198OApJ. . .238 . .585C The AstrophysicalJournal,238:585-595,1980June1 © 1980.TheAmericanAstronomicalSociety.Allrightsreserved.PrintedinU.S.A. earlier studyofnorthernplanetaries(Cohenand which representacontinuationandextensionofour investigation wasintendedbothtoenlargethesample southern planetarynebulaeandemission-lineobjects, could becomparedandtosurveyfornebulaebright rich inbrightplanetarynebulae,andthepresent enough forsubsequentstudyintheinfraredathigher of objectsforwhichradioandinfraredproperties Barlow 1974,hereafterPaperI).Thesouthernskyis spectral resolution. dust emissionapparentinthenear-infrared;(2)bona fide planetarynebulae(PN);and(3)objectsofvery objects ofcategoryD(Allen1973),typifiedbyhot low excitation(VLE),definedbySanduleakand observed intothreecategories:(1)stellaremission-line atory, whichisoperatedbythe Association ofUniversitiesfor Research inAstronomy,Inc.,under contractwiththeNational These authorsexcludelow-excitationnebulaewhich 25007 andgenerallystrong[On]23727emission. Stephenson (1972)ashavingHßstrongerthan[Om] have Wolf-Rayetnuclei.Insuchacase,theWolf- shown thatcounterpartsoftheVLEobjectsin trum. RecentlySanduleakandPhillip(1977)have Science Foundation. Rayet characteristicswilldominatetheopticalspec- Magellanic Cloudscontainstarswhoseabsolute 1 2 In thispaperwepresentinfraredobservationsof We havefounditusefultoseparatetheobjects VistingAstronomer,CerroTololo Inter-AmericanObserv- NAS/NRCSeniorPostdoctoral Associate. © American Astronomical Society • Provided by theNASA Astrophysics Data System 43 nebulae, and11emission-lineobjects,ofwhichmorethanhalfwerealsodetectedat20¿¿m.The the dominantgrainheatingmechanismfornebularelectrondensitieslessthanapproximately1.5 by thelatter.Auniformsampleofplanetarynebulaeforstatisticalanalysisiscreatedcombining nebulae caneasilybedistinguishedfromVLEbyvirtueofthesilicateemissiondisplayed the presentsamplewithourearliernorthernhemispheresurvey.ItisshownthatLaphotonsare dominates. Subject headings:infrared:sources—-nebulaeplanetary 10 /unbehaviorofthevariousclassesobjectsarecompared,anditisshownthatplanetary x 10cm~,whereasabovethisdensitydirectdustabsorptionofstellarcontinuumphotons Ten-micron photometryispresentedof23planetarynebulae,sevenverylowexcitation(VLE) INFRARED PHOTOMETRYOFSOUTHERNPLANETARYNEBULAE I. INTRODUCTION Department ofPhysicsandAstronomy,UniversityCollegeLondon NASA AmesResearchCenter,MoffettField,California Received 1979September21;acceptedDecember4 AND EMISSION-LINEOBJECTS 1 Michael J.Barlow 1,2 Martin Cohen ABSTRACT AND 585 also representapopulationdifferentfromthat chopping secondaryandfeedingamultifilter separated bytheir10gminfraredproperties. whether thesethreecategoriesofobjectscouldbe of planetarynebulae.Itwasouraimtodetermine magnitudes aretypicalofPopulationIobjects(i.e., the CTIO1.5mtelescopeequippedwithanf/30 M %—4to5)sothattheGalacticVLEobjectsmay chopping wasusedatafrequencyof21Hzwiththrows observed thosenebulaewhoseopticaldiameterscould multiaperture liquid-helium-cooledbolometer.We be accommodatedwithin9"or18"beams.Spatial the filtersystem,absolutefluxcalibrationforzero of 15"and25",respectively.Table1presentsdetails magnitude, andself-consistentmagnitudesforthree upon workbyJ.A.Frogel(1977,privatecom- standard stars.Thesemagnitudesarebased munication) whichwassupplementedbyus. emission-line objects,andTable3comprisesthoseof v errors, or3star inthecenterofaringnebulawhichhasbeen between 1and3/un).WealsoincludeAGCar,aBe the lOjumregion(D.K.Aitken1979,private of eightobjectsshowingD-typeemissionaccordingto biotic starRRTelalsoexhibitssilicateemissionin 2 (/mi) The objectsVe27,GGCar,He2-80,2-90,and He 2-106isasymbioticstar(Swings1973).Its In Figure1wepresentplotsofthefluxdistributions 6.19 (21) 6.32 (70) 6.45 (50) 1.65 (20) III. INFRAREDFLUXDISTRIBUTIONS a) StellarEmission-LineObjects 135 (400)167(110)209(140) 25.0 (17)53.3(31)68.1(50) 29.0 (26)29.4(33)17.4(5) 5.24 (21).3.71(4) 9.05 (30)7.73(7)<12.4 13.2 (25)11.7(23)13.9(4) 1.14(6) ...<10.7 19.2 (83)20.319.6(7) 4.63 (3) Vol. 238 <35.7 198OApJ. . .238 . .585C a a a a a a 3 a a a a a a a No. 2,1980 NGC 2792 IC 418 IC 2501.... NGC 2867 IC 2621.... He 2-47.... NGC 3132. distributions ofGGCarandVe27againindicate NGC 5315 IC 4191.... He 2-77.... IC 4406. NGC 5315. He 2-99. NGC 5189 NGC 5189. He 2-131... NGC 5882. He 2-108. He 2-101... bution ofHe2-90ismorebaffling,sincethefivedata self-consistent spectrum,comparedwithwhichwecan points between8.0and11.7¡amdefineasmooth silicate emissioninthe10/umregion.Thefluxdistri- Te I NGC 6153 NGC 5979. NGC 6369 IC 4637. H 2-1 He 2-138... NGC 3918 distribution between4.7and11.7fimistoosharply due to[Ne11]emission.Moreover,theslopeofflux only suggestthatthegreatlyelevated12.7/unpointis whose identityisasyetunknown. necessary toinvokesomenongraygrainmaterial peaked tobeablackbody.Thereforeitmay NGC 6537 He 2-260... Hb 5 NGC 6578. NGC 6567. M 1-38.... NGC 7009631 ±154 NGC 6537396 ±10328389 IC 3568141±30 IC 5117408 ±5432190 BD ±30°3639616 ±82519165 NGC 65721146 ±223 M 1-781000 ±66909±190 He 2-34isclassifiedasasuspectedVLEobjectby © American Astronomical Society • Provided by theNASA Astrophysics Data System Denotes measurementwith18"beams;allotherobservations9".FiguresinparenthesesasTable2. Nebula S(31.4GHz)(mJy) Object High-Frequency RadioMeasurements TABLE 4 Type PN PN PN PN PN VLE H ii? PN PN PN PN PN PN PN PN VLE PN PN PN VLE PN VLE PN PN VLE VLE PN PN VLE PN PN PN PN IR PHOTOMETRYOFPLANETARYNEBULAE Observed Fluxes,F(Jy),ofPN,VLE,andHe2-77 v 10.5 (12) 2.72 (3) 2.57 (4) 3.87 (4)3.83(13)2.13(9) 2.92 (5) 2.44 (3)1.42(6)1.05 <4.82 <2.64 <6.31 8.0 3.59 (8)5.43(11) 2.65 (18) 0.82 (17) 3.37 (15) 1.26 (5) 0.76 (4) 1.69 (8) 1.00 (6) 0.82 (4) 1.74 (14) 2.56 (12) 5.45 (17) 1.92 (8) 1.61 (14) TABLE 3 2.22 (5) 2.85’(4) 2.49 (6) 1.11 (4) 3.12 (11) 1.18 (4) 2.81 (7) 1.46 (5) <1.22 9.8 4 upper limitofAy<5.Moreover, thislimitisclearly which wehaveacquiredyields A=1.85,wherethe interpretation ofthetroughinspectrumcenteredat Webster andGlass(1974).Themostobviousinitial 2.2 /an.Opticalspectrophotometry ofCPD-56°8032 the (F—K)indexof4.3forthisobjectprovidesarigid for normal’’ratiosoíA/t(Rieke1974;Gillettetal. implied forthisfeaturewouldthenbei^0.5,which 9.8 /nnwouldbesilicateabsorption.Theopticaldepth narrow HaandH/?lines are assumedtooriginate too highbecauseofobvious thermaldustemissionat emission, whichshouldpeakat9.8/on.Itisconceiv- a peakat10.6/onnotreadilyidentifiableassilicate feature occursbetween8and13/¿m,characterizedby 1975) wouldsuggestanAofabout8mag.However, the rathersmooth,broademissionpeakwhichisob- able thatthepeakcouldinclude[Siv]emissionat Sanduleak andStephenson(1973).Aratherbroad emission featureknowntooccurinotherastronomical served andwhichdoesnotcorrespondtoanyresonant v vs[1 sil objects. Norshouldalowexcitationnebuladisplaythe [S iv]ion. 10.5 /¿m.However,[Siv]lineemissioncannotexplain v CPD —56°8032hasbeenclassifiedastypeWC10by 2.61 (19) 2.93 (24) 4.93 (19) 4.93 (31) 0.66 (4) 3.84 (17) 6.32 (23) 3.02 (31) 10.0 (70) 1.17 (7) 0.48 (3) 2.04 (12) 2.76 (13) 3.57 (37) 0.66 (7) 1.22 (14) 6.50 (30) 3.63 (33) 1.10 (16) 1.29 (11) 1.65 (8) 1.00 (7) 1.96 (15) <0.33 <0.34 <0.31 <0.38 <0.32 <0.50 <0.34 <0.34 <0.41 <0.43 10 2 Gum) 9.98 (44)12.6(33)19.2(27)30.0(9) 4.70 (30) 2.70 (8) 2.88 (15) 2.52 (10) 4.10 (11)3.65(43)7.97(24) 5.01 (20) 5.45 (10) 5.32 (22)6.16(26)10.4(17) 1.40 (6) 4.57 (17) 2.36 (9)2.46(17) 6.00 (110)4.92(16) 2.29 (13)2.00(9)2.67(6) 6.62 (17) 10.6 4.49 (12) 2.67 (10) 2.46 (10) 3.24 (12) 5.22 (18) 5.29 (42) 1.30 (5) 0.89 (5) 4.20 (19) 7.75 (25) 1.21 (6) 11.7 4.22 (6) 4.28 (6) 4.16 (7) 8.09 (13) 7.05 (12) 1.65 (3) 2.48 (6) 4.00 (10) 9.86 (26) <1.67 <1.44 12.7 29.7 (5) 41.0 (22) 2I.7’(8) 22.2 (4) 25.4 (6) 7.82 (4) 21.2 (13) 6.27 (4) 35.7 (7) 38.1 (20) 21.2 (7) 21.7 (8) 8.18 (4) 16.2 (9) 10.3 (4) 3.77 (4) 31.4 (3) 13.3 (4) <10.1 <15.3 <11.9 20 587 198OApJ. . .238 . .585C 588 under case(b)conditionsinthesameregionas circumstellar regiongivingrisetothebroaderper- narrow nebularforbiddenlines,ratherthanfromthe the silicateabsorptionhypothesis,itismostimprob- mitted linespectrum.Inadditiontotheseobjections around thiscarbon-richWCstar.Amoreplausible able thatoxygen-richsilicategrainscouldcondense interpretation oftheshapespectrumCPD having astrong7.7/xmfeature(seethecomposite continuum fluxdistributionofNGC7027,inparticular spectrum ofNGC7027inFig.1Russell,Soifer,and Willner 1977). distribution issimilartothatfoundforPopulationIBe attributable tofree-freeemission,andtheIRflux emission fromanopticallythickwindhaving infrared wavelengthscanbeinterpretedasdueto steepening oftheexcessfree-freespectrumatlonger stars byGehrz,Hackwell,andJones(1974).The thermal dustemissionfromAGCar.Webelievethat (Wright andBarlow1975).Thereisnoevidencefor approximately inverse-squarelawdensitydistribution — 56°8032inthe10fimregionisthatitsimilarto a planetarynebula. there isnojustificationfortheclassificationofAGCar and itsassociatedringnebula(ofdiametersomeW)as infrared fluxdistributionsradicallydifferentfromthat which narrowband9.8/¿m dataexistshowdeep for 12PN.ApartfromIC418, alloftheplanetariesfor emission. of AGCar,inthesensethattheyallshowstrongdust interpret thisphenomenon in termsofabsorptionby troughs atthiswavelength. However,wedonot AG Carshowsanexcessbetween2.2and11.7^m All oftheotherobjectsplottedinFigure1have Figure 2illustratestheflux distributionsobserved © American Astronomical Society • Provided by theNASA Astrophysics Data System Fig. 1.—Theinfraredfluxdistributionsofnineemission-lineobjects.Twoblackbodycurvesareshownforcomparison. b) PlanetaryNebulae COHEN ANDBARLOW A asdeducedfromtheopticaldepthofputative context thediscrepancybetweenexpectedvaluesof the discussionofCPD—56°8032above.Inpresent silicate dust,forbasicallythesamereasonasgivenin deduced byMilneandAller(1975)usingtheradio-Hß silicate features,versusthetotalextinctionatHß pared with0.8-3.5magobserved). method, isenormous(e.g.,10-30magexpected,com- representative ofthetruecontinuum,whereaseach the otherfivenarrow-bandfilterspotentiallyincludes nebulae. TheA8.0filtercanincludethestrong,broad an emissionlineorfeatureknowntooccurinplanetary wavelength wingofthis7.7/mifeature,togetherwith the 9.0¡im[Arm]lineand8.7¿¿mfeatureseenby the edgeof11.3/imfeaturefoundbyGillett, and Willner(1977).The/18.8filtercanincludethelong 7.7 /mifeaturefoundinNGC7027byRussell,Soifer, 210.6 filtercanincludethe10.5/un[Siv]lineaswell Gillett, Forrest,andMerrill(1973)inNGC7027.The v brackets thislatterfeature.The212.7filtercaninclude have establishedthepresenceinIC418ofabroad the 12.8/un[Nen]line.Finally,Willneretal(1979) silicon carbideemission.Itispossiblethatinall feature between10and14/nn,whichtheyattributeto Forrest andMerrill(1973).The211.7filterfully planetary nebulaethisfeatureunderliesthenarrower lines andfeatures.Ourobservedfluxdistributionfor fluxes arelower,duetotheuseofasmalleraperture. photometry ofWillneretal,althoughourabsolute IC 418(Fig.2)agreesverywellwiththespectro- distributions between8.0and9.8/¿m,itisclearthatwe ing sequence:Hb5,NGC6537, NGC5315,IC2501, may orderthewell-observed planetariesinthefollow- NGC 3918,6567,IC 2621,andIC418.This strength ofthe7.7and8.7 /an featuresrelativetoa sequence maybeinterpretable asoneofdiminishing Instead weregardthe9.8/¿mpointasmost If weexaminethesteepnessofdeclineflux Vol. 238 198OApJ. . .238 . .585C No. 2,1980 emission, respectively.Onemightthenpredictboth7.7 fluxes mightrepresent[Siv]andbroad11.3jum data forIC4191andNGC6153areparticularly exists forIC4191,NGC6369,or2867,buta and 11.3/mifeaturesathighresolution. limits toadjacentnarrow-bandfluxes.Theseelevated at 10.6and11.7jumcomparedwiththesevereupper trough at9.8/midefinitelyexistsforNGC6153.The general underlyingcontinuum.No9.8jumfluxpoint striking inthatbothshowremarkablystrongemission flux at8.8/Limismuchlowerthanthat9.8¿¿m.We 2-47, He2-131,and2-138(Sanduleak to silicateemissionpeakingat9.8fim,alongwith interpret thefluxdistributionofHe2-47asbeingdue an obviouswayfrombonafideplanetariesinthatthe Stephenson 1972)areplottedinFigure2.Theydiffer types. Comparisonofourown (unpublished)optical and astrong[Nen]line.Aitkenetal.alsohavefound have obtainedspectrophotometryoftheVLEobject interpreted inthesamemanner.Aitkenetal.(1979) bution ofHe2-131(andpresumably2-138)isalso strong [Nen]emissionat12.8/um.Thefluxdistri- 3 mtelescope,ofSwSt1and BD+30°3639supports spectra, takenwiththeLick image-tube scanneratthe (1969) astransitionalbetween Wolf-RayetandOf silicate emissioninthespectrumofSwSt1(HD M 1-26andfoundthatitalsoexhibitssilicateemission 167362) whosenucleusisclassified bySmithandAller The 10¡imfluxdistributionsoftheVLEobjectsHe © American Astronomical Society • Provided by theNASA Astrophysics Data System Fig. 2.—The8-20fimfluxdistributionsof12planetarynebulae,threeVLEandthepossibleHnregionHe2-77 c) VeryLowExcitationObjects IR PHOTOMETRYOFPLANETARYNEBULAE dominantly thespectrumofaVLEobject.Wefeelthat type nucleus(cf.SmithandAller1969).SwSt1reveals the nebularlines.Bycontrast,SwSt1showsonlyafew spicuously thatofacoolWCtypenucleus,withrich their view.ThespectrumofBD+30°3639iscon- 224650 and5696ofCm.These,otherlines,are broad-lined spectrumofCmandivinadditionto 2.3 Jy(Milne1979).Wehave acquiredopticalspectro- be immediatelydistinguishedfromPNonthebasisof albeit closertothelatter.Therefore,VLEobjectscan seen forBD+30°3639,aswouldbeexpectedanOf significantly narrowerthanthecorrespondinglines of thestrongestcarbonlinesinemission,principally patchy, irregularshapewith noobviouscentralstar 2) resemblesthatofplanetarynebulae,showingadip VLE objectsareoxygen-richwhereasPNcarbon- their 10/anfluxdistributions,anditwouldappearthat object ontheborderlinebetweenPNandVLEobjects, and moreoverhasarather large 5GHzradiofluxof at 9.8/an.However,morphologically He2-77hasa vations ofHe2-131withIUEbyM.J.Seaton(1979, rich. Thissuppositionhasbeenverifiedbyobser- BD +30°3639isdefinitelyaPN,butthatSwSt1an private communication),showingthatO/C>1inthis Kohoutek (1967)andits10¿anfluxdistribution(Fig. object. TheevolutionarystatusoftheVLEobjects remains tobeunderstood. photometry ofHe2-77which showsittobeheavily He 2-77isincludedinthecatalogofPerekand d) He2-77 589 198OApJ. . .238 . .585C -2 -2 assuming case(b)conditionstohold,yieldsA elevation oftheadjacent8.8and10.6jumfluxpoints obscured. AnanalysisoftheobservedHa/Hjßratio, would havetobeascribedlineemissionby[Arm] should emphasizethatHe2-77istheonlyobjectinour and [Siv],respectively,underthishypothesis.We sistent withtheobserveddipat9.8jim.The is aheavilyobscuredcompactHiiregion.Radio silicate absorption.WethereforesuggestthatHe2-77 sample forwhichthe9.8fimtroughisconsistentwith cloud wouldbeveryuseful. 10 jam,duetosilicateabsorption,ofi~0.6,con- 590 observations insearchofanassociatedmolecular v from PaperI,wehavecreatedasampleofPNfor had tosatisfythefollowingconditions:(1)both10and further analysis.Tobeincludedinthesample,anebula in §IIIc,toshowinfraredpropertiesradicallydifferent have beenobservedwithapertureslargerthanits 20 ^mdatahadtobeavailable;(2)thenebulamust were alsoexcluded.Thesethreeobjectsareknownto criteria, thenebulaeHD167362,Vy2-2,andHb12 optical diameter;(3)thenebulacouldnotbeaVLE from thoseofPN. object sincetheseareknown,followingthediscussion = 8.9mag.Thiswouldpredictanopticaldepthat density distribution(Marsh,Purton,andFeldman estimate cannotbemadeoftheirLaluminosities. have asteepradiospectralindex,indicativeofanr exhibit silicateemissioninthe10/nnregion.In sil classification alwaysappropriateforWolf-Rayetcen- Aitken etal.(1979)havefoundthatSwSt1andHb12 tral starsofplanetarynebulae. addition, AllenandSwings(1976)havefoundthatHb 1976). Sincetheirradioemissionisopticallythick,an various parameters.Themethodusedheretoderive 12 hasaWNtypenucleus,incontrasttotheWC for grainsemittingintheinfraredcontinuum(e.g.,as from thatofPaperIintwoways.(1)Inwe grain temperaturesfromour10and20¿¿mdatadiffers more realistictoassumeaAemissivitydependence assumed thatthegrainsemittedasblackbodies.Itis temperatures. (2)Broad-band10¿nnfluxeswereused assumed byTelescoandHarper(1977) in PaperI.Acomparisonofthebroadband10¿¿mand estimate thetruelevelofcontinuumat10/nn,due McCarthy, Forrest,andHouck(1978)).Theeffectof to theinclusionofnumerous strongemissionlinesand shows thatthebroad-bandfluxesseriouslyover- eight nebulaeontheleft-hand sideofFigure2,the features inthebandpass,asdiscussed in§III6.Forthe narrowband 9.8pimdataforthePNinTable3 ratio ofbroad-band10/tm fluxtonarrow-band including thisdependenceistolowerthederived 9.8 /nnfluxhasafairlynarrow range,withamean IV. NEBULARPARAMETERSANDHEATINGMECHANISMS By combiningthepresentIRphotometrywiththat In additiontothenebulaeexcludedbyabove We areleftwith30PN,forwhichwehavederived © American Astronomical Society • Provided by theNASA Astrophysics Data System COHEN ANDBARLOW -2 Aitken etal(1979),andIC5117NGC6302from value of2.3±0.4.Forthosenebulaenotpossessing These includeNGC7027andBD+30°3639from factor. Foranumberofnebulae,9.8/nnfluxescould by dividingtheirbroad-band10/¿mfluxesthis 9.8 jimphotometrywehaveconstructed/nnfluxes the total10/tmfluxofDanziger,Frogel,andPersson 6572 fromWillneretal(1979),NGC6790 be obtainedfrompublishedIRspectrophotometry. Gillett, Forrest,andMerrill(1973),IC418NGC broad-band 10/tmfluxesisagaintolowerthederived Grasdalen (1979:thefluxforNGC6302wasscaledto yields agraintemperatureof139K,comparedwith applied tothe9.8and20/tmfluxes(thelatterfrom by McCarthy,Forrest,andHouck(1978).Onepossi- wavelength fluxesbyTelescoandHarper(1977) grain temperatureof90-95Kderivedfromthelonger Becklin, Neugebauer,andWynn-Williams1973) continuum butinsteadmerelyrepresenttheinter- ments donotreallysamplethe“true”underlying bility forthisdiscrepancyisthatthe9.8/tmmeasure- 1973). Theeffectofusingnarrow-bandinstead continuum. Consequently,inthispaper,weshall continuum shortwardof20/¿m,therebyestablishing with 90Kgrainemissionleavesanexcessofsmooth to derivenebularluminosities.However,thatdistance shows thatfittingtheA>25/¿mdataforNGC7027 of the11.3firnandsiliconcarbidefeatures.However, 10-20 /tmgraintemperatures. representation oftheunderlyingcontinuumat9.8/im. therefore assumethatourdatagiveameaningful the existenceofarangedusttemperatures.Weshall Figure 3ofMcCarthy,Forrest,andHouck(1978) section ofthewings7.7/miandacombination for thiseffect.WehavederivednebularLaluminosities and thereforecanseriouslyoverestimatethedistances scale assumesthesameionizedmassforallnebulae cooler grainsemittingatlongerwavelengths,asdis- fluxes. TheseIRluminositiesthusdonotallowfor with aAemissivity,normalizedtothe9.8and20/¿m lated solelyonthebasisofenergyemittedbygrains from the5GHzfluxesofMilneandAller(1975) adopt thedistancescaleofAcker(1978),whichallows of nebulaewhichareopticallythickintheLyman cussed below. assumed meannebularelectrontemperatureofT Milne (1979),usingthemethodofRubin(1968)andan nebulae fromPaperI)grain temperaturesT,column column (3)givesthe9.8-20/¿m(or9.8-18for parentheses ifderivedfrombroadband10//mfluxes), ascension order,column(2)liststhe9.8/¿mfluxes(in Column (1)givesthenamesofnebulaeinright distances D(inkpc),andcolumns (6)and(7)listtheLa luminosities. Column(8)gives theratioL/for and infraredluminositiesL andL,inunitsofsolar (4) liststhe5GHzradiofluxes, column(5)givesthe e = 12,000K.Infraredluminositieshavebeencalcu- d IRLa La IR -2 For thecaseofNGC7027,aAgrainemissivity In PaperIweusedthedistancescaleofHiggs(1971) Table 5liststhevariousderivednebularparameters. Vol. 238 198OApJ. . .238 . .585C 102 2 2 No. 2,1980 J900 IC 2149 IC 418 IC 2501 IC 2165 IC 2621 IC 4191 IC 4593 NGC 5315.... NGC 3918.... NGC 6302.... IC 4634 NGC 5882.... NGC 6537.... Hb 5 NGC 6210.... NGC 6578.... NGC 6567.... NGC 6572.... NGC 6741.... Hu 2-1 NGC 6807.... NGC 6803.... NGC 6790.... each nebula.Column(9)liststheelectrondensitiesof IC 5117 NGC 7027.... NGC 6886.... NGC 6884.... NGC 6881.... BD +30°3639 the nebulaeft(FL)derivedfrom[On]lineratios Column (10)liststhermselectiondensities(rms) to allowthetotalinfraredluminositybeevaluated. derived byHiggs(1971).Thesearemodel-dependent, Telesco andHarper(1977)foundatotalinfraredflux been measuredoverawideenoughwavelengthrange and assumeT=12,000K. and nebulartemperaturescollectedbySabbadin was emittedineachoftheintervals1-17//m,17- and Minello(1978).Thesearedistance-independent. emission featuresandhotter dustatshorterwave- of2.4 x10"Wm",distributedsuchthatonethird deduced byTelescoandHarperduetothefactthat estimate ofLforNGC7027inTable5,produced 30 /¿m,and30-300¿¿m.Theratiooftotalinfraredto temperature of15,000KwhencalculatingL).The latter authorsadoptedahighernebularelectron La luminosityis3.15(thisdiffersfromtheratioof3.6 wavelengths. Anotherwayof lookingatthisistosay the 9.8and20pimfluxes,accountsfor60%oftotal e solely byfittinga139Kemissivityblackbodyto lengths and327oisemitted bycoolerdustatlonger infrared flux.Oftheremainder,8%isemittedby that a90KX~emissivity blackbodyfittedtothe 30 ¡imfluxaccountsfor73% ofthetotalinfraredflux, e IR La a Seetext,§IV. For onenebula,NGC7027,theinfraredfluxhas © American Astronomical Society • Provided by theNASA Astrophysics Data System Nebula (Jy)(K)(kpc)(L)L/L 0irl a F (9.8jum)T(9.8-20)(5GHz)DL vdLaIR 135 17.6 48.7 10.4 (0.34) (0.45) 11.1 (1.08) (0.55) (0.33) (0.50) (0.31) (0.86) (0.72) (0.99) (0.30) (0.38) (0.45) (0.60) (0.39) (0.86) 2.22 2.13 2.81 1.11 1.18 6.34 5.41 1.44 1.05 IR PHOTOMETRYOFPLANETARYNEBULAE 142 122 118 108 135 124 117 116 107 107 114 134 115 117 122 122 144 120 139 128 126 122 136 139 128 142 120 123 113 126 0.188 0.254 0.195 0.261 0.121 0.476 0.104 0.370 0.172 0.859 0.671 0.548 0.129 0.311 0.170 0.176 3.49 1.613 0.220 0.130 0.114 0.171 0.183 0.511 0.022 0.256 0.230 0.105 6.28 1.307 Nebular Parameters TABLE 5 0.36 2.25 3.1 0.65 2.9 2.8 2.25 0.8 2.9 0.67 1.0 2.8 0.65 1.1 1.3 2.2 3.6 5.1 1.24 1.9 1.05 1.95 1.9 3.1 1.2 1.6 1.3 1.6 1.6 1.09 a rangeofgrainsizes(Panagia,Bussoletti,andBlanco wavelengths. Undoubtedlyarangeofgraintempera- with theremainderemittedbyhotterdustatshorter although far-infraredfluxmeasurementsarerequired tures ispresent,whichcanmosteasilybeexplainedby planetary nebulae,theestimatesofLinTable5 for adefinitivediscussionoftheinfraredproperties probably representabout607ofthetotalinfrared can leadtovaluesofL/inexcessunitywithout tion bydustofdiffusenebularradiation,includingLa, continuum photonsbythedust.However,aratioof the needfordirectabsorptionofstellarLyman Lir/Z^ >3,suchasNGC7027possesses,mustin- ative IRpropertiesofplanetarios. 1977; Mathis1978).Thepointtobemadehereisthat tinuum photons,sincecompleteabsorptionofall dicate significantdustabsorptionofLymancon- luminosity, therebyallowingadiscussionoftherel- take theviewthatnebulae inTable5withvaluesof to Lymancontinuumphotons oft~0.3.Weshall diffuse radiationunaccompaniedbydirectabsorption definitely experiencesignificant dustabsorptionof Z'ir/Z'Loc greaterthanorequal tothatofNGC7027 Blanco (1977)ledtoanestimate ofadustopticaldepth analysis ofNGC7027byPanagia,Bussoletti,and of stellarphotonsisanunlikelyscenario.Adetailed IR 0 IRLa uv Telesco andHarper(1977)haveshownthatabsorp- 1387 2773 573 496 277 421 213 399 323 532 736 498 164 192 225 190 147 881 126 380 344 182 168 109 116 94 74 88 92 82 1422 5257 1114 281 216 282 420 424 200 326 396 454 325 101 408 604 613 851 160 118 165 146 68 36 68 42 91 81 91 95 0.91 0.38 0.39 0.24 0.87 0.36 0.64 0.31 0.58 0.54 0.50 0.46 0.87 1.15 1.71 1.70 0.29 0.69 2.02 1.03 1.00 1.36 2.93 5.07 1.06 5.28 1.53 1.10 1.90 1.19 3 n (EL) (cm") e 18000 20000 20000 3000 3000 2000 2000 4000 6000 7000 9000 9000 3000 3000 9000 5000 7000 5000 -3 n (rms) e (cm) 14000 37000 26000 10000 30000 13000 13000 12000 15000 10000 11000 4000 6000 8000 2000 8000 4000 6000 2500 9000 7000 3000 4000 3000 8000 7000 591 198OApJ. . .238 . .585C o 592 emitted longwardoftheLymancontinuumisprobably is socoolthatdustabsorptionofstellarphotons discussed inPaperI,thecentralstaroflastnebula Our high-frequencyradiodataforBD+30°3639 the dominantmechanismpoweringinfraredflux. 6790, NGC6807,IC5117,andBD+30369(as The remainderpossessvaluesofL/comfortably support theviewexpressedbyO’DellandTerzian tion ofLymancontinuumphotonsmaybeindicated. only two(J900andIC2501)havevaluesofL/ optically thin.)Oftheremaining25nebulaeinTable5, stellar continuumphotons.ThesenebulaeareNGC close toorlowerthanunitysuchthatthedominant approaching thatofNGC7027,sosomeabsorp- fluxes. Radiofree-freefluxesareproportionaltothe photons. grain heatingsourcemaybesupposedtoLa band 10fimflux,asdescribedinthe text.Thetickmarksonsymbolsdenotenebulaewhosevaluesof L/LinTable5indicatethatdust whose 9.8fimfluxhasbeenmeasured directly,andtheopencirclesrepresentnebulaewhose9.8fimflux hasbeenestimatedfromtheirbroad- 1970, thatthe5GHzradiofluxofthisobjectisnot NGC 7027.(b)SameasFig.3a,but forabsolutefluxes,whereDisthedistanceofanebula,inkpc. absorption ofstellarcontinuumphotons isimportant.Thesenebulaeare(fromlefttoright)NGC6807, 6790,IC5117,BD+30°3639,and infrared fluxesmaybeplottedversus5GHzradio lRLa IRLa IRLa As atestoftheLaheatinghypothesis,9.8gm Fig. 3.—(a)Theapparent9.8jimfluxes plottedagainsttheapparent5GHzfluxesof30planetarynebulae. Thefilledcirclesrepresentnebulae © American Astronomical Society • Provided by theNASA Astrophysics Data System COHEN ANDBARLOW fluxes. Thereforeweshouldexpectalinearrelationship the plotsandproducesaspuriousrelationshipbetween that theintroductionofdistancesartificiallystretches absolute infraredandradioproperties.Grasdalen powers theinfraredemission.InPaperIweplotted that theintroductionofabsolutequantitiesdoesnot the absolutefluxes.Inspectionofdiagramsshows quantities. Totestthishypothesis,wehaveplottedin between 9.8gmand5GHzfluxesifLapredominantly Figures 3aand36,respectively,theobservedfluxes (1979) hascriticizedthisprocedure,onthegrounds product ofnebularelectronandiondensities,asareLa being smallerfortheabsolutequantities.Thereis, values hasweakenedthecorrelationbetween9.8gm however, evidencethattheintroductionofabsolute stretch therangeofvaluesplotted,infact in thedistancescale.Withomissionoffive photons isindicated(thosewithtickmarksinFigs.3a nebulae forwhichsignificantdustabsorptionofstellar and 5GHzfluxes,presumablybecauseofinaccuracies Vol. 238 198OApJ. . .238 . .585C 2 24 2 deduced fromtheforbiddenlineratiosof[On]223726,3729. Higgs (1971).SymbolsasinFig.3.(b)Theratiosofthe9.8fimand5GHzfluxesplottedagainstelectrondensitiesplanetarynebulae No. 2,1980 exists betweenlog{«(rms)}andtheofratio with acorrelationcoefficientr=0.69;and that relationship(1),inconjunctionwiththeL/ with acorrelationcoefficientr=0.50.Wepropose for theremaining25nebulae: included whosevaluesof L/indicatedirect 9.8 fimand5GHzfluxesonlyifthosenebulaeare continuum photonsisthedominantheatingmech- equivalent diagramtoFigure2ofGrasdalen(1979), sity andtheratioof10jumtoradioflux.Hegoeson tant correlationexists,betweennebularelectronden- the majorityofplanetarynebulae. photons arethedominantgrainheatingmechanismin ratios inTable5,providesstrongevidencethatLa and 36),thefollowingleast-squaresfitsareobtained log {DF(9.Sfim)}=0.52+0.705GHz)} fluxes againstthermselectrondensitytakenfrom where wehaveplottedtheratioof9.8fimand5GHz anism inplanetarynebulae.InFigure4awepresentthe argue fromthisthatdirectabsorptionofstellar Higgs (1971;thesedensities,listedincol.[10]ofTable Table 5.WehaveexcludedBD-F30°3639fromFigure cussed earlierinthedescriptionofcolumn(9) model-dependent, wehaveplottedasimilardiagramin Grasdalen). SincethedensitiesfromHiggs(1971)are 5, arethesameasthoseusedinPaperIandby 4 forthereasonsmentionedearlier. forbidden lineratioof[On]2/13726,3729asdis- Figure 4busingelectrondensitiesderivedfromthe e lRLoc lRLa vV 4 log {i%(9.8¡im)}=0.57+1.01{F{5GHz)}(1) v Thecorrelationcoefficient Fig. 4.—{a)Theratiosofthe9.8/imand5GHzfluxes25planetarynebulaeplottedagainsttheirrmselectrondensities,takenfrom Inspection ofFigure4ashowsthatacorrelation Grasdalen (1979)hassuggestedthatamoreimpor- © American Astronomical Society • Provided by theNASA Astrophysics Data System 2 2 -(Z^-Zl/^)] r= 2 2 (Z -[(Z-*l»]}{ZJi[(Zyd ¡n]} ' log [F(9-8^m)/F(5GHz)] v Fig. 4a4b IR PHOTOMETRYOFPLANETARYNEBULAE (2) n -2 2 43 2 2 2 2 2 -3 4 bounded nebulasurrounded byaneutralshell.For {a) adensity-boundednebula, and(6)aradiation- behavior oftheneutralhydrogen opticaldepthinthe TazR, implyingtgcÆ . Inconsideringthe uniform densitynebulaofradiusR,nRisaconstant, clusions areobtained. tons. Whennebulaewithn>1.5x10cm~are nebulae (NGC6790,NGC6807,7027,andIC Lyman continuum,to,wemay distinguishtwocases: ing. Asthenebulaexpands,twillfall;e.g.,fora the Lymancontinuum,t,suchthatdirectgrain electron densitywillhavealargedustopticaldepthin nebulae. Ayoungplanetarynebulawithahigh electron density(notshownhere),thesamecon- and thustheirinclusionwouldbeexpectedtoyieldthe direct absorptionofstellarphotonstobeimportant, precisely theoneswhosevaluesofL!indicate included, acorrelationdoesexistbutthesenebulaeare model wherebygrainheatingispoweredbyLapho- 46 asshowingthatnosignificantcorrelationexists so foradust-to-gasratiowhichremainsconstant, heating bystellarphotonsdominatesdiffuseLaheat- observed correlations.When£/Lisplottedagainst absorption ofstellarphotonstobesignificant.Ifthese scenario fortheinfraredbehaviorofmodelplanetary between \ognandlog{F(9.8/¿m)/F5GHz)}for 0.36. Asimilarsituationholdsfortheplotof 7027, rfallsto0.42;andwiththeadditionalexclusion squares analysisyieldsacorrelationcoefficientofr log {«(FL)}versustheofratio9.8/tmand remaining 21nebulae.Forall25nebulaerrisesto ysis yieldsacorrelationcoefficientofr=0.04forthe of IC418,rfallsto0.26.WeinterpretFigures4aand cm, aswouldbeexpectedonthebasisofa 5 GHzfluxesinFigure46.Forall17nebulae,aleast- nebular electrondensitieslessthann^1.5-2x10 5117) areexcluded,thenaformalleast-squaresanal- meuv e e h uv uv lRh0L IRLa ev e = 0.64.WiththeexclusionofNGC6790and e We maythereforeoutlinethefollowinggeneral log[Ç (9.8/im)/F(5GHz)] v 593 198OApJ. . .238 . .585C 2 2 2 43 we thusfindthattoocR~.Therefore,asthenebula must balancerecombinations.Sincen^ozR~, case {a)ofanexpandingdensity-boundednebulawith expands anddensitydecreases,thefractionofphotons to =«Æ,wehaven^R~ocnasionizations absorbed bydustandL/willincrease.Forcase(b) absorbed bygaswilldecreasefasterthanthefraction 594 4-3 sufficient neutralhydrogensurroundingthenebulato of aradiation-boundednebula,therewillalwaysbe absorb theLymancontinuumcompletely.Therefore, as thedustopticaldepthfallsnebulaexpands, the fractionofLymancontinuumabsorbedbygas will increaseandT/Ldecrease.Eventually,at by stellarphotonswillfallbelowLaheatingand some transitionnebulardensity,directgrainheating 3- h ne thereafter Lawilldominate.Theobservationsindicate that case{b)holdsandthetransitionnebular hHe consistent withthefactthatplanetarynebulaehaving IRLa density isapproximately1.5x10cm".Thisresult P7 radiation-bounded. Theprobabilityofdustabsorp- densities ofthisorderaregenerallybelievedtobe tion ofLaphotonsdoesnotfallrapidlywithdensity, from anebula.Asmalldustopticaldepthistherefore since typicallyaLaphotonmustundergoseveraltens IRLa to thousandsofH-atomscatteringsbeforeescaping Panagia andRanieri1973).Eventually,however,as the nebulacontinuestoexpand,densitywillfall sufficient tocausesignificantabsorptionofLa(cf. a levelwheretheLascatteringopticaldepthis nebula withoutbeingabsorbed,andL/willfall sufficiently lowthatmostLaphotonswillescapethe in thepresentsampledonotextendbelow~2 density heresincethenebulardensitiesencountered once again.Itisnotpossibletoestimatethistransition make thetransitionfrombeingradiation-boundedto being density-boundedatadensityfarenoughbelow IRLa Acker, A.1978,Astr.Ap.Suppl.,33,367. Aitken, D.K.,Roche,P.F.,Spenser,M.,andJones,B.1979,Ap. Allen, D.A.1973,M.N.R.A.S.,161,145. Allen, D.A.,andSwings,J.P.1976,Astr.Ap.,47,293. ~1.5 x10cmthattheeffectsassociatedwithcase Cohen, ML,andBarlow,M.J.1974,Ap.J.,193,401(PaperI). Becklin, E.E.,Neugebauer,G.,andWynn-Williams,C.G.1973, Danziger, I.J.,Frogel,J.A.,andPersson,S.E.1973,Ap.{Letters), Gehrz, R.D.,Hackwell,J.A.,andJones,T.W.1974,Ap.J.,191, x 10cm.Itisassumedthatplanetaryhebulae Gillett, F.C.,Jones,T.W.,Merrill,K.M.,andStein,W.A.1975, Gillett, F.C.,Forrest,W.J.,andMerrill,K.M.1973,Ap.183,87. Grasdalela L.l979,Ap.J.,229, 587. Higgs, L.A.1971,CatalogofRadio ObservationsofPlanetary Koppen, J.1977,Astr.Ap.,56,189. J. 223925. Ap. Letters,15,87. 675. 184, L29. © American Astronomical Society • Provided by theNASA Astrophysics Data System Nebulae andRelatedOpticalData (Pub.Ap.Branch,NRC, Canada), Vol.1,No.1. COHEN ANDBARLOW REFERENCES due tothecompletedominanceofLaheating. (a) onL/,describedabove,arenotnoticeable require aknowledgeofboththecomposition without specificknowledgeoftheseparametersare mass ofdustcontainedintheplanetarynebulaeour dust andthegrainsizedistribution.Estimatesmade sample since,asdiscussedbyPanagia,Bussoletti,and Blanco (1977)andMathis(1978),suchestimates method ofKoppen(1977),whenappliedtothe10and likely togiveunreliableresults.Forinstance,the mass ofdustestimatedforthesamenebulabyTelesco of dustwhichdifferbyafactor100,whereasthe 30 /unfluxesofNGC7027,givesestimatesthemass and Harper(1977),assuminganinfraredemissivity conducting grainssuchasgraphitewereassumed. by afactorofabout15ifequally,ormore,plausible appropriate fordielectricgrains,wouldbeincreased importance offar-infraredobservationsplanetary nebulae atwavelengthslongerthan20/un,bothto IRLa confirm thepresentconclusionsconcerningtran- sition densityfromstellarcontinuumtoLadominated larger, lower-densitynebulae. grain heating,andinordertoextendthesample help andadvicewiththeoperationofCTIO infrared equipment;andtheDirectorstaffof Roche fordiscussions;J.A.Frogelhisabundant CTIO fortheirhospitality.M.C.thankstheDirector Associateship. M.J.B.thankstheNationalScience and staffofNRAOinTucsonfortheirhospitality Foundation forsupportundergrantAST76-22032. and expertise.M.C.thankstheNationalScience National ResearchCouncilforanNAS/NRCSenior grants AST75-13511and77-19896,the Foundation foritssupportofthisworkunder O’Dell, C.R.,andTerzian,Y.1970,Ap.J.,160,915. Panagia, N.,Bussoletti,E.,andBlanco,A.1977,CNOIsotopesin Milne, D.K.1979,Astr.Ap.Suppl,36,227. Milne, D.K.,andAller,L.H.1975,Astr.Ap.,38,183. Panagia, N.,andRanieri,M.1973,18thLiègeColloquium,Les Marsh, K.A.,Purton,C.R.,andFeldman,P.A.1976,Astr.Ap.,49, Perek, L.,andKohoutek,L.1967, CatalogueofGalacticPlanetary Mathis, J.S.1978,inProc.IAUSymposium76,PlanetaryNebulae McCarthy, J.F.,Forrest,W.J.,andHouck,R.1978,Ap.224, Rubin, R.H.1968,Ap.J.,154,391. Rieke, G.H.1974,Ap.J.{Letters), 193,L81. Russell, R.W.,Soifer,B.T.,andWillner, S.P.1977,Ap.J.{Letters), We havenotattemptedtomakeestimatesofthe In conclusion,wewouldliketoemphasizethe We thankD.K.Aitken,A.Allen,andP.F. Astrophysics, ed.J.Audouze(Dordrecht:Reidel),p.45. 211. Nébuleuses Planétaires,p.275. Nebulae (Prague:CzechoslovakInstitute ofSciences). 281 Observations andTheory,ed.Y.Terzian(Dordrecht:Reidel),p. 217, L149. 109. Vol. 238 198OApJ. . .238 . .585C No. 2,1980 WC1E 6BT,England Michael J.Barlow:DepartmentofPhysicsandAstronomy,UniversityCollegeLondon,GowerStreet,London Martin Cohen:NASAAmesResearchCenter,Mailstop245-6,MoffettField,CA94035 .1973,Ap.J.,185,899. Sanduleak, N.,andStephenson,C.B.1972,Ap.J.,178,183. Sabbadin, F.,andMinello,S.1978,Astr.Ap.Suppl,33,223. Smith, L.F.,andAller,H.1969,Ap.J.,157,1245. Sanduleak, N.,andPhillip,A.G.Davis.1977,Pub.A.S.P.,89,792. Swings, J.P.1973,Ap.Letters,15,71. © American Astronomical Society • Provided by theNASA Astrophysics Data System IR PHOTOMETRYOFPLANETARYNEBULAE Telesco, C.M.,andHarper,D.A.1977,Ap.J.,211,475. Wright, A.E.,andBarlow,M.J.1975,M.N.R.A.S.,170,41. Webster, B.L.,andGlass,I.S.1974,M.N.R.A.S.,166,491. Willner, S.P.,Jones,B.,Puetter,R.C,Russell,W.,andSoifer, B. T.1979,preprint. 595