J = 3–2 and 2–1 lines from the densemedium will be attenuated and scattered over larger areas thanthe true spatial extend of the dense clumps. This possibility hasbeen analyzed in more detail in the next section. Fortunately,observations of the CS J = 5–4 line allow to directly trace thedense clumps more safely (Table 8). In particular, we found thatthese lines can only be reproduced with denser gas components,n(H 2 ) = (4 ± 2) × 10 5 cm −3 , not resolved by the ∼10 ′′ beam ofthe IRAM-30 m telescope at ∼250 GHz. Note that the CS J =5–4 line widths are fitted if the turbulent velocity in the densergas is ∼0.2 km s −1 , a factor 2 lower than the one required by theC 34 S J = 3–2 and 2–1 lines (Fig. 7). Thus, a different spatialorigin for this line emission is favored.At this stage we have a general knowledge of the CS andHCS + excitation and abundance in the region. In the followingsections we concentrate in the photochemistry of these species.Only higher angular observations provide the appropriate linearscale to resolve the most important physical gradients inthe PDR edge. Hence, interferometric observations should allowa better comparison with chemical predictions.5.2. The PDR edgePdBI C 12 O J = 2–1, 1–0, C 18 O J = 2–1, and CS J = 2–1 observationsalong the direction of the exciting star (at δy = 0 ′′ )areshown in Fig. 3. Here we take these spectra as representative ofthe PDR edge and try to constrain its physical conditions througha combined analysis of photochemical and radiative transfermodels. Both models use a unidimensional plane-parallelTable 8. Two-component radiative transfer model parameters.ParameterValueT k20–25 Kn(H 2 )(3–7) × 10 4 cm −3dense component (2–6) × 10 5 cm −3(filling factor) 0.3v turb0.3–0.4 km s −1dense component 0.2–0.3 km s −1χ(CS)(7 ± 3) × 10 −9S/H(3.5± 1.5) × 10 −6description of the geometry. Although some physical processesrequire more complex geometries, the main physical and chemicalgradients across the illuminated direction can be consistentlydescribed in this way. Plane-parallel geometry was judged to bethe best approach for this edge-on PDR since H 2 and PAH emissionsare only observed at the illuminated edge and not deeperinside the cloud (Habart et al. 2005).In this analysis, we have used the PdBI CS J = 2–1 andC 18 O J = 2–1 lines. As low-J 12 CO optical depths are veryhigh, they do not trace the bulk of material. The intensity peak ofthese lines only provide a good estimation of the CO excitationtemperatures (i.e. a lower limit to the gas temperature). Sincethe asymptotic brightness temperature of CO J = 1–0 lines is∼30 K, we take this value as the minimum of T k in the PDR.We note that lower temperatures do not reproduce the observedline intensities. For the rest of the (warmer) positions closer tothe PDR edge, the gas temperature was determined by solvingthe thermal balance. The predicted gas temperature in thedensity peak is ∼50 K while it rises to ∼200–250 K in theH 2 emitting regions where the density is n H ≃ 10 3 –10 4 cm −3 .More exact temperature values require observations of higher-J CO lines at comparable spatial resolution. We are currentlyanalysing 13 CO J = 3–2 data from the SMA interferometer.Regarding the density structure, both the observed H 2 andPAH mid-IR emission, together with their spatial segregation,are much better reproduced with a steep density gradient thanwith an uniform density (Habart et al. 2005). The same densitygradient is needed to correctly reproduce the observed offset betweenthe small hydrocarbons (Pety et al. 2005a) and H 2 emission(where the density is not at its peak). Therefore, in order toreproduce PdBI observations of CS and C 18 O, a steep power-lawdensity gradient at the illuminated regions and a step-density inthe more shielded region have been assumed.The following methodology was carried out: a fullPDR model with Horsehead standard conditions (see Sect. 4.1)