1988ApJ. . .326. .368P 1 Universities, Inc.undercontractwith theNationalScienceFoundation. NGC 6302,andthepresence ofrotationallyexcitedOH,as by SeaquistandDavis(1983) for Vy2-2andreportedhere scale structureofthe1612MHz emittingregion,asreported are nowapparent.Theseinclude somesimilarityinthesmall- between NGC6302andVy2-2,sinceanumberofsimilarities Astronomy ObservatoryVeryLargeArray(VLA)and43 m telescopes, andbydiscussionsintheliterature,summarized by as revealedbyfollow-upobservationsattheNationalRadio Rodriguez etal(1985).Wewillfrequentlydrawcomparisons selves. TheonenewdetectionisthatofNGC6302. fusion intheGalacticplaneratherthannebulaethem- indicating thatthelineswereassociatedwithbackgroundcon- In eightofthe10cases,emissionwasspatiallyextended, and detectedOHlinesinthedirectionof10,includingVy 2-2. for OHassociatedwithplanetarynebulae.UsingtheNRAO are probablyOH-emittingproto-planetarynebulaealso. from twoOH/IRsourceswhich,althoughopticallyinvisible, (1987, hereafterPBZ)havedetectedradiocontinuumradiation infrared excesses.VeryrecentlyPottasch,Bignell,andZijlstra 140 foot(43m)telescope,weobserved76planetarynebulae the courseofasurveyearly-typeemission-lineobjectswith 2-2 byDavis,Seaquist,andPurton(1979,hereafterDSP)in lines, failedtodetectanyplanetarynebulae.Theonlydetection (Silverglate etal1979),concentratingprimarilyonthemain of OHinanopticallyvisibleplanetarynebulaisthatfromVy able main-lineemission.AlatersurveyfromArecibo observed toa5<7fluxlimitof600mJy,noneshoweddetect- nebulae wascarriedoutbyTurner(1971).Ofthenine Black 1983).ThefirstsurveyforOHemissionfromplanetary results availablehavegeneratedconsiderableinterest(e.g., nebulae isintheearlystagesofdevelopment,butlimited 1 The AstrophysicalJournal,326:368-375,1988March1 © 1988.TheAmericanAstronomicalSociety.Allrightsreserved.PrintedinU.S.A. TheNationalRadioAstronomyObservatory isoperatedbyAssociated Most ofthispaperisdevotedtoadiscussionNGC6302 In thispaperwereporttheresultsofanew,sensitivesurvey The studyofmolecularmaterialinthevicinityplanetary Subject headings:interstellar:molecules—nebulae:individual(NGC6302)planetary absorption. MHz lineandcontinuumemissiondetectionsofrotationallyexcitedOHlinesat5cmwavelengthin been detected. interesting object:anOH1612MHzmaserintheyoungplanetarynebulaNGC6302.Thismaybe a briefevolutionarystage,similartotheyoungandcompactplanetarynebulaVy2-2,whereOHhasalready results confirmthatevolvedplanetarynebulaearenotOHsourcesingeneral.However,wediddetectone © American Astronomical Society • Provided by the NASA Astrophysics Data System We alsoreporttheresultsoffurtherobservationsNGC6302,includingVLA1612 We reporttheresultsofasensitivesurveyplanetarynebulaeinallfourground-stateOHlines.Our I. INTRODUCTION A YOUNGPLANETARYNEBULAWITHOHMOLECULES:NGC6302 National RadioAstronomyObservatory,GreenBankWestVirginia National AstronomyandIonosphereCenter,CornellUniversity J. A.PhillipsandYervantTerzian Received 1987July9;acceptedAugust14 ABSTRACT H. E.Payne 368 AND 1 _1 -1 - 1 1 _1 uncertain aboutassociatingthe COwiththenebula. nebula detected inOH,byDSP.Thespectrum appearstypical that oftheionizedgasnor that oftheHi,theyremained and notinterstellar,”butbecause thevelocitymatchesneither s ,withalineprofilethat appears “definitelycircumstellar Zuckerman andDyck(1986) report COemissionat—46km nation linevelocityoftheionizedgastobe-29±2km s. to be—31kms.Rodriguezetal(1985)reporttherecombi- Schneider etal(1983)reporttheradialvelocityofnebula associated withthedustytorusbyRodriguezetal(1985). by neutralhydrogenatavelocityof—40kms^shownto be nebula. RodriguezandMoran(1982)firstreportedabsorption splittings nearer100kms“(BohuskiandSmith1974; 20 kmsforthetorus.Nebularopticalemissionlinesshow fined theflow.Theirmodelsuggestsanexpansionvelocity of Meaburn andWalsh1980h)atmostpositionsacross the be theexpandingremnantofasmallertoroidthatoncecon- (Meaburn andWalsh1980a,b).Alternatively,LesterDi- nerstein (1984)havesuggestedthattheIR-emittingdiskmay hypothesized stellarwindofanundetectedcentralstar approximately 400kmsbipolarflowsresultingfromthe neutral materialwhichmayplaysomeroleindirectingthe stein 1984).ThisIRemissionisassociatedwithadensetorusof small 3-100gminfraredcontinuumsource(LesterandDiner- position ofthiscompactradiosourcecoincideswiththata originates froma15"regioninthecenterofnebula.The and Bignell(1974)determinedthatitsstrongradioemission must beveryhotandhavehadamassiveprogenitor.The N-rich. Althoughthecentralstarhasnotbeenobserved,it optical nebulahasanextentofnearly4',butTerzian,Balick, type I(PeimbertandTorres-Peimbert1983),meaningHe- NGC 6302. ing by2ordersofmagnitude,andthebipolarmorphology significant differencesareradiocontinuumfluxdensitiesdiffer- JSS) forVy2-2andreportedhereNGC6302.Amongthe reported byJewell,Schenewerk,andSnyder(1985,hereafter Vy 2-2(PK45—2°1,Ml-70, VV 230)wasthefirstplanetary NGC 6302(PK349-bl°l)isabipolarplanetarynebulaof 1988ApJ. . .326. .368P © American Astronomical Society • Provided by the NASA Astrophysics Data System NGC 7662 NGC 7354 NGC 7293 Hb 12,VV286 IC 5217,HD212534... Hu 1-2 IC 5117 A63 NGC 7027 NGC 6803 Me 2-2 K3-62 CRL 2688 NGC 7009 NGC 6853 HM Sge Vy 2-2,M1-70,VV230 NGC 6790 NGC 6741 NGC 6720 NGC 6543 Ps 1,K648,M15 Ml-78 IC 4997,He2-464 He 2-459 Pc 23 NGC 6833 NGC 6826 BD +30°3639 IC 4846 He 2-429 K3-23 Hu 2-1,VV458 CN 3-1,VV391 NGC 6572 M3-35 SwSt-1, MWC288 NGC 6369 NGC 6302 IC 4634 NGC 6210 IC 4593 NGC 6058 A36 NGC 4361 Ml-26, MWC270 M2-9 LoTr 6 IC 3568 CTIO 1230-275 NGC 3587 NGC 3242 A31 NGC 2440 A21 NGC 2392 NGC 2371-2372 Ml-6 J900 Me 2-1 LoTr 5 NGC 2346 IC 2165 IC 2149 J320 IC 418 Ml-9 Ml-5 CRL 618 NGC 1514 IC 2003 NGC 650-651 NGC246 NGC 40 Source PKAscensionDeclinationSession Nane Designation(1950)1984 hms Observed Sources 298 +34°1123036 294 +43°1122155 261 +32°1102221.7 219 +31°1085132 205 +14°1072615 111 —2°123 23 57 106 —17°123 23 29.2 358-0°2 174245 349+1°1 171021.3 318 +41°1133757 234 +2°1073941 215 +3°1070649.7 212 +4°1070242.4 211-3° 063311.5 221 —12°1061924 107 +2°1223827.9 100 —8°1222936 100 —5°1222156 342 +27°1151923 312 +25°1133112 339 +88°!125308 214 —24°1052509.3 123 +34°1123147 197 +17°1072613.3 148 +57°11154 189 +19°1072225 194 +2°1062302 166+10°1 055241 184 —2°1054346 190—17°1 050249 161 —14°1035312 166-6°5 043930.8 165 —15°1040608 130—10°1 014125.3 118 —74°1004432.1 120 +9°100108l 58 —10°1201751.4 97 +3°213036.0 95 +0°l213009 93 +1°1211905.5 45 —2°1192159 48 +l°l191121.6 36 —57°1222654.4 89 —5°1213037 46—4°1 192854 44+1° 190722.2 86 —8°1213107 65 —27°1212734.5 83 —3°1210509.5 80-6° 210142.5 37 —34°1210127.7 71 —2°1201904.0 68 —2°1201155.5 60 —3°1195727.0 68 +3°194957.2 83 +12°1194327 64 +5°l193247.5 27 —9°1191345 96 +29°l175834.2 82 +11°1194821 55-3° 193941.0 37 —6°1192025.0 63 +13°1185143.3 51 +9°1184738.3 34+11°1 180940.6 43 +37°l164223.6 25 +40°l160923 33 —2°1190002 38 +12°1181513.0 64 +48°l160242 1 —6°2181257 10+18°2 170252.6 0+12°1 165835 2 +5°l172618 TABLE 1 Right Observing -21 0532Apr -11 3352Apr -09 0806Jul -00 3112Jul -30 5312Jul -30 1052Jul -37 0247Apr,Jul -23 4312Jul -10 0427Apr -21 4528Jul -27 32Jul -18 3032Jul -23 2652Jul -19 3743Jul -36 3606Jul -18 2311Apr -18 0526Jul -00 4329Apr -00 0309Apr -12 5740Jul -12 4415Apr,Jul -12 0844Apr 42 1536Apr 61 0128Apr 47 3246Jul 44 2229Jul 42 0203Apr,Jul 57 5424Jul 48 5001Jul 09 5700Jul 09 4800Jul 01 2447Apr 50 4252Jul 39 2440Jul 55 3933Jul 57 2034Jul 51 4039Apr 29 2449Apr,Jul 22 3500Apr 36 3745Jul 32 1928Apr 32 5133Apr 50 2410Jul 20 4707Apr,Jul 06 5025Apr 40 49Jul 30 2421Apr 66 3805Apr 09 0525Jul 32 5753Apr 23 5329Apr,Jul 21 0057Apr 02 5136Apr,Jul 1157 16Apr,Jul 26 0942Jul 82 5022Jul 29 3526Jul 46 0553Jul 24 2059Jul 16 3423Apr 16 3736Apr 55 17Jul 14 5357Apr,Jul 11 0025Apr 10 0747Apr,Jul 36 0112Apr,Jul 72°14'35" Apr 30 3842Jul 33 4352Jul 58 0431Jul 12 08Jul 13 2044Jul 17 4915Jul 10 3825Jul 1988ApJ. . .326. .368P _1 1 interference. significant fractionofthe1612 MHzdatawaslostbecauseof run and1.06kmsinthe July run.Duringtherun,a band. Thevelocityresolution was 0.88kms~duringtheApril _1 Observations weremadeby frequencyswitchingwithinthe half-power beamwidthoftheobservationswasabout 18'. aperture efficiencyatthe1720MHzlinewasabout55%. The 1 ture efficiencieswerenear60%inallfourlines,exceptthat the polarization. Systemtemperatureswerenear25K,andaper- state lineswereobservedsimultaneouslyinasinglelinear for additionalperiodsof30minutesto5hr.Allfourground- minutes. Sourceswithcandidatefeatureswerethenreobserved list isgiveninTable1.Eachsourcewasobservedfor 30 wider varietyofplanetarieswasaddedtothelist.Thesource compact radiocontinuumsourcesorCOemission,andthen a scope. Sourceswerefirstselectedforsignsofyouth,such as nebulae in1984AprilandJulyusingtheNRAO43mtele- and formaldehydeobservations,thecontinuumspectrum. ground-state survey,theVLAobservations,excitedOH mapping themwiththeVLAwillnotbepossible. at 2cm.Thecontinuumsourcesarelessthan1"insize,so optically thickat6cmandmayhavesignificantopticaldepth fluxes at6and2cmwavelengthsindicatethatbothobjectsare 349.2-0.2 iscomparabletothatofNGC6302.Continuum than 4timesthatofVy2-2,andthepeakOHfluxin of theirsample.ThepeakOHfluxin0.9+1.3is25Jy,more and Vy2-2,butthiswasaselectioncriterion,satisfiedbyhalf maser, whichistypicalofOH/IRstars.Inbothcasesthe 349.2 —0.2and0.9+1.3.Bothobjectscontaina1612MHz ning ofplanetarynebulaformation.Thedetectedobjectsare OH/IR stars.Theonsetofionizationcorrespondstothebegin- 1612 MHzspectrumshowsonlyonepeak,asinNGC6302 the expansionvelocitywouldappeartobeabout18kms. PBZ searchedforcontinuumemissionfromasampleofknown If thisvelocitycorrespondstothefarsideofnebula,then report atentativedetectionofCOatvelocityof-44kms“. redshifted emissioncomponent.KnappandMorris(1985) as at6035MHz.Thereisaweakindicationoneepochof allowed themtodetectalineeveniftheintensityweresame Their signal-to-noiseratioat6030MHzwouldnothave and wellwithinthevelocityextentof1612MHzemission. OH 6035MHzlineinemission,atavelocitynearthepeak along alineattheedgeofshell.JSSdetected confined toashell,withthe1612MHzlineemissionfalling tions bySeaquistandDavis(1983)foundtheionizedgastobe an ionizedwindwithconstantmassoutflow.VLAobserva- and Keenan1981).Theradiospectrumhasbeenmodeledby compact, highemissionmeasureradiosource(Kwok,Purton, environment (AitkenandRoche1982).Thenebulaisa 9.7 /¿m“silicate”featureindicativeofanoxygen-rich also aninfraredsource,andtheIRspectrumcontains optical spectrum(PerekandKohoutek1967).Thenebulais main-line features.Althoughtheobjectisstellarinappearance, it hasbeenclassifiedasaplanetarynebulaonthebasisofits ponent. Thereisstrongmaseremissionat1612MHzbutno of anOH/IRstar,exceptthatthereisonlyablueshiftedcom- 370 Three sigmaupperlimitsfor sources notdetectedarelisted We observed76planetarynebulaeandproto-planetary In thissectionwewilldiscussseparatelytheresultsof © American Astronomical Society • Provided by the NASA Astrophysics Data System a) TheGround-StateSurvey II. OBSERVATIONS PAYNE, PHILLIPS,ANDTERZIAN Me 2 IC 5217 Hu 12 IC5117 K362 M178 IC4997 Pc 23a HM3 484 K3233 3 IC 4634 29 IC 4593 A36 Me 2 LoTr 6 IC 3568 NGC 7662-55to 45127147110 NGC 7354-81to 19203190197183 NGC 7293-75to 25227217187207 a “ -194to -94 123132144141 -137to -37105111123 A31 “ -24to76135150132 -60to40126171150 A21 “ -89to11138141156120 Ps 1,K648,M15-48to5287939684 CRL 2688-79to211088499 - -124to-2477157107137 NGC 7009-86to14177207217193 J90 -100to0233217223227 M3-35 -213to-113168153135117 NGC 6853-74to26143133160137 M16a IC 2165 NGC 6833-142to-42168153159150 —100to100127137147107 NGC 6826-37to63150162141 IC418 J32 3 Sge-lOOtolOO117120107 BD +30°3639-63to37197203157 NGC 6803-17to83183180144 NGC 67907to107267237263223 ÏC 6122to222135156150 ~ -lOOtolOO120143140117 NGC 6741—108to—8174153165156 NGC 6720-25to25178186179 Hu 2-1,VV458-14to86135138147141 CN 3-1,VV391-25to75159181174141 SwSt-1 -57to43165183177189 NGC 6572-40to60223247237207 NGC 6543-100to0167180173183 NGC 6369-140to-40204174189171 -69to31183165147153 M“ 52to152180193220197 NGC 6210-66to34167132135120 -7to93183153 NGC 6058-27to73123126144117 -5to95846966 -! 8to108150144162132 -47to53183186168 LoTr 5-47to53132...138120 —80to20150156117 CTIO 1230-275-57to43153183165 NGC 4361-38to62147165159132 NGC 3587-36to64213667569 NGC 3242-100tol00140153147127 -11to90129150162168 NGC 2440Oto100111123114102 -35to65705969087 NGC 239214to114130133117 NGC 2371-2372-36to64330968793 NGC 2346-45to559087 ° -13to87150155152133 Ml-9 70to17018713713097 ~ -100tol00110137107 -11to89171180192162 ÍC 2149-83to17138150132126 -3to97159111120 ° -87to13123129144 Observedwith1.25MHzbandwidth andHanning-smoothed. IC 2003-70to30126140121 NGC 650-651-72to28117114132 NGC 246-100to0157197190160 NGC 40-62to38190177197183 1 Source Range1612166516671720 Name (kms')MHz Sources NotDetected -83 to17108147150141 Velocity TABLE 2 3 aUpperLimits(mJy) Vol. 326 1988ApJ. . .326. .368P -1 No. 1,1988 the 1612,1665,1667,and1720MHzdata.Anarrowalong the November-December. Adjacentbeamareasinthefourcardi- detected, briefoff-sourceobservationsweremadein1984 in Table2.Fortheremaining10directions,whichlineswere The on-sourcespectraareshowninFigures1and2,respec- elsewhere (Turner,Terzian,andPayne,1987). excitation. Wesubsequentlymadeastudyofconfusioninthe line ratios,whichareconsistentwithopticallythin,thermal were sufficientlydifferenttomakethistestinconclusive,butthe interstellar. InthecaseofNGC7027off-sourceprofiles identical) fluxscales.Fromtoptobottom,thespectradisplay tively. Inthesefigures,thespectraforallfourtransitionsare OH linenearthesesourcesandtheGalacticplane,reported OH emissionwasjudgedtobeinterstellaronthebasisof the OHemissionturnedouttobeextended,andtherefore nal directionswereobservedforthese10sources.Inmostcases features iswhatwouldbeexpectediftheOHassociatedwith are listedinTable3. et al(1983).TheresultsoffittingGaussianstothesespectra velocity scaleindicatestheopticallistedbySchneider shown onthesamevelocityscalewithoffset(butotherwise the mainlines:highest velocity componentisabsentat The shapeofthe1612MHzspectrumiswellreproducedin the the dustytorusinmodelofLesterandDinerstein(1984). absorption features.Therangeofvelocitiesspannedbythe OH strong 1612MHzemissionlinesatthevelocitiesof H i least threecomponentsinthe velocityrangefrom—45to two spectraindicatethattheremaybeasuperpositionof at tion. Differencesintheshapeofdominantfeaturethese at 1667MHz,andthelowest velocity componentisinabsorp- velocity componentisinabsorption 1665MHzandisabsent scale andonoffset,butotherwiseidentical,fluxscales.Thearrowthevelocitymarksofopticalnebula,asgivenbySchneider etal.(1983). 1720 MHztransition,exceptthatthelinesarenowinabsorp- 1665 MHzandisinabsorption at1667MHz,themiddle — 35kms.Thesamethree components seemidentifiablein The tworemainingdetectionsareNGC6302andVy2-2. Our OHobservationsofNGC6302revealtwoanomalously Fig. 1.—Ground-stateOHspectratowardNGC6302.Fromtoptobottomareof1612,1665,1667,and1720MHzlines,allonthe samevelocity © American Astronomical Society • Provided by the NASA Astrophysics Data System OH INNGC6302 tainty ofthecalibrationtwoobservations. than thatofDSP,differenceisprobablywithintheuncer- tion isclearlyanomalousthroughoutthisvelocityrange. tion at1665MHzandinemission1667MHz.Theexcita- our measurementofthe1612MHzlinepeakisabout10%less to asmallfractionofthe1612MHzlineintensity.Although spectrum andnolinesarepresentintheothertransitionsdown In Vy2-2,astrongmaserlineispresentinthe1612MHz Our OHspectrumofVy2-2isverysimilartothatDSP. Fig. 2.—Sameas1forVy2-2 LSR Velocity(km/s) 371 1988ApJ. . .326. .368P -1 1 beam isshownat20%,50%, and80%contours.Intheother beam, andhighercontours increasebyfactorsof2.The on alogarithmicintensityscale. Thelowestcontouris4Jy the 1612MHzcontinuummap inbothrepresentations, contour andgray-scaleplots. The upperleft-handfigureshows continuum source,whichisindicativeofstimulatedemission. particular, allofthelinefluxcomesfromregion the estimating thegainof43mtelescope.Itthereforeappears that allofthesingle-dishfluxispresentinVLAmaps. In trum thaninthe43mspectrum,probablyasaresultofover- channel mapoveraboxcontainingallofthecontinuumemis- Figure 3.ThereisactuallyslightlymorefluxintheVLAspec- sion. Thisspectrumisshownwiththe43m in map wasthenself-calibratedandCLEANed. band continuumdata(channel0,bandwidth2.34MHz).This by subtractingthesumoflineemissionfrombroad- CLEANed. Amoresensitivecontinuummapwasthenmade channel mapswherelineemissionwasdetectedwerethen channels andthensubtractedfromeachchannelmap.Thefive 4.54 kms“.Acontinuummapwasformedfromline-free so thevelocityresolutionandchannelspacingareboth nels wereobserved.SpectraHanning-smoothedonline, size ofapproximately2"5x4"4.Thirty-onespectrallinechan- were processedwithuniformweighting,resultinginabeam results wereobtainedbycombiningthesetwodatasets.Data in 1986JunetheA-Bhybridconfiguration.(Approximately one-fourth ofthesedatawerelostduetointerference.)Final continuum source.Anearlycompletesynthesiswasthenmade (long northarm)configurationconfirmedthecoincidenceon the skyof1612MHzlineradiationwithNGC6302 372 Maps aredisplayedinFigure 4(Plate4)ascombination A VLAspectrumwasformedbyintegratingeachline Initial VLAobservationsin1985JunetheB-Chybrid 1 o-statisticalerrors. 1665 (150) 1612 4640±360-62.010.143.780.34 (MHz) 0 3 b 4765 1720 (130) 1667 (150) 6035 6030 4750 6016 1720 1667 1665 1612 Line FromSchneideretal.1983. Numbersinparenthesesare3aupperlimits.Quotederrors Fullwidthathalf-intensity. 01 NGC 6302;OpticalLSRVelocity—31.4kms 01 © American Astronomical Society • Provided by the NASA Astrophysics Data System Vy 2-2;OpticalLSRVelocity—53.9kms 3 -270 ±12 -238 ±12 -100± 13 Peak Flux -53 ±10 183 ±10 113 ±13 550 ±20 110 ±13 110 ±10 140 ±20 93 ±17 (mJy) (250) (250) (40) b) VLÂObservations OH Detections TABLE 3 LSR Velocity -37.31 ±0.09 -37.82 ±0.11 -41.12 -51.60 -37.52 -45.15 -54.36 -42.63 -51.70 -40.5 -51.28 1 (km s“) ± 0.19 ±0.22 ±0.24 ±0.30 ±0.28 ±0.31 ±0.53 ±0.09 ±0.33 b Line Width 3.39 ±0.26 4.22 ±0.26 1 4.76 ±0.70 8.16 ±0.48 3.86 ±0.52 3.37 ±0.57 4.17 ±1.29 3.06 ±0.66 5.99 ±0.76 6.66 ±0.23 5.66 ±0.80 (km s“) PAYNE, PHILLIPS,ANDTERZIAN -1 -1 2 -1 -1 2 - x - the ground-stateemission.If theobservedabsorptionorig- excited OHabsorptioncoincide withthelow-velocityedgeof tively. Asseeninthefigure, thevelocitiesspannedby of about1.5and2.1inthe6030 and6035MHzlines,respec- optically thinratioof1.43,which wouldimplyopticaldepths -1 these spectra.ThefittedparametersarelistedinTable3. in the6035MHzspectrum,andis0.48kms the 6030 MHzspectrum.SingleGaussianswerefittedtoeach of are showninFigure3.Thevelocityresolutionis0.24km s 6035 MHzlineswerebothdetectedinabsorption.Thespectra thermodynamic equilibriumcase,are1:14:20.The6030 and OH. Theintensityratiosfortheselinesintheopticallythin, 6035 MHztransitionsoftheU,J=5/2excitedstate of lines weredetecteddowntoa3limitof0.25Jy. with avelocityresolutionof0.6kms.Thesefrequencies are on theedgeofbandcoveredbyCassegrainreceiver. No down toa3limitof0.29Jy. with avelocityresolutionof0.3kms.Nolinewasdetected n, J=1/2excitedstateofOHweremadesimultaneously, 2.5 MHzbandwidth. vations. Spectrawereobtainedbyfrequencyswitchingwithina receiver. Onlyonepolarizationwasavailablefortheseobser- 43 mtelescopein1986SeptemberusingtheCassegrainmaser (DSP), orthe1612/1665ratioof100for0.9+1.3(PBZ). 3/2 ratios aremuchlessthanthelowerlimitof150:1forVy2-2 tion inthemap.Thefluxratioat-40kms,peak times thatat1667MHz,anditdoesappearthesameposi- map inthetwoadjacentvelocitychannels.Atof the detectedfeature,fluxat1612MHzis248mJy,or2.7 l/2 flux of63mJyisseeninthechannelcenteredat—44.4kmsL detected onlyinasinglechannel.Anemissionfeaturewith Four sigmaemissionpeaksappearatthesamepositionin The UVcoverageforthesedataispoor.Lineradiation 1612 MHzspectrum,appearstobeabout15:1.Theseflux therefore fortheHiabsorptionfeatureaswell. to ruleoutaninterstellaroriginfortheOHemission,and mogeneous, eitherindensityorexcitation(orboth).The scale structure,andthemaseremissioncharacteristicsappear apparent associationwiththedarkdustlane,smallangular 1986 Junerunwheninterferencewaspresentat1612MHz. ground continuumradiation.Theemittinggasmustbeinho- position withthepeakofcontinuumemission,eventhough emission, whichoccursat-40kms,doesnotcoincidein the lineemissionresultsfrommaseramplificationofback- of Rodriguezetal(1985).Alsonotethatthepeakline with respecttothecontinuumsource.Thisgradientliesalong the darkdustlanevisibleinopticalphotographs,suchasthat positional gradientasafunctionofvelocitythelineemission lowest contourisnow16Jybeam,againincreasingbyfactors of thecontinuumsourceareshowninwhite. of 2forthehighercontours.Forclarity,contoursincenter center velocityofeachchannelisgivenattheupperright.The but thecontoursshowCLEANedspectrallinedata.The five mapsthegray-scaleplotagainshowscontinuumdata, The ratioofthefittedlineamplitudes is1.13ratherthanthe Finally, observationsweremadeofthe6016,6030,and Observations ofthe4750and4765MHztransitions Observations attheformaldehydefrequencyweremade Observations near5and6GHzweremadewiththeNRAO A briefVLAobservationat1667MHzwasmadeduringthe The mapsarepresentedinthisfashiontoemphasizethe c) ExcitedOHandFormaldehydeObservations Vol. 326 -1 1988ApJ. . .326. .368P continuum source are showninwhite. The gray-scalecontinuummapisalso shownintheotherfivepanels,butcontoursshowCLEANed spectral linedataatthevelocityindicatedupper right ineachpanel.Thelowestcontour isnow16mJybeam,againincreasingbyfactorsof2forthehigher contours.Forclarity,contoursinthecenterof a logarithmicintensityscale.Thelowest contouris4mJybeam“\andhighercontoursincreasebyfactorsof 2.Thebeamisshownat20%,50%,and80%contours. Payne, Phillips,and Terzian{see326,372) PLATE 4 Fig. 4.—VLA1612MHzlineandcontinuum maps.Intheupperleft-handpanel,continuummapisshown incontourandgray-scalerepresentations,bothon © American Astronomical Society •Provided bythe NASAAstrophysics Data System 1988ApJ. . .326. .368P -1 No. 1,1988 Rotationally excitedOH6030and6035MHzspectratowardNGC6302 on offset,butotherwiseidentical,fluxscales.Allfourspectraarethesamevelocityscale. inates inthesameregionasground-stateemissionat the molecules arecospatial. emission at—35.5kms,scaledcrudelybytheratiooftotal flux. Thecontinuumfluxwithinthecontoursof1612MHz deduce bycomparingthescaleinFigure3withcontinuum small, sothattheopticaldepthismuchhigherthanonewould same velocity,thenthecoveringfactorofabsorbinggas is tively suggestthattheground-stateandexcited-state OH the absorptionlinesandderivedopticaldepths.Wetenta- flux at1612and6035MHz,isconsistentwiththestrength of from theVLAdataonshort baselines. Fluxesof3.4±0.2and NGC 6302.Afluxof2.3±0.2 Jyat1612MHzwasobtained 3.5 ±0.2Jyat4800and6035 MHz, respectively,wereobtained Fig. 3.-^-{Top)OH1612MHzspectraofNGC6302obtainedattheNRAOVLAand43mtelescopesonoffset,butotherwiseidentical,fluxscales. (Bottom) We haveobtainedthreenewcontinuum totalfluxvaluesfor © American Astronomical Society • Provided by the NASA Astrophysics Data System d) ContinuumSpectrum OH INNGC6302 was checkedbyobserving3C295,usingfluxesobtainedfrom the averageofeightmeasurements.Thegaintelescope the averageoffourmeasurements,andfluxat6035MHz is in Figure7ofRodriguezetal(1985),reproducedhereas our correction isthemajoruncertaintyinthesemeasurements. Although allofourpointsare abovethiscurve,theyareconsis- the modelfittedtoprevious databyRodriguezetal(1985). agrees exactlywithaprevious measurement.Thesolidcurveis are verydifferent,however,andtheuncertaintyingain Baars eial(1977).Thedeclinationsof3C295andNGC6302 homogeneous modelwithelectron temperatureT=21,000K, tent withthismodel.Themodel isasimpleplane-parallel, Figure 5.Ourdataareshown asopencircles,oneofwhich Gaussians tothesourcedeflection.Thefluxat4800MHz is at the43mtelescopebyscanningacrosssourceandfitting e Our newmeasurementshavebeenaddedtothedatashown 373 1988ApJ. . .326. .368P _1 7-6 7-6e radiation, containsaveryhot centralsourceofionizingradi- an abundantsourceofinfrared andcentimeterwavelength lines arethestrongestof excited-state lines.NGC6302is the strongestofground-state lines,whilethe5cmmain (Henkel, Batrla,andGüsten 1986). AndonlyinVy2-2does one findanotherobjectwhere the1612MHzsatellitelineis with 5cmabsorptionisuniquetoNGC6302,amongGalactic objects, althoughseeninthemegamasergalaxyIC4553 by thedetectionstatisticsofJSS.Emissionat18cmtogether indeed associatedwiththenebula. man andDyck(1986),leadingustoconcludethattheCO is model ofLesterandDinerstein(1984).Itisalsoapparent that expansion velocityisabout20kms,inagreementwith the the kinematicsofsourceallowCOvelocityZucker- the nebularexpansion,thenonewouldconcludethat the peak ofthecontinuumemissioncoincideswithcenter of sible rotation(althoughnotlikely)aswellexpansion.If the et al.1986).Thesemayturnouttobecarbon-richratherthan oxygen-rich objects. IC 418(TaylorandPottasch1987)4997(Altschuler ary nebulaerecentlyfoundtocontainneutralhydrogen: planetary nebulae. previous surveys—OHisnotgenerallyassociatedwithevolved tinuum radiation. the gasinfrontofcontinuumsourcecanamplifycon- be duetothegeometryrequiredforstimulatedemission:only of thenebula.Theabsencearedshiftedfeaturemustinstead predicts acontinuumopticaldepthof0.93at1612MHz.This optical depthistoosmalltohidelineemissionfromthefarside and emissionmeasureEM=1.5x10cmpc.Thismodel measure EM=1.5x10cmpc. (1985), fromwhichthefittedcurveisalsotaken.Parametersofplane-parallel,homogeneousmodelareelectrontemperatureT=21,000 Kandemission 374 Detection of5cmOHlinesisalsoratherrare,asconfirmed The VLAdataforNGC6302givetheappearanceofpos- We failedtodetectground-stateOHfromtwootherplanet- Our ground-statesurveydoesnotaltertheconclusionof Fig. 5.ContinuumspectrumofNGC6302.Ourmeasurementsareshownasopencircles.ReferencestoothermaybefoundinRodriguez etal. © American Astronomical Society • Provided by the NASA Astrophysics Data System in. DISCUSSION PAYNE, PHILLIPS,ANDTERZIAN Frequency (GHz) pated byexpansiontothepointwhereOHisnolongerexcited, (there isnoextendedmolecularcloud)willeventuallybedissi- and thentothepointatwhichmoleculesaredissociated by ceased, andthemolecularmaterialassociatedwiththatwind which mustbebrief.Massflowfromamassiveredgiant has considered aspossiblecandidatesforproto-planetarynebulae. have provedtobeverysuccessful,andthesesourcesshould Lewis, andTerzian1988)todetectOHfromIRASsources represent objectsintransition.Recentsurveys(e.g.,Eder, and thusitispossiblethatobjectslikeVy2-2NGC6302 fore beunsaturated. instead duetothegeometryrequiredforstimulatedemission. According toElitzur(1982),the1612MHzmasermustthere- emission isnotduetoabsorptionbytheionizednebulabut NGC 6302,however;theabsenceofredshifted1612MHz nisms. shocks; hencethereexistmanypossibleexcitationmecha- ation, andshowssignsofexpansionwhichcouldgiveriseto Like Vy2-2,NGC6302isobservedinanevolutionarystage OH/IR starsmaybetheprecursorsofplanetarynebulae, One constraintcanbeplacedontheexcitationof Expansion velocity Ground-state OH CO emission 5 cmOH Continuum structure..... Continuum opticaldepth. Emission measure Continuum flux ‘ KnappandMorris1985. Property Comparison ofNGC6302withVy2-2 IV. CONCLUSIONS TABLE 4 1 Absorption lines Yes Bipolar High Low High 1612 maser ~20 kms“ NGC 6302 -1 a 6035 maser Yes Shell High High 18 kms Low 1612 maser Vy 2-2 Vol. 326 1988ApJ. . .326. .368P No. 1,1988 closely resembleatypicalplanetarynebula. radiation fromthehotcentralstar.Atthatpointitwillmore NGC 6302mustbeconsideredanotheruniqueobject,withits Vy 2-2isgiveninTable4.IntermsoftheOHradiation, emission. Observationsofthe6cmOHlineswouldalsobe required beforethoselinescanbeassociatedwiththe18cm own challenges.Interferometricmapsofthe5cmlinesare J. A.PhillipsandYervantTerzian:NationalAstronomyIonosphereCenter,DepartmentofAstronomy,Space Sciences interesting. ExistingCOobservationsmustbeseverelybeam- diluted, andinterferometricobservationsmightprovefruitful. Building, CornellUniversity,Ithaca,NY14853-6801 Aitken, D.K.,andRoche,P.F.1982,M.N.R.A.S.,200,217. .19806,M.N.R.A.S.,193,631. 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