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

J. R. Goicoechea et al.: The ionization fraction gradient across the Horsehead edge: an archetype for molecular clouds 781a) b) c)Fig. 7. Derived abundance profiles for the most significant ions studied in this work for different PAH and sulfur elemental abundances. Anenhanced UV radiation field 60 times the mean ISRF illuminates the cloud from the right. The metal abundance ([M] = 10 −9 ) and the cosmic-rayrate (ζ = 3 × 10 −17 s −1 ) are fixed in all models. a) Model with [PAH]=0and[S]= 3.5 × 10 −6 (low sulfur depletion). b) Model with [PAH] = 0and[S] =3.5 × 10 −8 (high sulfur depletion). c) Model with [PAH] = 10 −7 and [S] = 3.5 × 10 −6 .tel-00726959, version 1 - 31 Aug 2012particular, Fig. 6 right shows the predicted [e − ], [H 13 CO + ]and[DCO + ] abundances at the core peak (A V > 10) as a functionof ζ, and suggests that in the absence of PAHs, the cosmic rayionization rate range compatible with the observations of theHorsehead edge is ζ = (5 ± 3) × 10 −17 s −1 (solid-blue curves).If PAHs are included in the chemistry, the metal abundance hasto be increased accordingly to reproduce the observations. Forour [PAH] = 10 −7 model case, the required metal abundanceneeds to be above [M] ≃ 10 −6 to obtain ionization rates aboveζ ≃ 10 −17 s −1 (Fig. 6 right; red-dotted curves).Note that given the fact that the H 13 CO + formation in thePDR is not dominated by the 13 CO + H + 3 reaction, the H13 CO +abundance in the UV illuminated gas does not scale with ζ.Therefore, we cannot further investigate if ζ varies significantlyin the transition from diffuse regions to the shielded core(e.g., McCall et al. 2003; Padoan & Scalo 2005).6.3. High ionization fraction in the PDRThe bright [C ii]158 μm (Zhou et al. 1993) and[Ci]492 GHz(Philipp et al. 2006) fine structure line emission towards theHorsehead PDR, together with subtle chemical effects suchas the large [l-C 3 H 2 ]/[c-C 3 H 2 ] linear-to-cyclic abundance ratio(Teyssier et al. 2005) all show observationally that the ionizationfraction is higher in the UV illuminated edge than towardsthe cloud interior. Nevertheless, all those studies lackedthe angular resolution to properly measure the ionization fractiongradient.The abundances of reactive ions such as HOC + are alsopredicted to be enhanced in the UV illuminated gas, wherewe have shown that the ionization fraction is high, up to[e − ] ∼ 10 −4 , and that the HOC + formation is linked tothe availability of C + . On the other hand, the H 13 CO + abundanceincreases as the electron abundance decreases towards theshielded core. Therefore, we predict that the [HOC + ]/[H 13 CO + ]abundance ratio scales with the ionization fraction gradient,reaching the highest values in the PDR (Fig. 4). In particular,we derive a high [HOC + ]/[H 13 CO + ] = 0.3–0.8 ratio(or a low [HCO + ]/[HOC + ] ≃ 75–200 ratio) towards the PDR,similar to that observed in other PDRs such as NGC 7023(Fuente et al. 2003).7. Discussion7.1. The ionization fraction gradientStar forming clouds display different environments dependingon the dominant physical and chemical processes. These environmentsare, to a first approximation, similar to those studiedhere: (i) a low density cloud edge directly exposed to a UV radiationfield from nearby stars; (ii) a transition region or ridgewhere the H 2 density increases as the gas temperature decreasesdue to the attenuation of the external radiation field. UV photonscan still play a significant role depending on their penetrationdepths (e.g., cloud clumpiness, grain properties, etc.); and(iii) the denser shielded cores that may be externally triggeredto form a new generation of stars depending on their stabilityagainst gravitational collapse (e.g., Goicoechea et al. 2008).Assuming that the observed field-of-view in the Horseheadnebula is representative of the above 3 environments, our mapsand chemical models reveal that the ionization fraction follows asteep gradient in molecular clouds: from [e − ] ≃ 10 −4 at the edgeof the cloud (the “C + dominated” region) to a few times ∼10 −9 inthe shielded cores. The prevailing chemistry and the abundanceof atomic ions such as C + and S + determine the “slope” of theionization fraction gradient in the transition regions. In particular,sulfur (with a ionization potential of ∼10.36 eV) is a goodsource of charge behind the “C + dominated” region. Advectionand time-dependent effects may also modify the ionization fractiongradient with time. However, Morata & Herbst (2008) haveshown models for (uniform) physical conditions similar to thosein the Horsehead where [HCO + ](and[e − ] presumably) does notchange with time appreciably.Figure 7 shows the predicted ionization fraction gradient,with a scale length of ∼0.05 pc (or ∼25 ′′ ), and themain charge carriers for 3 representative models with fixedstandard metal abundance (our strong metal depletion case)and standard cosmic-ray rate. Panel 7a shows a model withoutPAHs and high gas-phase sulfur abundance ([S] = 3.5 ×10 −6 ; Goicoechea et al. 2006). The ionization fraction gradientin the core / transition / edge zones, is mainly determined bythe [HCO + +S + +M + +...] / [S + ] / [C + ] abundances respectively.Sulfur ions control the charge balance in the transition layers,and due to their high abundance and slow radiative recombinationrate with electrons, the ionization fraction is high, a fewtimes 10 −7 , and the gradient is smooth. This model qualitativelyagrees with the observed more extended emission and narrower