1992Apj. . .386. .587F the Astrophysical Journal, 386:587-603

1992ApJ. . .386. .587F for ResearchinAstronomy,Inc.,under contractwiththeNationalScience for ResearchinAstronomy,Inc.,under contractwiththeNationalScience Foundation. Astronomy Observatories,whichisoperated bytheAssociationofUniversities Astronomy Observatories,whichisoperated bytheAssociationofUniversities Foundation. zones inreflectionnebulae(Witt&Malin1989;Wittet al. including many,butnotall,reflectionnebulae;thepeculiar has beenseeninfilamentsidentifiedasHphotodissociation clouds (Guhathakurta&Tyson1989).NotablyenhancedERE & Witt1990);andpossiblyinhigh-latitudegalacticcirrus bipolar “RedRectangle”nebula(Schmidt,Cohen,&Margon and theionizedinteriorofPNNGC7027(Furton&Witt environments atopticalwavelengthsÀ<1/umastwodistinct Mattila 1979);theplanetarynebula(PN)NGC7027(Furton with apeakwavelengthinthe650-700nmrangeandwell- in theIband(Witt,Schild,&Kraiman1984;WittSchild components. Oneisaquasicontinuumobservedmostreadily without thelatter(Witt&Schild1988). defined onsetnear540nm(Witt&Boroson1990;Witt tively correlated,althoughtheformercanbefoundtoexist Schild 1988).Theintensitiesofthesetwocomponentsareposi- 1990) .Thesefacts,aswellthe observedcharacteristicsofthe 1989) andneartheinterfacebetweenmolecularenvelope 1980); thedarknebulaL1780(Chlewicki&Laureijs1987; The AstrophysicalJournal,386:587-603,1992February20 1986); theotherisabroad(FWHM~80nm)emissionfeature © 1992.TheAmericanAstronomicalSociety.Allrightsreserved.PrintedinU.S.A. 2 2 1 VisitingAstronomer,KittPeakNational Observatory,NationalOptical VisitingStudent,KittPeakNational Observatory,NationalOptical ERE hasbeendetectedinarangeofastronomicalobjects, Extended redemission(ERE)isseenindustyinterstellar 3-2_1 + extensive literaturesearch.WecomparetheEREbandobservedinplanetarynebulaeto known asextendedredemission(ERE)insevenoftheseobjects.WehavethereforeaddedPNstothegrowing ary nebulae(PNs)coveringawiderangeofC/Oratiosandhavedetectedthebroad,redemissionband not necessarilyproducedbythesamecarrierbecausewepresentexamplesofseveralPNswhereUIB’sare We claimthatthissupportstheidentificationofEREasphotoluminescencehydrogenatedamorphous theoretical atomiccontinuum,computedasdescribedbyBrown&Mathews,andscaledtomatchthe to haveanaverageintegratedintensityof1.9x10“ergscmssrandcontributeapproximately list ofastronomicalobjectswhichareknowntoemitERE.InPNswhereEREisdetected,thebandfound Subject headings:dust,extinction—infrared:interstellar:continuumplanetarynebulae:general detected andEREisnot. dust grains.OurdataalsosuggestthatEREandtheso-calledunidentifiedinfraredemissionbands(UIB)are nebulae inoursampleexhibittheEREbandabovedetectionlimit,whilemostcarbon-richdo. sence orabsenceofEREinPNsiscorrelatedwiththeC/Oratiosensethatnoneoxygen-rich abundance ratios,He/Hand;thesewerecompiled,alongwithC/Oratiomeasurements,froman such amodelforgivenPNaretheelectrontemperature,T,colorexcess,E(B—V),andhelium observed inreflectionnebulaeandfindtheyaresimilarbandprofileintensity.Wethatthepre- observed nebularspectruminthe320-400nmspectralregion.Theinputparametersnecessarytocompute 15% ofthefluxincontinuumbetween550and820nm.TheEREisdetectedasemissionexcessa e We havecompletedalong-slit,spectrophotometricsurveyoftheopticalcontinuumemission20planet- © American Astronomical Society • Provided by the NASA Astrophysics Data System Ritter AstrophysicalResearchCenter,DepartmentofPhysicsandAstronomy,TheUniversityToledo,2801WestBancroftStreet, 1. INTRODUCTION EXTENDED REDEMISSIONFROMDUSTINPLANETARYNEBULAE Received 1991June17;acceptedAugust28 12 D. G.FurtonandA.N.Witt Toledo, OH43606-3390 ABSTRACT 587 from polycyclicaromatichydrocarbon(PAH)molecules is length, butotherpotentialsourcescannotberuledout. Red (Watanabe, Hasegawa,&Kurata1982)matchestheobserved many models.Thephotoluminescencespectrumoflaboratory identification forthesourceofERE?Allcurrentmodels (HAC) grains. establishing therelationship betweenrelativeEREintensity likely sitesforpredominantly silicate dust.Theidentificationof formation ofcarbondust.The remainderwithC/O<1are responsible fortheobservedERE.PNsofferadvantage d’Hendecourt etal.(1986)haveproposedthatluminescence Geake 1964;Derham,Geake,&Walker1964); and luminescence verysimilartotheobservedEREhasalsobeen interstellar dustconsidermulticomponentgrainensembles(see a moreintensivestudyofEREinPN.HowcertainistheHAC to photoluminescencebyhydrogenatedamorphouscarbon broad EREfeature,havegenerallysupportedthesuggestion, Zuckerman &Aller1986)and arethereforeidealsitesforthe still bearsthechemicalsignatureofoutflowenvironment. that thedustassociatedwiththemislikelyoflocalorigin and seen inenstatiteachondrite(silicate)meteorites(Derham & ERE bandverycloselybothinstructureandpeakwave- samples ofHACnearmaximumluminescenceefficiency a roleinmost,HACisnotnecessarilycomponentincluded originally advancedbyDuley(1985),whichattributestheERE and theC/Oratioofdust forming environment. the EREcarriercantherefore beadvancedconsiderablyby Mathis 1990foranup-to-datereview),andwhilecarbonplays More than60%ofPNshaveC/Oratios>1(Kaler1981; How important isthedustcomponentresponsible forERE Some keyquestionsarisewhichmaybeanswerablethrough 1992ApJ. . .386. .587F intact (Lenzuni,Natta,&Panagia1989),theevolutionarypre- 588 cursors ofPN,highlyevolvedAGBstars,areconsideredmajor history ofatypicalsampleinterstellardust?PNspermitthe small fractionofthisdustappearstosurvivethePNoutflow study ofdustshortlyafteritsformation.Althoughonlyavery on agalacticscale,andwheredoesitariseintheprocessing sources ofgalacticdust(Gehrz1989).TheERE-brightRed case ofNGC7027(Furton&Witt1990),earlyobservations carbon-rich andoxygen-richobjects.Aswehaveshowninthe of newlyformedinterstellardust. Rectangle probablyisarepresentativeofthisclass.Studies cussed byMiller(1978)appeartoberelatedthepresenceof an excessquasi-continuuminthespectraofseveralPNdis- spectrophotometric observationsof20PNs,includingboth ERE inPNsmayrevealtowhatextentcarriersarepart discuss ournewobservationsandthereductionofdata. compute theoreticalopticalcontinuaforoursampleofPN. reddening corrections.Resultsarepresentedin§4,followedby the derivationofappropriatePNatomiccontinuaandtheir ERE intheseobjects,albeitunrecognizedatthetime.In§2we ature compilationofthenebularparametersnecessaryto also includeanAppendixwhichcontainstheresultsofaliter- a discussionin§5.Finalconclusionsarepresented6.We Section 3containsthedescriptionofmethodsanddatausedin vability. TheseobjectsarelistedinTable1.Wefirstselected of carbon-to-oxygenratio,brightness,angularsize,andobser- In ordertoaddressthesequestions,wehavecarriedout We selected20PNsascandidatesforthisstudyonthebasis © American Astronomical Society • Provided by the NASA Astrophysics Data System IC 351 NGC 40 IC 1747 NGC6720 .... NGC6543 .... NGC 1535.... BD +303639. NGC7662 .... NGC6790 .... J900 3" N610930 NGC7027 .... NGC7026 .... NGC6826 .... NGC2392 6" S500...675 NGC 2371/2S/W knot12303060 IC3568 5"N610 ... 430 NGC6210 5" S210... 420 NGC 43616" N49504050 NGC 32426" N210310430 NGC6572 9" S310... 310 a b Grating250. Grating400. 2.1. SelectionofObjects 2. OBSERVATIONS ab b Object Object SlitPosition1990Feb28Marl 1990 Mar23 ab Slit Position1988Sep12131415 4" S 4" S 4" S 4" S 4" S Centered Centered South rim 6" S 3" S 3" S 3" S FURTON &WITT Observing Log TABLE l 1860 1860 1260 1890 1200 1500 400 240 300 300 110 110 A. B. -2_1 17 minimized thisproblemwithourchoiceofnebulaeandby cm sHzsr,incontrasttothelineemissionwhichis continuum emissionhasanintensityontheorderof10“ergs wide andrepresentativerangeofC/Oratios.Wechoseonly following theobservingproceduredetailedin usually 10-100timesbrighter.Simultaneouslyrecordingsuch terize theseobjectsisobservationallydifficult.InmostPNsthe emission betweenthemanybrightlineswhichcharac- bright PNsbecausedetectingthefaintnebularcontinuum aries inordertoprovideasampleofnebulaewhichspanned roughly equalnumbersofcarbon-richandoxygen-richplanet- had previouslybeenstudiedforotherpurposessinceour detector withitsintrinsicallylargedynamicrange.Wehave a contrastinintensityisdifficult,evenwhenusingCCD choice ofobjects,andobservingprocedure,ensuresthatthis continuum emissioninthevisibleregionofspectrum.Our central star,100,000K,iscomparabletotheatomic than about6".Thislimitationpermittedustoacquirespectra We alsoselectedonlyPNswithanangulardiametergreater analysis reliespartlyonexistingdataforindividualnebulae. subsection. Additionally,itwasnecessarytoselectobjectsthat problem isavoidedentirely. star. Insomeplanetariestheblackbodyemissionfrom of thenebulawhiledirectlyexcludingemissionfromcentral runs withtheCCDGoldCamlong-slitspectrographmounted on the2.1mtelescopeatKittPeakNationalObservatory, 1988 theCCDwasaTexasInstruments(TI)chipwithusable 1988 September12-15and1990February28-March4.In We obtainedthedataforthissurveyduringtwoobserving Total IntegrationTime(s) Total IntegrationTime(s) 2580 2000 1800 1800 1200 1200 430 400 330 100 100 100 2.2. ObservationalProcedure 2400 1800 1200 1200 400 300 100 2400 1860 1800 1500 300 300 100 Vol. 386 1992ApJ. . .386. .587F -1 -1 each nightwiththepackage SENSFUNC, thenapplyingthis images, determiningtheinstrumental sensitivityfunctionfor extracting one-dimensional spectra fromthestandardstar sensitivity functiontoeach object framewiththepackage dimensional imagesintoone-dimensionalspectradonotsmear pixel columnonthechipareidentical(thisisnotcase for frames. Next,wedeterminedthetwo-dimensionalwavelength spectral featuresinwavelength. the rawimages),sothatfutureextractionswhichcollapsetwo- the one-dimensionalwavelengthsolutionsforeachindividual and thehelim-neon-argonspectra.Weappliedwavelength bias-subtracted, andflat-fielded,wecreatedablue red by theNationalOpticalAstronomyObservatories,runningon FORM. TRANSFORMalsotransformseachimageso that solution totheobservationsusingpackageTRANS- solution foreachgratingusingtheIRAFpackageIDENTIFY spectral imagebyaveragingthecorrespondingreducedCCD a Sun3/160workstation.AftereachCCDframewastrimmed, Facility (IRAF)version2.8,whichisdevelopedandmaintained No. 2,1992 remove anyilluminationpatterns. neon-argon lampwasused.Finally,weobtainedquartz,dome, we observedstandardstarsseveraltimesspacedthroughout from 10to200s;andapproximately20exposureswere and skyflatswithbothgratingstoflat-fieldthechip the night.Forwavelengthcalibrationofchip,ahelium- used, andthetotalaccumulatedintegrationtime. The individualexposuretimesvariedfromobjectto lated integrationtimegaveacceptablesignalinthecontinuum. observed, thedateofobservation,slitposition,grating grating. Table1isalogofourobservationslistingtheobjects reduction. Thisenabledustoobservethefaintcontinuumin nation, inourcaseduetoemissionlines,weaccumulateda obtained oneachnebulawithboththeblueandred and thenumberofexposureswaschosensothataccumu- that thebrightestemissionlinesdidnotoverexposechip, CCD. Foreachnebulatheindividualexposuretimewassetso the midstofbrightemissionlines,withoutoverexposing large numberofshortexposuresandaveragedthemduring center ofthenebulatoavoidcentralstar.SinceCCD suffered fromresidualimagesifexposedtoverybrightillumi- and, whenpossible,positioneditslightlynorthorsouthofthe nm/2 pixelsrespectively. (row) anddispersion(column)dimensions.Theresulting between 600and700nm.Inordertoincreasethedynamic coverage ofabout320to940nmwithanoverlapregion in theredwithgrating400(158linesmmandblazek=675 spatial andspectralresolutionswerel"5/2pixels0.92 the chipwasbinnedonread-outby2pixelsinbothspatial range oftheCCD,whilepreservingsignal-to-noiseratio, nm) withaslitwidthof400/mi(5"),givingcombinedspectral with grating250(158linesmmandblazek=380nm) format of315by800pixelsinthespatialanddispersiondirec- usable slitlengthof5".Weobservedeachnebulaintheblue previous onewithalong-slitformatof512by800pixels,and CCD wasalsoaTIchip,butofmuchhigherqualitythanthe tions, respectively,andausableslitlengthof4!1.In1990the We achievedthefinalcalibration oftheobservationsby We reducedthedatawithImageReductionandAnalysis As abasisfortheabsolutecalibrationofobservations, We orientedthespectrographslitineast-westdirection © American Astronomical Society • Provided by the NASA Astrophysics Data System 2.3. DataReduction EMISSION FROMDUSTINPLANETARYNEBULAE + 3 + 3 + -1 -21 make thisapproachpractical. culate anabsoluteintensityin thecontinuum.Thesequantities emitted radiationmustadditionally beknowninordertocal- are notknownwellenough,for eventhebrightestnebulae,to amount ofwavelength-dependent extinctionsufferedbythe nebula. Thedistancetothenebula, itsionizedvolume,andthe perature mustbeknownthroughouttheionizedvolumeof the protons, He,andions,aswelltheelectrontem- for agivenplanetarynebula,thenumberdensityofelectrons, first calculatedbySeaton(1960),thenimprovedandapplied ionized heliumionspercmrespectively.They’srepresent electrons, protons,singlyionizedheliumions,anddoubly E dv=N(H)N(e-)\ energy emittedintothecontinuuminalldirectionspercm which arisesprimarilyfromtherecombinationofhydrogen and finallysummarizedbyAller(1984)Pottasch(1984). specifically togaseousnebulaebyBrown&Mathews(1970), processes describedabove.Theseemissioncoefficientswere temperature-dependent emissioncoefficientsfortheatomic where N{e\N(H),iV(Heandarethenumber of the hydrogenandheliumions,so-called2-photonemis- s perHzcanbewritten sion fromthedecayofmetastable2sstatehydrogen.The and helium,free-freeemissionfromelectronsinthevicinityof optical spectrumofPNs,thereisanunderlyingcontinuum fairly largeportionofthenebula. Hz sr. reduced theobservationstounitsofintensity,ergscms point, wealsoremovedthenightskyemissionlinesfrom this functiontoappropriatelymodifytheobservations.At extinction functioncanbederived.FLUXCALIBthenuses extinction functionforKittPeak;orwhenthestandardstar for atmosphericextinction.TheIRAFdatabasecontainsa intensity profileacrossthefaceofnebulaandthatbecause observations usingthepackageBACKGROUND.Finally,we of theaveragingperformedduringextractionprocess, also importanttonotethatallofthePNsobservedexhibitan combining theone-dimensionalbluespectrumtomatch created acompositespectrumforeachobjectbyscalingand from theindividualnebularspectrabyexcludingthosecon- observations coveralargeenoughrangeinairmass,this standard wavelength-andairmass-dependentatmospheric spectra weshowinthispaperarespatiallyaveragedovera one-dimensional redspectrumobtainedfromeachnebula.Itis taminated centralpixelcolumnsfromtheextraction.We were abletoremovethelastvestigeofcentralstaremission FLUXCALIB. FLUXCALIBalsocorrectstheobservations tained appreciablenebularemission.Atthispoint,wealso dimensional imagesbyaveragingthepixelcolumnswhichcon- v c In ordertocalculateanabinitiomodelatomiccontinuum Besides thebrightemissionlineswhichcharacterize We extractedone-dimensionalspectrafromthetwo- + +yÁHe; Te)+)?v(He+;(1) iV(He) N(H) Xh^’ 3.1. AtomicContinuumEmission [vv(H; 3. ANALYSIS T) +y(2q; ev 589 1992ApJ. . .386. .587F + match theobservationsin320-400nmspectralregion extinction parametercmeasuresthelogarithmicat iV(He)/iV(H andAT(He/AT(Habundanceratios.The color excessE(B—V)ortheextinctionparameterc,and compute suchamodelaretheelectrontemperatureT, which isfreeofEREinallPNs.Theparametersnecessaryto literature. Theresultsofthiscompilationaresummarizedin compilation. Wethenemployedtheseparameterstocompute values foreachofthefourinputparametersfromliterature written forthispurpose.First,weselectedappropriateinitial nebulae inoursurveyusinganinteractivegraphicsroutine dix A. the firstfivecolumsofTables4and5discussedinAppen- these fourinputparametersforeachofthenebulaeincludedin Hß andisdescribedindetailbyOsterbrock(1989).Valuesfor normalized, reddenedenergydistributionwasthenscaledto where/[£(5—F)] isaninterstellarreddeningfunction.This wavelength range320-1087nm.Next,weselectedapointin law givenbySchild(1977).Schild’sreddeningcoversthe coefficients givenbyAller(1984)andtheinterstellarreddening our surveyweretakenfromanextensivecompilationofthe model. Thispointwaschosenwiththeaidofanemission-line a normalized,reddenedenergydistributionusingtheemission depended ontheselectionofthispoint.Wethenplottedboth the nebula.Theshapeofmodelcontinuuminnoway list forthatobject,determinedfromhigh-dispersionspectraof region between320and400nm,forthepurposeofscaling the continuumofobservedspectrum,inEREfree 590 the observedspectrumandmodelcontinuumassessed ing theirconstraints,couldproduceamodelcontinuumwhich is detected,nocombinationofnebularparameters,evenignor- eters; anymanipulationweperformedisakintofinetuning acceptable fitwasachieved.BecausethesePNsarewell-studied the goodnessoffitbyeye.Thisprocesswasrepeated,varying fit theobservedspectrumacross550-850nmspectral them withinexperimentalerror.Finally,inobjectswhereERE objects, weexercisedlittlefreedomvaryingtheinputparam- the inputparametersbysmallamountseachtime,untilan fact, representativeofthetrueatomiccontinuum,eventhough computed modelcontinuumforeachofthenebulaein our region, withoutrequiringacontributionfromtheEREband. known toexhibitfluctuationsandgradientsthroughout the quantities suchastheelectrontemperatureandreddening are sample. ThemodelcontinuumwecomputeforeachPNis, in Figures la-Ifshowthedereddenedobservedspectrum and form each nebula.Theoreticalspectra computedbyaveragingindi- volume ofmostnebulae.As previouslystated,ournebular vidual syntheticspectra,characterized bydifferingelectron spectra arespatiallyaveraged overafairlylargeportionof observed bycomputinganormalizedenergydistributionofthe temperatures andreddening, werenotfoundtobesubstan- e v tially differentthantheoretical spectracomputedbyselecting E dvccf[E(B cv We computedandfittedamodelcontinuumtoeachofthe We modeledthecontinuumemissionofPNswe + N(U) + AT(He) © American Astronomical Society • Provided by the NASA Astrophysics Data System y(He; T)+ ve 3.2. ModelContinuum T) +y(2^; ev + AT(He) + N(H) + 7(He; T)\dv ve FURTON &WITT (2) 3_21 -21_3 3 + (1970). Thesestudieswereessentially identicaltoours,except emission inexcessofthepredictedatomiccontinuumbetween continuum atselectedwavelengths inthevisible.Dueto model forthePNBD+30°3639. continuum matchestheobservednebularacross several PNsweremadeintheearly1970s(e.g.,Miller & intensity inNGC7027is110x10“ergscmsr,more These intensitieshavebeencorrectedforinterstellarextinction cm s"srwithameanof1.9x10~ergss. band intensityrangesfrom0.34x10"to4.6ergs the entirespectrum.Wewerenotabletoproduceasatisfactory about 550and850nm.Twelveoftheprogramnebulaedonot observations neededtobecorrected forlineemissioncontami- absolute faintnessofthecontinuum emissionandthefact they reliedoninterferencefilter measurementsofthenebular optical continuumtheoryproposed byBrown&Mathews Mathews 1972;Danziger&Goad1973)inordertotest the 0.15. Soingeneral,theEREbandcontributesabout15%of intensity rangesfromabout0.05to0.25withameanof integrated EREintensitytotheatomiccontinuum and excludetheexceptionalnebulaNGC7027.TheERE show theEREband;intheseobjects,predictedatomic in thefourthcolumnofTable2,andFigure2ashowsERE fourth andfifthcolumnsofTable2listtheintegratedERE columns ofTables4and5listthevaluesT,E(B—V\ continuum ratioisgreaterthanabout0.05.Ourresultsare which exhibitERE.Weestimatethatwecanconfidentlydetect other PN.Inallofthenebulaeinoursampleratio than 50timesgreatertheaverageEREintensityseenin profile observedinNGC7027. extend beyond820nm,duetothecrowdingofemissionlinesat files from550to820nm.AlthoughtheEREbandisknown which containERE. intensity andtheERE/atomiccontinuumratiofor PNs He/H, andweadoptedforeachobject. The the fluxincontinuumbetween550and820nmPNs the EREintensitylongwardof822nm.Theseresultsarelisted these observations,wewerenotabletoaccuratelydetermine the Paschendiscontinuityandlowspectralresolutionof measuring thecontinuumintensityinemission-line-free summarized inTables2,4,and5.Thesixthseventh ERE inthecontinuumemissionofPNs,ifERE/atomic integrated EREintensitybynumericallyintegratingthesepro- model continuumatthesamewavelength,andcomputed fied byeyewiththeaidofpublishedemission-linelists.We windows atleast2nmwidewhereverpossibleintheobserved band. WedeterminedthespectralprofileofEREbandby the integratedEREintensityforPNswhichexhibitedthis spectrum ofeachobject.Theseline-freewindowswereidenti- eters isappropriate. subtracted fromeachmeasuredintensity,theintensityof some suitableaveragetemperatureandreddening.Therefore, our useofliteraturevaluesforeachthemodelinputparam- e Photoelectric observationsofthecontinuumemission of In thesevenPNinwhichweobserveERE,integrated Seven ofthe20PNsinoursampleexhibitEREbandas We extractedthespectralprofileofEREandcomputed 5.1. Introduction 5. DISCUSSION 4. RESULTS r-h ooLO LO oo IC1747

IC3568 NGC40

NGC2371

Fig. 1/ Fig. 1.—The dereddened optical spectra of the planetary nebulae included in our survey; the dashed line represents the theoretical atomic continuum computed as described in the text. The ERE band is evident in (a), (b), (q), and (r) as excess emission between about 550 and 820 cm. The absorption feature near 760 is the terrestrial A band.

NGC4361

NGC6543

WAVELENGTH (nm) Fig. l/c

nation, these measurements were estimated to be accurate only continuum emission of PNs confirm the red excess noted in to within 10% or 20%. Nevertheless, a discrepancy between some planetaries by Miller. these observations and model predictions at wavelengths PNs are not the only astronomical objects which exhibit this longer than about 600 nm was noted in several nebulae; the so-called extended red emission. Schmidt et al. (1980) observed observed continuum emission was generally 10% to 20% this same emission in the bipolar nebula the “ Red Rectangle ”; brighter than the atomic continuum models predicted. The Witt & Boroson (1990) and Witt & Schild (1988) have nature of this excess was unknown at the time; however, Miller observed ERE in a number of reflection nebulae; and Chle- (1978) speculated that an additional source of continuum emis- wicki & Laureijs (1987) and Mattila (1979) have observed ERE sion might be responsible. Our new CCD observations of the in the dark nebula L1780. Also, Guhathakurta & Tyson (1989)

have apparently observed a red excess in certain high-latitude PN because the extinction to individual reflection nebulae is cirrus clouds. uncertain; however, generally reflection nebulae and PN In reflection nebulae Witt & Boroson find that the average exhibit similar ERE intensities. ratio of integrated ERE intensity to integrated scattered light We find the planetary NGC 7027 to be in a class by itself as intensity, excluding observations of the exceptional “ Red Rec- Witt & Boroson (1990) also characterize the “Red Rectangle.” tangle,” is about 15%. In PN we find that the average ratio of The ERE intensity in NGC 7027 is 50 times greater than the integrated ERE intensity to the integrated atomic continuum average ERE intensity observed in other PNs. Similarly, the intensity is also about 15%. It is not practical to compare the ERE intensity in the “Red Rectangle” is about 50 times absolute ERE intensities seen in reflection nebulae to that in greater than the average ERE intensity observed in other

Fig. U

reflection nebulae. In a previous paper we have shown that the 5.2. The C/O ratio in PNs ERE in NGC 7027 is emitted predominantly from a shell PNs provide unique environments in which to study ERE. which lies outside the ionized volume of the nebula and that The chemical nature of these objects can be, and indeed has the ratio of ERE intensity to atomic continuum intensity rises been, investigated in detail. Specifically, the gas-phase carbon- to about 0.5 in this shell (Furton & Witt 1990). Witt & to-oxygen ratio can be used to indicate what type of dust Boroson (1990) find that the ratio of ERE intensity to scattered predominates in a particular nebula. Most of the carbon and light intensity ranges from about 0.5 to more than 1.0 in the oxygen in the outflow of a given planetary nebula will react to “Red Rectangle.” Both of these objects are extremely lumi- form carbon monoxide; the type of dust that subsequently nous, compact, dusty, and carbon-rich. forms will depend on whether carbon or oxygen was initially The spectroscopic profile of the ERE band in PNs can be more abundant. Silicate dust will form in planetaries with compared to the profile of ERE band observed in the “ Red excess oxygen, C/O < 1, while carbon-rich dust will form in Rectangle” and other reflection nebulae. Figure 2a shows the nebulae with excess carbon, C/O > 1. The nature of the ERE profile observed in the PN NGC 7027, while Figure 2b resulting dust, if any, is unclear if C/O ~ 1. PNs exhibit a wide shows the ERE profile observed in the reflection nebula NGC range of C/O ratios, and there are roughly equal numbers of 2327 by Witt (1988). This profile is characterized by a steep carbon-rich and oxygen-rich nebulae. Therefore, PNs can be onset around 550 nm, a peak wavelength between 650 and 700 used as laboratories to test the identification of the ERE nm, and a tail toward longer wavelengths in all cases. This fact, carrier. and the preceding discussion, indicate that in general the There are several methods which can be used to determine nature of ERE in PNs and reflection nebulae is the same. Witt the C/O ratio in PN. The C/H ratio can be determined by & Schild (1988) have compared the ERE spectral profile to applying recombination theory to observations of carbon and laboratory spectra of the photoluminescence of H AC thin films hydrogen recombination lines. Aller & Menzel (1945) first produced by Watanabe et al. (1982) and found them to be quite derived the C/H ratio for a number of planetaries by observing similar. This supports the identification of the general pheno- the 426.7 nm C n recombination line, and Torres-Peimbert & menon of ERE as the photoluminescence of HAC dust. Peimbert (1977) used the 465.0 nm C m line. Subsequent

Fig. 2.—(a) The spectral profile of the ERE band observed in the planetary nebula NGC 7027. {b) The speciral profile of the ERE band observed in the reflection nebula NGC 2327 by Witt (1988).

© American Astronomical Society • Provided by the NASA Astrophysics Data System 1992ApJ. . .386. .587F No. 2,1992 from theobservedC/Hratios byassumingasolarO/Hratioor researchers (e.g.Kaler1981; French1983)haveusedother Determining thecarbonabundance fromtheopticalemission by adoptinganO/Hratio determined insomeotherway. C in,and465.9nmiv)to derivetheC/Hratioinlarge optical recombinationlinesof carbon(465.0nmCn,465.1 samples ofPNs.Theseresearchers theninferredtheC/Oratio © American Astronomical Society • Provided by the NASA Astrophysics Data System NGC 7027 NGC 7026 NGC 40 NGC 7662 NGC 6826 NGC 6790 NGC 6720 NGC 6572 NGC 6543 NGC 6210 NGC 4361 NGC 3242 NGC 2392 NGC 1535 J 900 NGC 2371 IC 3568 IC 1747 IC 351 Object BD +30°3639 EMISSION FROMDUSTINPLANETARYNEBULAE Carbon-to-Oxygen RatioandEREBandStrength 4.4 4 >2.8 0.9 0.47 7 0.52 0.41 0.6 0.87 0.45 0.36 0.83 0.6 0.8 0.58 0.76 0.4 0.9 0.8 0.8 0.37 0.84 0.5 3 0.1 0.85 0.4 2.2 0.12 0.5 0.25 0.5 0.9 3.9 2.8 3.5 1.6 1.2 1.7 1.4 1.2 1.3 1.9 1.9 1.7 1.1 1 1.7 1.7 1.1 1.2 1.1 1.4 1.0 1.5 C/O o C o O ? 6 3 9 3 3 3 3 7 3 3 26 3 22 22 26 26 22 3 26 30 22 26 22 8 22 26 22 3 22 26 28 22 26 22 22 22 26 2 26 22 3 26 25 22 26 22 26 26 Ref. 26 25 10 11 18 13 19 15 TABLE 2 (10"^ ergscnT^s"^sr“^) the opticallinesofcarbonare alwaysveryweak,usuallyonlya lines ofcarbonhastheadvantage thattheselinesarealmost ratio hasremainedachallenge formainlytworeasons.First, temperature, yettheuseofthis methodtodeterminetheC/O purely recombinationlines,so theyarerelativelyinsensitiveto few percentoftheH/?intensity. Second,calculationsofthe C/O ratiofromtheobserved line intensitiesanddeducedC/H a 4.6 0 0 0.42 0.34 ere - 3.0 1.8 1.0 110 ^RE^CONT. 0.16 0.12 0.05 0.20 0 0 0.20 Ratio -0.30 0.09 595 1992ApJ. . .386. .587F o ratio measurementsforeach of 11nebulaeinoursampleare individual nebulaerangefrom20%tooverafactorof2. dances andabundancepatterns,errorsintheabundances ture ofthePNsNGC1535(Barker1989),3242 with anassociatedreference number. Table6canbeusedto a statisticalsense.Whilethisproducesreliablemeanabun- type whichincludealargesampleofPNstendtotreatthemin strong carbonlinesintheUV.Furthermore,studiesofthis determinations cancomefromananalysisoftherelatively from ourpointofviewbecausereliablecarbonabundance do notreproduceUVlineintensitiesaswellthey empirical oradhocformulaeareusedtocomputetheioniza- in Table2.Themostnotable problemisthatthesetofC/O recall eachliteraturecitation. Theerrorsoruncertainties second andthirdcolumnsof Table2listthesevaluesalong ature; webelievethiscompilationisreasonablycomplete. The the C/Oratiodeterminationswewereabletofindinliter- optical lineintensities(Aller&Czyzak1983).Thisisimportant associated withC/Oratiodeterminations inPNsareapparent tion correctionfactors.Itisalsofoundthatmodelsofthistype N andTpropagatethroughthesemodelsmanytimes, and elementalabundances.Althoughthismethodhasthe ratio canthenbedeterminedfromtheionicconcentrations parameters alongwithalargeamountofatomicdata.TheC/O emission lineintensities.ThenebulardiagnosticsN,Tand advantage ofbeingcompleteorself-consistent,uncertaintiesin E(B —V),orc,mustfirstbecomputed,thenareusedasinput careful, thoroughstudiesofindividualPNs.PNsaremuchlike (1990). Finally,NGC7662hasalsobeenstudiedbyHarrington more weightinouranalysis.NGC40hasbeenanalyzed carefully becauseoftheobviouscomplications;however, planetary mustbetreatedindividually.Informationfromthe nebular diagnosticsandabundances,amongotherthings,each for themostpartbadassumptions.Todetermineprecise fingerprints: notwoareexactlythesame.Assumptionsof factors fromthemodelwhichbestreproducesobserved recently beensummarizedbyKeyes,Aller,&Feibelman (Barker 1988),andNGC76621986).Thespectrum parameters derivedintheseworks,specificallytheC/Oratios, tunately, onlyasmallnumberofnebulaehavebeentreatedthis uniform densityandtemperature,sphericalsymmetryare (1986). et al.(1982),andBD-l-30369byPwa,Pottasch,& Mo and physicalparametersoftheconspicuousNGC7027have detail byBianchi&Grewing(1987)andCleggetal.(1983). several oftheseplanetariesareincludedinourlist.Wegivethe UV, optical,andIRmustbeanalyzedsimultaneously.Unfor- Barker hasconductedaseriesofstudiestheionizationstruc- determine theionicconcentrationsandionizationcorrection prehensive worksinthisfield.Thegeneralprocedureistosolve many casesareasonablyreliableC/Oratiocanbedetermined the equationsofstatisticalequilibriumforasetelementsand and Aller&Keyes(1987)haveproducedthemostcom- and UVemissionlines.Kaler(1970),Aller&Czyzak(1983), tion stages,anduncertainoxygenabundances.Nevertheless,in transfer ineachline,uncertaincorrectionsforunseenioniza- ratios involveassumptionsaboutthedetailsofradiative gated throughdetailedmodelingofalargenumberoptical by thismethod. e e 596 1985), NGC6720(Barker1980,1982,1987),6826 We havecompiled,fortheplanetariesinoursurvey,all of The mostreliableabundancedeterminationscomefrom The bulkchemicalcompositionofPNscanalsobeinvesti- © American Astronomical Society • Provided by the NASA Astrophysics Data System FURTON &WITT continuous emissionistheopticalatomiccontinuumpredicted primarily silicatedust,whichappearstobeopticallyinert in moderately carbonrich,and0.1,highlyoxygenrich.Clearly, certain IRfeaturesversustheC/Oratiocanbefoundin NGC 2392,3242,4361,6210,6826)as by Brown&Mathews(1970).Ontheotherhand,carbon-rich the visibleintheseobjects,andthereforeonlysource of responsible fortheEREband.Oxygen-richnebulaecontain hypothesis thatthephotoluminescenceofHACdust is the shapeorslopeofaplotanyphysicalparameterversus the mostextremecase,NGC7026,wefoundtwoC/Odetermi- literature (e.g.,Martin1987;Cohenetal.1986).Wefeelthat NGC 6790)andtwodonot(NGC6543,7026).These region withoutrequiringacontributionfromtheEREband. the EREband,andone(BD+30°3639)maycontainERE.For band whilemostcarbon-richnebulaedoisconsistentwith the and willthereforehavenorealsignificance. the C/Oratiowilldependstronglyonone’schoiceof nations bytwodifferentmethodswhichgavevaluesof1.7, these determinationsismuchlargerthangenerallybelieved.In ratios forthissampleofPNsclearlyindicatesthattheerrorin graphs ofthistypearenotusefulhere.OurcompilationC/O of C/Oratiototesttheobservedcorrelation.Similargraphs results aresummarizedinTable3. carbon- oroxygen-rich,twoshowtheEREband(NGC6720, Two oftheeightcarbon-richnebulae(NGC40,J900)donot the observednebularcontinuumin550-850nmspectral these objects,thepredictednebularcontinuumdoesnotmatch IC 1747,NGC6572,7027,7662)definitelycontain nebular continuumacrossthevisiblespectralregioninevery these. ThisisevidentinthenebularspectrashownFigures distinctly oxygen-rich,andtheEREbandisnotseeninanyof to classifyeightnebulae(IC3568,NGC1535,2371, of thefournebulaethatweareunabletoclassifyaseither show theEREbandaboveourdetectionlimits.Additionally, as distinctlycarbonrich.Fiveoftheeightplanetaries(IC351, one oftheseobjects.Wearealsoabletoclassifyeightnebulae respectively. Infourcaseseventhisisnotpossible. determinations foragivennebulaaregreaterthan1,orless based onthelistedC/Oratios.Ifmajorityofratio classified eachnebulaaseithercarbon-richoroxygen-rich not evenconsistentlygreaterthanorless1!Wehave the EREbandandmostcarbon-richnebulaedo.Weareable C/O ratiointhesensethatoxygen-richnebulaedonotshow than 1,thenweclassifyitascarbon-rich,oroxygen-rich, la-lt. Thecalculatedatomiccontinuummatchestheobserved The conclusionthatnooxygen-richPNsshowtheERE One istemptedtoplottheEREbandstrengthasafunction The presenceorabsenceofEREinPNiscorrelatedwiththe 0 0 8 C 5 1 2 ? 2 0 2 5.3. CorrelationofEREwithC/ORatio C-rich/O-rich YesMaybe No ERE andC/ORatioSummary TABLE 3 ERE Vol. 386 1992ApJ. . .386. .587F No. 2,1992 §5.5. which isconsistentwiththeobservationswillbediscussedin minescence ofHACthinfilmsmatchthespectralprofile //m siliconcarbidedustfeatures,andthesetofso-called grains. ApossiblescenariofortheformationofHACgrains environments indicatesthatthisdustisintheformofHAC lead tothediscoveryofwellknown9.7//msilicateand11.2 ERE observedincarbon-richPNsandotherastrophysical carbon andhydrogen.Althoughtheexactnatureofthisdustis nebulae containopticallyactivedustcomposedprimarilyof most likelyresponsiblefortheUIBs. Dischler, Bubenger,&Koidl1983a,b)haveallbeenproposed al. 1989;Borghesi,Bussoletti,&Colangeli1987;Goebel have beenmeasuredbyIRAS,Aitken&Roche(1982),Roche, still uncertain,thefactthatlaboratoryspectraofphotolu- aceous materialconsistingofsmallhydrocarbonclustersis as possibleUIBcarriers.Inanycase,somesortofcarbon- (Duley 1985;Papoular,Reynaud,&Nenner1991;Papoularet dola, Tielens,&Barker1985),quenchedcarbonaceouscom- carbon grains(Blanco,Bussoletti,&Colangeli1988;Adamán- polycyclic aromatichydrocarbon(PAH)molecules(Leger& the hydrocarboncarrierhasyettobedetermined.Isolated different bondingarrangements,theexactchemicalnatureof vibrational transitionsofC—CandC—Hbondsinslightly unidentified infraredemissionbands(UIB)at3.28//m,3.4¡um, obtained mid-IRspectraofover70objects.Thesesurveyshave Aitken, &Whitmore(1983),andRocheAitken(1986)have and IRobservations ofawiderangeastrophysical environ- amorphous carbongrainswhich isconsistentwithUY,optical, /mi UIBinJ900,NGC3242,and6543;allofthese posites (QCC)(Sakataetal.1984,1987,1990),andHAC Although thesebandsaremostcertainlycausedbyvarious constant excitationconditions, butratherbyrelatedmaterials. exactly thesameregionofnebula.Itseemslikelythat the facts areconsistentwiththehypothesisthatUIBs pro- correlated withtheC/OratioinasmallsampleofPNs.These found theotherUIBs,especially7.7jumband,alsotobe positively correlatedwiththeC/Oratio.Cohenetal.(1986) the 3.3/miUIBtobecommoninPNs,withanintensitythatis been shownthatthe9.7//msilicatedustfeatureisobserved Puget 1984),amixtureofisolatedPAHsandamorphous 3.51 yum,5.62jam,6.95/mi,7.7/¿m,8.6and11.3/an. by Gillet,Low,&Stein(1967)theIRspectraofPNshave UIBs andEREarenotproducedbythesamematerial,under sion, mappedbyWoodwardetal.(1989),arenotemittedfrom duced bycarbon-richmaterial.Cohenetal.detectedmost of only inoxygen-richPN(Roche1989).Martin(1987)foundthat limits. Furthermore,ourstudyofNGC7027(Furton&Witt detected mostoftheUIBsinJ900,andMartin 3.3 the 3.3jumUIBinNGC6572,allofwhichdefinitelyshow the the UIBsinNGC6572,and7027,Martindetected received considerableattention.BesidesthemanyPNsthat objects donotexhibittheEREbandaboveourdetection ERE band.Mostnotable,however,isthefactthatCohenet al. 1990) showedthatatleastinthisPN,EREand3.3/miemis- The originoftheUIBshasbeencentersomedebate. Recently, atheoryforthe production andevolutionof Are EREandtheUIBsproducedbysamecarrier?Ithas Since thedetectionofstrong10fimemissioninNGC7027 5.5. HydrogenatedAmorphous CarboninPNs © American Astronomical Society • Provided by the NASA Astrophysics Data System 5.4. InfraredFeaturesandERE EMISSION FROMDUSTINPLANETARYNEBULAE -21 3 _1 ment wheretheevolutionofHACdustgrainsplaysanimpor- interstellar extinctionandforvariationsinextinction,the it pertainstoastronomicalobservations,andconcludethat good agreement. spectra ofvariousmixturesHACandPAHsfindingquiet the UIBsobservedinPNNGC7027withlaboratory UIBs, andforERE.Finally,Blancoetal.(1988)havesimulated review therelevantlaboratorydataonamorphouscarbon,as cospatial inthereflectionnebulaNGC2023.Jonesetal.(1990) (1989) showthatinfactEREandH2v=1-0S(l)emissionare efficiency innarrowHphotodissociationzones.Witt&Malin Toyoda 1989;Hsu1988)thationizedatomichydrogenreacts large HACgraincouldbereducedtoanumberofsmaller which HACdustgrainsmaybedestroyed.Thefirstissimply enhanced ERE.ThisisbecausethelargerHACgrainswhich grains inPNsdecreasesasthenebulaeexpandandage.It tant role.Lenzunietal.(1989)haveanalyzedtheIRspectraof HAC initsvariousformscanaccountsuccessfullyforobserved environment. Witt&Boroson(1990)arguethatthespatial cm ssr , whiletheaverageofratio ofintegrated intensity observedinoursample ofPNsis1.9x10”ergs model thePNBD4-30°3639). TheaverageintegratedERE emission inexcessoverthe predictedatomiccontinuum compact andextended,carbon-richPN,whichwedonothave, hydrogen inthisregion.TheresultingmixtureofHAC,PAH believed tobeasymptoticgiantbranchstarswithcopious hydrocarbons asbyproducts.TheprecursorsofPNsare vigorously anddestructivelywithHACproducesfree gen. Ithasbeenshownbyseveralgroups(Sugai,Yoshida,& number ofcollisions.Asecondmechanismforthedestruction collision timeisshorterthanthedynamicaltime.Arelatively with turbulentvelocitiesaslow1kmsthegrain-grain through grain-graincollisions.Lenzunietal.claimthateven should bethatyoung,compact,carbon-richPNsshow 234 PNs,andtheirdatasuggestthattheaveragesizeofdust grains whichbecomerehydrogenatedandgaininluminescence notion thatitarisesfromthephotoluminescenceofHAC distribution ofEREinreflectionnebulaeisconsistentwiththe ments hasbeenproposedbyJones,Duley,&Williams(1990). between 550and850nm; 12of20donot(weareunableto determine towhatextentERE ispresentinthisclassofastron- tinuum emissionof20carbon-richandoxygen-richPNs to clusters, andfreePAHscouldthenemittheobservedUIBs; HAC grainsmaybeerodedthroughreactionswiththeionized the ionizationfrontpushesthroughneutral,dust-richparts amounts ofdustintheircircumstellarshells(Roche1989).As of HACgrainsisthroughchemicalerosionbyionizedhydro- PAH clustersandevenisolatedmoleculesafterasmall are mostlikelyresponsibleforEREpreferentially altered drasticallythroughvigorousinteractionwithitslocal the interstellarmedium,butthatitsopticalpropertiesare omical objects.Wefindthat sevenof20PNsshowEREas are necessarytotestthishypothesis. ERE intensity.Observationsofalargenumberboth and thedestructionoflargerHACgrainsdecreases the of aPN,assuggestedbyHuggins&Healy(1989),theresident It issuggestedthatamorphouscarbonfairlyubiquitousin destroyed asPNsevolve.Therearetwopossiblemechanismsby 2 We havecompletedaspectrophotometricsurveyofthecon- PN probablyrepresentyetanotherastrophysicalenviron- 6. CONCLUSIONS 597 1992ApJ. . .386. .587F + associated withit.Theactualliteraturecitationforeachmeasurement isthenfoundinTable6. key forthereferencesincludedinTables2,4,and5.Each valueorparameterlistedinTables2,4,and5hasareferencenumber iV(He)/iV(H and/AT(Hadoptedforeachnebula arelistedinthesixthandseventhcolumnsofTable5.6is abundance determinationsthanTorE(B—V)measurementsforeachnebula.Thisislargelybecauseprecisehelium abundances in measurements issmallformostofthePNsinoursample.Therefore,weexercisedlittlefreedomchoosingvalues ofTand extinction parameterc.TheassociatedE(B—V)wascomputedusingtheconversionc=1A1E(B—V)givenbySeaton (1979).The the factthatheliumabundanceratiosarelesstightly constrained thanTandE(B—V)isofslightimportance.Thevalues abundance ratiosthantotheelectrontemperatureandamountofreddening.Forpurposecomputingournebular continuum, as ofTable4.Again,thereareseveralmethodsthatusedtodeterminetheseabundanceratios;however, arefarfewer the sixthandseventhcolumnsofTable4. E(B —V)tocomputethemodelcontinuumforeachnebula.TheTandvaluesweadoptedplanetary arelistedin Double entriesinthecolumnofE(B—V)valuesareformc/E(BV),indicatingthatreferenceistoameasurement ofthe of TandE(B—V)foundforeachobject.Eachentryfollowedbyanumberindicatingthereferencetothatparticular measurement. environments issimilarinbothintegratedintensityandband PNs aregenerallyoflessimportance.Specifically,theshapeatomiccontinuumaPNismuchsensitive tothehelium that thesamephysicalprocesswhichproducesEREinreflec- composed primarilyofolddust,ismostnotable.Thisindicates nized sitesofdustformation,andreflectionnebulae,whichare tion nebulae,mostlikelythephotoluminescenceofhydro- gle.” ThesimilarityofEREobservedinPNs,whicharerecog- ERE-bright asisthepeculiarreflectionnebula“RedRectan- profile. WealsofindthePNNGC7027tobeexceptionally nebulae andfindthatEREinthesedifferentastrophysical over thesamewavelengthrangeisabout0.15. ERE intensitytotheatomiccontinuumintegrated genated amorphouscarbondustgrains,producesEREinPNs. T andE(B—V)valueslistedinTable4weredeterminedviaavarietyofdifferentmethods,yetthevariance bothsetsof light toexcitethisluminescence. This isplausiblebecausePNsareknowntocontainlarge while oxygen-richnebulaedonot.Thisisconsistentwiththe ratio inthesenseJhatmostcarbon-richnebulaeshowERE literature C/Oratiodeterminations.Wefindthatthepresence either carbon-richoroxygen-richbasedonacompilationof amounts ofdustandthehotcentralstarsprovidesufficientUV primarily silicatedust,asevidentbythe9.7¡imfeature, hypothesis thatEREisproducedbycarbon-richdust.IR 598 and carbon-richnebulaecontainprimarilydust,as spectra ofPNshaveshownthatoxygen-richnebulaecontain or absenceoftheEREbandinPNsiscorrelatedwithC/O e e e e e e + Table 5liststheliteraturevaluesforAT(He)/iV(HandiV(Hefoundeachobject.Theformatof isthesame This appendixsummarizestheinformationpertainingtoourcompilationofnebularparameters.Table4listsliterature values We comparetheEREobservedinPNstothatreflection We areabletoclassifymostofthePNinoursampleas © American Astronomical Society • Provided by the NASA Astrophysics Data System FURTON &WITT APPENDIX role. mical ObservatoriesfortheirsupportofIRAF.Wegratefully ous interactionwithitslocalenvironment,playsanimportant physical objectswheretheevolutionofHAC,throughvigor- leaving behindsmallerPAHclustersandfreePAHs.Thesoup which areresponsibleforEREdestroyedthroughgrain- University ofToledo. support forthisprojectthroughgrantAST-8814987toThe also wouldliketothanktheNationalScienceFoundationfor through SINBADIIImaintainedbyCDSinStrasbourg.We UIBs. InthisaspectPNsrepresentanotherclassofastro- grain collisionsorchemicalerosionbyionizedhydrogen, acknowledge valuableassistanceinourliteraturesearch providing supportandobservingtime;theentirestaffat of HAC,PAHclusters,andfreePAHsthenemitstheobserved that asaPNexpandsandages,thelargerHACdustgrains Central ComputerServicesoftheNationalOpticalAstrono- evident bytheso-calledunidentifiedinfraredemissionbands ed bythesamecarrier.WedetectEREinmost,butnotall, carbon-rich planetaries,butseveraloftheUIBsarepresentto ed bycompoundsrichincarbon. some extentinamuchlargerpopulationofPNs.Wesuggest between 3and12/zm.TheseUIBsarealmostcertainlyproduc- We wouldliketothankKittPeakNationalObservatoryfor Our resultssuggestthatEREandtheUIBsarenotproduc- 1992ApJ. . .386. .587F NGC 6210 NGC 4361 NGC 3242 NGC 2392 NGC 1535 IC 3568 NGC 2371 NGC40 IC 351 Object IC 1747 © American Astronomical Society 4 2.3 26 0.96 23 0.94 3 0.999 24 0.81 4 0.99 4 0.96 23 0.79 23 0.80 3 T (10K)Ref.E(B-V) 0.83 13 0.85 26 0.92 4 0.98 3 1.0 26 1.18 4 1.07 23 1.97 23 1.16 24 1.01 23 1.13 23 1.09 3 1.58 23 1.3 26 1.19 24 1.41 23 1.065 3 1.03 9 1.09 3 1.1 26 1.43 4 1.14 23 1.45 26 1.22 4 1.17 23 1.1 26 1.10 23 1.2 1 1.08 4 1.04 23 1.1 26 1.2 26 1.08 4 1.22 4 1.2 26 1.01 23 1.2 23 1.2 3 e 0.06 26 0.08 26 0.03 /0.02 23 0.106/0.07 4 0.0 /3 0.02 26 0.16/0.11 21 0.07 26 0.1 26 0.06 26 0.08 /0.0523 0.02 /0.014 0.33 /0.2221 0.0 /3 0.06 14 0.07 26 0.03 26 0.17 26 0.14 /0.109 0.10 /0.06823 0.13 /0.0921 0.0 /3 0.03 14 0.04 26 0.06 26 0.06 26 0.17 /0.1223 0.02 /0.014 0.43 /0.2921 0.08 26 0.14 26 0.13 /0.0923 0.58 /0.3921 0.29 /0.203 0.08 26 0.05 26 0.50 /0.3420 0.50 /0.3429 0.76 /0.5223 0.5 12 0.10 26 0.70 /0.4813 0.97 /0.6621 0.65 /0.443 0.33 1 0.44 26 0.38 26 0.19 /0.1323 0.25 /0.164 0.24 /0.1621 0.20 26 0.15 26 0.58 26 0.22 14 0.18 26 0.17 26 0.33 /0.2223 0.62 26 0.67 /0.4621 0.28 /0.193 0.39 14 0.14/0.09 3 0.72 26 0.29 /0.2023 0.27 /0.184 0.19 26 0.10 26 0.24 26 1.19 /0.8121 T andE(B—V) e TABLE 4 Provided bythe NASA Astrophysics Data System Rcf. 4 0.95 0.85 1.6 T(10K) E(B-V) 1.38 1.18 1.38 1.0 1.2 1.0 1.2 e Values adoptedinthiswork 0.08 0.0 0.02 0.05 0.05 0.1 0.40 0.35 0.6 0.19 1992ApJ. . .386. .587F J 900 BD +30°3639 NGC 7662 NGC 7027 NGC 7026 NGC 6826 NGC 6790 NGC 6720 NGC 6572 NGC 6543 Object © American Astronomical Society • Provided by the NASA Astrophysics Data System 0.86 0.95 0,8 1.44 1.06 1.15 0.94 0.95 0.91 0.9 1.21 1.18 0.9 0.91 1.31 1.36 1.35 1.32 1.42 1.28 1.06 1.3 1.25 1.40 1.60 1.43 1.24 1.37 0.75 1.4 0.89 0.96 0.69 0.70 0.85 0.76 0.80 0.81 0.90 0.80 T (10K) 0.82 0.83 1.12 1.22 1.03 1.07 1.28 1.7 1.13 1.2 1.1 1.06 1.06 1.11 1.0 1.05 1.05 1.07 1.12 1.03 1.21 1.05 c 24 23 23 26 23 4 26 3 24 4 23 23 26 6 5 5 24 4 3 23 23 26 5 24 23 23 26 23 3 23 26 23 23 26 24 3 3 4 23 23 26 27 24 4 8 7 23 23 26 5 5 26 5 5 5 23 23 3 Ref. 5 5 5 17 TABLE 4—Continued 0.18/0.12 0.71 /0.48 0.83 /0.56 0.57 0.48 0.46 /0.31 0.24 0.24 0.24 0.24 0.17/0.12 0.17 /0.12 0.40 /0.27 0.2 /0.14 0.15 0.11 0.11 1.78 /1.21 0.93 0.93 0.66 /0.45 0.99 /0.67 1.74 /1.18 0.83 0.54 0.03 /0.02 0.18/0.12 0.03 /0.02 0.02 0.0 0.82 /0.56 0.33 /0.22 1.24 /0.84 0.56 0.52 0.25 /0.17 0.29 /0.20 1.25 /0.85 1.22 /0.83 0.099 /0.067 0.29 /0.20 0.07 0.06 0.07 0.34 /0.23 0.31 /0.21 0.25 0.31 0.29 1.20 /0.82 0.12 /0.08 0.22 /0.15 0.22 /0.15 0.07 0.0 0.03 E(B -V) 1.25 /0.85 600 23 21 26 26 23 8 3 21 26 26 26 23 21 26 26 6 26 23 3 4 21 26 26 23 21 26 26 23 14 3 21 26 26 23 3 21 26 26 23 4 21 7 26 26 26 23 21 26 26 3 26 23 21 26 26 26 Ref. 3 4 0.92 1.4 0.92 1.2 0.95 1.3 T(10K) E(B-V) 0.85 1.1 1.05 1.2 e Values adoptedinthiswork 0.4 0.3 0.1 0.97 0.65 0.02 0.65 0.06 0.5 0.04 1992ApJ. . .386. .587F 1900 BD +30°3639 NGC 7662 NGC 7027 NGC 7026 NGC 6826 NGC 6790 NGC 6720 NGC 6572 NGC 6543 NGC 2392 NGC 2371 NGC 1535 NGC 3242 NGC 6210 NGC 4361 NGC 40 IC 3568 IC 1747 IC 351 Object © American Astronomical Society • Provided by the NASA Astrophysics Data System + 0.115 0.060 0.065 0.104 0.044 0.035 0.065 0.049 0.059 0.069 0.124 0.073 0.072 0.151 0.160 0.10 0.151 0.101 0.111 0.100 0.098 0.112 0.108 0.106 0.104 0.103 0.063 0.097 0.103 0.102 0.112 0.118 0.101 0.110 0.111 0.10 0.111 0.162 0.115 0.102 0.074 0.02 0.088 0.055 0.054 0.044 0.063 0.044 0.043 0.074 0.05 0.05 0.144 0.043 0.097 0.118 0.079 0.081 0.093 0.173 He/H 0.103 0.103 0.069 0.051 0.08 21 26 21 3 26 21 6 26 21 26 3 26 21 26 21 26 21 3 26 21 26 21 3 21 3 7 26 26 3 21 26 21 26 21 26 21 26 21 26 21 3 3 3 9 26 21 26 16 26 3 3 21 26 21 Ref. 16 3 16 16 16 16 3 12 16 1 13 Abundance Ratios + 0.054 0.038 0.037 0.018 0.064 0.063 0.042 0.059 0.044 0.045 0.003 0.038 0.044 0.020 0.015 0.001 0.002 0.017 0.001 0.002 0.060 0.014 0.022 0.024 0.001 0.0005 0.005 0.002 0.0 0.002 0.100 0.103 0.031 0.07 0.024 0.057 0.038 0.107 0.05 0.058 0.008 0.021 0.015 0.013 0.001 0.006 0.019 Ö.008 0.008 0.054 0.031 He7H TABLE 5 601 21 26 3 21 21 6 21 26 3 26 21 26 3 21 26 21 16 26 7 26 26 21 21 21 26 21 21 26 26 16 16 16 26 3 21 26 16 3 3 9 3 21 26 21 26 21 16 3 26 16 Ref. 16 +++ 0.065 0.1 0.06 0.07 0.11 0.1 0.13 0.1 0.1 0.1 0.15 0.01 0.07 0.06 0.04 0.12 0.05 0.10 0.15 He/H /F 0.05 Values adoptedinthiswork 0.035 0.02 0.05 0.04 0.015 0.002 0.005 0.02 0.001 0.005 0.004 0.1 0.03 0.04 0.05 0.02 0.01 0.004 0.01 0.05 1992ApJ. . .386. .587F .1983b,SolidStateComm.,48,105 .1989,ApJ,340,921 .1988,ApJ,326,164 .1987,ApJ,322,922 .1986,ApJ,308,314 .1985,ApJ,294,193 .1982,ApJ,253,167 .1980,ApJ,240,99 .1979,ApJ,227,863 .1983,ApJS,51,211 Garay, G.,Gathier, R.,&Rodriguez,L.F.1989,A&A,215,101 d’Hendecourt, L.B.,Leger,A.,Olofson, G.,&Schmidt,W.1986,A&A,170,91 Duley, W.1985,MNRAS,215,259 Dischler, B.,Bubenzer,A.,&Koidl,P. 1983a,Appl.Phys.Lett.,42,636 Derham, C.J.,Geake,J.E,&Walker,G.1964,Nature,203,134 Derham, C.J.,&Geake,J.E.1964,Nature,201,62 Danziger, I.J.,&Goad,L.E.1973,Mem.Soc.R.Sei.Liège,sér.6,5,153 Cohen, M.,Allamandola,L.J.,Tielens,A.G.Bregman,Simpson, Clegg, R.E.S.,Seaton,M.J.,Peimbert,M.,&Torres-Peimbert,S. 1983, Chlewicki, G.,&Laureijs,R.J.1987,inPolycyclicAromaticHydrocarbons Aller, L.H.,&Menzel,D.H.1945,ApJ,102,239 Aller, L.H.1984,PhysicsofThermalGaseousNebulae(Dordrecht:Reidel) Furton, D.G.,&Witt,A.N.1990,ApJ, 364,L45 French, H.B.1983,ApJ,273,214 Flower, D.R.1982,MNRAS,199,15P Feibelman, W.A.1982,AJ,87,555 Brown, R.L,&Mathews,W.G.1970,ApJ,160,939 Borghesi, A.,Bussoletti,E.,&Colangeli,L.1987,ApJ,314,422 Blanco, A.,Bussoletti,E.,&Colangeli,L.1988,ApJ,334,875 Bianchi, L.,&Grewing,M.1987,A&A,181,85 Benvenuti, P.,&Perinotto,M.1981,A&A,95,127 Barlow, M.J.1983,inIAUSymp.103,PlanetaryNebulae,ed.D.R.Flower Aller, L.H.,&Keyes,C.D.1987,ApJS,65,405 Aller, L.H.,Czyzak,S.J.,Buerger,E.G.,&Lee,P.1972,ApJ,172,361 Aller, L.H.,&Czyzak,S.J.1981,Proc.Nati.Acad.Sei.,78,5266 Barker, T.1978,ApJ,219,914 Allamandola, L.J.,Tielens,A.G.M.,&Barker,J.R.1985,ApJ,290,L25 Aitken, D.K.,&Roche,P.F.1982,MNRAS,200,217 602 J. P.,Whittborn,F.C,Wooden,D.,&Rank,D.1986,ApJ,302,737 (Dordrecht: Reidel),105 and Astrophysics,ed.A.Leger,L.d’Hendecourt,&N.Boccara(Dordrecht : MNRAS, 205,417 Reidel), 335 © American Astronomical Society • Provided by the NASA Astrophysics Data System Number 30 Zuckerman&Aller1986 29 VanBlerkom1971 28 Seaton1983 27 Rudyet.al.1991 26 Pottasch1984 25 Koppen1983 22 Kaler1981 21 Kaler1970 20 Jacoby,Quigley,& 24 Kaler1986b 23 Kaler1986a 9 Barker1989 7 Barker1987 4 Barker1978 2 Aller&Czyzak1981 8 Barker1988 6 Barker1986 5 Barker1979 3 Aller&Czyzak1983 17 Garay,Gathier& 19 Harrington&Feibelman1983 18 Harringtonet.al.1982 16 French1983 15 Flower1982 14 Feibelman1982 13 Clegget.al.1983 12 Bianchi&Grewing1987 11 Benvenuti&Perinotto1981 10 Barlow1983 1 Alleret.al.1972 Reference Africano 1987 Rodriguez 1989 References toTables2,4,and5 FURTON &WITT REFERENCES TABLE 6 .1990,ApJ,353,543 .1987,ApJ,320,L63 .1986b,ApJ,308,337 .1981,ApJ,249,201 .1986a,ApJ,308,322 Sakata, A.,Wada,S.,Tanabe,T.,&Onaka, T.1984,ApJ,287,L51 Rudy, R.J.,Rossano,G.S.,Erwin,P., &Puetter,R.C.1991,ApJ,368,468 Roche, P.F.,Aitken,D.K.,&Whitmore, B.1983,MNRAS,204,1017 Roche, P.F.,&Aitken,D.K.1986,MNRAS, 221,63 Jones, A.P.,Duley,W.W.,&Williams,D.1990,QJRAS,31,567 Jacoby, G.H,Quigley,R.J,&Africano,J.L.1987,PASP,99,672 Roche, P.F.1989,inIAUSymp. 131, PlanetaryNebulae,ed.S.Torres- Pwa, T.H.,Pottasch,S.R.,&Mo,J. E. 1986,A&A,164,184 Pottasch, S.R.1984,PlanetaryNebulae:AStudyoftheLateStagesStellar Papoular, R.,Reynaud,C,&Nenner,1.1991,A&A,247,215 Osterbrock, D.E.1989,AstrophysicsofGaseousNebulaeandActiveGalactic Guhathakurta, P.,&Tyson,J.A.1989,ApJ,346,773 Papoular, R.,Conrad,J.,Giuliano,M.,Kister,&Mille,G.1989,A&A, 217, Miller, J.S.,&Mathews,W.G.1972,ApJ,172,593 Lenzuni, P.,Natta,A.,&Panagia,N.1989,ApJ,345,306 Leger, A.,&Puget,J.L.1984,A&A,137,L5 Koppen, J.1983,A&A,122,95 Keyes, C.D.,Aller,L.H.,&Fiebelman,W.A.1990,PASP,102,59 Kaler, J.B.1970,ApJ,160,887 Hsu, W.L.1988,J.Vac.Sei.Technol.,A6,1803 Huggins, P.J.,&Healy,A.1989,ApJ,346,201 Harrington, J.P.,&Feibelman,W.A.1983,ApJ,265,258 Goebel, J.H.1987,inPolycyclicAromaticHydrocarbonsandAstrophysics, Gillet, F.C.,Low,G.,&Stein,W.A.1967,ApJ,149,L97 Gehrz, R.D.1989,inInterstellarDust,ed.L.J.Allamandola&A.G.M. Miller, J.S.1978,inIAUSymp.76,PlanetaryNebulae:Observations and Mattila, K.1979,A&A,78,253 Mathis, J.S.1990,ARA&A,28,37 Martin, W.1987,A&A,182,290 Harrington, J.P.,Seaton,M.J.,Adams,S.,&Lutz,H.1982,MNRAS,199, X X X X X X X X X X X Nuclei (MillValley:UniversityScienceBooks),209 Theory, ed.Y.Terzian(Dordrecht:Reidel),71 Peimbert (Dordrecht:Reidel),117 Evolution (Dordrecht:Reidel) 204 ed. A.Leger,L.d’Hendecourt,&N.Boccara(Dordrecht:Reidel),329 Tielens (Dordrecht:Kluwer),445 517 E(B-V) X X X X X X X X X X X X abundances C/O Helium X X X X X X X X X X X X X X Vol. 386 1992ApJ. . .386. .587F No. 2,1992 .1983,inIAUSymp.103,PlanetaryNebulae,ed.D.R.Flower .1974,MNRAS,187,73P Witt, A.N.1988,inDusttheUniverse,ed.M.E.Bailey&D.Williams Van Blerkom,D.J.1971,ApJ,166,343 Torres-Peimbert, S.,&Peimbert,M.1977,Rev.MexicanaAstron.Af.,2,181 Watanabe, I.,Hasegawa,S.,&Kurata,Y.1982,Jpn.J.Appl.Phys.,21,856 Sugai, H.,Yoshida,S.,&Toyoda,H.1989,Appl.Phys.Lett.,54,1413 Seaton, M.J.1960,Rep.Prog.Phys.,23,313 Schmidt, G.D.,Cohen,M.,&Margon,B.1980,ApJ,287,L51 Schild, R.E.1977,AJ,82,337 (Cambridge: CambridgeUniv.Press),1 (Dordrecht: Reidel),129 © American Astronomical Society EMISSION FROMDUSTINPLANETARYNEBULAE Provided bythe NASA Astrophysics Data System .1988,ApJ,325,837 Zuckerman, B.,&Aller,L.H.1986,ApJ,301,772 Woodward, C.E.,Pipher,J.L.,Shure,M.,Forrest,W.J.,&Sellgren,K.1989, Witt, A.N.,Schild,R.E,&Kraiman,J.B.1984,ApJ,281,708 Witt, A.N.,&Schild,R.E.1986,ApJS,62,839 Witt, A.N.,Stecher,T.P.,Boroson,A.,&Bohlin,R.C.1989,ApJ,336,L21 Witt, A.N.,&Malin,D.F.1989,ApJ,347,L25 Witt, A.N.,&Boroson,T.1990,ApJ,355,182 ApJ, 342,860 603