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

886 J. Pety et al.: Are PAHs precursors of small hydrocarbons in photo-dissociation regions?tel-00726959, version 1 - 31 Aug 2012also play a role in gas chemistry: some laboratory experimentsand theoretical calculations suggest that PAHs may fragmentinto small carbon clusters and molecules under photon impact(C 2 ,C 3 ,C 2 H 2 , etc.) (Joblin 2003; Le Page et al. 2003; Allainet al. 1996b,a; Leger et al. 1989; Scott et al. 1997). In addition,investigation of the lifetimes of interstellar PAHs implies thatphoto-dissociation may be the main limiting process for theirlife in the interstellar medium (Verstraete et al. 2001).It is therefore appropriate to wonder whether PAHs couldfragment continuously and feed the interstellar medium withsmall hydrocarbons and carbon clusters. This hypothesis isattractive for the following reasons:i) Cyclopropenylidene (c-C 3 H 2 ) is widely distributed in theinterstellar medium (Matthews & Irvine 1985; Matthewsii)et al. 1986; Cox et al. 1988; Lucas & Liszt 2000).Recent works have shown that the diffuse interstellarmedium is more chemically active than previously thoughtwith molecules as large as C 3 (Goicoechea et al. 2004;Oka et al. 2003; Ádámkovics et al. 2003; Roueff et al.2002; Maier et al. 2001) and c-C 3 H 2 (Lucas & Liszt 2000)widely distributed. The abundances of C 3 and c-C 3 H 2 aretightly connected to those of smaller molecules, C 2 andCCH respectively, with abundance ratios of [C 2 ]/[C 3 ] ∼10−40 (Oka et al. 2003) and [CCH]/[c-C 3 H 2 ] ∼ 27.7 ±8 (Lucas & Liszt 2000).iii) Thorburn et al. (2003) have found a correlation between theabundance of C 2 and the strength of some (weak) DiffuseInterstellar Bands (DIBs).As PAHs are present in the diffuse interstellar medium, couldthey contribute to form both the small carbon clusters (C 2 ,C 3 ) and larger hydrocarbon molecules which could be theDIB carriers?Unfortunately, studies of the PAH emission bands in thediffuse interstellar clouds where the carbon clusters have beendetected is extremely difficult because of the low column densities,and also because the bright background stars used forvisible spectroscopy prohibit the use of sensitive IR cameraswhich would be saturated. Photo-Dissociation regions (PDRs)are the first interstellar sources in which AIBs have been foundand for which the PAH hypothesis has been proposed (Sellgren1984; Leger & Puget 1984). It is therefore interesting to investigatethe carbon chemistry in these sources. Fossé et al. (2000)and Teyssier et al. (2004) have discussed medium spatial resolution(30 ′′ ) observations of various hydrocarbons in nearbyPDRs. They show that CCH, c-C 3 H 2 and C 4 H are ubiquitous inthese regions, with abundances almost as high as in dark, wellshielded clouds, despite the strong UV radiation. Fuente et al.(2003) also report high abundances of c-C 3 H 2 in NGC 7023and the Orion Bar. Heavier molecules may be present in PDRsas Teyssier et al. (2004) report a tentative detection of C 6 Hinthe Horsehead nebula. PDRs and diffuse clouds therefore seemto share the same carbon chemistry, but because of their largerH 2 column density and gas density, PDRs offer more opportunitiesto detect rare species.Teyssier et al. (2004) and Fuente et al. (2003) propose thatthe presence of carbon chains is in favor of a causal link betweensmall hydrocarbons and PAHs, but they lack the spatialresolution to draw firm conclusions. In the present work, wepresent high spatial resolution observations of one source studiedby Teyssier et al. (2004), the Horsehead nebula, obtainedwith the Plateau de Bure interferometer. We describe the observationsin Sect. 2. We show the interferometer maps in Sect. 3.Section 4 presents a comparison with chemical models.2. Observations and data reduction2.1. The Horsehead nebulaThe Horsehead nebula, also called Barnard 33, appears as adark patch of ∼5 ′ extent against the bright HII region IC 434.Emission from the gas and dust associated with this globule hasbeen detected from mid-IR to millimeter wavelengths (Abergelet al. 2002, 2003; Teyssier et al. 2004; Pound et al. 2003). Fromthe analysis of the ISOCAM images, Abergel et al. (2003)conclude that the Horsehead nebula is a PDR viewed edgeonand illuminated by the O9.5V star σOri at a projecteddistance of 0.5 ◦ (3.5 pc for a distance of 400 pc, Anthony-Twarog 1982). The far-UV intensity of the incident radiationfield is G 0 = 60 relative to the average interstellar radiationfield in Draine units (Draine 1978). The gas density, derivedfrom the excitation of molecular lines and from the penetrationdepth of the UV-radiation, is a few 10 4 cm −3 (Abergel et al.2003). From a combined analysis of maps of both CO andatomic carbon, Lis & Guesten (2005) obtain similar figuresfor the gas density. Habart et al. (2004, 2005) have modeledthe emission of H 2 (from narrow band images of the H 2 rovibrationalline), PAHs and CO, and conclude that i) the gasdensity follows a steep gradient at the cloud edge, rising ton H = 10 5 cm −3 in less than 10 ′′ (i.e. 0.02 pc); and ii) this densitygradient model is essentially a constant pressure model (withP = 4 × 10 6 Kkms −1 ).The edge of the Horsehead nebula is particularly welldelineated by the mid-IR emission due to PAHs, with abright 7.7 µm-peak (hereafter named the “IR peak”) reaching25 MJy/sr at α 2000 = 05 h 40 m 53.70 s ,δ 2000 = −02 ◦ 28 ′ 04 ′′ .Figure 1 shows the region observed with the IRAM PdBI centerednear the “IR peak”. Two mosaics (one for hydrocarbonlines and the other for the CO lines) have been observed. Theirset-ups are detailed in Table 1.2.2. Observations2.2.1. c-C 3 H 2 and C 4 HFirst PdBI observations dedicated to this project were carriedout with 6 antennae in the CD configuration (baseline lengthsfrom 24 to 229 m) during March-April 2002. The 580 MHzinstantaneous IF-bandwidth allowed us to simultaneously observec-C 3 H 2 and C 4 H at 3 mm using 3 different 20 MHz-widecorrelator windows. One other window was centered on theC 18 O(J = 2−1) frequency. The full IF bandwidth was also coveredby continuum windows both at 3.4 and 1.4 mm. c-C 3 H 2and C 4 H were detected but the weather quality precluded useof 1.4 mm data.We observed a seven-field mosaic in a compact hexagonalpattern with full Nyquist sampling at 1.4 mm andArticle published by EDP Sciences and available at http://www.edpsciences.org/aa or http://dx.doi.org/10.1051/0004-6361:20041170