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

896 J. Pety et al.: Are PAHs precursors of small hydrocarbons in photo-dissociation regions?tel-00726959, version 1 - 31 Aug 2012Fig. 9. Spatial variation of the total hydrogen density in models A toF. In model E and F, the density increases as a power law of scalingexponent 4 in the first 10 ′′ and then is kept constant at a value of2 × 10 5 cm −3 .AsinFig.8,thex-axis origin has been set so that [H ⋆ 2 ]peaks at the position of the observed 2.12 µm, H 2 line peak (i.e. δx =10 ′′ ).abundance has a large effect. When the sulfur abundance issolar, the small carbon chains C 2 , CCH, C 2 H 2 ,C 3 H, C 3 H 2and C 4 H react with S + to give C 2 S + , CCS + ,HC 2 S + ,C 3 S + ,HC 3 S + and C 4 S + . In this main destruction path of the smallcarbon chains, one hydrogen atom is released impairing the reformationof the carbon chains. When S is higly depleted as inModel E, this destruction mechanism is superseded by otherpathways involving C + . Those pathways form carbon chainions which in turn contribute to the formation of other carbonchains. Overall, model E (i.e. low S/H) performs better in thecomparison with observed abundances. The only exception isthe n(c-C 3 H 2 )/n(CCH) ratio at the “cloud” position. We willthus use model E only for comparison with the observations.At the IR peak (median point at δx = 15.5 ′′ ), the CCHabundance is correctly reproduced while c-C 3 H 2 and C 4 Habundances are underestimated by at least a factor of 3.Discrepancies are much higher both at the “cloud” (point tothe right at δx = 27 ′′ ) and the “IR edge” (point to the left atδx = 9 ′′ ) positions. In the UV-illuminated edge, the modeled[CCH] has a quite shallow increase with δx while the modeled[c-C 3 H 2 ]and[C 4 H] share the same steep abundance profile. Incontrast, the observed (“IR edge”) abundances are very similarfor the 3 species, reflecting the very good spatial correlation betweenthe different hydrocarbons (see Fig. 4). This discrepancyis independent of our knowledge of the total hydrogen densityas it is also seen when comparing abundances relative to CCH.In summary, none of our models is able to correctly reproducethe relative stratification of H 2 and small hydrocarbons.Comparison of model A and C shows that to reproduce theobserved offset between hydrocarbon and H 2 peaks, we needa high total hydrogen density (10 5 cm −3 ). By varying the profiledensity (model E and F), a shallow total hydrogen densityincrease at the PDR edge is needed to reproduce the profileof the 2.12 µm H 2 line. However, the shallower the totalFig. 10. Comparison between our two best models (curves) and observed(points with error bars) abundances of the small hydrocarbons.CCH is shown as a green solid line, c-C 3 H 2 as a dashed red line andC 4 H as a blue dotted line. The top and bottom panels respectivelyshow abundances relative to the total hydrogen density and CCH. Thedashed vertical line shows the position where the total hydrogen densityprofile changes from a steep gradient to a constant.hydrogen density increase, the larger the modeled offset betweenH 2 and the hydrocarbons. A good compromise is providedby a total hydrogen density profile increasing as a powerlaw with a scaling exponent 4 on the first 10 ′′ and then constantatavalueof2×10 5 cm −3 . Habart et al. (2005) showthat this model essentially corresponds to a constant pressuremodel (with P = 4 × 10 6 Kkms −1 ). Model E with low sulfurelemental abundance performs better than model F with solarabundance. Nonetheless, even model E does not succeed inArticle published by EDP Sciences and available at http://www.edpsciences.org/aa or http://dx.doi.org/10.1051/0004-6361:20041170