[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 779tel-00726959, version 1 - 31 Aug 2012Fig. 5. Left: H 13 CO + and DCO + spectra along the direction of the exciting star at δy = 15 ′′ (histograms). Radiative transfer models using theoutput of PDR models for a fixed cosmic-ray ionization rate (ζ = 3 × 10 −17 s −1 ) and varying metallicities. Thin blue curves for [M] = 5 × 10 −8and no PAHs; thick grey curves for [M] = 10 −9 and no PAHs; dashed red curves for [M] = 10 −6 and [PAH] = 10 −7 . Modeled line profiles havebeen convolved with the appropriate Gaussian beam at each observed frequency (the angular resolution for each line are quoted in Tables 1 and 2).Right: same as left but for a fixed metal abundance ([M] = 10 −9 ), no PAHs and varying cosmic-ray ionization rate ζ. Thin blue curves for a modelwith ζ = 5 × 10 −18 s −1 ; thick grey curves for ζ = 3 × 10 −17 s −1 ; dashed red curves for ζ = 10 −16 s −1 .At the illuminated edge of the cloud, PAH − is predominantlydestroyed by UV photons through electron detachment,PAH − + hν → PAH + e − (12)and through recombination with atomic cations which arevery abundant in the PDR (e.g., Bakes & Tielens 1998;Wolfire et al. 2008). As a consequence, the abundance of ionssuch as S + in the PDR decreases with respect to models withoutPAHs. This effect is important to determine the elemental abundancesand depletion factors. Despite the higher PAH − destructionrates in the PDR, the high electron density and relativelylow UV field in the Horsehead allows PAH − to form efficientlythrough electron attachment (reaction 11). Hence the resultingPAH − abundance is also high in the PDR. On the other hand, thepredicted abundance of positively charged PAH + in our modelsis ∼500 times smaller than the abundance of PAH − (due to fastelectronic recombination) and therefore PAH + do not seem toplay a major role in the ionization balance inside the cloud (seealso Lepp & Dalgarno 1988; Wakelam & Herbst 2008).5.3. The role of the cosmic-ray ionization rateCosmic rays affect the ionization state and the physics of molecularclouds, being the dominant source of heating and ionizationin the gas shielded from interstellar radiation fields. Indeed, secondaryUV photons are created in cloud interiors by H 2 electroncascades following H 2 excitation by collisions with cosmicrays (Prasad & Tarafdar 1983). Therefore, cosmic rays maintaina certain ionization degree in the shielded gas and play a fundamentalrole in the ion-neutral chemistry by setting the abundanceof key ions (Herbst & Klemperer 1973).Most studies based on the interpretation of observedmolecular ions set a range of a few 10 −17 to a few10 −16 s −1 for the cosmic-ray ionization rate (Le Petit et al. 2004;van der Tak 2006; Dalgarno 2006 and references therein).However, it is still debated whether or not ζ depends on environmentalconditions (e.g., galactic center vs. disk clouds) orif it varies from source to source (e.g., from dense molecularcores to more translucent clouds). In many ways, PDRs offer aninteresting intermediate medium to analyze the transition betweentranslucent and dark clouds.In terms of our observations, the DCO + and H 13 CO + abundancesdirectly scale with ζ in the UV shielded gas. Indeed, theseions are direct products of the H + 3destruction (through reactions2 and 3), and the H + 3 formation is proportional to ≃ζ n H.However, ζ and the metal abundance cannot be constrained independentlyfrom the inferred DCO + and H 13 CO + abundancessince both parameters control the ionization fraction, and thusthe destruction of these ions through reactions 5 and 6.6. Results: observational constraintsIn this section we compare the synthetic and observed H 13 CO +and DCO + spectra as a function of cloud position. We then explorethe range of metallicities and cosmic-ray ionization ratescompatible with the H 13 CO + and DCO + inferred abundances(see Table 6). The influence of PAHs is also investigated. We finallycompare the [HOC + ]/[H 13 CO + ] ratio obtained towards theHorsehead with the values derived in other PDRs.6.1. Constraints to the metals abundanceFigure 5 left shows the spectra along the direction of the excitingstar (histograms) and radiative transfer models using theoutput of several PDR models for a fixed ionization rate (ζ = 3 ×10 −17 s −1 ) and varying metallicities. In particular, the model with[M] = 10 −9 (and no PAHs) displays a notable agreement withboth the DCO + and H 13 CO + spatial distribution and with the inferredpeak abundances towards the core (Table 6). In addition,Fig. 6 left shows the predicted ionization fraction and [H 13 CO + ]and [DCO + ] abundances at the core peak (A V > 10) as a functionof [M] (blue-solid curves). These models (no PAHs, fixed ζ)show that the upper limit metallicity compatible with observationsis [M] ≤ 4 × 10 −9 , which implies an ionization fraction of[e − ] = (7 ± 1) × 10 −9 at the core peak. Higher metal abundancesincrease the ionization fraction (see Fig. 6 left), which translatesinto weaker lines than observed (Fig. 5 left: thin-blue curves).Therefore, the gas-phase metal abundance is depleted by ∼4