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Astronomy & Astrophysics manuscript no. ms7009 (cid:13)c ESO 2008 February 5, 2008 ⋆ Deuterium fractionation in the Horsehead edge J. Pety1,2, J.R. Goicoechea2, P. Hily-Blant1, M. Gerin2, and D. Teyssier3 1 IRAM,300 ruede la Piscine, 38406 Grenoble cedex, France. e-mail: [email protected],[email protected] 2 LERMA, UMR 8112, CNRS, Observatoire de Paris and Ecole Normale Supérieure, 24 Rue Lhomond, 75231 Paris cedex 05, France. e-mail: [email protected], [email protected] 3 European SpaceAstronomy Centre, Urb.Villafranca delCastillo, P.O. Box 50727, Madrid 28080, Spain. 7 e-mail: [email protected] 0 Received 22 December 2006 / accepted 22 January 2007 0 2 ABSTRACT n a Context. Deuterium fractionation is known to enhance the [DCO+]/[HCO+] abundance ratio over the D/H∼10−5 J elemental ratio in thecold and dense gas typically found in pre-stellar cores. 4 Aims.WereportthefirstdetectionandmappingofverybrightDCO+ J=3–2andJ=2–1lines(3and4Krespectively) 2 towards the Horsehead photodissociation region (PDR) observed with the IRAM-30m telescope. The DCO+ emission peaks close tothe illuminated warm edge of thenebula (<50′′ or ∼0.1pcaway). 1 Methods. Detailed nonlocal, non–LTE excitation and radiative transfer analyses have been used to determine the v prevailing physical conditions and toestimate theDCO+ and H13CO+ abundancesfrom theirline intensities. 0 Results. A large [DCO+]/[HCO+] abundance ratio (≥ 0.02) is inferred at the DCO+ emission peak, a condensation 0 shieldedfromtheilluminatingfar-UVradiationfieldwherethegasmustbecold(10–20K)anddense(≥2×105cm−3). 7 DCO+ is not detected in thewarmer photodissociation front, implying a lower [DCO+]/[HCO+] ratio (<10−3). 1 Conclusions. According to our gas phase chemical predictions, such a high deuterium fractionation of HCO+ can only 0 beexplained if the gas temperature is below 20K, in good agreement with DCO+ excitation calculations. 7 Key words.ISMclouds – molecules – individual object (Horsehead nebula) – radio lines: ISM 0 / h p Molecules are enrichedin deuterium over the elemental many chemical and physical processes competing for ef- - o D/H abundance (1−2×10−5,Linsky et al.2006)in many ficient fractionation, models are easier to compare with r different astrophysical environments. These include cold, observations for sources with well described physical con- t s dense cores (Guelin et al. 1982), mid–planes of circum- ditions. In this letter, we report the detection of very a stellar disks (van Dishoeck et al. 2003; Guilloteau et al. bright DCO+ lines in the Horsehead edge. In particu- : v 2006), hot molecular cores (Hatchell et al. 1998), and even lar, the mane of the Horsehead nebula is a PDR viewed i PDRs (Leurini et al. 2006). Multiply deuterated species nearly edge-on (inclination < 5◦) illuminated by the X were first detected several years ago, e.g. D2CO in warm O9.5V star σOri (Abergel et al. 2003; Philipp et al. 2006). ar gas (Turner 1990) and NHD2 in cold gas (Roueff et al. Habart et al. (2005) showed that the PDR has a very 2000). Solomon & Woolf (1973) and Watson (1974) first steep gradient, rising to nH ∼ 2×105cm−3 in less than ′′ proposed that deuterium fractionation is mostly caused 10 or 0.02pc, at a roughly constant thermal pressure of by gas-phase ion–molecule reactions. Smith et al. (1982) ∼ 4×106Kcm−3. The newly detected DCO+ lines arise and Roberts & Millar (2000a) confirmed that the deuter- from a condensation adjacent to the PDR, first detected ation of H+ at low temperatures (< 25K) and of CH+ by Hily-Blant et al. (2005). According to its submillimeter 3 3 at higher temperatures (up to ∼ 70K) are important pre- continuumemission,thiscore(B33-SMM1)is0.13×0.31pc cursor reactions in the subsequent deuteration of other long and has an average H2 density of ∼ 104cm−3 and species. Roberts & Millar (2000b), Walmsley et al. (2004) a peak density of ∼ 6×105cm−3 (Ward-Thompson et al. and Flower et al. (2006) succeeded in reproducing the 2006). amount of several multiply deuterated molecules in cold gas by adding to pure gas-phase chemistry the accretion (freeze-out)ofgas-phasemoleculesontothesurfacesofdust 1. Observations and data reduction grains.Finally the observeddeuteriumfractionationinhot The DCO+ J=3–2line was observedduring 2 hours of ex- cores is thought to result from the liberation of deuterated cellentwinterweather(∼0.7mmofwatervapor)usingthe molecules, trapped in ice mantles in the prestellar phase. first polarization (i.e. nine of the eighteen available pixels) Although it has been studied thoroughly for 30 years, of the IRAM-30m/HERA single sideband multi-beam re- deuterium chemistry is not yet fully understood. With ceiver. We used the frequency-switched, on-the-fly observ- Send offprint requests to: e-mail: [email protected] ing mode. We observed along and perpendicular to the di- ⋆ Based on observations obtained with the IRAM Plateau de rection of the exciting star in zigzags (i.e. ± the lambda Bureinterferometerand 30mtelescope. IRAMissupportedby and beta scanning direction). The multi-beam system was ◦ INSU/CNRS(France),MPG (Germany), and IGN (Spain). rotatedby9.6 withrespecttothescanningdirection.This 2 J. Pety et al.: Deuterium fractionation in theHorsehead edge Table 1. Observation parameters. The projection center of all the data is α2000 =05h40m54.27s, δ2000 =−02◦28′00′′. Molecule Transition Frequency Instrument #Pix.a Feffa Beffa Resol. Resol. Int. Timea,b Noisec Obs.datea GHz arcsec kms−1 hours K H13CO+ J=3–2 260.255339 30m/HERA 9 0.90 0.46 13.5′′ 0.20 5.9/11.3 0.06 Mar.2006 H13CO+ J=1–0 86.754288 30m+PdBI 2 0.95 0.78 6.7′′ 0.20 2.6/4.5 0.10 Sep.2006 DCO+ J=3–2 216.112582 30m/HERA 9 0.90 0.52 11.4′′ 0.11 1.5/2.0 0.10 Mar.2006 DCO+ J=2–1 144.077289 30m/CD150 2 0.93 0.69 18.0′′ 0.08 5.9/8.7 0.18 Sep.2006 C18O J=2–1 219.560319 30m/HERA 9 0.91 0.55 11.2′′ 0.11 – 0.26 May2003 Continuum at1.2mm 30m/MAMBO 117 – – 11.7′′ – – – – a Those columns apply to the 30m data but not to the PdBI data for the H13CO+ J=1–0 line. b Two values are given for the integration time: theon-source time and the telescope time. c Noise valuesestimated at theposition of theDCO+ peak. Fig.1. IRAM-30m integrated intensity maps. Maps have ◦ been rotated by 14 counter–clockwise around the projec- ′′ ′′ tioncenter,shownasthegreencrossat(δx,δy)=(20 ,0 ), to bring the exciting star direction in the horizontal direc- tionandthe horizontalzerohasbeensetatthe PDRedge, Fig.2. Cut alongthe directionof the exciting star atδy = ′′ delineatedbythedashedblueverticalline.Thesynthesized 15 . beamisplottedinthebottomleftcorner.Valuesofcontour levels are shown on each image lookup table. The emission of all lines is integrated between 10.1 and 11.1kms−1. pointing of the rasters was observed in frequency-switched ′′ ′′ mode.This resultedin a140 ×75 map,Nyquist sampled along the direction of the exciting star but slightly under- ensured Nyquist sampling between the rows except at the sampledintheorthogonaldirection(i.e. rowsseparatedby edges of the map. The DCO+ J=2–1 was observed dur- 6′′ insteadof4.75′′).Thenoiseincreasesquicklyatthemap ing 11.3 hours using the C150 and D150 single-side band edgeswhichwereseenonlyby afractionofthe HERApix- receivers of the IRAM-30m under ∼ 8.5mm of water va- els.Wefinallyusedafrequency-switched,on-the-flymapof por. We used the frequency-switched, on-the-fly observing the H13CO+ J=1–0line,obtainedatthe IRAM-30musing mode over a 160′′×170′′ portion of the sky. Scanned lines the A100 and B100 3mm receivers (∼ 7mm of water va- and rowswere separatedby 8′′ ensuring Nyquist sampling. por) to produce the short-spacings needed to complement A detailed description of the C18O J=2–1 and 1.2mm a 7-field mosaic acquired with the 6 PdBI antennae in the continuum observations and data reductions can be found CD configuration (baseline lengths from 24 to 176 m). in Hily-Blant et al. (2005). We estimate the absolute posi- The data processing was done with the GILDAS1 ′′ tion accuracy to be 3 . softwares (Pety 2005). The IRAM-30m data were first We also use a small part of the H13CO+ (J=1–0 and calibrated to the T∗ scale using the chopper wheel A J=3–2) data, which were obtained with the IRAM PdBI method (Penzias & Burrus 1973), and finally converted and 30m telescopes. The whole data set will be compre- to main beam temperatures (Tmb) using the forward and hensively described in a forthcoming paper studying the main beam efficiencies (Feff & Beff) displayed in Table 1. fractionalionizationacrosstheHorseheadedge(Hily-Blant The resulting amplitude accuracy is ∼ 10%. Frequency- etal.2007,inprep).Inshort,the H13CO+ J=3–2line was switched spectra were folded using the standard shift-and- observed under averaged winter weather (∼3.5mm of wa- tervapor)inrastersalongthe directionoftheexcitingstar 1 Seehttp://www.iram.fr/IRAMFR/GILDAS formoreinforma- using the first polarization of the unrotated HERA. Each tion about theGILDAS softwares. J. Pety et al.: Deuterium fractionation in theHorsehead edge 3 addmethod,afterbaselinesubtraction.Theresultingspec- Table2.Einsteincoefficients,upperlevelenergiesandcrit- trawerefinallygriddedthroughconvolutionbyaGaussian. ical densities for the range of temperatures considered in this work. 2. Results and discussion Molecule Transition Aij Eup ncrit Figure 1 presents the DCO+ J=2–1 and J=3–2 and (s−1) (K) (cm−3) the C18O J=2–1 integrated intensity maps, together with H13CO+ J=1–0 3.9×10−5 4.2 ∼2×105 H13CO+ J=3–2 1.3×10−3 25.0 ∼3×106 1.2mm continuum emission. All maps are presented in a DCO+ J=2–1 2.1×10−4 10.4 ∼6×105 coordinate system adapted to the source geometry, as described in the figure caption. The DCO+ emission is con- DCO+ J=3–2 7.7×10−4 20.7 ∼2×106 centrated in a narrow, arc-like structure, delineating the left edge of the dust continuum emission. A second maximum is found at the extreme left of the map, associ- the CS J=5–4 excitation (Goicoechea et al. 2006) and ated with a smaller dust continuum peak. Figure 2 shows with the value derived from dust submm continuum emis- the H13CO+ and DCO+ spectra in a cut along the direc- sion (Ward-Thompson et al. 2006). The weakness of the tion of the exciting star at δy = 15′′ (horizontal dashed H13CO+ J=3–2 line compared to the DCO+ J=3–2 line line of Figure 1). This cut intersects the DCO+ emission is caused in part by its larger Einstein coefficient (a factor peak which is close to the illuminated edge of the nebula ∼1.7 larger)and its higher energy level (see Table 2). This (<50′′ or∼0.1pc).Toourknowledge,this isthebrightest implies that the H13CO+ J=3–2line is more subthermally DCO+ emission(4K)detectedinaninterstellarcloudclose excited than the analogous DCO+ line for the derived to a bright H2/PAH emitting region (Habart et al. 2005; densities and temperatures. Note that we have not in- Pety et al. 2005). The 15′′ spatialshift betweenthe DCO+ cluded collisions with electrons in this excitation analysis. andC18O/continuumemissionpeakslikelyresultsfromthe In fact, the expected ionization fraction in such a cold and steep thermal gradient.The regionwhere the DCO+ emis- dense condensation is usually low, < 10−7 (Caselli et al. sion is produced, is probably cooler than the region where 1999). The derived DCO+ and H13CO+ column densities the C18O lines and 1.2 mm continuum emission peak, i.e. toward the DCO+ peak are ≃ (0.5 − 1) × 1013cm−2 cooler than 30K, the minimum temperature needed to ex- (i.e., [DCO+]≃[H13CO+]≃ (1 − 2)×10−10). Assuming a plain the intensity of the C18O J=2–1 lines in the cloud 12C/13C=60 isotopic ratio (Milam et al. 2005), we finally edge (Goicoechea et al. 2006). find a [DCO+]/[HCO+]≥0.02 abundance ratio. In order to constrain the [DCO+]/[HCO+] abun- In order to understand the observed deuterium frac- dance2 ratio from the observed line emission, we as- tionation in the dense gas close to the Horsehead PDR, sumed that both species coexist within the same gas we have modeled the steady state deuterium gas phase (implying the same physical conditions). This assump- chemistry in a cloud with a proton density nH = n(H)+ tion is mainly justified by the spatial coincidence of 2n(H2)=4×105cm−3 illuminatedbyaFUVfield60times the H13CO+ and DCO+ emission peaks (i.e. where the mean interstellar radiation field. We used the Meudon Tmb{DCO+(2−1)}/Tmb(cid:8)H13CO+(1−0)(cid:9)≃1).Besides, PDR code3, a photochemical model of a unidimensional we used the H13CO+ lines to determine the line–of–sight PDR (see Le Bourlot et al. 1993; Le Petit et al. 2006,for HCO+ columndensity. Indeed, the direct determinationof a detailed description) and its associated chemical reac- the HCO+ columndensity fromits rotationalline emission tion network. As this network only includes singly deuter- is hampered by the large HCO+ line opacities and their ated species, we added the D2 and HD+2 species and as- propensitytosufferfromself–absorptionandlinescattering sociated reactions from Flower et al. (2006). Nevertheless, effects(Cernicharo & Guelin1987).Inaddition,largecriti- these additional reactions do not affect much the pre- caldensitiesforHCO+ (anditsisotopologues)areexpected dicted DCO+ abundances. Only H2, HD and D2 form evenforthelowest–J transitionsduetoitshighdipolemo- on grain surfaces because the used chemical network al- ment: ∼ 4 D (Table 2). Hence, thermalization will only lows only H and D atoms to accrete onto dust grains. We occur at very high densities. For lower densities, n<ncrit, chosethefollowinggasphaseabundances:D/H=1.6×10−5, subthermalexcitationdominatesasJ increases.Therefore, He/H=0.1,O/H=3×10−4,C/H=1.4×10−4,N/H=8×10−5, in order to accurately determine the mean physical condi- N/H=8×10−5, S/H=3.5×10−6 (Goicoechea et al. 2006), tions and the [DCO+]/[H13CO+] ratio at the DCO+ peak, Si/H=1.7×10−8, Na/H=2.3×10−9 and Fe/H=1.7×10−9. we have used a nonlocal, non-LTE radiative transfer code We firstinvestigatedthe role ofgas thermodynamics in including line trapping, collisional excitation and radia- theHCO+ deuteriumfractionation.Todothis,westopped tive excitation by cosmic backgroundphotons (Goicoechea tosolvethethermalbalancewhentheFUVabsorptionwas 2003; Goicoechea et al. 2006). Collisional rates of H13CO+ large enough so that the temperature reaches a minimum and DCO+ with H2 and He have been derived from the value that we kept constant in the most shielded regions HCO+–H2 rates of Flower (1999). of the PDR. Figure 3 shows the predicted temperature Assuming a maximum extinction depth of profiles as well as the [H2D+]/[H+3] and [DCO+]/[HCO+] AV ≃50 along the line-of-sight where DCO+ abundance ratios as a function of the cloud depth for a peaks (Ward-Thompson et al. 2006), the observed minimum value of Tk=15, 20, 30 and 60K. The predicted DCO+ J=2–1 and J=3–2 line intensities are well repro- [DCO+]/[HCO+] ratio scales with the [H2D+]/[H+3] ratio, duced (with line opacities ∼1.5) only if the gas is cold as expected when DCO+ gets fractionated by the reac- (10–20K) and dense (n(H2) ≥ 2× 105cm−3). This high tion of CO with H2D+ and mainly destroyed by disso- density is consistent with the one required to reproduce ciative recombination with electrons (see e.g. Guelin et al. 2 [DCO+]=n(DCO+)/n(H2). 3 Publicly available at http://aristote.obspm.fr/MIS/ 4 J. Pety et al.: Deuterium fractionation in theHorsehead edge thusofferstheopportunitytostudyingreatdetailthetran- sition from the warmest gas, dominated by photodissocia- tionprocessesandphotoelectricheating,tothecoldestand shieldedgaswherestrongdeuteriumfractionationistaking place. Therefore, the Horsehead edge is the kind of source needed to serve as a reference for PDR models (Pety et al. 2006) and offers a realistic template to analyze more com- plex galactic or extragalactic sources. Acknowledgements. WethankM.Guelinforusefulcommentsandthe IRAMPdBIand30mstafffortheirsupportduringtheobservations. JRGwassupportedbyanindividualMarieCuriefellowship,contract MEIF-CT-2005-515340. References Abergel,A.,Teyssier,D.,Bernard,J.P.,etal.2003,A&A,410,577 Brown,P.D.&Millar,T.J.1989, MNRAS,237,661 Fig.3. Chemical models for different minimum gas Caselli, P., Walmsley, C. M., Tafalla, M., Dore, L., & Myers, P. C. 1999,ApJ,523,L165 temperatures: 15, 20, 30 and 60K. The density is Cernicharo,J.&Guelin,M.1987, A&A,176,299 nH=4×105cm−3and the illuminating radiationfield is χ= Flower,D.R.1999,MNRAS,305,651 60. Temperature profiles and predicted [H2D+]/[H+3] and Flower,D.R.,PineauDesForêts,G.,&Walmsley,C.M.2006,A&A, [DCO+]/[HCO+]abundanceratiosareshownasafunction 449,621 of AV. The [DCO+]/[HCO+] ratios inferred from observa- Gerlich,D.,Herbst,E.,&Roueff,E.2002,Planet.SpaceSci.,50,1275 ′′ Goicoechea, J. R. 2003, PhD Thesis. Universidad Autonoma de tions in the cold condensation at δx∼40−45 and in the Madrid ′′ warm PDR gas at δx ∼ 10−15 are shown respectively Goicoechea, J.R.,Pety,J.,Gerin,M.,etal.2006,A&A,456,565 with the blue and red arrows. Guelin,M.,Langer,W.D.,&Wilson,R.W.1982,A&A,107,107 Guilloteau, S.,Piétu,V.,Dutrey,A.,&Guélin, M.2006, A&A,448, L5 Habart, E., Abergel, A., Walmsley, C. M., Teyssier, D., & Pety, J. 1982). 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Therefore, the observed [DCO+]/[HCO+] ≥ 0.02 506 abundance ratio can be reproduced using gas phase chem- Leurini,S.,Rolffs,R.,Thorwirth,S.,etal.2006, A&A,454,L47 istry only if the gas cools down from the photodissociation Linsky,J.L.,Draine,B.T.,Moos,H.W.,etal.2006,ApJ,647,1106 front to ≤ 20K, in good agreement with the DCO+ ex- Milam,S.N.,Savage,C.,Brewster,M.A.,Ziurys,L.M.,&Wyckoff, S.2005, ApJ,634,1126 citation calculations. Note, however,that CO freeze–out is Penzias,A.A.&Burrus,C.A.1973,ARA&A,11,51 believedtofurtherenhancethe[DCO+]/[HCO+]ratioover Pety, J. 2005, in SF2A-2005: Semaine de l’Astrophysique Francaise, the values predicted by pure gasphase fractionationby in- ed.F.Casoli,T.Contini,J.M.Hameury,&L.Pagani,721–+ creasing the abundance of H+3 and H2D+ (Brown & Millar PetSye,mJa.i,nGeodiecol’eAchsterao,pJh.ysRiq.,ueGFerrainn,caMis.e,,eatstarlo.-p2h0/0066,1i2n58S8F2A-2006: 1989;Caselli et al.1999).TheC18OJ=2–1emissionshown Pety,J.,Teyssier,D.,Fossé,D.,etal.2005, A&A,435,885 inFigure2substantiallydecreasesattheDCO+ peak.This Philipp,S.D.,Lis,D.C.,Gu¨sten,R.,etal.2006, A&A,454,213 behavior is reminiscent of CO depletion but it could also Roberts,H.&Millar,T.J.2000a,A&A,364,780 come from a combination of lower excitation and of opac- Roberts,H.&Millar,T.J.2000b,A&A,361,388 Roueff,E.,Tiné,S.,Coudert,L.H.,etal.2000,A&A,354,L63 ity effects. Future observations of molecular tracers of gas Smith,D.,Adams,N.G.,&Alge,E.1982,ApJ,263,123 depletion are needed to constrain the dominant scenario. Solomon,P.M.&Woolf,N.J.1973,ApJ,180,L89+ TheDCO+ linesstayundetectedinthewarmgaswhere Turner,B.E.1990,ApJ,362,L29 HCO+ (not shown here) and H13CO+ still emit. Indeed, van Dishoeck, E. F.,Thi, W.-F.,& van Zadelhoff, G.-J.2003, A&A, DCO+ cannotbeabundantinthephotodissociationfront, 400,L1 Walmsley,C.M.,Flower,D.R.,&PineaudesForêts,G.2004,A&A, where the large photoelectric heating rate implies warm 418,1035 temperatures (Tk > 50K), because the reaction of H2D+ Ward-Thompson, D., Nutter, D., Bontemps, S., Whitworth, A., & with H2 dominates and implies a low H2D+ abundance Attwood, R.2006,MNRAS,369,1201 ([H2D+]/[H+3] ≃ 2 ×10−4). From the upper limit of the Watson, W.D.1974, ApJ,188,35 DCO+ emission at δx =10-15′′(AV ∼ 1), we estimate a low abundance ratio [DCO+]/[HCO+]< 10−3 in the FUV photodominated gas, in agreement with the model predictions. ThesmalldistancetotheHorseheadnebula(∼400pc), itslowFUVilluminationanditshighgasdensityimplythat many physical and chemical processes, with typical gradi- ′′ ′′ entlengthscalesrangingbetween1 and10 ,canbeprobed ′′ inasmallfield-of-view(lessthan50 ).TheHorseheadedge