1991ApJ. . .376. .150C moving throughagas-richenvironment. by Hua&Louise(1990).Usingtheradialvelocitiesderived much attentionuntilGieseking&Solf(1986,hereafterGS) (CJA). Eventriple-shellnebulaearerare;onlyseven are however, hardlyanynebulahasmorethanthreevisibleshells from thefollow-upechelleobservations,wefindthatsome of was stillnotdetected. Jacoby, &Arendt(1987,hereafterCJA)didshowtwoansae faint staticenvelopeoutsidethebrightmainnebula,while dation. of NGC6751.6751isapparentlyatriple-shellnebula detected byGSbutalsoadditionalfaintfilamentsextending exposure CCDimagesshowclearlynotonlyallthesystems corresponding tothebipolaroutflow,butfaintenvelope neither systemwasregisteredintheonlyphotograph(Duncan discovered spectroscopicallyabipolarmassoutflowand are theambientinterstellarmaterialionizedbycentral the outerfilamentsbelongtoahaloofNGC6751,whileothers static envelope.Thesefaintfilamentshavealsobeendetected out toaradiusgreaterthan100",fourtimesatextendedas the Astronomy Observatories,whichisoperated bytheAssociatedUniversitiesfor Research inAstronomy,Inc.,under contract withtheNationalScienceFoun- and attheEuropeanSouthernObservatory, LaSilla,Chile. our observingprogramformultiple-shellPNs.Ournewdeep- 1937) availabletoGS.TheCCDimagepublishedbyChu, The AstrophysicalJournal,376:150-160,1991July20 © 1991.TheAmericanAstronomicalSociety.Allrightsreserved.PrintedinU.S.A. 2 1 The multiple-shellphenomenonisprevalentinPNs; The planetarynebula(PN)NGC6751hadnotattracted VisitingAstronomer,KittPeakNational Observatory,NationalOptical Intrigued byGS’sdiscovery,wehaveincludedNGC6751in Basedonobservationscollectedat theKittPeakNationalObservatory dance ratioenhancementinthe[Nii]-brightknotsmainnebula.Fromphysicalstructureof Subject headings:nebulae:individual(NGC6751)—internalmotionsplanetary shells inNGC6751wederiveinformationabouttheaveragemass-lossratesatvariousstages. He abundancedifferencebetweentheplanetarynebulaandambientmedium;wealsofindanN/Oabun- . NGC6751isapparentlyamultiple-shellnebulamovingthroughgas-richenvironment.Wefindan low-dispersion spectratodeterminethephysicalconditionandabundancesofeachfeatureinwhole additional fainthalosextendingouttoamuchlargerradius. of thisnebula,whichdetectnotonlythemorphologicalcounterpartsaforementionedfeaturesbutalso nebula NGC6751byGieseking&Solf.Tostudythephysicalstructure,wehaveobtaineddeepCCDimages We havefurtherobtainedlong-slitechellespectratodeterminetheinternalmotionandradialvelocity, An intriguingbipolarmassoutflowandafaintenvelopeweredetectedspectroscopicallyintheplanetary © American Astronomical Society • Provided by the NASA Astrophysics Data System 1 THE MULTIPLE-SHELLSTRUCTUREOFPLANETARYNEBULANGC6751 1. INTRODUCTION nebulae: structure Astronomy Department,UniversityofIllinois,1002W.GreenStreet,Urbana,IL61801 Astronomy Department,WilliamsCollege,Williamstown,MA01267 Kitt PeakNationalObservatory,P.O.Box26732,Tucson,AZ85726 Instituto deAstrofísicaCanarias,E-38200LaLaguna,Spain Received 1990September28;accepted1991January21 Arturo Manchado Karen B.Kwitter George H.Jacoby 2 You-Hua Chu ABSTRACT AND 150 telescope atKittPeakNational Observatory(KPNO)on1986 September 21.A512x Tektronix CCDwasused.The NGC 6751(Aller&Czyzak1983),wehaveobtainedlow- features canberesolvedandstudieddifferentially. dispersion, long-slitspectraofNGC6751sothatitscomplex chemical abundancesineachshellreflecttheconditionof mass lossandtheformationmechanismofshells.The extended historyofthestellarmassloss,sinceeachshellpre- chado, Pottasch,&Mampaso1988;thispaper).Thetriple- known: NGC2022,6720,6751,6826, medium in§6. NGC 6751anditsinteractionwiththeambientinterstellar morphology, kinematicproperties,andphysicalconditions supplement theexistingsingle-aperturespectroscopicstudyof atmosphere ofthecentralstarattimemassloss.To it isimportanttostudythekinematicsandphysicalconditions shell nebulaeprovideauniqueopportunitytoexaminean 6891, NGC7662,andA78(CJA;Moreno&López1987;Man- and abundancesarereportedin§§3,4,5,respectively. nebula NGC6751.Theobservationsaredescribedin§2. The and low-dispersionspectroscopicstudiesofthetriple-shell It isimportanttostudyeachtriple-shellnebula.Furthermore, sumably marksanerawithadistinctmodeorrateofmassloss. Finally wediscussthemass-lossandformationmechanism of of eachshellinordertodeterminethecharacteristics The CCDimagesofNGC6751 wereobtainedwiththe2.1m In thispaperwereporttheresultsofourimageryandhigh- 2. OBSERVATIONSANDDATAREDUCTION 2.1. Imagery 1991ApJ. . .376. .150C -1 1 -1 s. Thespectra weretakeninthe[Om]line tominimizethe lar tothedispersion.Theinstrumental FWHMis9.3±0.3km lent to~4.5kms“alongthe dispersionand0"33perpendicu- 2x2 binning,andtheresulting pixelsize(30pm)wasequiva- at 5000Â.TheUVflooded800 x800T12CCDwasusedwith camera. Theresultingreciprocal dispersionwas2.47Âmm on 1987September1-2. with theechellespectrographon4mtelescopeatKPNO Therefore, weobtainedfourlong-slitspectraofNGC6751 which canbestudiedwithhigh-dispersionspectroscopy. phological featuresinanebulaisthroughthekinematics, ments inthesouthwestquadrant;however,noadditional panel) features.TheHa,[Nn]and[Om]full-frameimagesare nebulosity isseen. images tolookforcounterpartsofthenorthwestirregular fila- with a30"southoffsetfromthepositionofotherthree ments. Thelastimage,ashorterexposurein[Nn],wastaken reproduced inFigure2(Plate2)toshowthefaintouterfila- levels toaccentuatethebright(toppanel)andfaint(bottom duced inFigure1(Plate1).Eachimageisdisplayedatfour exposure times.Fourimagesthroughinterferencefilterswere was 3'.Thelowreadoutnoise(6electrons)oftheTektronix pixel sizewas27//m,correspondingto0"35.Thefieldofview sure times. obtained. SeeTable1Aforthefiltercharacteristicsandexpo- CCD madeitpossibletodetectfaintnebulosityeveninshort -1 We useda79linemmechelle gratingandthelong-focus A directwaytodeterminetherelationshipamongmor- The central80"xofthefirstthreeimagesarerepro- 4 [Nii]6585,155 2 [Oin]5004,275 3 [Nii]6585,1510 1 Ha6563,115 4 38"E,52"NO20 2 ...020 3 35"W020 2 19"W34.560 1 ...105°30 1 ...104°10 Number Line(Â)(minutes) Number CentralStarAngle(minutes) Number CentralStarAngle(minutes) © American Astronomical Society • Provided by the NASA Astrophysics Data System Low-Resolution SpectroscopyofNGC6751 Echelle ObservationsofNGC6751 2.2. EchelleObservations Images ofNGC6751 Nebular A,ExposureTime Offset fromPositionTime Offset fromPositionTime TABLE 1A TABLE 1C TABLE IB Filter MULTIPLE-SHELL STRUCTUREOFNGC6751 Exposure Exposure -1 nebula andthe ansaethatcorrespondtoGS’s bipolaroutflow. The firstslitpositionwasselected tocoverboththecentral the brightcentralnebulaand the faintouterhalos,respectively. The pixelsizeis15pmby60 pm,correspondingto3.45Âby sion alongthedispersionand theshortinspatialdirection. effective resolutionof4.1Â.Thedatawererecordedwith the the spectraarereproducedinFigure3(Plate3).Thefirsttwo resentative lineprofilesareplottedinFigure5. ESO CCD11chipin1030x160arrays,withthelongdimen- second echellelineimageispresentedinFigure4.Severalrep- and thefaintouterpart,respectively.Acontourmapof spectra weredisplayedtwicetoshowthebrightinnernebula 1"35. Theseeingwasabout1"6. 300 grismgivesareciprocaldispersionof230Âmmand an D’Odorico 1985)ontheESO3.6mtelescopeatLaSilla in Object SpectrographandCamera(EFOSC;Dekker & long-slit spectroscopywasconductedusingtheESOFaint intrinsic thermalwidthsandtoavoidthecontaminationof gas inthecentralnebulaaswellhalo,low-dispersion times aregiveninTableIB. telluric lines.Theslitpositions,orientations,andexposure 1988 March.Theslitusedwas0"5wideand3'long. blue levels areat90%,75%,50%,25%,10%,5%,2.5%,and1%ofthepeakvalue, 818 counts. Two slitpositions,shownin Figure 2c,weretakentocover The slitpositionsarealsoshowninFigure2c.imagesof In ordertoinvestigatethephysicalconditionofionized Fig. 4.—Contourmapoftheechellogramdisplayedin3b.Thecontour 2.3. Low-DispersionSpectroscopicObservations ( km/s) 151 1991ApJ. . .376. .150C Chu, Manchado,Jacoby,&Kwitter (see376,151) © American Astronomical Society •Provided bythe NASAAstrophysics Data System Fig. 1.—CCDimagesofNGC6751 in theHa,[Om],and[Nn]lines,displayedatfourlevelsofintensity.North isatthetop,andeasttoleft. Ha (0111)(Nil) PLATE 1 19 91ApJ. . .376. .150C the high-andlow-resolutionspectra are shown. Chu, Manchado,Jacoby,&Kwitter (see376,151) PLATE 2 Fio 2.—(a)HaimageofNGC6751. Northisatthetop,andeasttoleft,(b)[Nii]imageofNGC6751. (c)[Om]imageofNGC6751.Theslitpositions © American Astronomical Society •Provided bythe NASAAstrophysics Data System [oiin Fig. 2c 1991ApJ. . .376. .150C Chu, Manchado,Jacoby,&Kwitter(see376,151) centered onthecentralstaralongnorth-southdirection.Theechellogramwasdisplayedattwointensitylevelstoshowbrightinnernebula andthefaint central star,centeredontheirregularfilamentsinnortheast. envelope, (c)Slitposition,havinga35"offsetfromthecentralstar,centered onthefainthalo,{d)Slitposition,havingaoffsetof38"eastand52"northfrom star, atapositionangleof105°.Theechellogramisdisplayedintwointensitylevelstoshowthebrightinnernebulaandfainterenvelope andansae,{b)Slit 1 Fig. 3.—[Oin]echellogramsofNGC6751.Everypanelcoversabout350kms"withthesamebeginningandendingvelocities,(u)Slitcentered onthecentral © American Astronomical Society •Provided bythe NASAAstrophysics Data System 350 km/s PLATE 3 1991ApJ. . .376. .150C 152 CHU,MANCHADO,JACOBY,&KWITTERVol.376 ([no offset],1"),(16':3SE,4"),(18':3S,(24'.'33'.'3),(22"3N,(43"SW, envelope, theouterhalo,andirregularfilamentsin the The secondslitpositionwasselectedtocoverthestatic northeast. Table1Cgivestheoffsetandpositionangleof the 776), (67"NE,27"). and size(offsetfromcentralstar,slitlength)ofthesespectraare(13"9SE,2"3), slit positionsandtheexposuretimes. ent morphologicalfeaturesinFigures1and2.Inorder of increasing radius,ordecreasingsurfacebrightness,these fea- (2) apairof6"longansaeprotrudingfromthemainnebula at tures are(1)thebrightcircularmainnebulawith25"diameter, position anglesof104°and284°,(3)anapproximatelyeast- west-oriented 42"x28"ovalenclosingtheinnernebula and the ansae,(4)afaint62"x54" envelope,(5)afaintincomplete extending to105"radiusinthe northeastquadrant.Thesefea- knotty ringataradiusof—46", and(6)faintirregularfilaments consists ofasmoothinnerring (16"across)andaknottyouter tures areindividuallydescribed below. Fig. 5.—High-resolutionlineprofilesatselectedpositions.Theslitposition The structureofNGC6751iscomplex.Therearesixappar- The circularmainnebula(see toppanelsofFig.1)actually © American Astronomical Society • Provided by the NASA Astrophysics Data System 3. MORPHOLOGYOFNGC6751 HELIOCENTRIC VELOCITY (km/s) -1 1 _1 curved andknotty.Theseansaeareatthesamelocationwhere ring (25"across).Theknotsintheouteraremostcon- “ knot”atthewesterntipofansaeisaforegroundstar.The GS detectedthebipolaroutflow.Notethatbrightest than justionizationandexcitationstratificationsinanebula. between the[Om]and[Nn]imagesprobablymanifestsmore brightness decreasesoutwardwithlittleperturbationfromthe spicuous inthelow-excitationlineof[Nn].The[Om]surface ansae areenclosedinanoval(seethebottompanelsFig.1) whether theansaeandovalbelongtosamesystem,with aligned alongasimilardirectiontotheansae.Itisnotclear the ansaebeingcondensationsinoval. [N ii]-brightknots.Asweshallshowlater,thiscontrast envelope isidenticaltothestaticdetectedspectro- lines. Itappearspatchierinthe[Nn]line.Thesizeofthis brightened andhasradialfilamentsinbothHa[Om] scopically byGS. located at—46"fromthecentralstarofNGC6751;these irregular filamentsinthenortheastquadrantatalargerdis- west-southwest-south. Theothergroupconsistsofthefainter, knots arebestseeninthesectorsnortheast-east-southeastand groups accordingtolocation,morphology,andspectroscopic envelope. Thesenebulositiescanbeseparatedintotwodistinct intensity ratiosinbothgroupsaresignificantlyhigherthan behavior. Onegroupconsistsoftheringknottymaterial those intheenvelope. tance, about60"-100"fromthecentralstar.The[Nn]/Haline the echelleobservations.Althoughcoverageisnotade- the internalmotioncanbewelldiagnosedfromvelocity quate toallowthree-dimensionalspatiokinematicmodeling, dispersions andtheradialvelocities. (after correctingforinstrumentalandthermalbroadenings)in ing atauniformvelocity,sincetheintrinsicvelocitydispersion sion velocityof42.5kms.However,theshellisnotexpand- an expanding-shellstructure.Thepeak-to-peaklinesplitis profile extractednearthenebularcenter(seeFig.5)is the shellislarge,FWHM=28±3kms“.Thevelocity asymmetric—steeper towardthesystemicvelocity.These 85 ±2kms,whichistraditionallyinterpretedasanexpan- velocity structuresmostlikelyresultfromagradient between thesetwopossibilities,oneneedstomapthevelocity or alargeamountofturbulencewithintheshell.Todistinguish metry inthenebula. models ofanexpandingshellwithbuilt-invelocitygradient. Additional constraintscanbeprovidedbytheexpectedsym- structure overtheentiremainnebulaandcompareit with ities, givingthebow-shaped line imageatiltedlook(Fig.3a) not followtheexpansionpatternofaforementionedshell. theless, theknotsappearasbright blobsathigherradialveloc- expansion ismostlytangentialtothelineofsightand the These knotsareattheperipheryofmainnebula,where the emission lineshouldappearnearthesystemicvelocity;never- seen inotherplanetaries,such asNGC3242,6826, 7009, andNGC7662(Balick, Preston,&Icke1987).Itispos- or adouble-hornedimpression (Fig.3b).Thisphenomenonis The ansae(seethemiddletworowsofpicturesinFig.1)are The faintenvelope(seethebottompanelsinFig.1)islimb- In Figure2therearestillfaintnebulositiesoutsidethe All sixmorphologicallyidentifiedsystemsweresampledin The mainnebulashowsabow-shapedlineimage,indicating The knottyouterring(25"across)inthemainnebulamay 4. KINEMATICPROPERTIESOFNGC6751 1991ApJ. . .376. .150C -1 _ 1 -1 _ 1 field inthenebulacanplacemoreconstraintsonthisambi- located inaninclinedequatorialbandwithinathickshellwith geometric projectioneffect.Forexample,iftheknotsare built-in velocitygradient,thisconfigurationmayexplainthe note thatthevelocityanomalycanalsobecausedbypurely sible thattheseknotsexpandfasterthantheshell;however,we curved inthelineimage.Likeknottyring,interpreta- guity. observed radialvelocities.Acompletemappingofthevelocity curved filamentwithauniformexpansionvelocity.Itisinter- the curvatureoflineimagemayindicatearealdeceleration from thefaintestpartofinnerring.Itisnotclearwhether esting tonotethatintheechellelineimageansaeprotrude tion dependsonthethree-dimensionalgeometryofansae; No. 1,1991 indicates anexpansion.Ourslitpositionsareeitheralongthe of expansionvelocityatlargerradiusortheprojectiona extended thanthemainnebula,oralonglongeraxis,where nebula atthesystemicvelocity.Thelinebroadeninginoval tinct velocityfeature;itappearsasanextensionofthemain this anticorrelationinintensityhasanyphysicalsignificance. the ansaedominateemission.Notmuchinformationabout shorter axisoftheoval,whereovalisnotmuchmore in thenortheast.See Fig.2cfortheslitposition. {a) Halinealongtheslitpassingthrough thecentralstar,(b)[Om]linealongslitpassingthroughenvelope andhaloofNGC6751theirregularfilaments central starare19.4,13.4,16.8,14.1,and12.6kms,respec- echellograms. Theobservedlinewidthsextractedat18"south, the ovalcanbederivedfromourspectra. 24" south,22"north,southeast,and26"northwestfromthe ity of10±1kmsforthis62"x54"envelope. profile andthermalbroadening,wederiveanexpansionveloc- radius, broaderlineprofilesatsmallerradii,canbeexplained tively. Theanticorrelationbetweenthelinewidthand distance tothecentralstarrangesfrom41"46".Assum- ring at46"radius.ThelineisnarrowwithanobservedFWHM by aslowexpansion.Aftercorrectingfortheinstrumental The ringisdetectedforabout15"alongtheslit,ofwhich but losesomeaccuracyindeterminingtheexpansionvelocity. of 12.7kms,correspondingtoanintrinsicturbulent the ring,wherewebenefitfromhighersurfacebrightness FWHM of6.2kms.Theslitpositionisneartheperiphery The ansae,correspondingtoGS’sbipolaroutflow,appear The east-west-oriented42"x28"ovaldoesnothaveadis- Fig. 6.—Surfacebrightnessprofiles extracted fromthelow-dispersionspectra.Theeffectiveslitpositionsfor the extractedspectraarelabeledbelowprofiles. The staticenvelopedetectedbyGSisclearlyinour The thirdechelleobservationwastakenonthefaintknotty © American Astronomical Society • Provided by the NASA Astrophysics Data System MULTIPLE-SHELL STRUCTUREOFNGC6751 _1 -1 -1 -1 -1 -1 projected tothelineofsightat41"star,then ing thattheFWHMof6.2kmsisduetoexpansion (Kid) °fthiscomponentis13+1kms,whilethatofthe files arenarrow,withanobservedFWHMof13.8kms, lar filamentsatthenortheastquadrant.Theextractedlinepro- large velocitydifferencebetweenamainnebulaanditshalo main nebulaofNGC6751is—42±2kms.Althougha This componentisunlikelytobepartofNGC6751because corresponding toanintrinsicturbulentFWHMof8.2kms. ments probablyhaveaninterstellarorigin;theywouldbe faint haloatthenorth,andFirregularfilamentsin averaged, wefirstinspectedthespectravisuallylineby part oftheplanetarynebulaNGC6751.Theseirregularfila- central starmakesitunlikelythattheirregularfilamentsare has beenseenintheHelixnebula(Walsh&Meaburn1987), the largedifferenceinradialvelocity.Theheliocentricvelocity due totheuncertaintyingeometriccorrection. the expansionvelocitycanbeapproximatedas(FWHM/2) northeast. ThespectraareshowninFigure7. knotty outerringinthemainnebula,Aansaeonboth central star,Clisthesmoothringinmainnebula,C2 the morphologicalfeaturesdefinedin§3,CSiscenteredon the HasurfacebrightnessprofilesinFigure6.Comparedwith respectively. Thelocationsofthesespectraarelabeledagainst spatial extentsare8'.T,10'.'8,lO'.'S,SH,21'.'6,4'.'0,and9'.'5, to asCS,Cl,C2,A,E,H,H',andF,wereextracted;their shared similarspectralcharacteristics.Eightspectra,referred along theslit,thengroupedtogetheradjacentspectrathat H iiregionionizedbythecentralstarofNGC6751. the lackofvelocityprofilevariationatdifferentdistancesto sides, Etheenvelope,Hfainthaloatsouthwest,H' from theHa/H/?ratio(E_=0.45)isslightlylowerthanthat (Ha, Hß,andHylines)inthemainnebula,sinceitsspectra have thehighestsignal-to-noiseratio.Theextinctionderived BV x {sin[cos(41/46)]}=7±2kms.Thelargererroris To determinethespatialextentsoverwhichspectracouldbe Finally, thelastslitpositionwaschosentocoverirregu- The extinctionisdeterminedfromtheBalmerdecrement 5. PHYSICALCONDITIONSOFTHEIONIZEDGAS 5.1. ExtractionofSpectralLineStrengths 153 1991ApJ. . .376. .150C © American Astronomical Society 154 Provided bythe NASA Astrophysics Data System

Fig. 7.—Low-resolution spectra of NGC 6751, (a) at position CS, (b) at Cl, (c) at C2, (d) at A, (e) at E, (/) at H, and {g) at F 1991ApJ. . .376. .150C © American Astronomical Society • Provided by the NASA Astrophysics Data System 155 1991ApJ. . .376. .150C 16a (in regionsA,E,andH'),weassumeareasonabledepth and width; fortheenvelopeandhalo,weassumeaspherical density. Fortheansae,weassumeitsdepthissameas its À6116 lineratio,whenevertheselinesaredetected.When the each oftheeightregionsarepresentedinTable2. the abundanceanalysisbelow. poor signal-to-noiseratioof its 4363Âline.InregionF,only electron temperatureisnot well determinedbecauseofthe for thehigh-excitationregion.InregionofC2,[O m] shell geometry. use theHßfluxtocalculaterootmeansquareelectron extinction isslightlylowerthanthevalue=0.6givenby implies thattheextinctionismostlyofinterstellarorigin.Our within theerrorlimits,withthisamountofextinction.The The Bahnerdecrementsintheotherspectraareallconsistent, 0.46 ±0.01derivedfromHa/Hyfortheextinctioncorrection. uncertainty; weassumeanelectron temperatureof8000Kfor region andfromthe[Om]lineratio(¿4959+¿5007)/¿4363 Hß flux(Pottasch1984).Thedereddenedlinestrengthsfrom E_ =0.39derivedfromthe6cmradiocontinuumand Aller &Czyzak(1983),butslightlyhigherthanthevalueof tion ofself-absorptioninH/?;therefore,weadopttheE_= derived fromtheHy/H/?ratio(E_=0.51).Thisisanindica- absence oflargeextinctionvariationoverthewholenebula [S ii]lineshaveapoorsignal-to-noiseratioorarenotdetected [N ii]electrontemperature canbedeterminedwithlarge [N ii]lineratio(A6548+A6584)/>15755forthelow-excitation bv BV BV 156 F(Hß) x10.... He i¿6678 Ha ¿6563 [S ii]¿6731 [S ii]¿6717 Hß ¿4861 He i¿4471 Hy ¿4340 HÖ ¿4101 H8 ¿3889 [Nil] ¿6584 He i¿5876 [Nil] ¿6548 [O I]¿6363 [O in]¿4363 [Cl hi]¿5537 [Cl in]¿5515 [Fe in]¿5412 [Fe in]¿5270 [N I]¿5199 [O in]¿5007 [O in]¿4959 [S ii]¿4070 [Ne in],He¿3969 [Ne m]A3869 [On]A3727 [Sm] ¿6312 [O I]¿6300 [Nil] ¿5755 The electrondensityisdeterminedfromthe[Sn]A6731/ The electrontemperatureshavebeencalculatedfromthe Note.—Colon denotesupperlimit. Line © American Astronomical Society • Provided by the NASA Astrophysics Data System 5.2. ElectronDensityandTemperature -0.36 -0.36 -0.35 -0.34 -0.34 -0.34 -0.31 -0.02 -0.30 -0.30 -0.23 -0.21 -0.15 -0.12 -0.09 -0.08 -0.03 -0.15 m 0.00 0.135 0.18 0.18 0.10 0.13 0.21 0.22 0.23 0.26 1164 1832 +1 279 ±1 606 ±1 154 ±1 100 ±2 185 ±30 153 ±7 55 ±5 76 ±4 50 ±4 31 +6 33 ±5 11.4 ±1.5 18 ±3 18.2 ±1.8 7 ±1 0.92: 3.6 ±0.7 2.8 ±0.8 2.3 ±0.8 1.8 +0.8 cs CHU, MANCHADO,JACOBY,&KWITTER 2790 1118 279 381 190 100 157 63 46 47 25 27 98 17 11.3 17.5 4.8 0.74 ±0.3 0.67 ±0.3 0.87 ±0.4 2.2 ±0.3 6 Line IntensityRatiosforNGC6751 3.1 3.0 2.1 ±0.3 8.5 3.9 1.1 1.7 Cl ±2 ± 1.5 ±0.6 ± 1 ± 1 ±3 ±0.3 ± 10 ±6 ±0.4 ±0.4 ±0.8 ± 3 ±2 ± 1 ± 1 ±4 ±4 ±0.4 ±3 ±4 ± 5 ± 7 1040 TABLE 2 288 ±5 291 ±5 595 ±6 191 ±4 100 ±2 125 ±8 26 ±1 20 ±1 96 ±2 42 ±2 25 ±5 21 ±8 32 ±9 38 ±6 18 ±1 3.9 ±0.8 4.0 ±1 2.3 ±0.4 2.2 ±0.4 2.0 ±0.6 5.9 ±1 1.5 ±0.5 C2 + + + 0+ envelope, andhalo(A,E,H, andH')cannotbedetermined, which thesignal-to-noiseratioisquitehigh,Heabun- in Table3.Theheliumabundancesarederivedwiththecorrec- mulism ofionizationcorrectionfactors(Torres-Peimbert & because the[On]linesare notdetected,hencewecannot quite reliable.Thetotalnitrogen abundanceintheansae, ¿4471, implyingthatthehelium abundanceofHe/H=0.121is dance calculatedfrom¿5786 agreesverywellwiththatfrom tion ofcollisionaleffects(Clegg1987).IntheClspectrum, of electron temperatureoftheenvelope(E),uncertaininitself, is of thelargeerrorin[Om]electrontemperature. the high-excitationionsmaybesomewhatuncertainbecause ansae maybequiteuncertain.InregionC2,theabundances of adopted fortheansae(A),hencederivedabundancesin the He fromthehigh-excitationregion.Theelectrontemperature the low-excitationregion,and0,SClNe program thatsolvestheequationsofstatisticalequilibriumfor Peimbert 1977).Theionicandelementalabundancesarelisted compute theionicabundancesfromlinestrengthsusinga T of8000Kinthehalo(HandH')haslargeuncertainties. The 10-level atoms.WeassumethatO,0NandSarefrom lines isconsistentwithanelectrontemperaturenearlytwiceas is remarkablyhigh.Althoughtheerrorin4363Aline Table 3.Notethattheelectrontemperatureinenvelope(E) high asthatofthemainnebula. strength issomewhatlarge,theabsenceof[On]and[Sii] e Element abundancesarecalculatedusingthestandardfor- After derivingtheelectrontemperaturesanddensities,we The derivedelectrondensitiesandtemperaturesarelistedin 200 328 676 ±20 213 ±10 484: 100 ±15 66.6 20: 55 50 18.4 ±5.5 15.3 ±4.5 19 ± 10 ± H ±8 ± 10 ± 6 1051 ±10 278 ±6 345 ±3 100 ±5 43.4 83 ±4 21.62: 35 38 15 ±3 7.7 +2 5.45: ± 10 ± 8 5.3. Abundances 440 265 211 ±4 702 +14 211 ±60 326 ±6015±41212100 141 100 ±8 28 28 8.52 6.36 ±3 H ±2 ±2 ±8 ± 8 ±6 250 ±25 568 ±20 554 ±25 129 ±20 140 ±42 100 ±30 38 ±10 2.70 H' 278 ±10 547 ±11 553 100 ±8 187 ±9 166 100 21.5 82 ±6 42 ±8 58 ±6 84 ±8 11.4 ±3.4 9.3 ±3 Vol. 376 ± 12 ± 10 ± 10 1991ApJ. . .376. .150C + + + derive theionizationcorrectionfactorfornitrogenusing relation N/=00. constant withinthenebula,is~20%lowerinnortheast conclusions aboutabundancevariationinthenebulacanbe inferred fromthekinematicproperties. irregular filaments.Thislowerabundance,closertothesolar drawn fromtheresults.First,heliumabundance,fairly error fortheN/Oratioinmainnebulashouldbeless than from thecentralpart(CS)andsmoothinnerring(Cl)to in Table3thattheN/Oratioincreasesbynearlyafactorof2 N/0, inwhichthetemperaturedependenceofabun- large errors,buttheratioofthesetwoabundancesismore The elementalabundanceofNorOindividuallycanhave value, isconsistentwiththeinterstellaroriginofthesefilaments No. 1,1991MULTIPLE-SHELLSTRUCTUREOFNGC6751157 classified thestar asWC6.Usingtheintegrated H/?fluxofthe (Barker 1986). and the[On]linesarewellmeasuredformainnebula. The temperatures fromthe[Nn]linesareprettywelldetermined, the knottyouterring(C2)inmainnebula.Theelectron dance nearlycancelsoutthatofthe0abundance.Wecansee variation intheN/OratiohasbeenobservedNGC7662 reliable. ThisisbecauseN/Ocanbewellapproximatedby sities (e.g.,Méndez&Niemela 1981),wederiveaspectraltype spectrum showninFigurela. Basedontheratiosoflineinten- nebulae (Peimbert&Serrano1980). theless, wenotethatourheliumabundanceis20%higher than not resolvethefeaturesininnernebulaasoursdid.Never- Czyzak (1983),becausetheirsingle-apertureobservation did of WC4.Usingthecriteria linewidths,Smith&Aller(1969) theirs; ourvalueisclosertothosefoundintypeIplanetary 10%. ThisvariationinN/Oratioisprobablyreal.Similar The secondconclusionconcernsthevariationinN/Oratio. Despite theaforementioneduncertainties,tworelativelyfirm It ishardtocompareourresultswiththoseofAller & We canclassifythecentral star ofNGC6751usingthe +5 +5 5 5 0+5 +5 +5 +5 +5 5 +5 + + + + + lognJTW)] ... N/H x10... 0/H x10. t [Nn] lognTSn] N/H x10 O/H x10 O/H x10... 0/H x10... t [Oin] Ne/H x10 S/H x10... Cl/H x10. Ne/H x10 S/H x10.. N/0 He/H HeH(6678) ... He/H(4471) .. He/H(5876) .. Note.—Colon denotesupperlimit. © American Astronomical Society • Provided by the NASA Astrophysics Data System Region 6.1. CentralStar 6. DISCUSSION 41.6 60 16.7 17.3 12.48 4.81 0.51 0.097 3.4 0.1195 0.28 0.11 0.087 1.15 ± 1.2 CS 0.025 Physical ParametersandAbundancesofNGC6751 34.1 53 20 17.4 17.8 0.85 ±0.025 0.146 3.4 0.0085 0.57 6.6 5.83 0.33 0.121 ±0.005 0.1155 0.1210 0.1215 1.0 ±0.025 1.2 Cl TABLE 3 38.3 61 33.43 10.66 19.4 0.8 ±0.025 0.85: 0.285 3.2 0.71 3.1 0.55 0.13 0.133 2.58 C2 1 = 1 - l nebula andthevisualmagnitudeofcentralstarlistedby H/? and[Om]fluxes(Kaler&Jacoby1991)suggeststhatthe However, theStoytemperaturerelationbasedonratioof Pottasch (1984),wederiveaZanstratemperatureof36,000K. NGC 6751.TheradialvelocityoftheHnregionand chance tointeractwiththeinterstellarmedium,too.Bothcom- with thismedium.Sincethehaloisporous,envelopegetsa medium (Smith1976;Isaacman1979).IfNGC6751ismoving is expectedtoslowdown,orbebraked,withrespectthe they sharethesameionizationsource—thecentralstarof velocity of^el=—42±2kmsexhibitedinthemain ponents shouldshowsomedecelerationfromthesystemic the Hiiregion,haloofNGC6751wouldinteractdirectly through adensemediumofwhichthedensestpartshowsupas velocity variationinNGC6751furthersuggestthattheHn its distance:usingtheradial velocityoftheHnregionto has beenbrakedthemostanditsaveragevelocityis^hel nebula. Indeed,theringofknotsinhalo,beingoldest, region hasbeeninteractingwithNGC6751. 220 kms"and assumingadistanceof8kpc fromtheSunto derive akinematicdistance.Assuming aflatrotationcurveat with anHnregion,wehave an additionalwayofdetermining theless, thevelocityvariationsupportsqualitativelythatNGC since thedensityinambientmediumisunknown.Never- toward morepositivevelocitiesattheedges.Thesouthernedge Figures 4and5,thelineimageofstaticenvelopeiscurved stellar temperatureisgreaterthan50,000K. uncertain (Lutz1989).SinceNGC 6751isphysicallyassociated being brakedbythemedium. 6751 isindeedmovingthroughtheinterstellarmedium and of theenvelopehasaradialvelocityV=—38kms^ — 35kms.Theenvelopeshowsdeceleration,too;asseen in hel 6.2. InteractionbetweenNGC6751andItsAmbientMedium The proximityoftheHnregiontoNGC6751suggeststhat A planetarynebulamovingthroughtheinterstellarmedium Unfortunately, wecannotmodelthebrakingquantitatively, Distances toplanetarynebulae havealwaysbeenquite 38.5 2.3 0.02 60.6 61 2.01 2.7 1.5 43.9 38.4 33.7 82 15.78 0.8Ü 2.8 0.22 0.4 H 43 + 01 49 26.1 12.4 10.5 19.7 0.30 2.2 0.30 7.44 0.067 ±0.03 0.067 0.4 1 15 1991ApJ. . .376. .150C l -1 with thearrowsindicateexpansion velocityinkms~. (attheadopteddistanceof2 kpc). Numbersinthickoutlineassociated has aslowexpansionvelocity.Itshighexcitationishardto ery tothestar.Thejuxtapositionofansaeperiphery continuous jetintheEskimonebula(Gieseking,Becker,&Solf mass outflow.Ifthisoutflowstartedoutfromthestar,itwould within theshell,andbulkexpansionvelocityis~42km understand, sinceitsvelocityprofilesarenarrow,implying ated interiortotheshellinsteadofnearstar.Theenvelope of themainnebulamayalsoimplythatoutflowisacceler- have lastedonlyashortperiodoftime,asopposedtothe loss fromthecentralstar.Theansaecorrespondtoabipolar higher N/Othantheshell,indicatingaseparatemodeofmass s. Theembedded[Nn]knotsneartheperipheryhave 158 1985), wherethebipolarmaterialisdetectedfromperiph- schematic drawingofourmodelNGC6751isgivenin dynamical ageofeachmorphologicalfeatureinTable4.A derived byMaciel&Pottasch(1980)andDaub(1982). Figure 8. ture. Forclarity,welistthesize,expansionvelocity,and abundance variationinNGC6751tomodelitsphysicalstruc- 6751. Thisdistanceissimilarto,butsmallerthan,the2.7kpc the Galacticcenter,wederiveadistanceof2.0kpctoNGC Fig. 8.—ProposedmodelforNGC6751. Dimensionsofeachregionarein The mainnebulaisathickshellwithvelocitygradient We usetheinformationonmorphology,kinematics,and © American Astronomical Society • Provided by the NASA Astrophysics Data System 6.3. PhysicalStructureofNGC6751 7 7 5 Hii ...0.0750.0217 Halo 87.40.00370.01670.42634004.6x10’ Envelope 48.31-0.020.068-0.007100.2328500(2.7-26)x10’ Ansae 410.002...0.023 Knots ...10.004 Main nebula25"0.060.01742.50.1227002.2x10’ a d=2kpc. -1 Region Diameter€(M)(kms(pc)(yr)yr“ 0 CHU, MANCHADO,JACOBY,&KWITTER a Masses andMass-LossRateofNGC6751 TABLE 4 Mass VRtM ej¡p -1 -1 -1 1 The shellthicknesscanbeapproximated asthespatialextent over whichthelineprofilesare notsplit,orthespatialwidthof apparent shellthicknesstothe shellradiusofthemainnebula. of 25".Thisfillingfactorismuch smallerthantheratioof the spectrumCSandassuming adepthequaltothediameter factor, themasscanbederived.Fromanddura- calculate thedepth.Fromdensity,volume,andfilling calculate thefillingfactororassumeaofunity and more. was ejected;(5)afastermasslossstartedabout3200years ago stellar mass-lossrate. tion oftheshellformation,wecancalculateanapproximate lines. Wemayeitherassumeadepthalongthelineofsight and Hß fluxesandtheelectrondensitiesdeterminedfrom[S n] stress againthattheseagesmaybeuncertainbyafactorof 2or to formthemainnebulathatisexpandingat~42kms. We angle oftheansaeisunknown);(4)aringN-richmaterial bipolar ejectionatanunknowntime(becausetheinclination episodes ofmassloss:(1)isotropicslow(<10kms)wind listed inTable4. nebula; (3)theansaewereformedbyashortdurationof the nextepisodeoffastermasslossstartedtoformmain structure ofNGC6751,weseechronologicallythefollowing tenuous stellarwindwithaterminalvelocityof2000-3000km by thepopular“two-windinteractionmodel,”inwhicha but theinnermostshellhasalsobeenacceleratedbyfast started 28,000yearsago,andlasteduntil3200when unknown; (2)anotherisotropicslow(~10kms)wind started 64,000yearsago,butthedurationofthisepisodeis Purton, &FitzGerald1978;Balick1987).Fromthephysical s“ sweepsuptheenvelopeofpreviousslowwind(Kwok, triple-shell structureofNGC6751clearlycannotbeexplained stellar windfromwithin.Theapproximatedynamicagesare All shellshavebeendeceleratedbytheslowermaterialoutside, nor anaccelerationmechanismhasbeentakenintoaccount. (radius)/(expansion velocity).Thisapproximationisprobably uncertain byafactorof2ormore,sinceneitherdeceleration compute thedynamicageofeachshell,approximatedas Knowing theexpansionvelocityandlinearsize,wecan through thehalotointeractwithenvelope. quite porous,sincetheinterstellarmaterialapparentlysieves tenuous thatonlythecondensationsareseen.Thehalomaybe small turbulences.Shockexcitationisnotlikely.Thehaloso For themainnebulaweobtainafillingfactorof0.017,using We cananalyzetheshellstructurequantitativelyfrom Planetary nebulanucleicanlosemassinmanyways.The We adoptthekinematicdistanceof2kpcforNGC6751. 6.4. HistoryandPropertiesofMassLoss Vol. 376 1991ApJ. . .376. .150C -3 1 -71 6 -715 41 No. 1,1991 0.017 M. ent phasesinthedredge-up process. TheobservedN/Oand in thenebula.ThisN/Oratio variationcouldbeduetodiffer- electron densityandH/?flux,wederiveadepthofabout1"for nucleosynthesis modelsafterthesecondand thirddredge-up He/H ratioscan becomparedtothetheoretical predictionsof .TheN/Oratio isafactorof2higherinthe N/O ratiosinNGC6751.TheHe/Hratiovariationisbetween knotty outerringinthemain nebula (C2)thantheotherplaces The N/Oratiovariation,on theotherhand,iswithin the planetarynebulaandambientinterstellarmedium. The 0.00025 M.Wecountabout15-20knotsinFigure1;their widths oftheknots,indicatingthattheseknotsareindeed a unityfillingfactor.Thisdepthisofordersimilartothe the lineimageatsystemicvelocity.FromFigure3,wesee He/H ratiowithintheplanetary nebulaitselfisprettyconstant. mined, sincethe[Sn]lineswerenotdetected.Assuminga three-dimensional ones.Themassofeachknotisabout 0.007 M. is assmallthatintheouterhalo,masswouldbeabout halo arevisible.Thetotalmassinthisisabout0.016M. a veryporousstructure.Onlythedensestspotsinouter total masswouldbeabout0.004M. interstellar medium. filling factorisconsistentwiththosecommonlyseeninthe along thelineofsightandderiveafillingfactor0.075.This nebula, themasswouldbeabout0.015M;iffillingfactor static envelope.Ifthefillingfactorissimilartothatinmain of about30cm,andatotalmass0.068Minthe unity fillingfactor,wemayderivearootmeansquaredensity an apparentshellthicknessof3"-4",whichisabout25%-30% phases (Becker &Iben1980);theabundances intheknotty each shell.Wemakeanaprioriassumptionthatthereisno estimate theaveragemass-lossrateduringformationof only 6%-7%.Themassintheshellofmainnebulaisabout of theradius.Therefore,fillingfactorinshellmustbe consistent withthetotalmassof0.05-0.1Mderivedfrom the fluxes (Phillips&Mampaso1988).AdoptingDraine’s(1990) minus thedynamicageofenvelope,thatenvelope gap fromoneepisodeofmasslosstothenext.Theduration electron densitiesandH/?fluxes.Changingtheadopted dis- assume adust-to-gasratioof0.01,thenebularmassis0.1M , grain emissivity,wederiveadustmassof0.001M.If we yr “forthemainnebula.Thesemass-lossratescanbecom- mass-loss ratesof4.6x10Myrforthehalo,(3- nebula, andthatofthemainnebulaisagedifference would betheageofenvelopeminusmain the masslossforhalowouldbedynamicageof rates byafactorof1.35,andthemasses1.82. tance from2to2.7kpcwillincreasetheagesandmass-loss pared tothoseofOH/IRstars,afewtimes10“ 26) x10Myrfortheenvelope,and2.210“ between theouterandinnershellsurfaces.Wederiveaverage 0 q 1989). Theagreementisreasonablewithintheuncertainties. 10“ Mqyr“(Habing,teLintelHekkert,&vanderVeen 0 0 0 0 0 0 0 0 0 q0 As describedin§5.3,therearevariationstheHe/Hand The spectrumC2correspondstothe[Nn]knots.Using The electrondensityinthestaticenvelopecannotbedeter- The smallfillingfactorintheouterhalo(H),0.0037,implies Knowing themassanddynamicageofeachshell,wemay For theirregularfilaments(F),weusewidthasdepth We mayalsoderivethenebularmassfromIRÁSfar-IR © American Astronomical Society • Provided by the NASA Astrophysics Data System 6.5. AbundanceVariations MULTIPLE-SHELL STRUCTUREOFNGC6751 1 1 5-1 71 1 fore discussonlytheothersixnebulae.Forconvenience,we NGC 7662allhavetypeIIdouble-shellstructureintheinner call theoutermostshell“halo,”andinnertwoshellsouter planetary nebulathatclearlyhasadifferentevolutionary shell ”and“innershell.” status (Ibenetal.1983)thantheothernebulae.Wewillthere- expanding atavelocitysimilartothatoftheinnershell;NGC (the mainnebula). NGC 6751areintermediatebetweenthesetwotypes;itsouter 0.5% asbrighttheinnershell(CJA).Thetwoshellsof two shells,withtheoutershellbeingabout20%asbright has twoshells(thehaloandtheenvelope)expandingsub- the innershell,whileNGC6720hasatypeIdouble-shell sonically. NGC2022and7662havetheoutershell shell (theenvelope)isaboutl%-2%asbrighttheinner structure intheinnershells,withoutershellbeinglessthan experienced brakingfromthis medium. main nebulaeareinsideanovalwhoserelationshiptotherest 6826 andNGC6891havetheoutershellexpandingfasterthan the thirddredge-up. the smoothinnerringofmainnebula(Cl)matchwellwith 8818192. densest partshowsupasthe Hnregion,andthehalohas moving throughadenseinterstellar medium,ofwhichthe from thatoftheplanetarynebula by55kms",andtheHe/H 0.01-0.02 Mq,andthemass-lossrateincreasesfroma few mass lossinacomplexway.Theeachshellisabout high, nearlytwicethetemperatureinmainnebula. The velocity (~10kms“),butitselectrontemperatureis very change invelocityandmass-lossrate. have thetwoinnershellsexpandingsupersonically.Ifsub- slower thantheinnershell(Chu1989).Allofthesefivenebulae the innershell;andNGC6720hasoutershellexpanding outer ring(C2)matchwelltheseconddredge-up,andthoseof ratio islowerintheHnregion. NGC6751isapparently halo isporous,anditsexpansionvelocitylowerthan7 km of thenebulaisunknown.Theenvelopehasasmallexpansion ansae atpositionanglesof104°and284°.Thethe nebula. Thebipolarmassoutflowcorrespondstothepairof km s“LThe[Nii]-brightknotsembeddedintheperipheryof evidence thattheredgiantwindmaygothroughsudden of NGC6751.Theradialvelocity ofthisHnregionisdifferent very likelyinterstellarmaterialilluminatedbythecentral star times 10“Myrduringtheformationofmainnebula. times 10“Mqyr"duringtheformationofhalotoa few the progenitorofPNnucleusmusthavegonethrough s“. and theyhaveahigherN/Oratiothantheotherpartsof physical conditionofthetriple-shellplanetarynebulaNGC red giant’swind,theenvelopeandhaloofNGC6751giveclear sonically expandingshellsarethemasslostbyprogenitor the mainnebulamayfollowadifferentpatternofexpansion, 6751. ThemainnebulaofNGC6751isexpandingatabout42 o Morphologically, NGC2022,6826,6891,and There aresevenknowntriple-shellnebulae.A78isareborn Y. H.C.acknowledges thesupportofNSF grantAST- Kinematically, NGC6751istheonlytriple-shellnebulathat The shellstructureofNGC6751givesclearevidencethat We havestudiedthemorphology,internalmotion,and The irregularfilamentstothenortheastofNGC6751are 6.6. ComparisonwithOtherTriple-ShellNebulae 7. SUMMARY 159 1991ApJ. . .376. .150C 160 CHU,MANCHADO,JACOBY,&KWITTER Draine, B.T.1990,inTheInterstellarMediumGalaxies,ed.H.A.Thronson Dekker, H.,&D’Odorico,S.1985,FaintObjectSpectrographandCamera Daub, C.T.1982,ApJ,260,612 Clegg, R.E.S.1987,MNRAS,229,31P Chu, Y.-H.,Jacoby,G.H.,&Arendt,R.1987,ApJS,64,529(CJA) Chu, Y.-H.1989,inIAUSymposium131,PlanetaryNebulae,ed.S.Torres- Gieseking, F.,&Soif,J.1986,A&A,163,174(GS) Gieseking, F.,Becker,I.,&Soif,J.1985,ApJ,295,L17 Duncan, J.C.1937,ApJ,86,496 Habing, H.J.,teLintelHekkert,P.,&vanderVeen,W.E.C.J.1989,IAU Becker, S.A.,&Iben,1.1980,ApJ,237,111 Aller, L.H.,&Czyzak,S.J.1983,ApJS,51,211 Barker, Y.1986,ApJ,308,314 Balick, B.,Preston,H.L.,&Icke,V.1987,AJ,94,1641 Balick, B.1987,AJ,94,671 & J.M.Shull(Dordrecht:Kluwer),483 Operational Manual(Garching:ESO) Peimbert (Dordrecht:Kluwer),105 Kluwer), 359 Symposium 131,PlanetaryNebulae,ed.S.Torres-Peimbert(Dordrecht: © American Astronomical Society REFERENCES Provided bythe NASA Astrophysics Data System Torres-Peimbert, S.,&Peimbert,M.1977,Rev.MexicanaAstr.Af.,2,181 Kwok, S.,Purton,C.R.,&FitzGerald,M.P.1978,ApJ,219,L125 Isaacman, R.1979,A&A,77,327 Walsh, J.R.,&Meaburn,1987,MNRAS,224,885 Smith, L.F.,&Aller,H.1969,ApJ,157,1245 Smith, H.1976,MNRAS,175,419 Pottasch, S.R.1984,PlanetaryNebulae(Dordrecht:Reidel) Peimbert, M.,&Serrano,A.1980,Rev.MexicanaAstr.Af.,5,9 Moreno, M.A.,&López,J.A.1987,A&A,178,319 Méndez, R.H.,&Niemela,V.S.1981,inIAUSymposium99,Wolf-Rayet Manchado, A.,Pottasch,S.R.,&Mampaso,A.1988,A&A,191,128 Maciel, W.J.,&Pottasch,S.R.1980,A&A,88,1 Lutz, J.H.1989,inIAUSymposium131,PlanetaryNebulae,ed.S.Torres- Kaler, J.B.,&Jacoby,G.H.1991,ApJ,372,215 Iben, I.,Jr.,Kaler,J.B.,Truran,W.,&Renzini,A.1983,ApJ,264,605 Phillips, J.P.,&Mampaso,A.1988,A&A,190,237 Hua, C.T.,&Louise,R.1990,A&A,235,403 (Dordrecht :Reidel),457 :Observations,Physics,Evolution,ed.C.W.H.deLoore&A.J.Willis Peimbert (Dordrecht:Kluwer),65