198 6MNRAS.221. . .63R Mon. Not. R. Astr. Soc. (1986

198 6MNRAS.221. . .63R investigating planetarynebulae(pn).Theshapeof the10-/xmcontinuumemissioncangive wavelength regionrendersvaluabledataonelement abundances.Previousobservationshave important informationonthedustcontentof nebular shell,whilstlineemissioninthis Spectroscopy inthe8-13/xmatmosphericwindowhasprovedtobeaveryusefultool 1 Introduction verified byopticalandUVspectroscopy(Seaton1983); gasphaseC/Oratioscalculatedusingthe shown thatmanyplanetarynebulaecanbeclassed aseitheroxygenrichorcarbonby AR). Thisprocedureforclassifyingplanetarynebulae accordingtotheirC/Oratiohasbeen matching thedustemissionspectrawithemissivitycurves derivedfromO-rich(silicate)orC-rich London WC1E6BT. * Presentaddress:Department ofPhysicsandAstronomy,UniversityCollegeLondon, GowerStreet, latter techniquesareinexcellentagreementwiththeassignments oftheplanetarynebulaeasCor all planetarynebulaeinthis way.The10-/xmspectraofsomenebulaearedominated byemission O richonthebasisoftheir 8-13/xmspectra.However,CorOdominancecannot bederivedfor (silicon carbide)circumstellarshells(seeAitken&Roche 1982andreferencestherein;hereafter © Royal Astronomical Society • Provided by theNASA Astrophysics Data System The infraredspectralpropertiesofplanetarynebulae Mon. Not.R.astr.Soc.(1986)221,63-76 NSW 2121,Australia Parkville, Vic3052,Australia Accepted 1986January31.Received15;inoriginalform1985December2 David K.AitkenPhysics(RAAF)Department,UniversityofMelbourne, Patrick F.Roche*Ànglo-AustralianObservatory,POBox296,Epping, Those planetarieswithstrongcontinuumemissionareclassifiedaccordingtotheir dust emissionspectraasoxygenorcarbonrich.ThespectrumofNGC6302shows Summary. Spectrabetween8-13/xmof15planetarynebulaearepresented. number ofavailable8-13-/xmspectrato49.Thefar-infraredpropertiesplanet- by thelow-resolutionspectrometeronIRAS.Thedatapresentedherebring a verystrongemissionlineat7.65/xm,confirmingtherealityoffeatureseen that thedifferentgrainmaterialsseenin8-13/xmspectraalsoshowdiffer- dence isclearlyseenincolour-colourplotsconstructedfromtheIRASdata,so differences inthedustchemistryfoundfrom8-13-/xmspectra.Thisdepen- ary nebulae(givenbytheIRASPointSourceCatalog)arefoundtoreflect ences inthedustemissivityatlongerwavelengths. 198 6MNRAS.221. . .63R in thenarrowunidentifiedfeaturesat11.25and8.65/xm,togetherwithlong-wavelengthwing chemistry thatproducesthemisalsounknown,althoughBarlow(1983)hasarguedthe of the7.7¡mmfeature.Becauseoriginandexcitationthesefeaturesisnotyetclear,dust Name P.K.Telescope planetary nebulaeshowingthefeaturesarecarbonrichwithC/O>2andDuley&Williams indeed, theremaynotbeasignificantpopulationofwarmgrains(Roche,Aitken&Whitmore CN 3-1 aromatic compounds.Insomeotherplanetaries,the10-/xmemissionfromdustisveryweak,and NGC 6741 (1981) andLeger&Puget(1984)arguethattheemissionfeaturesarisefromcarbon-based M 1-12 M 2-56 NGC 6886 Table 1.Logofobservations. NGC 6884 NGC 6881 He 2-459 Hu 2-1 Hb 6 Hb 4 M 1-6 1983); intheseobjects,too,thedustchemistrycannotbedetermined.Nevertheless,of31 NGC 6302 19W32 IC 5217 64 p,F,RocheandD.K.Aitken with opticallythicksilicateemission, (c)planetarynebulaewhosespectraaredominatedbythefamily ofunidentified Figure 1.8-13-jüimspectraof(a) planetarynebulaeshowingthe11.2-/umSiCfeature,(b)bipolar planetarynebulae emission featuresand(d)planetary nebulaewithweakcontinua.Notethattheobjects weakcontinuaare plotted onalinearordinatescale whilsttheothersareplottedlogarithmically. © Royal Astronomical Society • Provided by theNASA Astrophysics Data System "e ^ 1Q-18 ^ r £ o 10-16 - 10- 211 235 359 349 118 100 68 60 07 03 74 51 38 33 82 +09.1 -02.1 +01.1 -05.1 +07.1 +02.1 -02.1 -07.2 +12.1 +02.1 -03.1 -03.1 +08.1 +01.1 +01.3 VMIIf' AAT UH UKIRT UKIRT UKIRT UKIRT UKIRT UKIRT UH UKIRT UH m UKIRT UKIRT IRTF 10 12 À(/um) (a) (arcsec) 5.9 4.3 4.3 4.3 4.3 7.6 7.6 4.3 4.3 4.3 7.6 7.6 4.2 4.3 3.8 MI-6 (x80) M1 -12 Hu 2-1 (x4) (+5) l4 1982 1982 1982 1982 1982 1982 1982 1982 1981 1982 1985 1984 1983 1982 1982 Date July July June June June June June July June Nov May July Dec June Sep 1 10 10 10 size (arcsec) Nebula 5X22 5X8 <10 <10 44 ¥ 6 6 6 4 4 5 6 3 7 3 H y}tv-v 10 ®®®oeeeee*oe,®®ceee et000jj À(jum) (b) Distance 5 (kpc) 0.65 3.2 2.8 3.6 3.1 2.0 1.4 3 1.4 1.45 1.3 1.6 3 12 19W32 M2-56 14 dia. (mpc) Nebula 120X190 70X320 <68 139 60 41 42 70 48 47 81 93 33 19 Infrared spectroscopy of planetary nebulae 65

A(/Um) (d) À(/um) Figure 1 - continued planetary nebulae for which ground-based 8-13-^tm spectra are available, seven can be classed as O rich, six as C rich, 10 are dominated by the narrow emission features and the remaining eight have continua too weak to permit classification in this way; a further four bipolar nebulae show evidence of silicate dust in emission and/or absorption in their 8-13-/xm spectra implying grain formation in an O-rich environment (see AR). Where the signal-to-noise ratio is adequate, data from the low-resolution spectrometer on IRAS confirm the presence of silicate or silicon carbide grains, notwithstanding the contradictory statement by Pottasch et al. (1984). Here we report spectroscopic observations at 8—13 pxn of a further 14 planetary nebulae, and investigate the far- infrared properties of planetary nebulae as a function of their dust chemistry. 3

© Royal Astronomical Society • Provided by the NASA Astrophysics Data System 198 6MNRAS.221. . .63R 2 The spectrapresentedherewereobtainedwiththeUCLliquid-heliumcooledarrayspectrometer Telescope (UH),the3.9-mAnglo-Australianor3.0-mNASAInfrared Facility (IRTF).TheobservationaldetailsaregiveninTable1,togetherwithnebularsizesand 2 Observations The planetarynebulaeconsideredherearemostlycompact(<10arcsec)andmostwereselected 3 Objects on the3.8-mUnitedKingdomInfraredTelescope(UKIRT),2.2-mUniversityofHawaii A2—0.09 ¡Jimwasemployed.ThespectraareshowninFig.1. accurate to15percent.Thespectralresolutionwas0.23¡¿mexceptforHu2-1,where distances fromAcker(1978).Fluxcalibrationiswithrespecttostandardstars,andconsidered from thephotometricsurveyofCohen&Barlow(1974).However,integrated8-13-/xmfluxes measured inourspectrometerbeamare,withonlyafewexceptions,considerablylowerthan 66 P.F.RocheandD.K.Aitken those measuredbyCohen&Barlowandthe12-^cmfluxesgiveninIRASPointSource effect frommeasuringessentiallyamonochromaticfluxinbroad-bandfilter. likely thatforthosenebulaeinwhichthefluxisdominatedbyemissionlines,theresome them byreferencetotheirdustemission. way, wecanclasstheplanetariesasrequiringemissionfrom: fitting withvariousgraincomponentsderivedfromobservationsofastronomicalsources.Inthis cedures asAR.Axminimizationisemployedtodeterminethegoodnessoffitobtainedbycurve Catalog. Alargepartofthediscrepancycanprobablybeattributedtobeamsizeeffects,butitis the Msupergiant¡jlCep,orTrapeziumregioninOrion; (iii) asmoothgraincomponent,withQ=1/2; (ii) ‘siliconcarbide’grains,usingtheemissivitycurvefromcarbonstarYTau; We havefittedthecontinuumspectraofremainingplanetariesfollowingsamepro- Of theobjectsinpresentsample,fivehaveveryweakcontinuaandweareunabletoclassify (i) ‘silicate’grains,wheretheemissivitycurveusedisderivedfromcircumstellarshellof © Royal Astronomical Society • Provided by theNASA Astrophysics Data System Table 2.Best-fitparameters:fraction,temperature. M 2-56 M 1-6 Name H 1-12 notesî 1.opticaldepth usingemissivitycurvederivedfromtheTrapezium. NGC 6302 NGC 6741 He 2-459 Hu 2-1 19W32 NGC 6886 NGC 6881 M 2. " " ”¿j.cep. Graphite Black body 1.00/520 1.00/320 1.00/350 0.79/175 0.83/175 0.76/210 1.00/390 0.66/154 0.29/128 0.60/150 0.22/132 0.70/150 Orion Bax 0.40/550 O.34/490 0.71/295 0.30/250 0.78/355 0.86 1.26 '9.7 SiC 0.21/110 0.17/200 0.24/140 0.90 0.57 '9.7 2 z X/Nf X/N 1.9 1.0 1.7 2.8 0.8 0.8 2.6 0.4 1.5 2.1 5.6 2.4 f 43 43 43 43 51 37 39 37 36 21 21 22 198 6MNRAS.221. . .63R 2 spectrum oftheSEionizationfrontinOrionnebula,oracombinationthese. characteristics; thebestfitstospectraaresummarizedinTable2. M 1-12hasbeenlesswellstudied,butclassedasavery-low-excitationobject(VLE)by Ml-6 hasbeendiscussedbyKwok&Purton(1983),whoproposethatitisaveryyoungplanetary. 3.1 M1-6and1-12 Sanduleak &Stephenson(1972).Thesetwoplanetarioshavequitesimilar8-13-/xmspectra.Both dust. Thespectralshapesofthedustemissionarewellfittedbycarbon-starexcess-emissivity show strongemissioninthe12.8-)um[Nen]fine-structureline,andthermalfromwarm infer thatthegrainsinthesetwonebulaearecarbonrich. curve, andtheyresemblethearchetypalC-richplanetarynebulaIC418(Willneretal.1979).We 3.2 Hu2-1 Although thisplanetarynebulahasastrong,well-measuredcontinuum,thenatureof emitting dustisnotclear.Fitstothealmostfeatureless,butslightlyconvexcontinuumare oxygen-rich dustonthebasisofdust-emissionfeatures.However,fromsampleplanetary ambiguous, andthevalueof^isacceptablewheneithersilicateorsilicon-carbideemissivity nebulae thatcanbeclassified,thoserequireaSiCcomponentinvariablylarge are wellfittedbythesilicatecurvewithonlyasmallcontributionfromsmoothcomponent.Itis curves areusedwithasmoothgraincomponent.Weunabletodistinguishbetweencarbon-or The bipolarnebula19W32isaradio-discoveredplanetary(Isaacman,Wouterloot& therefore likelythatthedustemissioninHu2-1isfromC-richgrains,and,fact,thisplanetary amount ofthefeaturelessgraincomponentinfits,whereasoxygen-richplanetarynebulae has beenfoundtobecarbonrichbyLutz(1981). 3.3 19W32andM2-56 The spectralshapesarelikethoseofthewell-known bipolar nebulaeM2-9andMz3(AR) Habing 1980)atadistanceofabout3kpctowardstheGalacticCentre(Kohoutek1983),whilst The fitstothe10-/xmspectrumrequireasilicateabsorption ofopticaldepthrintherange1.2- Trapezium inOrion(Forest,Gillett&Stein1975) than thatobtainedfromthecircumstellar M 2-56hasbeendiscussedbyCohen&Barlow(1974)andAR.Thespectraofthesetwoobjects spectrum ofM2-56isconsistentwith,butmuchhigher qualitythan,thatpresentedbyAR. are dominatedbystrongwarmcontinua,andnoionic-emissionlinesweredetected.The A goodfittothespectrum ofM2-56isobtainedusingthe/xCepcurvewhich yieldsr7—0.6, envelope ofpuCep,whichisrepresentativetheinterstellar extinction(Roche&Aitken1984). better fitstothe10-/xmspectrumareobtainedusing silicate-emissivitycurvederivedfromthe corresponding toA—10mag. TheIRASPointSourceCatalogshowslittleevidence ofacooldust optical data,sothatasubstantial amountofsilicateabsorptionmustbeproduced localtothestar. are wellfittedbywarmdustundergoingabsorption bysilicategrains.19W32suffersahigh component suffering—10mag ofvisualextinction.Cohen&Kuhi(1977)derive anextinctionof component, andtheinfrared spectrumcanbefittedreasonablybyawarm greybodydust interstellar extinctionandKohoutek(1982)gives^4^=9 magfromtheopticalspectrum.Much 1.5, correspondingtoA^20mag.Thisvalueisconsiderably morethanthatderivedfromthe 9 7 9 K v We nowdiscusstheplanetarynebulaeinthissampleaccordingtotheir8-13-/u,mspectral (iv) thenarrowunidentifiedemissionfeaturesat7.7,8.65and11.25/¿m,derivedfrom © Royal Astronomical Society • Provided by theNASA Astrophysics Data System Infrared spectroscopyofplanetarynebulae67 198 6MNRAS.221. . .63R 2 A^5 magfromopticaldata,sothatheretoothereissomelocalobscuration.Thecharacteristic The fivenebulaegroupedherehavespectrathatshowthenarrowdustemissionfeaturesat11.25 3.4 NGC6741,6881,He2-459,6886and6302 spectral shapesofthesenebulaeprobablyarise,atleastinpart,fromtemperaturegradientsa objects, butallfivespectracanbefittedadequatelybythenarrowemissionfeaturesandasmooth excitation withstrongemissioninthe[Nen]linetoNGC6302whichisofverymuchhigher required andthatthesebipolarnebulaehaveO-richdust. ring ordiscofdustgirdlingthehotcentralstars.Fromfits,weconcludethatsilicategrainsare The fluxmeasuredinour3.8-arcsecbeamis1/12thatdeterminedbyIRAS.However,Lester& with thatshownhere;theidentificationoffeatureseennear7.7/xminIRASspectrumas excitation. Theequivalentwidthofthenarrowfeaturesvariesconsiderablyamongstthese and 8.65/xmprominently.TheycoverawiderangeofparametersfromHe2-459,whichislow the 7.65-jnm[Nevi]lineisconfirmedtogetherwithotherionicanddustemissionfeatures. grain component.The7.5-24-/umspectrumofNGC6302hasbeenmeasuredbytheIRASLow- Dinerstein (1984)haveshownthatthelO-^tmemissionisextendedoveraregionofabout12x8 Resolution Spectrometer(Pottaschetal.1985)andtheoverallshapeofspectrumagreeswell arcsec inthecoreofnebula. v 68 P.F.RocheandD.K.Aitken The spectraofthesefiveplanetarynebulaearedominatedbyfine-structurelineemission.Their 3.5 HB4,Hb6,CN3-1,NGC6884andIC5217 continua areveryweak,andwecannotclassifythembydustemissioncharacteristics.Hb6does inadequate toinvestigatethedustemissionindetail. have adetectablecontinuumthatrisestowardslongwavelengths,butthesignal-tonoiseratiois free fluxobtainedbyextrapolationfromtheradiomeasurements(Karsas&Latter1962),and planetary nebulaehave10-fxmcontinuawhichareatmostonlyafewtimestheestimatedfree- 4, Hb6andCN3-1havediameters<0.05pc,adoptingthedistancesofAcker(1978).All presumably young,nebulae;itmaybethatthereislittledustclosetothecentralstars.Neverthe- making someallowanceforbeam-sizeeffects.Therecanbelittlewarmdustinthesecompactand, less, theinfraredluminositiesderivedfromIRASdataarestillconsistentwithdustheatingby there islittledirectheatingfromthestellarcontinuum. Lya radiationwithintheionizedregion.WithexceptionofCn3-1,alow-excitationobject, The 8-13-/xmspectrapresentedherecoverarangeof propertiessimilartopreviouslypublished 4 Discussion type Iobjects(seePeimbert&Torres-Piembert1983). ThetypeInebulaegenerallyshowdefinite examples. However,someofthenebulaewarrantfurther discussion. NGC 6741havebeenclassed astypeIorpossibleplanetarynebulae(Peimbert &Torres bipolar structureandareenhancedintheirnitrogen and heliumabundancesthroughdredge-up spectroscopically at10/xm, sixobjects,NGC5315,Hb5,Mz3,M2-9(AR)and NGC6302and of processedmaterial.Oftheapproximately40planetary nebulaethathavebeenstudied that 19W32,andperhapsM 2-56,mayalsobeclassedastypeIplanetaries. emission fromtheWC6central starmaketheHe-abundancedeterminationdifficult. Itislikely Peimbert 1983),although this classificationforNGC5315maybesuspectasthe stronghelium The planetarynebulaeinthissamplewithweakcontinuaareallfairlycompact,andindeed,Hb Planetaries producedbythemoremassiveprogenitor stars(^2.4M)havebeenclassedas 0 © Royal Astronomical Society • Provided by theNASA Astrophysics Data System 198 6MNRAS.221. . .63R presumably closetotheexcitingstars,althoughcoolerdustmayextendsomedistanceaway.It much morelikethoseofthetypicalplanetarynebula.ThewarmdustinM2-9andMz3is for NGC6741intheir11arcsecbeam,agreementwiththeIRASdatum.Mz3andM2-9areof seems likelythatthedustinothertypeInebulaeismoremixedwithgas;forexample, nebulae, Hb5,NGC6741and6302,liesinthedetailsoftheirdustemissionspectra.The evolved thantheothertypeIplanetarynebulaediscussedhere.Whethertheywillevolveinto NGC 6302wasdiscussedaboveandCohen&Barlow(1974)measuredsomefourtimesourflux (AR), whilstthefluxdistributionsofremainingfoursourcesimplymuchcoolerdustand narrow unidentifiedfeatureshavenotbeendetectedintheformer,whilsttheyarestrong high-excitation objectslikeNGC6302or6741isunknown. low excitation,havingoftenbeenconsideredtobeproto-planetarynebulae,andareprobablyless 19W32) arecharacterizedbywarmdustemissionandshowsubstantialabsorptionopticaldepths silicate grainstofittheir8-13-/¿mspectracanbeclassedasoxygenrich,suggestingthattheyare different excitationconditionsmayalsocontribute. latter grouping.Thisprobablyresultsfromdifferencesinthegrainchemistry,althoughvery particular interestbecausethisgivesinformationontheconditionsinwhichemissionfeatures Analysis oftheultravioletandopticalspectrasuggeststhatsomeothertypeIplanetary 1983; Peimbert&Torres-Piembert1983)and,asnotedbyBarlow(1983),thosewhichrequire are produced.Recentworkontheunidentifiedemissionfeaturesproposedthattheyarisefrom nebulae mayalsohaveC/0<1,e.g.Aller(1983)givesC/O—0.5forNGC6741. produced bystarswithinitialmassesgreaterthanabout5M(seeRenzini&Voli1981). Torres-Piembert &Pena(1982)giveC/O—2.5forNGC5315.However,thesedeterminationsof small (—10Â)grainscomposedprincipallyofcarbon(e.g.Leger&Puget1984).Barlow(1983) has arguedthattheplanetariesshownarrowunidentifiedemissionfeaturesarecarbon who onlyconsideredthedoublyandtriplyionizedstagesofcarbonoxygenintheiranalysis the carbontooxygenratioareratheruncertain.ThiswasacknowledgedbyBarraietal.(1982) rich withC/0>2.ThiswouldappearnottobethecaseforNGC6302and6741,although the abundancesinNGC6302.Inviewofextremelyhigh-excitationlinesseeninfrared spectrum ofNGC6302(thestrongline[Nevi]requiresalargefluxenergeticphotons Cohen etal.(1986)haveshownagoodcorrelationbetween thestrengthofnarrowemission chemical composition,andinparticulartheC/Oratio ofthenebulargas(seeSeaton1983). amounts ofoxygenandcarbonareinveryhighionization stateswhichwerenotcorrectly carbon richwithafulleranalysis.Itiswellestablished thatthegrainchemistryreflects accounted forintheanalysisofopticalandultraviolet data;NGC6302maywellturnouttobe >126 eV),theC/Oratioof0.26(Barraietal.1982)maybesuspect.Itisprobablethatlarge carbon rich. features inthe4-8-/xmregionandC/Oratio,so thatifthedustinNGC6302wasindeed width thaninotherplanetaries observedtodate(Aitken&Roche1984).More observations are anomalously strongwhen compared tothatat11.25pm\italsohasamuchhigher equivalent ejected fromtheplanetarynebulacentralstar,itstrongly suggeststhatthegasinnebulais 0 required toinvestigatethis further,butitispossiblyrelatedtoabundanceanomalies inthis paper. object. The8-13-/¿mspectral propertiesofNGC6302willbediscussedinmore detail inafuture These objectsshowtwodistincttypesof8-13-^tnispectra.Mz3andM2-9(and2-56 The bipolarplanetarynebulaediscussedherehaveenhancednitrogenabundances(seeAller A furtherdifferencebetweenMz3,M2-9,2-56and19W32theotherbipolarplanetary The C/Oratiointheplanetarynebulaewhichshownarrowunidentifiedfeaturesisof The spectrumofNGC6302isremarkableinthatthe dustemissionfeatureat8.65pmis © Royal Astronomical Society • Provided by theNASA Astrophysics Data System Infrared spectroscopyofplanetarynebulae69 198 6MNRAS.221. . .63R + + 5 -3 5 Lineemission 70 P.F.RocheandD.K.Aitken The intensitiesofthefine-structurelines[Am],[Su]and[Nil]arelistedinTable3. relative strengthsofthethreelinesareinagreementwithknownexcitationnebulae. from thewholenebulae. Because mostofthesourcesarelargerthanspectrometeraperturesused,lineintensities effects arequiteuncertainwithoutknowledgeofthespatialdistributionandionizationstructure quoted arelessthanthetotallinefluxesfromthoseplanetarynebulae.Correctionsforbeam-size of thenebulae,sowehavenotattemptedtocorrectourobservedlineintensitiesthoseexpected S andNe.WehaveemployedthecollisionstrengthsfromcompilationbyMendoza smaller thanthebeamsusedinthiswork.ForthesewecancalculateionicfractionsofA, In calculatingtheionicfractions,wehaveassumedthatnebulardensitiesarelessthan critical densitiesofthefine-structurelevels(i.e.^10 cm);thisisareasonableassumptionfor most oftheplanetarynebulae,butsomemore compactobjectsmayhavedensities (1983), andhavecalculatedtheionicabundancesusingradiomeasurementsgiveninTable4. IRAS PointSourceCatalog, andalltheobjectshavebeendetectedat1225 /im, andmostat The InfraredAstronomicalSatellite(IRAS)hasmeasured thefar-infrared(12-100-/im)fluxofall 6 ComparisonwithIRASresults approaching thatvalue. NGC 7027whichwasmissedbythesurvey.Wehave retrievedthephotometricdatafrom of theplanetarynebulaeforwhich8-13-/xmspectra areavailable,withtheexceptionof with thefluxin8-13-¿um spectra.Theexceptionsarethosenebulaelarger thanthebeam 60 and100¿um.Inmostsources, the12-jumphotometricpointmeasuredbyIRAS agreeswell employed forthespectroscopic observations,andthosewithveryweakcontinua. Inthelatter sources, the12-¿umIRAS band isoftencontaminatedbystronglineemissionand theeffective Nevertheless, therearesixsourcesforwhichthespatialextentsquotedinliterature © Royal Astronomical Society • Provided by theNASA Astrophysics Data System -172 192 Table 3.Observedlineintensities. Name NGC 6886 NGC 6741 M 1-12 M 1-6 NGC 6884 NGC 6881 He 2-459 Hu 2-1 Hb6 Hb4 CN 3-1 NGC 6302 Notes IC 5217 2 Observed8-13pmfluxinthespectrometerbeamincludingemissionlines. 4 Lineintensitiesareaccurateto20percent,exceptthe[NeVI]line 3 [NeVI]intensityinNGC63025x10“Wcm“pm". 1 Lineintensitiesin10~Wem”?fluxWcnr*uif. which isprobablyaccurateonlyto50percent. 19 <2.0 <1.0 [AIII] <2.0 <0.8 <2.0 <1.0 0.5: 2.3 5.9 1.5 5.6 9.6 17 15 25 47 27 25 22 <1.0 [SIV] <2.0 <0.8 0.9: 1.4: 9.1 13 13 26 20 [Nell] <1.5 <1.0 <1.5 <2.0 <2.0 0.8: 2.3: 6.0 8.0 Average* 8-13¿im flux 100 18 17 70 29 37 30 36 5 6 8 9 8 flux IRAS IZtm 670 18 25 17 56 25 58 95 43 28 46 38 39 (3) Type X X X C? 198 6MNRAS.221. . .63R n wavelength isshiftedtowards15becauseofthesteeplyrisingfluxtolongwavelengths. functions forgrainemissivitiesthatfollowß=l//l^=0,1and2arealsoplotted.Itisclear nebulae areidentifiedaccordingtotheirdustcharacteristicsinthe8-13-pjnregion.Planck there isaseparationinthefar-infraredpropertiesofplanetarynebulaeaccordingtotheir 8-13-^m characteristics.Thiseffectismostobviousonthe12/25:25/60-^mplot,butpresentto greybody linewhilstthebipolarnebulaewithevidenceofconsiderableopticaldepthsintheir cluster neartheft=1/2line,thosedominatedbyunidentifiedemissionfeatureslie that showsilicatesinemission,tendtoliebetweenthe/?=1andft=2lines,thosewithSiCemission some extentinthe25/60:60/100-jumfigure.Informerdiagram,O-richplanetarynebulae, emitting dust.Theplanetarynebulaewithweakcontinualieabovethegreybodyline,butthisis presumably aconsequenceofcontaminationbylineemissionandthelow25/60-/umcolour silicate emissionshowagreatdealofscatter,probablyduetolargerangetemperaturesinthe band wavelengthwillbenearer15than12pm. temperature; mostofthefluxinIRAS12-pmband ariseslongwardsof13pmandtheeffective contribute substantiallytotheIRASdata.Contamination fromemissionstructureislargestat temperature ofthecooldustcontinuum,butemission linesanddustemissionfeaturescanalso bands. Themagnitudesof thesecomponentsarequiteuncertain,butthey are likelytobe Houck &McCarthy1981) in somenebulaeandacontributionfromionic-lineemission inallfour bands fromsilicateorsiliconcarbidegrainsthose producingthenarrowemissionfeatures. so includesthestrongionicemissionlinesof [Am],[Siv],[Nv]andtheemission planetary whichForrestet al. (1981)foundtodisplaythebroad25-pm‘carbon star’ featureand considerably smallerthan the effectsat12pm.Indeed,IRASmeasurements ofIC418,a silicate bandandthebroad emissionfeaturesnear25pmseeninsomecarbon stars(Forrest, Similarly, therewillbeacontributiontothe25-pmIRAS fluxesfromtheemissionin19-pm 12 pmwherethedustcontinuumemissionisweakest. The 12-^tmbandhasaFWHMof7pmand The IRASdataarepresentedintheformofcolour-colourplotsFig.2,whereplanetary The positionsoftheplanetariesincolour-colour plotsdependonthespectralshapeand © Royal Astronomical Society • Provided by theNASA Astrophysics Data System Table 4.Ionicfractions. NGC 6884 NGC 6881 Hu 2-1 M 1-12 M 1-6 NGC 6302 He 2-459 NGC 6886 CN 3-1 NGC 6741 Hb 6 Hb 4 Name Nebulae largerthantheaperturearebracketted. IC 5217 1 Milne&Aller(1975) Ionic fractionsrelativetoadoptedabundancesofA- (a) Radiofluxat10GHz. 3 Isaacman(1984) (<0, (0, (0, (0. (0, (0, <0. <0, <0, <0. <0, 0, 0, 06 07 03) 06: ) 07 11) 06) 19) 13 14 10 24) 34 (<0.01) (0.04) (0.11) (0.01) (0.25) (0.16) <0.01 <0.02 0.02 0.10 0.10 0.01 0.16 Infrared spectroscopyofplanetarynebulae (<0.02) 2 Purtonetal(1982) 4 Kwok&Purton(1983) (0.07) (0.88) -1 (0.03) (0.01:) (0.05:) <0.03 <0.06 Ne 0.64 0.27 0.04 1.10 0.51 We 5GHz 3500 (mJy) 120 170 105 105 180 220 240 45 86 50 86 55 6.80, S=7.00,Ne-8.05 1 1 1 1 1 1 1 2a 4 2a 3 2a Ref 3 71 198 6MNRAS.221. . .63R 72 P.F.RocheandD.K.Aitken bottom right-handcorner. 8-13 pun.Theplanetarynebulae arecodedaccordingtothecharacteristicsoftheirdustemission. 0=oxygen-rich, Figure 2.Colour-colourplotsof theIRASfluxesofplanetarynebulaethathavebeenmeasured spectroscopicallyat emissivity aredrawnacrosstheplots forn=0,1,2andtemperaturesareindicated,Atypicalerror barisshowninthe silicate; •=bipolarsilicates;+= carbon-rich,SiC;★=unidentifiedfeatures;x=weakcontinuum. Linesofconstant © Royal Astronomical Society • Provided by theNASA Astrophysics Data System Log(60/100) (b) Log (25/60) (a) 198 6MNRAS.221. . .63R 2 18 nebulae inhissampledoesnotappeartoshowanysignificantexcessemissionat25pm(see Moseley (1980)foundtohaveanarrowerfar-infraredfluxdistributionthantheotherplanetary fluxes, andthelargesteffectisonO-richplanetarynebulathatshow9.1-pmsilicate Pottasch etal1984). feature. TheseparationbetweenthesilicateandSiCplanetarynebulaeisenhancedoverFig.2, spectra toderiveabetterestimateofthetrue12-^tmcontinuumlevel.Thisreduces12-/xm were appliedtothelatterselection.Also,contributionsfromlineemissionin13-15-¿cm results, atleastinpart,becausenocorrectionsforemissionfeaturesorawarmdustcomponent but theincreasedseparationbetweenthesetwogroupsandremainingplanetarynebulae structure isquiteuncertain. region areunknownand,moreseriously,thecontaminationof25-pmbandforemission that theemittingdusthasanemissivityfallswithincreasingwavelength;contributionsfrom warmer dustwouldpushthepointsaboveline.Thefar-infraredemissivityofsilicatematerials planetary nebulaenearthen=2locusinFig.2(a)iskeepingwithbulkofinfrared follows aQ=l/2lawbeyond20pm(e.g.Draine&Lee1984),sothatthelocationofO-rich with SiCemission.Ontheotherhand,emissivityofamorphouscarbongoesasQ=\/À(e.g. calculations byDraine&Lee(1984)showamorecomplexemissivitystructure.However,neither the 1/Àline.Thefar-infraredemissivityofgraphiteisoftentakentofallas1/2,althoughrecent emission comingfromsilicategrains.TheC-richplanetarynebulaeshowingSiClienear of thesepossibleemissivityfunctionsprovideaclosefittotheobserveddataforplanetarynebulae carbon grains.Becauseemissionfromgrainsoverarangeoftemperatureswillbroadenthe Borghesi, Bussoletti&Colangeli1985andreferencestherein).Thepositionoftheplanetary nebulae showingSiCemissionsuggeststhatthecontinuumisproducedbyamorphous nebulae isrequiredtoinvestigatetheemissivity. observed spectrum,betterknowledgeofthetemperaturestructuredustinplanetary pm fluxratioofalltheplanetarynebulaeinsampleconsideredhere.Fromitsposition planetary nebulaeasclearlythe12/25:25/60-/u,mplot.Inprinciple,longerwavelength quality 8-13-^tmspectrumshouldshowsilicateemission. be asstronglyaffectedbyemissionlinesanddustfeatures,theysamplethefallingsideof points shouldgiveabetterestimateofthegrainemissivitylaws,becausedataareunlikelyto various dustemissionfeatures(silicates,SiC,etc.)between 8and30pm,whilstFig.2(b)traces Planck distribution.TheclearseparationseeninFig. 2(a) ispresumablylargelyproducedbythe the longerwavelengthdustemissivity.Theerrorson the 60-and100-/LtmIRASfluxesarelarger 12/25:25/60-/zm colour-colourdiagram,wepredictthatitisanO-richplanetary,andahigher than at12and25/xmfortheplanetarynebulaeinthis sample,andthismaybluranyseparation carbide features(seeFig. 3) andlietotherightof«=0lineinFig.2.It is thereforenot emissivity, aplotofthe12/25against25/60-pmcolours ismoreusefulasadiagnosticoftheC/O planetary nebulaelieintherange«=1.5-2,while O-richobjectsshowmorescatterandlie ratio inplanetarynebulae. nearer to«=1-1.5.AlthoughFig.2(b)maybebetter forinvestigatingthefar-infrareddust according todustchemistryinFig.2(b).However, inthe25/60:60/100-¿xmplot,C-rich high observed10-^mfluxes, whereasthatofPottaschetal.iscomprisedoptically wellstudied Pottasch etal.(1984).Our sampleisbiasedtowardscompactplanetarynebulae withrelatively and rathermoreextended objects.Manyofthecompactplanetariesshowsilicate orsilicon We cancorrecttheIRAS12-pmmeasurementsforemissionstructureseenin8-13pm That thesilicateandSiCplanetarynebulaeliebelowgreybody(n=0)linedemonstrates The very-low-excitationobjectCN3-1(Sanduleak&Stephenson1972)hasthelowest12/25- The 25/60:60/100-/x,mdiagramdoesnotshowtheseparationbetweenO-richandC-rich The colour-colourdiagrams shownherecanbecomparedwithsimilarplots presentedby © Royal Astronomical Society • Provided by theNASA Astrophysics Data System Infrared spectroscopyofplanetarynebulae73 198 6MNRAS.221. . .63R 2 74 P.F.RocheandD.K.Aitken the right-handsideof12/25:25/60-/xmplot;similarly,compactsourceslietowards surprising thatthecolour-colourdiagramforoursamplehasalargerfractionofobjectstowards upper rightinthe25/60:60/100diagram. 25-/xm fluxof27JyleadingtoaveryhighemissionopticaldepthfortheadoptedQ=\/À high fluxes.AsnotedbyMoseley(1980),IC4997hasanopticaldiameterof2arcsec,anda emissivity assumingthatthefar-infraredfluxarisesfromwithinopticalnebula.Vy2-2and respectively (Seaquist&Davis1983;Kwok,PurtonKeenan1981).Thebrightnesstempera- SwSt 1areevenmoreextremewithdiametersoftheionizedregions0.4and0.8arcsec tures at25¡xmforsourcesofthoseangulardiameters are120and100K,whichverycloseto possibly ejectedbythepre-planetarystar. the temperaturesrequiredforfitstoIRAS data.Clearlythefar-infraredfluxisnot optically thick,(ifitwas,wouldrequireveryhigh opticaldepthsat10¡xm)sothatinthose 7 Conclusions objects, muchofthefar-infraredfluxmustarisefrom materialoutsidetheionizednebulae,and two showingstrongSiCemission features;theclassificationofHu2-1islesssecure becausethe NGC 6881,He2-459, 6886andNGC6302displaythenarrowunidentified features at11.25 local extinctionbysilicate grains,indicatingthattheyareO-richbipolarnebulae. NGC6741, emission structureisveryweak. 19W32andM2-56havesmoothspectrarequiring thepresenceof according totheirdustemission spectra.M1-6,1-12andHu2-1arecarbonrich, withthefirst Of the15planetarynebulaeforwhich8-13-^mspectra arepresented,ninecanbeclassed A furtherconsiderationoftheIRASfluxesisthatmanyverycompactnebulaehave © Royal Astronomical Society • Provided by theNASA Astrophysics Data System Figure 3.Histogramofnebulasizeaccordingto8-13-/¿mspectralproperties. NEBULA DIAMETER(PC) 198 6MNRAS.221. . .63R n 5 not seemtobethecaseforNGC6302whichhasbeenshownhaveanenhancednitrogen infrared spectrum.Theremainingfiveplanetarynebulae;Hb4,Hb6,Cn3-1,NGC6884andIC this objectmaybeindoubtasaconsequenceoftheextremelyhigh-excitationlinesseen abundance andC/O—0.25fromopticalultravioletstudies,thedeterminationin and 8.65¿¿m.Mostobjectsthatshowthesefeaturesappeartobecarbonrich,butwhilstdoes found fromthe8-13-/xmspectraarereflectedinfar-infraredfluxdistributions.Thisisbest have beenobservedspectroscopicallyat10pm.Thedifferentclassesofdust-emissionspectra 5217 haveveryweakcontinua,andthespectraaredominatedbystrongfine-structureionic feature areseparatedfromC-richobjects.O-richplanetarynebulaeliewithintheregionbounded seen ona12/25:25/60-/¿mplotwhereO-richplanetariesthatshowthe9.7-^msilicateemission emission. by rc=lto2,wheretheemissivityisgivenQ-\/Xconsistentwithcalculatedandmeasured emissivities ofsilicates.Thosewiththe11.2-/¿msiliconcarbideemissionlieclosetoalinegivenby planetaries withthenarrowemissionfeaturesat11.25and8.65¿¿m,thoseveryweak nebulae showagreatdealofscatterasexpectedfromdustwithlargetemperaturerange.The continua, generallyhavefar-infraredspectrawellfittedbyemissionfromgrainswithm=0.Aplot emission, haveemissivitiesintherange/r=1.5-2whilstO-richobjectsshowmorescatterand chemistry ofthesources.Atlongerwavelengths,C-richplanetarynebulaewithSiC of the25/60:60/100-^mIRAScoloursdoesnotshowasclearaseparationaccordingtodust lie closerto1-1.5. 2=1/2°, inkeepingwiththeemissivityfunctionofamorphouscarbon.TheO-richbipolar within theionizedregionsdelineatedbyopticalorradiosizes.Thisprovidesgoodevidencethatin IRAS pointsareveryclosetothebrightnesstemperaturesat25¿¿mifemissionarisesfrom high far-infraredfluxes.InthecaseofSwSt1andVy2-2,temperaturesrequiredbyfitsto the ionizednebulae. ground, 15aredominatedbythenarrowemissionfeaturesandprobablyhaveC/O>2,nineshow at leastsomeofthemostcompactsources,bulkfar-infraredfluxisemittedfromoutside References Acker, A.,1978.Astr.Astrophys.Suppl.,33,367. grains andsohaveC/O<1whilsttheremaining13continuatooweakforclassification. 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