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

890 J. Pety et al.: Are PAHs precursors of small hydrocarbons in photo-dissociation regions?tel-00726959, version 1 - 31 Aug 2012Fig. 3. Same as Fig. 2 except that maps have been rotated by 14 ◦ counter-clockwise around the image center to bring the exciting star directionin the horizontal direction as this eases the comparison of the PDR tracer stratifications. Maps have also been horizontally shifted by 20 ′′ toset the horizontal zero at the PDR edge delineated as the vertical red line. Horizontal red lines delimit the two lanes that have been verticallyaveraged to produce the two series of cuts shown in Fig. 5.σ Ori far to the right side of Fig. 3. The cuts have been takenalong the σ Ori direction (i.e. PA = −104 ◦ ). The main peak forall hydrocarbons is located near an offset of δx ≃ 12−15 ′′ atless than 5 ′′ of the H 2 peak. The ISO-LW2 peak is located halfwaybetween hydrocarbons and H 2 peaks. Intense 12 CO emissionin both the J = 1−0 andJ = 2−1 lines is also detected inthe same region, while the C 18 O(J = 2−1) emission arises farther(at least 5 ′′ ) inside the cloud.AsshowninFig.6,the 12 CO (J = 2−1) emission (convolvedat the same angular resolution as the 12 CO J = 1−0transition) is very bright (≥50 K at 10.6kms −1 , the line peakvelocity) and more intense than 12 CO (J = 1−0) in the mostexternal layers of the PDRs, directly facing σOri. The line intensityratio T b (1−0)/T b (2−1) rises from ∼0.3 to∼0.8 fromWest to East. Combined with the high brightness temperaturedetected for both lines, the higher brightness temperature ofthe 12 CO(2−1) line is a clear sign of the presence of warmand dense gas. We have estimated the kinetic temperature usingan LVG model. We assumed that the emission is resolvedand fills the beam. We explored the kinetic temperature dependenceupon the density by solving for 5 different protondensities going from 1.6 × 10 4 cm −3 to 10 5 cm −3 . Under thesehypotheses, the 12 CO line intensity ratio and brightness temperatureconstrain the kinetic temperature to increase from 60 K inthe inner PDR (15 ′′ = 0.03 pc from the PDR edge) to morethan 100 K in the outer layers for proton densities larger than4 × 10 4 cm −3 . For lower proton densities, the kinetic temperaturestill starts from 60 K in the inner PDR but increases muchmore stiffly. The kinetic temperature derived from single dishobservations (Abergel et al. 2003) is lower, in the 30−40 Krange and corresponds to the bulk of the cloud, rather than tothe warm UV-illuminated edge.3.2. AbundancesWe have computed the CO and hydrocarbon column densitiesat three representative positions in the maps: the “IR peak”where the PAH and hydrocarbon emission is the largest, the“IR edge” 10 ′′ West which represents the region with the mostintense UV-radiation and a “Cloud” position behind the IR filament.Table 3 lists the derived column densities and abundancesrelative to the total number of protons for theseArticle published by EDP Sciences and available at http://www.edpsciences.org/aa or http://dx.doi.org/10.1051/0004-6361:20041170