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

576 J. R. Goicoechea et al.: Low sulfur depletion in the Horsehead PDRFig. 10. IRAM-PdBI C 18 O J = 2–1 spectra along the direction of the exciting star at δy = 0 ′′ (histograms). Radiative transfer models using the outputof PDR models for C 18 O (blue curve) for a density gradient and physical conditions discussed in the text. Lower panel shows inclination effectsassuming that the PDR is inclined relative to the line of sight by a ϕ = 5 ◦ angle. Modeled line profiles have been convolved with an appropriateGaussian beam corresponding to each synthesized beam. Intensity scale is in brightness temperature and abscissa in LSR velocity.tel-00726959, version 1 - 31 Aug 2012Fig. 11. IRAM-PdBI CS J = 2–1 spectra along the direction of the exciting star at δy = 30 ′′ (upper panel) andδy = 0 ′′ (lower panel). Radiativetransfer models using the output of PDR models for CS (red curve) for a density gradient and physical conditions discussed in the text (assumingthat the PDR is not inclined relative to the line of sight). Modeled line profiles have been convolved with an appropriate Gaussian beamcorresponding to each synthesized beam. Intensity scale is in brightness temperature and abscissa in LSR velocity.In the most external layers of the cloud, still dominatedby the FUV radiation field, CS is predominantly formed byHCS + dissociative recombination and principally destroyed byphotodissociation and charge transfer with H + . Once the gas isshielded, OCS + dissociative recombination and reaction of Cwith SO also contributes to CS formation, while its destruction isnow governed by ion-molecule reactions, mainly with HCO + butalso with H 3 O + . These last two reactions with abundant molecularions return HCS + again. The peak abundance of HCS + occurat A V< ∼ 2 mag, where it is formed by reaction of CS + with H 2 anddestroyed by dissociative recombination. For this reason, an orderof magnitude change in k DR (HCS + ) clearly modifies its peakabundance in the outer PDR layers. In the more shielded regions,HCS + destruction is dominated by dissociative recombinationand reaction with atomic oxygen to form HCO + and OCS + .Since the predicted CS abundance scales with S/H, and CS formationis dominated by HCS + dissociative recombination, wehave used our CS/C 34 S/HCS + observations and modeling to estimateS/H.Figure 14 shows results of a grid of photochemical modelsfor different sulfur elemental abundances from S/H = 10 −8to 2 × 10 −5 , using the latest HCS + and OCS + dissociative recombinationrates. CS and HCS + abundances with respect to H 2are shown as a function of S/H attwodifferent PDR positions(A v ∼ 10 and ∼2 mag respectively; see Fig. 9). Densities at thesepositions are the same, n(H 2 ) = 10 5 cm −3 , but we have takendifferent PDR positions in order to plot the HCS + maximumabundance and to get the CS/HCS + ratio closer to observations.Inside the cloud, the predicted maximum HCS + abundances area factor ∼3 lower than observed. Horizontal shaded regions markthe CS and HCS + abundances derived from observations and radiativetransfer modeling. For clarity, HCS + abundances havebeen multiplied by a factor of 1000. Finally, the vertical shadedregion shows the estimated sulfur elemental abundance in theHorsehead derived from the overlap region between observedand predicted abundances. We derive S/H ∼ (3.5 ± 1.5) × 10 −6as the mean value for the PDR. Note that CS is used for the upperlimit and HCS + for the lower limit. However, according tothe inferred HCS + abundance, larger sulfur abundances are stillpossible.6. DiscussionOur multi-transition single-dish and aperture synthesis observationsand modeling of CS and related species allow us to