19 90ApJ. . .351L. .49D -1 The AstrophysicalJournal,351:L49-L52,1990March10 © 1990.TheAmericanAstronomicalSociety.Allrightsreserved.PrintedinU.S.A. 78 analysis ofinfraredemissionspectrafirstrecordedbyGillett, the occurrenceofsuchspeciesderivesprimarilyfrom the interstellarmediumisnowgenerallyaccepted.Evidencefor near 3040(3.29),1615(6.2),1310(7.6),1150(8.7),and885(11.3) cm (/un)andcanbeassociated,inprinciple,withvibrational Forrest, andMerrill(1973).Prominentfeaturesoccurator modes inaromaticspecies(DuleyandWilliams1981;Léger and Puget1984;Allamandola,Tielens,Barker1985) although theexactclassofmoleculehasnotbeenidentified. sources (e.g.,theOrionridge)suggeststhatspecies Localization ofthesefeaturestoregionsadjacentenergetic responsible mayhaveatransientexistenceand,infact,could be theproductsofdissociationorsputteringcarbon grains (DuleyandWilliams1986;Duley1988). has beendiscussedbyAllamandola,Barker,andTielens(1987) intensities oftheemissionfeaturesareastrongfunction that spectralfactorssuchastheshape,width,andrelative and Barker,Allamandola,Tielens(1987).Theyconclude ambient interstellarradiationfield.Thepredictionsarein in emissionwithinindividualobjects(Tokunagaetal.1988; general agreementwiththeobservationsofspatialvariations Geballe etal.1989;Roche,Aitken,andSmith1989),although behavior ofseveraltheweakerfeatures[e.g.,2940(3.40), there isstillconsiderableuncertaintyconcerningthedetailed focused oncompactfusedaromaticringmoleculessuch as which therearelaboratoryspectra(Allamandola,Tielens, and CH,coroneneC24Handovalene2 for 2890(3.46), 2850(3.51),and2800(3.57)cm’H/mi)]. close tothe20-50carbonatomsthatseemberequired to stabilize thePAHagainstdestructionbyinterstellarradi- Barker 1985;LégerandPuget1984).Suchmoleculeshave cloud conditions. ation fieldforextendedperiods (~10-10yr)underdiffuse atoms. Itisworth emphasizingthatthestable PAHswithin contain only15-20carbonatoms, andWoodwardetal.(1989) conclude thatinNGC7027, the PAHscontain13-27carbon Rectangle thesmallesthydrogenated PAHswouldappearto 160234 The presenceofpolynucleararomaticmolecularspeciesin Excitation andemissionofradiationbyinterstellarPAHs Attempts atachemicalidentificationoftheIRemittershave Geballe etal.(1989)findthat intheOrionbarandRed © American Astronomical Society •Provided bythe NASAAstrophysics Data System Subject headings:infrared:general—interstellar:grainsmoleculeslineidentifications consistent withthestructureoflaboratoryhydrogenatedamorphouscarbons(a-C:H).Benzeneandsim- , tetracene,andperhapspentacene.Anoriginforthesemoleculesincarbondustisindicated of two-tofive-ringlinearPAHspeciesinamorphouscarbons;specificallythederivativesnaphthalene, features. plest aromaticringmolecule,cyclopropenylidene,maybeobservableinsourceswhichshowtheUIRemission We showthattheIRemissionspectruminobjectssuchasOrionridgeiscompatiblewithpresence SIMPLE LINEARPOLYCYCLICAROMATICHYDROCARBONMOLECULESANDTHE INFRARED EMISSIONFEATURES:MOTHBALLSINTHEORIONRIDGE? I. INTRODUCTION nebulae: general Received 1989August2;acceptedDecember20 Department ofPhysics,YorkUniversity W. DuleyandA.P.Jones ABSTRACT L49 -1 -1 -1 1 x 1 1 these sizerangesarecatacondensedandnotcompactstruc- tures. Inthiscontext,wenotethatalowerlimit(Geballeetal. ecules thatcanbepresent.Indeed,forthesmallerstablePAHs the hydrogenatedPAHsseverelyconstrainsrangeofmol- the numberofcarbonatomsperPAHmustbeeven.Fora generic PAHCH,ifn=14(threerings),onlytwostructures are possible,forn=16(fourrings)onlyonestructureexists, with n>20havemanytensofisomers. and forn=18(fourrings)fivestructuresarepossible.PAHs 1989), oranupperlimit(Woodwardetal.onthesizeof emission features.Inparticular,apoorcorrelationexists but notexact,spectroscopicmatchtothemajorinterstellarIR spectra ofcompactmoleculessuchascoroneneprovideaclose, 885 cm;therelativelysharpnatureofinterstellar between laboratoryandastronomicaldataintheregionnear cm featurewouldseemtoimplyalimitedrangeofemitters. The recentdetectionofadditionalsharpemissionfeaturesat ecules areresponsiblefortheIRemissionandthatthesemol- 905 cm(11.05/un)and787cm'(12.70/mi)ledRoche, ecules haveonlyatransientexistenceinlocalizedregionsof the from asinglespeciesofmoleculeorrelativelysmallnumber nm ecules ischaracterizedbyaclosematchtoallthemajor .Theabsorptionspectrumofthesemol- Aitken, andSmith(1989)tonotethat“weareseeingemission observed IRemissionbands(Table1).Specifically,thespec- of dominantspecies.” gives (onitsown)analmostperfectrepresentationof the trum ofthesimplethree-ringlinearPAH,anthraceneCH , observed emissionspectrumwithlinesat3048,1621,1450, (Bellamy 1964).Thefeature at1905cm"(5.25gm) the CHbendingvibration(884 cm"^andtheobservedenergy (Allamandola 1989)isalsoobserved inthespectrumofanthra- view ofthesensitivity CH bendenergytoenvironment of theinterstellarpeak(885cm" ) isparticularlysignificantin cene. Wealsonotethatthe2940 cm"and2850features are consistent with theanharmonicityof C-Hstretching 1317, 1147,and884cm".Agreementbetweentheenergy of 14 10 As notedbyDuleyandWilliams(1981)theabsorption II. LINEARPOLYCYCLICAROMATICHYDROCARBONSPECIES In thisLetterweproposethatsmalllinear,fusedringmol- AND THEINFRAREDEMISSION 19 90ApJ. . .351L. .49D -1 -1 -1 -1 -1 disappear (weaken) onsubstitutionordehydrogenation. energetic conditionsthatexistinobjectssuchastheOrion meric material.Erosionoftheseclustersisexpectedunderthe whether IRemissionoccursbeforeorafterejectionofthese from thetwo-tofive-ringlinearpolyacenesisthereforeconsis- highly excitedandthefirsttoberemovedduetheirlow ridge whereclusterswithfewerthanfiveringsarethemost tent withthedissolutionofHACdust.Itisuncertain,however, sists ofacollectionPAHclusterslooselybondedbypoly- thermal heatcapacity(Duley1988).Observationofemission by RobertsonandO’Reilly(1987)Smith(1984),HACcon- this model,whichisbasedontheanalysisoflaboratorydata phous carbon(HAC)recentlyproposedbyDuley(1987).In ible withthemodelforstructureofhydrogenatedamor- (i.e., )mayaccountforsomeoftheobservedUIR emission, especiallyintheneutralregion.Thus,forfirst that smallPAHswithabout10carbonatomspermolecule time thereappearstobeclearobservationalevidenceforsmall PAHs inaplanetarynebula. between 860and940cmcannowbeunderstoodasarising and anthracene(Geballeetal.1989). mations availableinsuchsimplemolecules. cally narrowbecauseofthelimitedrangemolecularconfor- vibration (2-1and3-2transitions)inbenzene,naphthalene, (904 cm),thethreesimplestlinearpolyacenemolecules. three-, andtosomeextentfour-ringmoleculeswillbeintrinsi- L50 absorbs atthisenergy(Fig.1).Wenotethatemissionbytwo-, the 905cminterstellarfeature;however,anthracenealso naphthalene (782cm),anthracene(884andtetracene from thesuperpositionofemissionCHbendingmodesin Emission frompentaceneat908cmcouldalsocontributeto Tetracene .... Anthracene .. Pentacene .... Naphthalene , Interstellar .. Fig. 1.Infraredabsorptionspectrum ofanthraceneinKBr.FilledcirclesdenotetheobservedUIRfeatures; across(minussign)denotespectrallinesthat Woodward etal(1989)inastudyofNGC7027conclude The existenceofrelativelysmallPAHmoleculesiscompat- The spectrumreportedbyRoche,Aitken,andSmith(1989) ‘ Comparisonofenergiesmajorabsorptionbandsinsimplepolyaceneswithinterstellarspectra.LaboratoryspectraarefromNujolsolutions. Species © American Astronomical Society •Provided bythe NASAAstrophysics Data System 3043 3043 3048 3048 3040 1910 1905 1775 1754 3 Absorption BandsandInterstellarEmissionFeatures 1621 1593 1615 1538 1530 1504 DULEY ANDJONES Absorption andEmissionFeatures(cm^ 1450 1460 TABLE 1 1315-1250 1296 1297 1317 1271 1- -1 -1 -1 -1 -1 -1 -1 Witteborn etal(1989)couldbeduetothepresenceofanthra- weakening thisbandwhileleavingmostotherfeatures of the884cmfeatureto891asseen,forexample,by of slightlyincreasingtheenergy884cmfeature.Ashift unchanged. Table2showsthatsubstitutionalsohastheeffect polyacene .However,substitutionhastheeffectof important confirmationofthepresencesmallhydrogenated excitation offouradjacentCHoscillators(Bellamy1964). Detection ofanemissionbandatsuchenergywouldbe contribute tothe800-900cmcontinuum. Neither featuresarepresentintheinterstellarspectra; show absorptionfeaturesnear727cmcorrespondingtothe however, alowconcentrationofthesePAHmoleculescould chrysene (fourrings),CH,lieat873cmand864. the nonlinearPAHsphenanthrene(threerings),CH,and energy sideofthe885cmfeature.TheCHbendingmodesin the “plateau”emission(deMuizonetal1986)onlow- tinuum betweenthe787and885cmemissionpeaksorto range ofabsorptionfeaturesthatcouldcontributetothecon- absorption bands(Table2).However,substituentsyielda tetracene haslittleeffectonthe884cmor1620 abundant cyclopropenylidenemolecule,CH(Leach1989). and C3H3productswhicharelikelytobetheprecursorsof rapidly bedestroyedbystellarphotonsofenergygreaterthan linear PAHspecieswithfewerthan20carbonatomswill may beemittingbeforeleavingtheHACsurface. cene (Fig.1)andtheUIRspectrumsuggeststhatmolecules good agreementbetweenthespectrumofcondensedanthra- fragments fromdust(Duley1988;Allamandola1989).The 182 140 32 ~ 5eV(Leach1989).ThisrapiddestructionshouldyieldCH2 2 It issignificantthatspectraofanthraceneandtetracene Substitution ofsidegroupsnaphthalene,anthracene,and On ejectionfromthegrainsurfaceitislikelythatsmall 1270 1162/1147 1122 1123 1150 1000 990 996 958 958 954 959 908 904 905 836 884 885 782 787 Vol. 351 732 741 727 19 90ApJ. . .351L. .49D 1 No. 2,1990 1 1 cene derivatives.SubstitutionwithCHgroupswouldalso produce afeatureinthe2940cm'(3.4/rm)region. important indicationofthesourcePAHmolecules responsible fortheUIRfeatures.Theacceptedmodel diamond-like moleculargroupsbondedthroughapolymeric structure ofHAC(Smith1984)involvesaromaticand structure. Suchstructures,e.g.,pendantbenzeneina-C:H, 3 have beenconfirmedbylaboratoryexperiments(Tamoretal. -1 resulting naphthaleneoranthracenemoleculewouldeither involve thebreakingofsingleC—Cbonds(Fig.2).The retain aCH„group,orbedehydrogenated,atthesesites.Any would yieldmoleculesinwhichtheproportionoffouradjacent Naphthalene CH oscillatorsisreducedandthecorresponding727cm' the terminatingringsinlinearPAHs.Thus,dissociationevents PAH-matrix bondisstatisticallymorelikelytobeononeof 1989). LiberationofPAHmoleculesfromthismaterialwill Anthracene emission featuresuppressed.Indeed,itwouldbeabsentif bonds werebrokenatbothendsofthemolecule(Fig.2).The nonobservation ofa727cm'emissionbandmaythen the dissolutionofHAC.IfIRemissionbythesespeciesoccurs provide clearevidencethatthePAHUIRemittersderivefrom 1 bond breakingasshownformingagas phasemoleculewithonlyone,two,or to polymer()inHAC.Liberation ofanthracenewouldlikelyinvolve three adjacentC—H oscillatorsandno727cmemission band. The absenceofa727cm'emissionfeaturemayprovidean Fig. 2.—Schematicrepresentationof naphthaleneandanthracenebonding -1 SubstituentObservedEnergy(cm) © American Astronomical Society •Provided bythe NASAAstrophysics Data System Effect ofSubstitutiononAbsorptionSpectra H 2- methyl 9-methyl 2-methyl 9,10-dimethyl 2-ethyl 1- methyl 9-vinyl 9-carbonitrile 9-acetyl TABLE 2 1593 1599 1601 1621 1635 1622 1619 1626 1621 1620 1620 885 884 892 887 893 881 892 899 MOTHBALLS INORIONRIDGE? 848 842 847 770-790 782 811 778 803 815 810 790 781 789 740 725 737 740 723 744 734 732 1 1 1 1 1 1 1 1 bonding attheseterminalsites(Fig.2). feature wouldofcoursealsonotbeobservedbecause before themoleculesleavesurface,then727cm 1 strongest oftheselines(inabsorption)occurat1530(6.5), that havenotyetbeenobservedininterstellarspectra.The cm“ ‘(/un).Someofthesefeaturescouldbepresentinthespec- HD 44179(Wittebornetal.1989).Otherssuchasthe954cm trum ofNGC7027(Russell,Soifer,andWillner1977)in line maybeconfusedwithioniclines.Forexample,Aitkenand Roche (1982)havereportedafeatureat~10.5fiminthe 1270(7.9), 1162(8.6),1000(10.0),954(10.5),and727(13.8) compatible withthestructureofHAC(Fig.2),namely,that An explanationforthelackorweaknessofthesefeaturesis spectra ofseveralnebulaewhichhasbeenattributedtoStv. end ringsubstitutionsinlinearPAHsleadtoaweakeningor disappearance ofthesebands.Forexample,2-methyl-,2-ethyl-, a considerablyweakened954cm'feature.Di-substituted and 2-(tert-butyl)-anthracenesshowno1000cm“feature with onesubstituentoneachringshowthesamebehaviorand (2,5-dimethyl,1,6-dimethyl-,and2,6-dimethyl-) also thepresenceofsolo,duo,andtrioCHbendingmodes. anthracene) leadstothelossorweakeningof1162,1000, Substitution onsmalllinearPAHspecies(naphthaleneand presence ofa~885cm'(soloH)bandinsubstitutednaph- and correspondingappearanceofthe787cmband, and 954cm'features,aweakeningofthe727feature thalenes wherenosuchbandispresentinnaphthalene.We features inmono-substitutedanthracenesiscompatiblewith also notethataweakeningofthe1450,1147and884cm' 44179. Theabsenceof1000cm“and727cm'emissionlines the observedUIRemissionfeaturesinNGC7027andHD CH„ substituentsoraredehydrogenatedatterminalbonding in interstellarspectrasuggeststhattheemitterseithercontain sites. and suggestthatthisfeaturemayalsobeobservableinemis- absorption featureat~21fim(483and47cm',respectively) (three rings)wouldappeartobemostabundant,followed by two- tofive-ringlinearpolyacenescouldbeimportantconstit- uents inemissionobjectssuchastheOrionridge.Anthracene sion. perhaps pentacene(fiverings).Itisunknownwhetherthis rep- naphthalene (tworings)andthenbytetracene(four and from anthraceneviaphotodissociation,orwhether the resents adecompositionsequencewithnaphthalenederived in thedust.Ifphotodissociationisimportant,thenratio observed relativeabundancesrepresenttheinitialabundance 7(787)//(885) whichmeasurestherelativeconcentrationoftwo- and three-ringmoleculeswillbesensitivetoambientcondi- tions. ThemodelforHACproposedbyRobertson and dant thanlargerclusters.Onthebasisofthismodel, a O’Reilly (1987)predictsthatsmallringclustersaremoreabun- reduction in7(787)//(885)would indicateanenhanceddestruc- features acrosstheOrionridge usingthedataofGeballeetal tion ratefornaphthalenerelative toanthracene. (1989) andRoche,Aitken, Smith(1989)showsthatthe optical depth,t,forpumping ofthisemissionincreasesmore or lessmonotonically fromposition4to20" southofposition uv Figure 1showsseverallinesinthespectrumofanthracene We notethatbothnaphthaleneandanthraceneshowan The interpretationoftheIRspectragivenheresuggeststhat An analysisofthespatialdependence oftheIRemission III. FORMATIONANDEXCITATION L51 19 90ApJ. . .351L. .49D 4 436 23 21 3 162 17 W. DuleyandA.P.Jones:DepartmentofPhysics,YorkUniversity,4700KeeleStreet,NorthYork,Ontario,Canada, M3J1P3 1 .1988,M.N.R.A.S.,234,61P. Duley, W.W.,andWilliams,D.A.1981,M.N.R.A.S.,196,269. Duley, W.1987,M.N.R.A.S.,229,203. de Muizon,M.,Geballe,T.R.,d’Hendecourt,L.B.,andBass,F.1986AvJ Daunt, S.J.,andShurvell,H.F.1976,Spectrochim.Acta,32A,1575. Birks, J.B.1970,PhotophysicsofAromaticMolecules(London:Wiley Bellamy, L.F.1964,TheInfraredSpectraofComplexMolecules(London* Barker, J.R.,Allamandola,L.J.,andTielens,A.G.M.1987,Ap.(Letters), Allamandola, L.J.,Tielens,A.G.M.,andBarker,J.R.1985,Ap.(Letters), Allamandola, L.J.,Barker,J.R„andTielens,A.G.M.1987,inPolycyclic Allamandola, L.J.1989,InternationalSchoolofPhysics“EnricoFermi", abundance n=3.7x10". Aitken, D.K.,andRoche,P.F.1982,M.N.R.A.S.,200,217. object. Thiscorrespondsto<0.07oftheavailablecarbon ”h ~10cm".Thenn(anthracene)/n<1.8x10"inthis observed value. than 1.8x10cminthisregionortwouldexceedthe conclude thatthespacedensityofPAHcannotbemuchlarger 1.7 x10nL,theobservedUVopticaldepthimplies L52 cm ifself-shieldingdominates.Expresseddifferently,onecan cH 450 pc(WittandLillie1978),thepathlengththroughfrom H <7(anthracene) =8.4x10“cm,n(anthracene)~1.810" position 4to20"southis1.35x10cm.With by thesemolecules.WiththeOrionridgeatadistanceof interest toexaminewhethertcouldarisefromself-shielding anthracene andnaphthaleneareknown(Birks1970),itisof uv 4. Thevalueoftforboththe3040and884cm"featuresis H uv uv ~2 at20"south.SincetheUVabsorptioncrosssectionsfor (Letters), 306,L105.’ Interscience). 315, L61.’ Methuen). 290, L25. and N.Boccara(Dordrecht:Reidel),p.255. Aromatic HydrocarbonsandAstrophysics,ed.A.Léger,L.B.d’Hendecourt Course CXI,SolidStateAstrophysics,inpress. Extinction bydustwillalsocontributetot.WithA~ uvm © American Astronomical Society •Provided bythe NASAAstrophysics Data System DULEY ANDJONES REFERENCES 1 1 Woodward, C.E,Pipher,J.L.,Shure,M.,Forrest,W.J.,andSellgren,K.1989 Witteborn, F.C.,Sandford,S.A.,Bregman,J.D.,Allamandola,L.J.,Cohen, Witt, A.N.,andLillie,C.F.1978,Ap.J.,222,909. Tokunaga, A.T.,Nagata,Sellgren,K.,Smith,R.G.,Onaka,NakadaY Tamor, M.A.,Wu,C.H,Carter,R.O.,andLindsay,N.1989,Appl.Phvs. Smith, F.W.1984,J.Appl.Phys.,55,764. Russell, R.W.,Soifer,B.T.,andWillner,S.P.1977,Ap.J.(Letters),217,L149 Roche, P.F.,Aitken,D.K.,andSmith,C.H.1989,M.N.R.A.S.,236,485. Robertson, J.,andO’Reilly,E.P.1987,Phys.Rev.B,35,2946. Gillett, F.C,Forrest,W.J.,andMerrill,K.M.1973,Ap.183,87. Léger, A.,andPuget,J.L.1984,Astr.Ap.,137,L5. Leach, S.1989,inIAUSymposium135,InterstellarDust,ed.L.Allamandola Geballe, T.R.,Tielens,A.G.M.,Allamandola,L.J.,Moorhouse,A.,and Duley, W.W.,andWilliams,D.A.1986,M.N.R.A.S.,219,859. from theNSERCC. postdoctoral fellowship.Thisworkwassupportedbygrants sources. ring molecule,maybeatracerofPAHdecomposition(Leach addition toQbranch-emission(DauntandShurvell1976). emitter sincegaseousmoleculeswillshowPandRbranchesin Emission bycyclopropenylideneC3H2,thesimplestaromatic band shouldbeaclearindicatorofthephysicalstate 1989) andshouldbesearchedforinaromaticIRemission isexpectedat675cm"Thecontourofthisemission in objectswithstrong787cm"emission.Emissionfrom thalene suggeststhatbenzene,CH,mayalsobeobservable 6 Ap. J.,342,860. M., Wooden,D.H.,andGraps,A.L.1989,Ap.J.,341,270. Sakata, A.,andWada,S.1988,Ap.J.,328,709. Letters, 55,1388. and A.G.M.Tielens(Dordrecht:Kluwer),p.155. Brand, P.W.J.L.1989,Ap.J.,341,278. A. P.J.isgratefultoSERCandNATOfortheawardofa The possiblepresenceofaPAHmoleculeassmallnaph-