Astronomy & Astrophysics manuscript no. 0534 February 2, 2008 (DOI: will be inserted by hand later) Carbon budget and carbon chemistry in Photon Dominated Regions D. Teyssier1,2, D. Foss´e2, M. Gerin2, J. Pety2,3, A. Abergel4, and E. Roueff5 1Space Research Organization Netherlands, P.O.Box 800, 9700 AV Groningen, The Netherlands, under an ESA 4 external fellowship. 0 2Laboratoire d’Etude du Rayonnement et de la Mati`ere, UMR 8112, CNRS, Ecole Normale Sup´erieure et 0 Observatoire de Paris, 24 rueLhomond, 75231 Paris cedex 05, France 2 3Institut deRadioastronomie Millim´etrique, 300 ruede la Piscine, F-38406, St Martin d’H`eres, France n 4Institut d’AstrophysiqueSpatiale, Universit´eParis-Sud, Bˆat. 121, 91405 Orsay Cedex, France a 5LUTH,UMR8102 du CNRS,Observatoire deParis, Place J. Janssen, 92195 Meudon Cedex, France J 5 Received 17 October2003/ Accepted 9 December2003 1 1 Abstract. WepresentastudyofsmallcarbonchainsandringsinPhotonDominatedRegions(PDRs)performedat v millimetrewavelengths.OursampleconsistsoftheHorseheadnebula(B33),theρOphL1688cloudinterface,and 9 thecometary-shaped cloud IC63. UsingtheIRAM30-m telescope, theSEST and theEffelsberg 100-m telescope 0 at Effelsberg., we mapped the emission of C H, c-C H and C H, and searched for heavy hydrocarbons such as 2 3 2 4 3 c-C H,l-C H,l-C H ,l-C H andC H.Thelargescalemapsshowthatsmallhydrocarbonsarepresentuntilthe 3 3 3 2 4 2 6 1 edge of all PDRs, which is surprising as they are expected to be easily destroyed by UV radiation. Their spatial 0 distributionreasonablyagreeswiththearomaticemissionmappedinmid-IRwavelengthbands.C Handc-C H 2 3 2 4 correlate remarkablywell, atrendalready reported in thediffuseISM(Lucasand Liszt 2000). Their abundances 0 relative to H are relatively high and comparable to the ones derived in dark clouds such as L134N or TMC-1, / 2 h known as efficient carbon factories. The heavier species are however only detected in the Horsehead nebula at p a position coincident with the aromatic emission peak around 7µm. In particular, we report the first detection - of C H in a PDR. We have run steady-state PDR models using several gas-phase chemical networks (UMIST95 o 6 andtheNewStandardModel) andconcludethatbothnetworksfail inreproducingthehighabundancesofsome r t of these hydrocarbons by an order of magnitude. The high abundance of hydrocarbons in the PDR may suggest s a that thephoto-erosion of UV-irradiated large carbonaceous compounds could efficiently feed the ISMwith small : carbon clusters or molecules. This new production mechanism of carbon chains and rings could overcome their v destruction by the UV radiation field. Dedicated theoretical and laboratory measurements are required in order i X to understand and implement theseadditional chemical routes. r a Key words. ISM: abundances – clouds – molecules – structure - individual objects (Horsehead nebula, IC63, ρOph)– Radio lines: ISM 1. Introduction Band carriers (DIB) which are most likely large organic molecules (Herbig 1995). Carbon, neutral or ionized, is Carbon is the fourth most abundant element in the in- also one of the main reactants in interstellar chemistry terstellar medium (ISM), and also the most versatile for networks,due the large number of organic molecules, but building molecules. Carbon chemistry can therefore be also to its versatility allowing it to participate to numer- considered as the core of interstellar chemistry. Of the ous chemical reactions at any temperature, from the very nearly 130 molecules now observed in various sources, cold dense cores, to warm and hot gas. Therefore under- about 75% have at least one carbon atom, while one standing the carbonchemistry is of major importance for fourtharehydrocarbons.Moreover,theheaviestandmost astrochemistry, and for star formation. complex molecules are organic molecules with carbon. This statistic does not take into account the Polycyclic A large fraction of the chemical reactions in ISM net- Aromatic Hydrocarbons (PAHs), nor the Diffuse Infrared works involve the numerous hydrocarbons present in the ISM. Such molecules were first reported in circumstel- Send offprint requests to: D. Teyssier lar shells, where the chemistry is particularly favorable e-mail: [email protected] to their formation (C4H – Gu´elin et al. 1978, C5H – 2 D. Teyssier et al.: Carbon budget and carbon chemistry in PDRs Cernicharoetal.1986),butalsoinmoleculardarkclouds, wheretheyareefficientlyshieldedfromtheinterstellarra- diation(C H –Thaddeus1981etal.,C H–Gottliebetal. 3 2 2 1983). Since then, hydrocarbons of increasingly greater number of carbons have been reported in several objects (e.g.C H–Gu´elinetal.1987,C H–Dickensetal.2001). 6 8 In the diffuse gas, molecules like CH, CH+ and CN are known since the 40’s, and carbon clusters (C , C ) are 2 3 nowalmostroutinelydetectedinthevisibletowardsbright stars (Maier et al. 2001, Roueff et al. 2002, Oka et al. 2003)aswellasinthefar-IR(e.g.Cernicharoetal.2000). Using radio telescopes, Lucas & Liszt (2000) have also shownthatC Handc-C H areubiquitousindiffusegas, 2 3 2 confirming previous work by Cox et al. (1989). A natural question therefore arises: if carbon chains are present in the diffuse ISM, what does happen in the Photon-DominatedRegions (hereafter PDRs)? As indif- fuse clouds, the chemical processes are dominated by the radiationbutthegasisdenser.Theradiationfieldcanalso σOri beconstrained(intensityanddirection)and,withthenew data obtained by ISO, an accurate picture of the mid-IR emission due to the Aromatic Infrared Band (AIB) car- riers at these cloud interfaces has emerged. Knowledge aboutthe distribution ofcarbonchainand ringsin PDRs ishoweveryetscarceandlimitedtotheworksofFuenteet Fig.1. Composite colour image of the Horsehead nebula al. (1993, 2003), who reported the observation of C H at 2 PDR obtained by the VLT in the B, V and R bands. The somepositionsinNGC7023,theOrionBarandNGC7027, arrowindicatesthedirectionoftheilluminatingstarσOri. and Ungerechts et al. (1997), who presented maps of the Image courtesy of ESO. OrionBarinthemillimetretransitionsofC Handcyclic- 2 C H . In this paper we extend the study of hydrocarbons 3 2 in PDRs through an extensive inventory of carbon chains and rings of up to six carbon atoms. Our aims are to get a better view of their distribution on large scale with respectto otherstandardtracers,andto derivetheircon- tribution to the total carbon budget in comparison with values measured in other regions. Thepaperisorganisedasfollows.WepresentinSect.2 the three sources studied here, while the observations are describedinSect.3.We then analysethe spatialdistribu- tion of the mapped hydrocarbons (Sect. 4.1) and derive the molecular column densities of the observed species (Sect. 5.1). These results are compiled in the form of a carbon budget and compared to similar observations re- ported in dark clouds and in the diffuse ISM (Sects. 5.2 and 6.2). We finally compare the inferred abundances to numericalPDRmodels (Sect.12)andpresentourconclu- sions in Sect. 7. 2. Presentation of the sources 2.1. The Horsehead nebula Fig.2. Visible image from the Digital Sky Survey in the area of IC63. The cometary shape originates from the In the visible, the Horseheadnebula (B33,Barnard1919) strong radiation field emitted by the close-by HD5394 appears as a dark patch seen in silhouette against the ionized Hα emission emanating from the IC434 Hii re- stars 20′ South-West. gion. The large-scale maps reported by e.g. Maddalena et al. (1986, CO(2–1)) or Lada et al. (1991, CS) show its connection to the L1630 cloud, which belongs to the D. Teyssier et al.: Carbon budget and carbon chemistry in PDRs 3 Fig.4. Integrated emission maps (T∗, Kkm/s) of the molecular emission in the Horsehead nebula PDR for several A species. Contours are: c-C H (2 -1 ) 0.1 to 0.7 by 0.15, C H(1–0) 0.3 to 2.1 by 0.3, C H(9-8) 0.025 to 0.2 by 0.03, 3 2 1,2 0,1 2 4 C18O(2–1)1to6by1.Alsodisplayedisthecontinuumemissionbetween5and8.5µm(aromaticemission)asmapped by ISO (from Abergel et al. 2002, contours are 10 to 25 by 5 MJy/sr), and at 1.2mm (contours are 3 to 53 by 10 mJy/11′′ beam) mapped by the MAMBO 117-channel bolometer. The white squares indicated the position labelled as CO-peak (top) and IR-peak (bottom). Orion B molecular complex (see e.g. Abergel et al. 2002 Little attention was paid to this object until recently. for an overview of the region). In the scenario proposed In the CO and isotope maps of Kramer et al. (1996), the by Reipurth & Bouchet (1984), the Horsehead nebula is nebula is heavily diluted in the 2′ beam and only ap- interpreted as an early Bok globule emerging from its pears as a small intrusion of L1630 into the Hii region. parentcloudvia the erodingincidentradiationfield emit- Theselarge-scaledataalreadyindicatedmeandensitiesof ted by the close-byO9.5Vstar σOri. The Horseheadneb- order 3×104cm−3 (Zhou et al. 1993) and column den- ula thus corresponds to a condensation resisting the il- sities compatibles with A ∼ 3 (Kramer et al. 1996). V ′′ lumination and it presents on its western border a PDR Observations at higher spatial resolution (10-20 ) were whose detailed shape is displayed on Fig. 1. Assuming a onlyreportedveryrecently.Poundetal.(2003)presented distance of 400pc (Anthony-Twarog 1982), and consider- BIMA CO(1–0) and discussed in details the possible ori- ingaprojecteddistancetoσOriof0.5◦,the incidentradi- gin and evolution of the dark cloud. They reveal a com- ationfieldis oforderG =100(Zhouetal.1993,Abergel plexvelocitystructurebuttheirdataarelikelyaffectedby 0 etal.2003).Thiscanbeseenasrelativelyweakcompared non-negligible optical depth effects. Abergel et al. (2003) tootherwell-knownPDRsofthesolarneighborhood(e.g. report IRAM 30-m maps obtained in the (J=1–0) and theOrionBar,NGC7023).ThisPDRwasmappedbyISO (J=2–1) transitions of CO, 13CO and C18O. They infer ′′ (6 resolution)infiltersaround7and15µm(Abergeletal. densities compatible with the values reported at larger 2002, Abergel et al. 2003). It reveals a very thin filament scale, and show clear selective photo-dissociation of the (10′′, or ∼0.02pc at 400pc) whose narrow size suggests considered species at the PDR edge. that the PDR is seen perfectly edge-on (see also Fig. 4). 4 D. Teyssier et al.: Carbon budget and carbon chemistry in PDRs carbonis insolidphase andmightbe locatedonPAHs or larger grains. IC63 has also been mapped with ISOCAM in the CVF mode allowing imaging spectroscopy between 5 and 16.5µm (see Foss´e et al. 2000). Fig. 5 shows the emis- sion observed around the 6.2µm AIB feature, which cor- responds to a stretching mode of the C-C bond. The aro- maticemissionfollowsthePDRborderandexhibitseveral peaks just behind the PDR and inside the molecular tail. 2.3. The ρ Ophiuchi L1688 cloud The Ophiuchi molecular complex is a close-by star for- mation region (distance 135±15pc, Habart et al. 2003). While its so-called “North” area is considered inactive, the West counterpart contains two main massive cores, L1688 and L1689, heated on large scales by the Sco-OB2 association. We concentrate here on the L1688 complex, which hosts numerous pre-stellar cores (e.g. Motte et al. Fig.3. Close-up of the western border of L1688 mapped 1998), as well as hundreds of YSOs (Greene & Young by ISOCAM around 7µm (Abergel et al. 1996). The 1992, Bontemps et al. 2001), very likely responsible for illuminating star HD147889 direction is indicated by the strong luminosity in the infrared. an arrow. The black squares indicate the positions of ISOCAM observations revealed bright interlaced fila- the cut where molecular data have been collected. The mentsofwidth∼0.03pccoincidingwiththewesternbor- tilted rectangle delimitates the areamapped in 13CO and der of the cloud (Fig. 3, see also Abergel et al. 1996). It C18O.Offsetsaregivenwithrespecttoα =16h25m58s, corresponds to a PDR (hereafter called L1688-W) illumi- 2000 δ =−24◦21′00′′. natedbytheB2VstarHD147889.AccordingtoAbergelet 2000 al. (1999), this star is located in the centre of a spherical cavity exhibiting irregular edges. The filamentary struc- ture is interpreted as the edge-on regions of this cavity. Assuming thatthe starandPDRs areina commonplane 2.2. IC63 perpendicular to the line of sight,Habartet al. (2003)in- fer an incident radiation field of G = 400. Based on the 0 IC63isareflexionnebulaassociatedtotheB0.5IVpevari- AIB and the H (1–0) S(1) line emissions, these authors able star γ Cas (HD5394, G0 = 1100) located 230±70pc predict a density2 plateau of n =4×104cm−3 inside the H from us (Vakili et al. 1984). This strong radiation field cloud, followed by a decrease of the form n (r)∝r2.5 to- H hascreatedaPDRatthewesternborderofthecometary- wards the radiating star, where r is the radial dimension shaped molecular cloud reported by Jansen et al. (1994). across the PDR. This drop occurs at ∆α∼−100′′ on the IC63 is associated to another reflexion nebula, namely cut indicated in Fig. 3. IC59, located at a projected distance of 20′ (or 1.3pc), On large scales, the mid-IR emission shape is well re- as is illustrated on Fig. 2. According to Blouin et al. producedby the COandisotopesmapsobtainedbyLada (1997),IC59wouldbelyinginthebackgroundoftheIC63- & Wilking (1980) and Wilking& Lada (1983). Their data HD5394 pair. Jansen et al. (1994), on the other hand, revealed a complex velocity structure and, in particular, claim that the illuminating star would be closer to the significant self-absorption in the 13CO species. On these observer,so that IC63 is partially seen face-on. scales, 12CO data are compatible with A in the range V The molecular component appears elongated and 50-100mag. seems well aligned with the direction of γ Cas. Jansen et al. (1994) report that the CO and CS emissions are confined into a cometary cloud of size ∼1′×2′, and as- 3. Observations sumeanevensmallerareaof30′′×20′′ forallotherspecies The data presented in this paper have been gathered be- than CO. This has particular consequences in terms of tween September 1999 and May 2002 at the IRAM 30-m, beamdilution. These authorsderiveT =50±10K and n = 5±2×104cm−3, with a posskiibnle density gradi- the CSO,the SEST andthe Effelsberg100-m1 telescopes. H2 They are concentrated on a series of molecules including ent along the cloud major axis. The inferred H column 2 CO and its main isotopomers, as well as a collection of densities translate into A = 6.3±2.5mag. Finally, an V small carbon chains and rings. The details on the con- analysisofthecarbonbudgetinthissourceindicatesthat the total gas phase carbon abundance (5.4×10−5) is only 1 The 100-m telescope is operated by the MPIfR (Max- 13% of the solar one, which suggests that the bulk of the Planck-Institut fu¨r Radioastronomie) D. Teyssier et al.: Carbon budget and carbon chemistry in PDRs 5 Molecule Transition Frequency HPBW sidered lines are compiled in Tab. 1. We give here some (GHz) (arcsec) additional information for each source. In the following 12CO J=2–1 230.538000 11 the prefixes c- and l- will respectively refer to the cyclic 13CO J=2–1 220.398686 22/44 and linear isomers of the considered species. C18O J=1–0 109.782160 22/44 J=2–1 219.560357 11 3.1. Horsehead nebula CS J=2–1 97.980950 24 C H N=1–0, J=3/2–1/2 2 The Horsehead nebula PDR was mapped at the 30- F=2–1 87.316925 28/56 m telescope in the (J=1–0) and (J=2–1) transitions of F=1–0 87.328624 28/56 N=1–0, J=1/2–1/2 C18O, as well as in c-C3H2(21,2–10,1), C2H(1–0) and F=1–1 87.402004 28/56 C4H(9–8) (Fig. 4). System temperatures at 3mm were F=0–1 87.407165 28/56 in the range 100-120K. Some weaker lines were probed N=3–2, J=7/2–5/2 at dedicated positions (Figs. 8 and 9) in CS, c-C H(2 – 3 1,2 F=4–3 262.004260 28(a) 1 ), l-C H(9/2–7/2), l-C H (3 –2 ), l-C H (11 − 1,1 3 3 2 3,0 2,1 4 2 0,11 F=3–2 262.006480 28(a) 10 ), HN13C(1–0), CH CCH(5 –4 ) and C H(59/2- 0,10 3 k k 6 N=3–2, J=5/2–3/2 57/2).Exceptfromthe on-the-fly maps ofC18O,allspec- F=3–2 262.064990 28(a) tra wereobtainedinthe FrequencySwitching mode using F=2–1 262.067460 28(a) an autocorrelator spectrometer providing 80kHz resolu- c-C H N=2 –1 ,J=5/2–3/2 3 12 11 tion elements. Additional data were obtained in C H(3– F=3–2 91.494349 27/54 2 2) at the CSO (AOS spectrometer, T ∼ 800K), and in F=2–1 91.497608 27/54 sys l-C H (1 –0 ) and c-C H (1 –1 ) at the Effelsberg N=2 –1 ,J=3/2–1/2 3 2 0,1 0,0 3 2 1,0 0,1 12 11 100-mtelescope(T ∼90K).Moreover,a1.2mmcontin- F=1–0 91.692752 27/54 sys F=2–1 91.699471 27/54 uumemissionmapwasobtainedatthe30-musingthenew l-C H J=9/2–7/2 117-channel MAMBO bolometer (11′′ resolution). Using 3 F=5–4 (f) 97.995166 25/50 a fast-mapping mode (Teyssier & Sievers 1999), we could F=4–3 (f) 97.995913 25/50 cover areas of 8.5′×7.5′ in about 30minutes, with differ- F=5–4 (e) 98.011611 25/50 entchoppingthrowsandatdifferenthourangles,reducing F=4–3 (e) 98.012524 25/50 efficientlysomemappingartifactsinherenttotheEKHin- c-C3H2 11,0–10,1 18.343145 54(b) versiontechniqueappliedhere(Emersonetal.1979).Only 2 –1 85.338898 28/56 1,2 0,1 partofthis mapis displayedhere (Fig.4)andfurtherre- l-C H 1 –0 20.792590 48(b) 3 2 0,1 0,0 sults from these data will be discussed in a forthcoming 3 –2 216.27875 11 3,0 2,1 paper. C H N=9–8, J=19/2–17/2 4 The data were first calibrated to the T∗ scale using F=9–8 85.634006 28/56 A F=10–9 85.634017 28/56 the so-called chopper wheel method (Penzias & Burrus N=9–8, J=17/2–15/2 1973). The final adopted scale however depends on the F=8–7 85.672581 28/56 sourcesize.IntheparticularcaseoftheHorseheadnebula, F=8–8 85.672583 28/56 we applied the correction factor introduced in Abergel et l-C4H2 110,11−100,10 98.245016 24 al. (2003) taking into account the error beam of the 30- CH3C2H J=5–4 m telescope. At 3mm, we assumed hydrocarbon emission K=2 85.450730 28 areas similar to the C18O one. K=1 85.455622 28 K=0 85.457272 28 C6H 2Π3/2 J=59/2–57/2 e 81.777893 28 3.2. IC63 2Π J=59/2–57/2 f 81.801247 28 3/2 HN13C J=1–0 IC63 was mapped at the 30-m in CO(2–1) and C H(1–0) 2 F=0–1 87.090735 27 using the on-the-fly mode (Fig. 5). Orthogonal coverages F=2–1 87.090859 27 were optimally combined using the PLAIT algorithm by F=1–1 87.090942 27 Emerson & Graeve (1988). In addition to C18O(2–1), the HC N J=9–8 81.881468 28 3 same lines as in the Horsehead were probed along a cut Table 1. Line parametersfor the allthe species anddata crossing the PDR (see Fig. 9). All other observing pa- reported in this paper. When two spatial resolutions are rameters were similar to the Horsehead ones. Dedicated indicated, we refer to the HBPW’s corresponding to the positionswerealsoobservedatthe100-minc-C H (1 – 3 2 1,0 IRAM30-mandtheSESTtelescopesrespectively.(a):ob- 1 ). 0,1 served at the CSO. (b) Observed at the Effelsberg 100-m After calibrating to the T∗ scale, the lack of large- telescope. See Sect. 3 for details. A scaleinformationintheprobedmoleculesmotivatedusto adoptthewidely-usedmainbeambrightnesstemperature T assuming beam efficiencies at the IRAM 30-m of η mb b (= B /F in the IRAM nomenclature) = 0.79 and 0.49 eff eff 6 D. Teyssier et al.: Carbon budget and carbon chemistry in PDRs at3mmand1.3mmrespectively.Anadditionalcorrection was considered to account for the source dilution in the beam (see the descriptionby Jansenet al.1994)applying deconvolvedsizes as inferred in Sect. 4.3. 3.3. L1688-W The L1688-W cloud was observed at the SEST in July 2000.Wemappedthe13CO(2–1)andC18O(1–0)lineemis- sionsinanareaindicatedonFig.3.Theareawehavecov- eredisjustattheedgeofthelarge-scalemapofWilking& Lada(1983).Thisfigurealsoshowsthepositionofthecut performed in Frequency Switching for some of the small hydrocarbonspecies listedabove.Fig.6 displaysthe inte- gratedintensitymapsofthetwoCOisotopomers.Thecor- responding observing parameters can be found in Tab. 1. AsforIC63,thedatawerefinallyscaledtomainbeam brightnesstemperature,assumingSEST beamefficiencies of η =0.78 and 0.56 at 3mm and 1.3mm respectively. b 4. Results 4.1. General trends Despite the strong incident radiation fields (G ∼ 100- 0 1000) compared to the diffuse ISM (G ∼ 1), the small 0 carbon chains and rings are detected in all objects. At first glance, this is surprising as small hydrocarbons are expected to be rapidly destroyed by such UV radiations. The spatial intensity variations of all hydrocarbons mea- sured in this study correlate well. This result was al- readyreportedbyLucas&Liszt(2000)inthediffuseISM for c-C H and C H. Fig. 7 illustrates this trend in the Fig.5. Maps of IC63. Top: Integrated emission of the 3 2 2 Horsehead nebula. Some additional information is given ISOCAM CVF spectra between 5.98 and 6.54µm after below for each of the sources. subtraction of a 1-order baseline. Contours are 70 to 310 by 40 MJy/sr. The dashed box indicates the area covered by the molecular millimetre data displayed be- 4.2. Horsehead nebula ∗ low. Bottom: Integrated emission maps (T , Kkm/s) of A ThemapsobtainedintheHorseheadnebulaaredisplayed 12CO(2–1) (contours 5 to 40 by 5) and C2H(1–0) (con- inFig.4.Theyshowthatthehydrocarbonlarge-scaledis- tours 0.10 to 0.35 by 0.05). The white squares indicate tributioncorrelateswellwiththePDRstructureastraced the position of the IR peaks. byISOor12CO(seemapsbyAbergeletal.2003).Inpar- ticular the sharp edge, although smoothed by the beams, is observed in all tracers. Onsmallerscales,differentbehaviourshavetobemen- tioned. It is indeed interesting to note the different po- dances probed at the IR peak will not coincide with the sitions of the respective hydrocarbon and CO (and iso- peak emission of the hydrocarbons.This issue will be ad- topomers) peaks. Fig. 7 shows that the correlation be- dressed in a companion paper presenting interferometric tween C18O and C H is worse than with c-C H , though observationsofthe samemolecules (resolution∼5′′,Pety 2 3 2 the former lines are the strongest and thus offer better et al. 2003). signal-to-noise ratio. Moreover, the hydrocarbon peak is Other species were also probed at positions of inter- locatedveryclosetothepeakofaromaticemissionaround est. They include heavier and rarer hydrocarbons,as well 7µm(hereaftercalledIRpeak),suggestingachemicallink as some cyanopolyynes or density tracers such as CS (see between AIB carriers and the small hydrocarbons. More Tab.1)andwerechoseninordertocompleteourinventory conclusivecomparisonswiththeISOmid-IRdataarenev- of the carbon budget. In the Horsehead, these deep inte- ertheless limited by the 30-m spatial resolution. This has grations were unfortunately performed prior to the large- important consequences on the physico-chemical analysis scale mapping, so that the position observed at the IR along cuts crossing the PDR as, for instance, any abun- peak does actually not correspond to the maximum hy- D. Teyssier et al.: Carbon budget and carbon chemistry in PDRs 7 Fig.7. Scatter diagrams in the Horsehead nebula. The scatter around the zero emission points is indicative of the noise in the data. the various tracers in the Horsehead is not so striking in the IC63 maps asthis sourceis barelyresolvedby the 30- m observations and the large beam (28′′ compared to the 6′′ offered by ISO) significantly smoothes out the details at the PDR border. Smoothing the ISO data to the 30-m Fig.6.ChannelmapsoftheC18O(1–0)emission(Kkm/s, resolutionhowever shows a very good agreementbetween T∗) in the L1688-W PDR, as well as integrated intensity A the probed emissions as the two main infrared peaks get maps of C18O(1–0) and 13CO(2–1) ((a) and (b)) in the smoothed into one single maximum coincident with the velocity interval 3.4-4.2km/s, corresponding to the emis- molecular emission feature. On the other hand, the CO sionassociatedto thePDRasseenby ISO.Thevelocities and C H peaks coincide fairly well, and are located just 2 of each channel map is indicated in the upper left cor- North to the south-most aromatic peak at 6.2µm (indi- ner. Offsets are given with respect to α =16h25m58s, 2000 cated on Fig. 5). From the observed integrated emission, δ2000 =−24◦21′00′′. a beam-deconvolved size of ∼ 40′′×30′′ (FWHM) can be inferred for 12CO and C H (the North-East tail is not 2 accounted here). In IC63, the deep integrations were done along a cut drocarbon emission, as is seen on Fig. 4. The CO peak crossing the south-most AIB emission peak (indicated in position was inferred from the 12CO intensity map ob- Fig. 9), as well as some extra positions inside the cloud servedbyAbergeletal.(2003).Althoughnotdisplayedin tail. C H and c-C H are in most cases detected, the lat- 4 3 2 Fig. 8, HN13C was detected at both positions, and HC N ter including transitions at 18 and 85GHz (see Tab. 1). 3 was observed at the hydrocarbon peak. l-C H , l-C H C H(3–2) is only marginally detected at some positions. 3 2 4 2 2 andCH C H werenotdetected at either ofthe positions, The non-detections are similar to the ones reported in 3 2 while c-C Handl-C Hweredetectedonly atthe IRpeak. the Horseheadnebula,butinclude aswellbothisomersof 3 3 All other molecules were detected to a better than 3σ C H, HN13C and C H. 3 6 level. We also emphasize the detection of C H at the IR 6 peak which is to our knowledge the first detection of this 4.4. L1688-W molecule reported in a PDR. It could however not be ob- served at the CO peak. In L1688-W, the molecular cloud exhibits a complex ve- locity field, as was already reported by Wilking & Lada (1983). It is illustrated on Fig. 6 for the area covered in 4.3. IC63 this study. We associate the PDR border as seen by ISO The IC63 maps are shown in Fig. 5, as well as a cut il- (Abergel et al. 1999) to the molecular emission in the lustrating the radial profiles of more species across the velocities range 3.4 to 4.2km/s (see integrated intensity PDR border (Fig. 10). The correlation observed between maps in Fig. 6). Along the cut displayed in Fig. 3, the 8 D. Teyssier et al.: Carbon budget and carbon chemistry in PDRs 5. Analysis of the physico-chemical conditions We have used the available collection of species to derive the physical and chemical conditions at work in the three sources,andinparticularassessatentativecarbonbudget in these PDRs (see Sect. 6.1). 5.1. Column densities As mentioned in the previous section, one should bear in mind thatbeamdilution effects areexpected toaffect the measuredintensity.Thisisespeciallytrueinourcasesince we lookatpositions locatedcloseto sharpedges.To first- order,thismainlymeansthatsomeofthecolumndensities derived below may be under-estimated. The assumptions and methods used to compute the quantities of interest are described in details in Appendix A. We discuss in the following some specific hypothesis on a source to source basis. The results are gathered in Tab. 2. 5.1.1. Horshead nebula From the 12CO maps of Abergel et al. (2003), we derived T ∼40±2K at the PDR location.Using this tempera- kin ture,thehydrogennumberdensitiesdeducedfromLVG Fig.10. Normalized line intensities across the PDR modelling are of the order of some 104cm−3. fronts of the Horsehead nebula and IC63. Horsehead: iso- Following the method described in Appendix A.2, we declinationcutperformedthroughtheIR-peak(seeFig.4) inferredC Hexcitationtemperaturesbetween5and10K, at δ2000 = −2◦28′00′′. All intensities are normalized to while τtot2never exceeded 1.5. We cross-checked these re- the peak (respectively 0.24, 0.9, 2.3 and 8.7K from top sults using the additional N=3–2 transitions observed at to bottom, and 24MJy/sr for the ISO LW2 profile). The somepositions.TheN=3–2/N=1–0lineintensityratioare illuminatingsourceliestowardsthenegativeoffsets.IC63: consistent with T in the range 4.5–8K, in good agree- ex cut performed along the direction indicated by open cir- mentwiththe HFS outputs.The resultsshowsimilarval- cles on Fig. 9. All intensities are normalized to the peak ues as the one reported by Fuente et al. (1993, 2003) (respectively0.03,0.27,0.5and0.2Kfromtoptobottom, in NGC7023 and the Orion Bar. The constant density and 310MJy/sr for the ISOCAM CVF profile). The illu- assumed to conduct the c-C H and C H analysis (see 3 2 4 minatingsourceliestowardsthenegativeoffsets.Allerror Appendix A.3) was taken to be 2×104cm−3. bars are ±1σ. Upper limits indicate non-detections at a 2σ level. 5.1.2. IC63 As discussed previously, the relatively small size of the emitting area compared to the mapping beams requires a correction factor to be applied to the line intensities. We used the approach described by Jansen et al. (1994) to convert T into brightness temperatures within a mb source filling 1000arcsec2 for 12CO and the hydrocar- brightest hydrocarbon emission is detected at positions bonemission,and400arcsec2 forthe13CO(2–1)emission. towards the illuminating star where C18O has already Under these assumptions, the 12CO observations indicate started to decay significantly (Fig. 11). The emission at T ∼48±2K,inexcellentagreementwithJansenetal.’s negative offsets can nevertheless be due to the particular kin finding of 50±10K. Since only one C18O transition was geometry of L1688-W(Abergelet al. 1999),implying that available in this source, we made the additional assump- the PDR is partly seen face-on at probed positions inside tionofavolumedensityn =5±2×104cm−3,basedon the cavity. H2 the results of Jansen et al. (1994). The same density was In L1688-W, a somewhat smaller number of species applied to the analysis of the c-C H and C H column 3 2 4 was investigated in deeper integrations. Apart from the densities. The inferred C18O column density at (0′′,0′′) data presented in Fig. 11, observations of c-C H and l- offsetis however3 times lowerthanthe value reportedby 3 C H were unfruitful down a 5mK (1σ) noise level. these authors.This is likely due to the 10′′ offset between 3 D. Teyssier et al.: Carbon budget and carbon chemistry in PDRs 9 Fig.8. Sample spectra of heavier hydrocarbons and other species obtained at the so-called CO peak (indicated by dashed 12CO contours, 30% and 80 to 95% of the peak, from Abergel et al. 2003) and IR peak (indicated by full line ISOcontours15to30MJy/sr,fromAbergeletal.2002).AlltemperatureareinK,T∗ scale.The samescalesareused A for both positions to ease the comparison between the various molecules. The missing channels in the C H spectrum 6 were removed due to spurious features in the backend response. For this line, we also display the Gaussian fit to the line. Offsets are given with respect to α =05h40m59.0s, δ =−02◦25′31.4′′. 2000 2000 their position and ours, sufficient to explain such rapid ulations confirm the small optical depth at the probed variations in regards of the source size (see Tab. 2). positions. The C H analysis indicates that 80% of the observed 2 points exhibit τ <1 and are consistentwith T = 5.8– tot ex 5.2. Fractional abundances 8K. The inferred abundances are somewhat higher than the results inferred by Jansen et al. (1994) at a position 5.2.1. Systematics 10-20′′ away from ours. The molecular abundances with respect to H are com- 2 piledinTab.2.Appendix A.6describesthemethodsused 5.1.3. L1688-W to derive the H column densities in each of the sources. 2 Thehydrocarbonabundancesappearreasonablyconstant The CO kinetic temperatures were derived using an ad- across the different objects and positions, although to a ditional C18O(2–1) map obtained at the CSO (Gerin, lesserextentinL1688-Wwhereabundancesdonotalways private communication). We calculated Tkin ∼ 21±2K peak at the intensity maxima. We will discuss this point in the region of interest. As already reported by Lada later. This constancy confirms the general trend already & Wilking (1980), the CO lines however exhibit signif- reportedinthediffusegas(Lucas&Liszt2000).Although icant self-absorption. Despite the dilution effects men- errorbarsremainhigh(essentiallyduetouncertaintieson tioned above, the temperatures in this source are thus N(H )),the observedvaluesresultinaverageabundances 2 lower limits, implying upper limits for the molecular col- of [C H]= 1.9±0.4×10−8, [c-C H ]= 1.4±0.2×10−9 2 3 2 umn densities. and [C H]=1.1±0.2×10−9. 4 While the C H HFS analysis reveal excitation tem- Abundanceratiosbetweenmoleculesshowasomewhat 2 perature similar to the ones found in IC63, we used the similartrendandseemtobehierarchicallydistributed.On volume density profile derived by Habart et al. (2003) to average,thefollowingratiosarefound:[C18O]/[C H]∼25 2 constrainthec-C H andC Hcolumndensities.Thesim- (although with large scatter), [c-C H ]/[C H]∼1 and 3 2 4 3 2 4 10 D. Teyssier et al.: Carbon budget and carbon chemistry in PDRs Fig.9. Sample spectra of weaker hydrocarbons and other species obtained at some dedicated positions in IC63 dis- played over contours of the 6.2µm ISO integrated emission map. It illustrates in particular the non-detection of the cyclic and linear forms of C H around 92 and 98GHz respectively. The probed positions are marked with filled and 3 ∗ open circles. The open circles indicate the cuts displayed in Fig. 10. All temperature are in K, T scale. Offsets are A given with respect to α =00h59m00.7s, δ =60◦53′19′′. 2000 2000 [C H]/[c-C H ]∼15.Thelatterresultisofthesameorder to the proximity of the probed positions with respect to 2 3 2 as the value reportedin the diffuse gas (ratioof 28±8)by the beam sizes. The C18O abundances are significantly Lucas & Liszt (2000). lower than the value reported by Jansen et al. (1994). This discrepancy arises from the large H column densi- 2 ties assumed at positions probed outwards the cloud. 5.2.2. Horsehead nebula In this source, it is interesting to note that the position 5.2.4. L1688-W where the hydrocarbon emission is the largest coincides withapeakoftheH columndensityastracedbythecon- FirmconclusionsaremoredifficulttodrawinL1688-Was 2 tinuumemissionat1.2mm(Fig.4).This trenddoeshow- the available sample remains restricted to few points. If ever not hold for species such as C18O (although within our picture of the variations of the H column density 2 large error bars), CS and HN13C, whose abundance max- is correct, the highest hydrocarbon abundances are not ima do not coincide with this H column density peak. found at their corresponding intensity peaks but closer 2 Note also that the fraction of the beam filled by the to the illuminating star (Fig. 11). Again, this can be due cloudissmalleratthe hydrocarbonpeak thanatthe CO to the particular geometry of the PDR partly seen face- peak, thus lowering the intensities measured at the bor- on. Still, the C18O abundances experience a drop much der. This is confirmed by interferometric observations of steeper thanthe hydrocarbonsinthe directionofthe illu- the PDR border which show an enhancement of the hy- minating star. drocarbon abundances at the aromatic peak (Pety et al. 2003). 6. Carbon chemistry 5.2.3. IC63 6.1. Carbon budget In IC63, the small relative variations of abundances ob- Thecompilationofmolecularabundancesallowsustode- served between the different offsets is very likely related riveatentativecarbonbudgetrepresentativeofPDRcon-