1956ApJ. . .124 . . .93M persion of110A/mmatH7.Thisopticalarrangement permittedustoobtainplatesin prism spectrographwiththeSchmidtcameraof6-inch focallength,whichgivesadis- gives adispersionofabout170A/mmatH7.Atthe 100-inchtelescopeweusedthetwo- inch reflectorweusedatwo-prismspectrographwith the3-inchSchmidtcamera,which graphs atthe60-and100-inchreflectorsMount WilsonObservatory.Atthe60- NGC 2165,and2440.Thefaintestobjectonourlist,2022,waschosenbe- cause ofitshighexcitation. photographic spectrophotometryhavebeencarriedoutwiththeCassegrainspectrographsat60- and 100-inchreflectorsoftheMountWilsonObservatory.Theresultsextensiveobservationsarere- NGC 1535,2022,2392,and7662thesomewhatirregularstruc- selected positioninthenebulaandexposurescarefullyguidedThelineintensities,expressedon ported forNGC40,1535,2022,2392,and7662Additionalmaterialhasbeenob- fordetailedanalysis.Previousphotographicspectrophotometricstudies(Berman have demonstratedthevalueofphotoelectrictechniquesformeasurement scale /(H/3)=100,aregivenforlinesfromtheBaimerlimittoX9000 tained forIC351,1747,J320,NGC2165,and2440Ineachobjecttheslitwasplacedona For obviousreasons,mostofthemwereratherbright.Weincludedtheringobjects placed forthelongexposuresrequiredtorecordmoderatelyfaintlinesandcontinua. ture, NGC40.AfewadditionalobservationsweresecuredofIC351,1747,J320, estimates onscaleplates.Themostinterestingplanetarynebulafromthestandpointof stronger lines.Wyse(1942)attemptedtomeasuretheweakerlineswithaidofeye- measured byphotographicphotometry.Italsoispossibletoselectsmallerregionsinthe the richnessofspectrumandrangeinexcitationisNGC7027(Bowen,Minkowski, and oftenmuchfainter,objectscanbecompared. Aller 1955).Similarspectrophotometrictechniqueshavebeenappliedtoothernebulae. intensities ofthestrongerlines.Forpresent,atleast,weakerlinesmustbe ties havebeenaccuratelymeasuredmayserveasphotometricstandardstowhichother, of emission-lineintensitiesintypicalgaseousnebulae.Ontheonehand,fortheoretical 1930; Plaskett1931;Page1936,1942;Aller1941,1951)havebeenconfinedtothe hydrogen andotherpermanentgases.Onthehand,nebulaeinwhichlineintensi- studies, itisnecessarytoknowtheemissionpertypicalunitvolumeinradiationof © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem The observationsdiscussedinthispaperwereall securedwithCassegrainspectro- Measurements ofthelineandcontinuousspectraseveralbrightplanetarynebulaebymethods We selectedanumberofnebulaerangingfromlowtohighexcitationwhichwerewell Recent investigations(FillerandAller1954;MacRaeStockCode1956) For anumberofastrophysicalproblemsitisdesirabletohavereliablemeasurements SPECTROPHOTOMETRY OFPLANETARYNEBULAE Carnegie InstitutionofWashington,CaliforniaInstituteTechnology II. THEOBSERVATIONALTECHNIQUE Mount WilsonandPalomarObservatories R. MinkowskiandL.H.Aller University ofMichiganObservatory Received February13,1956 I. INTRODUCTION ABSTRACT and 93 1956ApJ. . .124 . . .93M 94 R.MINKOWSKIANDL.H.ALLER rately uponaselectedportionofthenebulaforentireexposure.Inthisway,differ- intensity rangeofoverathousandfold,itisnecessarytosecureobservationsrangingin good definitionfromthenearinfraredtoBaimerlimit.Sincebothspectrographswere ployed. TheJSplateseriesreferstothe60-inchCassegrainspectrograph;E been madetoobtainintegratedintensitiesofthenebularimages.Sincelineshavean equipped withauxiliaryguidingeyepieces,itwaspossibletokeeptheslitplacedaccu- plate sensitivity,atmospherictransparency,andtelescopereflectivityplusspectrograph to the100-inchCassegrainspectrograph.Forphotographicandvisualregions, exposure timefromafewsecondstomanyhours. ences ofexcitationindifferentpartsthenebulamaybecompared.Noattempthas lish therelationbetweenphotographicblackeningandintensityaswelleffectsof The sixthandeighthcolumnsgivetherangeofexposuretimesonnebulacomparison foreachplatesecured. standards withthe32-inchcameraandcoudéspectrograph,adjustingexposure to developnebularandcalibrationplatetogether.Wephotographedthe transmission. Thefirststepisaccomplishedbyasuitablephotometricstandardization; the secondiscarriedoutmostexpeditiouslybyphotographicsuitablecomparisonstars. obtain theproperdensitiesforallrelevantwavelengths.Fortunately,development times tobecomparablewiththoseatthetelescope.Thuscalibrationexposuresranging ring oftheout-of-focusimageonslit.Thusweprovidedadiffuseillumination are severalmagnitudesfainter,weadoptedthefollowingdevice.Asmallpieceofground curve withexposuretimeisverysmall.Inasinglecalibrationitdifficultto impress thephotometricstandardsonalargerpiececutfromsameplateandthen 103a-F emulsionwasused;fortheinfrared,hypersensitizedI-Nplateswereemployed. photoelectrically inthelaboratorybyEdisonPettit,underconditionscloselysimulating procedures werekeptthesameforallplates. from aminutetomanyhourshadbesecured.Thechangeinslopeofthecalibration- a Lyrae,whichwasobservedbyWilliams(1938), WilliamsandHall(1941),Kienle, slit withasourceofknownenergydistribution. parison staroutoffocusbyaprearrangedamountandplacedportiontheluminous ment withoneanother.Redward ofX3700theenergydistributioncorresponds toacolor quartz wasplacedovertheslitofspectrograph.Wethenranimagecom- Williams andHall.Themostfrequentlyusedcomparison ,ePersei,hasbeenmeas- Cephei, andrjUrsaeMajoris,whichhavebeenobserved byKienle,Strassl,andWempe, Strassl, andWempe(1938),BarbierChalonge (1940),aswellePersei,ß ured byKienle’sgroupand byBarbierandChalonge,whoseresultsarein goodagree- by BarbierandChalonge,orbothgroups. Williams forXX3700-9000andbyphotoelectric photometry forthenearinfraredby quate toavoidtheintermittencyeffect. rapid trailingbackandforthofthefocalstellarimage ontheslitwithafrequencyade- ly atthetelescope.Forthispurpose,stellarspectraobtainedwithdiffuserwerecom- temperature of26000°K; shortwardofX3700thecolortemperatureis23000° K.The pared withspectraofthesamestarsobtainedwithout thediffuserandwidenedbya those obtainedatthetelescope.Inaddition,calibrationwascheckedphotographical- © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem Table 1givesthenebulaeandcomparisonstarsobservedspectrographsem- To measuretheintensitiesofnebularlinesandcontinua,itisnecessarytoestab- Because ofthesmallsizeplatesusedinspectrographs,itwasnecessaryto The energydistributioninVegawasmeasuredby photographicphotometrybyR. The standardenergy-distributionstarswhichserved forcomparisonstarsinclude To comparethestandardstars,ofknownenergydistribution,withnebulae,which The transmissionofthequartzdiffuserasafunctionwavelengthwascalibrated III. PHOTOMETRICCALIBRATION 1956ApJ. . .124 . . .93M NGC 7027 NGC 7027 NGC 2440 NGC 2392 J 320 IC 2165 NGC 2022 NGC 1535 IC 351 NGC 40 © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem * A2-magdiaphragmwasplacedin theopticalsystemforthisexposure f A1-magdiaphragmwasplacedin theopticalsystemforthisexposure 21 052 21 052 1 7 39 OMO^ 6 194 3 44 7 263 4 119 Spectrophotometric ObservationsofPlanetaryNebulae 39 3 2 8 +42 01 +42 01 -18 05 +21 02 + 1039 +34 55 + 902 -12 57 -12 52 + 72°11 JS 771 JS 768 JS 766 E 1757 E 1755 E 1754 E 1753 E 1752 E 1751 JS 776 JS 765 E 1746 JS 764 JS 762 E 1748 E 1747 E 1731 E 1730 E 1729 JS 774 JS 772 E 2284 E 2282 JS 769 E 2269 E 2270 E 2238 E 2237 JS 770 JS 775 E 2268 JS 774 JS 771 JS 769 JS 775 JS 766 JS 763 JS 762 JS 761 JS 758 E 2236 JS 759 JS 757 JS 756 JS 754 E 2283 JS 755 JS 753 Plate TABLE 1 I-N I-N I-N 103a-F 103a-F 103a-F 103a-F I-N I-N I-N 103tf-F 103ö-F I-N I-N 103a-F I-N 103a-F 103ö-F 103a-F 103ö-F 103a-F 103+F 103ö-F 103a-F lOSa-F 103a-F 103a-F 103a-F 103a-F 103a-F 103a-F 103a-F 103a-F 103a-F 103a-F 103a-F 103a-F 103a-F 103#-F 103a-F 103a-F 103a-F 103a-F 103a-F 103a-F 103a-F 103ö-F I-N Emul s m s m m m mr m m m m m m m m m m m m m m m m m m m m m m m 80, 25105 4m 4 80, 251040“ 360“ 67“, 25“ 390“ 410“ 330“ 360“ 60“ 336“ 13“ 13“ 60“, 60“f 430“ 120“ 360 480 10“, 150“ 8, 2 116“ 520“ 60, 30154 510 435 322 91 194“ 60 225 90 120“ 120“ 60, 207 480 60, * 420 420 420 180, 90 120 180 145 64, * 110 70 180 100 150 Exposures a Lyrae a Lyrae a Lyrae a Lyrae a Lyrae ß Cephei e Persei a Lyrae a Lyrae a Lyrae e Persei e Persei a Lyrae e Persei e Persei e Persei e Persei e Persei e Persei e Persei e Persei € Persei e Persei e Persei ß Cephei e Persei € Persei e Persei 7] UrsaMa- ß Cephei ß Cephei ß Cephei e Persei e Persei e Persei e Persei e Persei e Persei e Persei e Persei e Persei ß Cephei ß Cephei ß Cephei ß Cephei e Persei e Persei Comparison jor Star s 2“, 20 60“, 10“ 60“ 10“, 3“ 60“ 60“ 60“ 60“ 60“ 60“ 10“ 60“ 60“ 60“ 60“ 60“ 60“ 60“ 10“, 1“ 60“ 60“ 60“ 60“ 60“ 60“ 60“ 15“, 60“ 60“ 60“ 60“ 60“ 60“ 60“ 60“ 60“ 60“ 60“ 60“,15“ Exposures 60“ 60“ 60“ 60“, 15“ 60“ 60“ 10“ 60“ 15“ 1956ApJ. . .124 . . .93M NGC 7662 96 NGC 7662 Balmer discontinuity,D,is0.06.ForßCephei(whichservedasacomparisonstarfor and Chalongeresults. NGC 40),onlytheBarbierandChalongedataareavailable.TheKienleforrjUrsae draw theenergy-distributionenvelopeoverstellarlinesand,usingdataofBar- Majoris givealesssteepslopethandotheBarbierandChalongedata.Iftwocurves the highermembersofBalmerseriesmaybegivenonlyimperfectly. bier andChalonge,makesomeallowanceforthiseffect.Nevertheless,theintensitiesof are forcedtofitatX5000,thedeviationisaboutAlogI=0.043900.Inregion nebular continuum. a fewminutestoabout1hour.Itwasmanifestlyimpossiblygivethestarand from afewminutestoabout8hours,whereasthoseonthecomparisonstarvaried of thebrightringdidnotagreewiththosemeasured attwodifferentpointsoftheouter nebula thesameexposureforobservationsintendedtoreachfainterlinesand the confluenceofstellarlinesatBalmerlimitprovidessomedifficulty.Onemay XX 5000-6000bothsetsofcurvesareingoodagreement.WehaveadoptedtheBarbier wave lengthitispossibletofindthepointonnebular imagewheretheintensitywas error inthealignmentofspectrogramsmicrophotometer, amountingtoabout fainter ring.Thediscrepanciesweresystematicand correspondedtoaslightsystematic at asnearlypossiblethesamemeanzenithdistance.Exposuresonnebularanged 40 ¡JLbetweenHaandX3727.Oncethealignment errorhasbeenestimated,foreach were encounteredwasNGC7662.HeretheBalmer decrementmeasuredatthecenter of HaorthehighnumbersBalmerseries. The onlynebulainwhichdifficulties each lineandasmoothcurve drawnconnectingthepoints,onemaydetermine thesmall correction neededtoreduce allintensitiestothesameselectedpointsin nebula.The actually measured.Ifaplot ofintensityversusdistancealongtheslitisnow madefor intensity ofHß,forexample,ismeasuredatadifferent pointinthenebulathanisthat the MountWilsonphotographicallyrecordingmicrophotometer. series totheinfrared.Allourspectrogramshavebeentracedparalleldispersionwith plates, whichcanresultinconsiderabledifficulties. Ifthealignmentisnotperfect, the otherobjectsitwaspossibletosecureobservationsonlyfromlimitofBalmer © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem The observationalprocedureconsistedinobservingthenebulaandcomparisonstar In thecomparisonofnebular-lineintensitieswithstellarenergydistribution, The ultravioletspectrumofNGC7027wasobservedatMcDonaldin1946,butfor Great carehadtobetakenavoidevenatinyerror inalignmentofthesesmall-scale h 23™5 23 5 +42° 16' +42 16 R. MINKOWSKIANDL.H.ALLER JS 773 JS 767 JS 765 JS 760 E 2203 E 2205 E 2225 E 2224 E 2222 E 2204 E 2202 E 2220 E 2226 Plate TABLE 1—Continued I-N I-N I-N I-N I-N I-N I-N I-N I-N 103a-F 103a-F 103a-F 103a-F Emul m m m m m m ms m m mr m m mr m 450 420 480 480 425 6 60, 30l2 60, 24815 15,30 15, t*61 120 120, 40 15, t*30 360 Exposures e Persei e Persei e Persei e Persei e Persei e Persei e Persei e Persei e Persei e Persei e Persei Comparison Star r r m r r r r m m m m Exposures 60 60 60 60 60 60 60 15 15 15 10 1956ApJ. . .124 . . .93M Here A(neb.)istheintensityofnebularline,^4aconstantindependentwave datum, theratio/x(neb.)//\(*),where been secured. ate exposurewouldhave beendesirable. point betweenthetwointensitysystems.Itnowappears thatmoreplatesofintermedi- lines. Thederivedintensitiesaretobeputonacommon systembycombiningthere- sures givingthestrongerlines;longerexposures beingnecessarytogettheweaker for thefewmiscellaneousobservationswehavesecuredofNGC1535andotherobjects. length foreachplate,i\(*)istheintensityincontinuumofcomparisonstarat width shouldbesufficiently widethatthetopoflineontracerepresents the lines commontothetwoplateshavebeused. To bedeterminedistheshiftofzero sults fromthevariousplates.Tocombineresults exposuresoftwodifferentlengths, for atmosphericextinctionisnotserious.Onthe nights devotedtothelongexposures objects) thezenithdistancesofstarandnebula areaboutthesame,correction available. so smallthattheyhavebeenneglected.Themaximumerrorincomparinglinesthe the transparencyofskyseemedconstantthroughout. mospheric extinctionfromnighttonight,sowehavedeemeditbestusefortheentire servers sincethesolarinvestigationsofC.G.Abbot(1908).Onanumbernightson observe thecomparisonstaratsamezenithdistanceasnebula;and,for the wavelengthX,andd\/dxisdispersionontracing;Txtransmissionof maximum correctionforanylineatpointinNGC7662wasabout20percent.For equation (1)themeanvalueofzduringanexposureforallobservationsthathave effects ofatmosphericextinctionandhavefoundthatitissufficientlyaccuratetousein Mount WilsonobservationsofNGC7027,theatmosphericextinctioncorrectionsare difficult matter.ForthereductionofobservationsNGC7(327obtainedat the brightestcentralstripcorrectionsarenegligible.InNGC40andparticularlyin series ofourobservationsasetmeancoefficients adoptedfromthesix-colorphotome- Pettit orbyHaroldJohnson.Thesemeasuresindicateasizablefluctuationintheat- which wesecuredobservations,photoelectricmeasureswerecarriedoutbyEdison McDonald Observatoryin1946,Popper’s(1937)valuesforMountHamilton,corrected the effectsofdifferentialatmosphericextinctionaresmall.Theyrelativelylarge and zdenotesthezenithdistanceofstarornebula. the quartzdiffuser(determinedbyPettit);k\isatmosphericextinctioncoefficient; NGC 2392condensationsappearsoconspicuousaseasilytopermitanaccuratealign- for thealtitudeofMountLockeinaccordancewithproceduressuggestedbyFowle try ofStebbinsandWhitford(1945).Since(except forNGC1535andafewother X 6400regionwithlinesinthe3700isabout0.01logarithm. ment ofthespectrauponmicrophotometer.ThemeasurementsNGC7027, (1933), havebeenused.Unfortunately,nodirectmeasuresoftheextinctionwerethen 1535, andNGC2022donotindicateanyevidenceofalignmenttroubles. © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem The selectionoftheproper slit-widthfortheseobjectsisaseriousproblem. Theslit- The reductionofthemicrophometrictracingsgivesfundamentalobservational During alongnebularexposure,sayabout8hours,svaries.Wehavecalculatedthe The determinationoftheappropriateabsorptioncoefficient,k\,isasomewhatmore Each exposureyieldsaseriesofrelativeintensities ofemissionlines,theshorterexpo- We mustnowdiscussthecorrectionforatmosphericextinction.attemptedto The atmosphericextinctionatMountWilsonhasbeenstudiedbyanumberofob- For thesubsequentobservationsofNGC40,2392,2022,and7662 ex—secsec A (*)=AhPt”1 PLANETARY NEBULAE 97 (i) 1956ApJ. . .124 . . .93M ment isnotperfectandthe nebularknotsaresmall,differentpartsofthe knots willbe the microphotometeralways uponthesamepointinnebula—and,since thealign- nebular structurearemuch moremarked.Itisimpossibletosettheanalyzing beamin accidental errorsarepresumedtobegreater,but alsobecausetheirregularitiesin renders thesemeasureslessreliable.Theprobable errors arelarger,notonlybecausethe probably asgoodcanbeattainedwithanarrow slitandphotographicphotometry slit-width usedatthe100-inchwas0'i68,correspondingto25/xonplateforNGC7027 98 R.MINKOWSKIANDL.H.ALLER without amuchmoreextendedprogram.Thescatter ofthevaluesinouterring ments intheouterringshowedsomescatter,muchofwhichmustcertainlybeattributed would haveobliteratedmanyoftheweakerlines. Aller 1955).Theseintensitiesrefertothebrightestcondensationinnorth-preceding secured forthestrongerlinesbywideningspectrographslit,butlossofresolution with itsveryrichspectrum.Fortheotherobjectsslit-widthwasI'i3,corresponding agreement. and wehavemeasuredonlythebrightestpartofbrightring. outer ring.Themuchsmallerscaleofthe60-inchplatesprecludedsuchadetailedstudy, the resultsfor100-inchand60-inchspectrograms showsareasonablygoodover-all Fewer linesweremeasuredonthe60-inchspectrograms. Nevertheless,acomparisonof sible tomeasuretheintensitiesofstrongerlines withrespecttothoseofmoderate way astocutdownthebeamby1or2mag.Thus,keepingexposuretimeat portion ofthebrightring.Wetraced100-inchspectrogramsatfivepoints—thecen- to 49¡JLontheplatefor100-inch,and3''7,corresponding42jit actual intensityofthelineintegratedoveritswidth.Thephotometricaccuracyincreases fainter north-followingpartofthenebula.Theintensitiesmeasuredinthis lines theactualerrorsmaybesubstantiallygreater,particularlyforweakestlines, 60-inch. Fromtheagreementobtainedbetweenvariousplates,internalprobable employed there,itwaspossibletointroducediaphragmsintotheopticalpathinsucha to theextremelymottledstructureofouterpartsnebula. intensity, withoutresorttothemethodofcombining intensitiesotherwiseemployed. stronger lineswithhisphotoelectricscanner. during thebestobservingseason.OlinWilsonhasstudiedinternalmotionswith almost anyotherplanetary.Itisabrightobject,favorablyplacedforlongexposures part ofthenebula.Ourplatesalsopermitteddeterminationintensitiesin per centforlinesofintermediateintensitymeasuredonstrongerplates.Forindividual decreases, andfaintlinescannolongerbeseparatedfromthecontinuousspectrum.The as theslitiswidenedbeyondthisminimumwidth;butatsametimeresolution tude hasbeenmeasuredphotoelectricallybyLiller(1955),whileCodethe direct photographssecuredatPalomarbyoneofus(Aller1956).Theintegratedmagni- where errorsoftheorder30-50percentarepossible.Smallermighthavebeen telescope fixedatsomeconvenientinterval(15minutes wasactuallyused),itpos- multislit; isophoticcontourshavebeentracedfromhisslitlessspectrogramsand are lesscertainbutshownodefinitesignsofstratificationeffectsinthenebula. error isabout15percentfortheweakerlinesmeasuredonseveralplatesand5 ter ofthebrightring,innerportionsouterand © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem Table 3givestheintensitiesderivedfrom60-inchplates.Withspectrograph Table 2givestheintensitiesmeasuredon100-inchspectrograms.Themeasure- The resultsforNGC7027havebeendescribedelsewhere(Bowen,Minkowski,and The slitofthespectrographwasplacednorthcentralstar,acrossbrightest The intensitymeasurementsinthebrightringshould bereasonablyreliable—atleast, The RingNebulainAndromedaNGC7662hasbeenmoreintensivelystudiedthan IV. RESULTSFORINDIVIDUALNEBULAE 1956ApJ. . .124 . . .93M 3868 7 3858 0 4026 1 4068 6+40763 4143 8. 3967 4+39701 4101 7 4120 8 4180 9 4187 0 4471 4541 4634 3888 9 4199 9 4219 . 4227 4 4237 4267 4340 4379 4640 6 4647 5+46491 4658 4363 4388 0 4685 4711 3 4724 2+47256 4740 2 4922 0 4861 3 4959 0 5006 9 5426 6 5875 6 5411 5 5537 8 5801 4 6086 9 6562 9 6678 2 6716+6730 8862 8 5517 7 6300 3+63121 6363 8 8196 6+82364 8665 0 8750 5 7065 3 7135 8 7319 9+73302 7530 5 7751 0 7592 8 8045 6 © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem Wave Length 6 2 4 5 7 /[Fe VII]\ \[Cav] / He II Hi m He I He I He I He II H He I He II He I N III/ N IIl\ He II Hß N III He I [S II] [Fev] [Ne III] [Ne IIl]+HI Cm He II Ha C m+oII He I He I He I He II He II H C II [O HI] Civ [A IV] [Ne IV] H H [A IV] [O III] [O III] [Fe VI] [Am] [Cl in] [Cl III] [O I] [S II] [Cl iv] [A III] [Cl IV] Civ [O I]+[Sill] [0 ii] 7 Line IntensitiesinNGC7662Measured Ident. AT THE100-lNCHREFLECTOR 30 36 98 10 8 2 1 3 0 2 05 0 75 6 1 3 5 2 3 4 6 2 3 1 3 10?8 TABLE 2 236 710 155 30 36 11 6 14 9 10 2 -5?4 0 75 8 0 3 0 3 0 7 Ó 8 0 45 2 9 2 1 3 6 6 8 6 0 3 3 1 6 5 2 5 2 7 7 1 1 6 1 1 0 1 8 1700 * 51 390 / 5 \ 2 550 100 of Ring 33 89 23 2 44 17 pr 18 13 8 16 Center 0 31 0 33 0 16 0 62 0 21 2 8 0 20 0 14 0 4 0 2 0 0 8 8 0 1 8 0 33 0 68 4 9 3 1 0 0 6 0 9 2 6 5 9 2 4 0 8 2 1 4 1 7 6 0 2 1 15 2 6 1 3 1 9 1 5 1 2Ó0 161 42 48 7 22 5 36 13 3 18 1 +5':4 8 4 0 4 0 8 9 8 2 6 1 2 5 9 4 8 0 7 3 1 4 5 4 Ó 7 5 1 1 8 2 0 1 7 7 0 1 1 8 1 1 4 1 1 340 110 +io':8 21 9 85 26 5 11 0 9 2 5 1 6 3 4 8 4 9 2 4 2 0 5 8 1 6 1 6 1 3 1956ApJ. . .124 . . .93M 3835 3797 3819 3791 3726 0 3703 8 3712 0 3721 8 3728 8 3734 3 3750 2 3757 2 3759 9 3770 7 3683 3691 5 3697 2 3686 9 NGC 7662. 100 traced atdifferentwavelengths.Thesedifficultiesseemedtobeparticularlygreatfor brightest portionofthebrighterringsouthnucleusbutalsoincludedportions the fainterouterringateitherside.Theorientationdifficulties,whichturnedouttobe so troublesomeinNGC7662,wereminimizedforthisobjectbecauseabrightknot © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem As inourobservationsofNGC7662,theslitspectrographwasplacedon Wave Length 4267 1 4363 2 4471 5 6716+30 4200 4340 4 4388 0 4541 6 4640 6 4685 7 4711 3 4740 2 4959 0 6563 . 6678 2 5411 5 5875 6 6300 3+63121 4861 3 Hi He I Hi H H H Hi H i+HeI Hi Hi O III Hi OlII Hi O III Hi [O HR [O ii]/ Line IntensitiesinNGC7662Measured Ident R. MINKOWSKIANDL.H.ALLER at the60-InchReflector TABLE 2—Continued 404 370 100: 46 49 20 19 4 4 2 28 0 0 2 9 9 3 5 8 1 0 66 0 8 0 7 0 55 2 1 0 5 tors NGC 2392 TABLE 3 05 4143 8 4026 1 4101 7. 3756 3759 3967+3970 4068+76 3721 8 3726+29 3734 3750 3770 3797 3819 3835 3868 3888 3924 3 0 0 83 4 2 2 1 1 1 1 1 1 ■5?4 4 9 7 7 of Ring Center 4 6 0 8 5 7 0 73 7 1 6 39 28 87 19 10 0 6 4 0 3 9 6 3 3 5 2 7 1 1 6 1 +5r4 3 8 5 0 1 0 +io:8 2 1 1 0 1 2 1 4 1 4 1 6 1 0 1 3 1956ApJ. . .124 . . .93M photometer withitsaid. 4 nearthecenterofbrightringandatpointschosenoneitherside,allexpressed the nebulosityfellonslitonesidesothatplatecouldbealignedinmicro- 60-inch platearelistedforcomparisoninTable5. the scaleHß=100incenterofbrightring.Alsointensitiesderivedfrom © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem We givetheintensitiesmeasuredonEplatesatseveralpositionslistedinTable 4340 4471 4541 4658 4711 4724 4740 tion ofX4686withrespecttoH/3!The 4227lineiscertainlyreal;noidentificationhasbeenfound 4200 0 4363 4634 4640 4685 4861 4959 0 5006 9 6650 6678 2 6716+30 7751 0 3968 4026 4069 4076 3+4068 4101 7 4143 8 4227 5411 5 5875 6 6300 3+63121 6562 9 7319 9+73302 7592 8. the sky.X3770maybeblended.[Nn] isstrengthenedat—O''15withrespecttoHaNoticethepeculiardistribu- 7135 8 3727 3734 3 3750 2 3760 0 3770 3797 3835 3868 3888 9 3703 8 3712 0 * Thesignonthesettings+15",+10", etc.,isarrangedsoastoagreewiththeconventionforNGC7662in Wave Length 7 1 Line IntensitiesinNGC2392ESpectrograph -15" 212 21 64 75 10 9 13 2 16 7 17 4 6 0 6 9 5 0 7 0 PLANETARY NEBULAE101 100-Inch Reflector* TABLE 4 227 -10" 23 72 70 11 7 15 5 13 0 17 3 5 8 4 1 8 2 5 0 8 9 2 9 5 8 7 Bright Ring Center of 1100 380 326 r 100 0 21 2 44 26 17 3 13 5 30 57 28 P 10 0 71 17 78 3 4 3 9 2 9 8 7 2 2 2 0 3 2 4 5 4 4 4 2 2 5 1 7 1 1 0 1 1 1 1 1 185 + 10" 49 54 19 6 15 6 12 6 3 8 4 2 4 9 7 4 0 2 6 6 6 8 5 7 7 0 194 + 15" 53 56 18 13 4 14 6 9 1 6 0 3 0 6 0 5 8 1956ApJ. . .124 . . .93M 3341 3203 lines insomeofthebrightknotshavebeenmeasured. WegiveinTable7thevaluesfor 3868 3727 3428 amorphous backgroundhas beensubtractedsothatthevaluesreferto knotsthem- in Table6.Notethegreatstrengthof[Nev]lines. Inaddition,theintensitiesof tained withthequartzspectrographatCrossley reflectoroftheLickObservatory. and 4340werefoundtobe 3.0,3.4,1.2,and1.8,respectively.Heretheintensity ofthe main bodyofthenebula. In aweakerknottherelativeintensitiesofXX3727, 3868,4101, used. The[Nev]linesaremissing,andX3727is relatively muchstrongerthaninthe one oftheseknotsonthescaleH7=50.H/3was notmeasurableontheslitlessplate 4724 4740 2 4861 3 6300+6312 The photometricproceduresandreductionmethods aredescribedelsewhere(Aller1951). main bodyofthenebula.In1944and1945anumber ofslitlessspectrogramswereob- some ofwhichattainafairsize.Theknotsshow lower levelofexcitationthandoesthe star. Uponthisamorphousmassaresuperposed numerousknotsandcondensations, Measurements oflineintensitiesinthebrightestportion ofthebrightringarecompared 6716+30 5411 5 5875 6 6678 2 level islikewiseunusual,inthatstrong[Nev]linesareobservedsimultaneouslywitha structure—and, indeed,maynotbeshell-likeatall.Theouter“ring”consistsofanum- in theneighborhoodof30000°K(WilsonandAller1954). It seemstocorrespondaspectralclassofabout09.5andanexcitationtemperature ber ofknotsandcondensations.Theinnerringisbrokenirregular.excitation an amorphousmasswhoseintensitygraduallyfallsoffwithdistancefromthecentral strong X3727[On].Thespectrumofthecentralstarhasbeenstudiedinsomedetail. 102 R.MINKOWSKIANDL.H.ALLER © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem The remarkablefeaturesofthisnebulaaremany.Itcertainlyisnotasimpleshell At firstglance,itseemsthattheouterportionofnebulositymayberegardedas Intensities ofLinesIntermediateStrengthinNGC2392as Comparison ofIntensitiesinNGC2392asMeasuredwith He II [Ne V] [Ne Hl] [O II] [Ne V] Ident the CrossleyReflectorandatMountWilson (100 0) 19 3 0 6 9 9 9 4 6 / (MtW) Measured withthe60-InchReflector 78 71 4363 4200 0 4227 4471 4640 4541 4658 4712 /(L) (Bright Ring) 84 36 26 76 78 TABLE 6 TABLE 5 4686 4861 4340 4101 3970 18 3 2 3 9 2 1 1 H/3 He II H m [Ne III] 7 4026 3727 3888 3770 3797 3835 4069+76 4101 7 Ident. I (Mt.W.) 100 44 30 57 28 55 2 17 0 19 8 /(L) 88 60 30 53 23 6 3 2 8 4 1 5 4 6 7 1956ApJ. . .124 . . .93M knot andthatoftheamorphousbackground. more definitivedataontheknotsandcondensations(Aller1956). selves. Theintensitiesderivedfromtheslitspectraincludebothcontributionof and gavesomewhatdiscordant values;X3869isblendedwith[Nem],3889 isconfused trinsically fainter,andforagivenexposuresmallernumberoflinesisrecorded. while todiscusstheoldermaterialfurther.Isophoticcontoursofnebulaprovide are strong.Thehighernumbers oftheBaimerseriesweremeasuredononly oneplate plate only;theseareindicated byanasterisk.Thelinesof[On],[N[S n],andHei take as100.Someofthestrongestandsome weakestlinesweremeasuredonone possible toobservethisnebulaonlywiththe60-inch reflector.Theslitwasset4"north ing inviewoftheeffortexpended.Becausefar northerndeclination(+70°),itwas of thecentralstar;tracingsweremadeatthreedifferent pointsinthenebula. the usualprominentelementshavebeenmeasured. Theresultsobtainedaredisappoint- © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem Fewer lineshavebeenobservedthaninNGC7662or7027.Thenebulaisin- Table 8givesthemeasuredintensitiesatthree points,theintensityofHßbeing Since muchsuperiorslitlessspectrogramsarenowavailable,itdoesnotseemworth This low-excitationnebulaisintrinsicallyratherfaint, andonlythestrongestlinesof 4101 4861 3868 3727 3727 3750 3770 3797 3835 3969 4340 4959 3970 3889 6300 6562 9 6716+30 5007 5755. 5876 Intensities inanOuterKnotofNGC2392 [Ne III] [OII] [Ne III] Ion +H H H HS H H/3 Ha+[N II] N11+Sky He I PLANETARY NEBULAE103 [O II] [O III] [O I] [S II] [O III] t Intensities inNGC40 Ident. 168 26 73 TABLE 8 TABLE 7 NGC 40 4101 4686 4340 541 0 172 0 Weak Knot 34 93 33 3 43 7 11 12 18 19 8 15 0 4 9 7 8 206 0 800 0 100 0 Center 43 0 22 5 26 6 29 5 20 0 54 6 11 9 11 6 15 3 13 1 He II H Hô 7 Ion 1500 0 341 166 0 Strong Knot 40 0 25 0 32 0 24 21 0 97 0 73 0 10 13 18 13 0 18 0 9 8 7 30 50 26 1956ApJ. . .124 . . .93M hm 4541 4685 4740 4861 4959 0 line isalmosttwiceasbrightHß,accordingtoourpresentmeasures.Relativeother 4340 4363 4711 5006 9 6562 9 5411 5 5875 6 104 R.MINKOWSKIANDL.H.ALLER slightly largerthanincertainoftheknotsNGC2392oransae7009. lines, itisstrongerthaninotherplanetaries,althoughthe\3727/Hßratioonly although theindividualintensityvaluesarenotingoodagreement.TheX3727[On] with HeI,whileHaisstronglyblended[Nn].M.L.White(1952)estimatedthe the redlineistobeattributed[Nn]. The [Om]linesareveryweakinthisnebula. one ofusin1938withaslitlessspectrograph(Aller1941)showedalsosteepdecrement, /(L), measuredonaspectrogram securedwiththeCrossleyquartzslitless spectrograph though theLickplatedid notregisterthefainterlines,itdoesshowthat ultraviolet both the60-inchand100-inchreflectors.For comparisonwegivetheintensities, when theyareobservedwithalargetelescope. BD+30°3639 hasacomplexnuclear as thoughthenebulawasaffectedbyinterstellarabsorption.Observationsobtained of abasiccylindricalsymmetry. does nothaveasphericallysymmetricstructure,althoughtherearesomevaguehints in theextremelycomplexnebulaNGC40.Directphotographsindicatethatthis average 6584[Nn]/Haratiotobe0.76inthisnebula;henceabouthalftheradiation at theLickObservatory in 1944.Thetwosetsofintensitiesaregoodagreement. Al- study ofalow-excitationnebulaisplanned,itwouldbebettertoobservesomeobject the positionsof[O11]knots.Henceconditionsexcitationvaryfrompointto more, thepositionsofcondensationsobservedin[0in]donotalwaysagreewith the highest-excitationregular-shapednebula. a particularlystrongX4686.Earlierwork(Aller1951) hadsuggestedthatitwasperhaps spectrum; thecentralstarofAnonISMS™seemsto haveaperfectlycontinuousspectrum. study becauseofitshighexcitation.Itisarather regularringnebula,characterizedby greater. Theseobjectsaresmall,acircumstance that providesnoparticulardifficulty such asBD+30°3639,orAnon1815,whosesurfacebrightnessesareverymuch © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem The NiandN2linesseemtobeconcentratedtowardthecenterofnebula.Further- Even afterallowingfortheseblends,theBaimerdecrementseemstobetoosteep, Further longexposuresonthisnebulawouldappearunprofitable.Ifanintensive Although itisanintrinsicallysomewhatfaintobject, NGC2022wasselectedfor Our adoptedintensities,/(Mt.W.)(seeTable9), arebasedonplatessecuredwith Hi H/3 He II He I H He II He II O III O III [A IV] [A iv] [O III] t Ident. J(Mt. W.) 306 0 100 0 189 Ó 101 0 41 7 16 7 13 2 10 5 Line IntensitiesinNGC2022 3 4 9 5 6 550 0 100 0 197 0 /(L) 39 1 86 0 Ü 2 NGC 2022 TABLE 9 4101 3428 7 3868 3888 3967 3132 8 3203 1 3312 3 3345 8 3444 0 3727 . He II Hi m Oil! O III Om [Ne V] [Ne III] [Ne III]+H [NeV] [O II] Ident. I(Mt W) 36 7 21 4 19 4 8 3 /(L) 44 0 35 45 0 53 20 26 10 15 15 8 8 8 0 1956ApJ. . .124 . . .93M larities inbothrings.Inspiteofitsunfavorabledeclination,theobjectwasincluded secured withtheJSspectrographat60-inchtelescope. our program,andexposuresrangingfrom7minutesto5hours22were image inthislineislarge.Tentativeintensitiespreviouslypublishedbyoneofus by Stoy.Thepresentsetofintensitiesshouldbemuchmorereliable,eventhoughthe [Ne v],[Om],andHenlinesarestrong.Then]X3727radiationispresent,the intensities ofsomethelinesinfarultravioletmaybeoverestimated. other representativesofitsclass.Evenso,large-scaleslitlessspectrogramsrevealirregu- with theintensityofH/3inbrighterringchosen as100.Thehigh-excitationlines both inthebrightinnerringandouterfainter ring.TheyaregiveninTable10, (Aller 1941)werebasedonaplateofquestionablephotometriccalibrationtakenin1934 high-excitation objectsconsidered—inaccordance withourexpectations.Furtherexpo- one ofusshowsreasonablygoodagreement.Ultraviolet spectrashowthatthelinesof are missingfromtheouterring. published intensitiesfor the strongerlines(Aller1951).Table11contains theresults for IC351andJ320.They arebasedononeplatepernebula. given eye-estimatesofthe intensitiesofallthelineshecouldobserve,and one ofushas NGC 2440onlylinesofintermediatestrengthhave beenmeasured.Wyse(1942)has O in,X3444,3346,andHen3203arepresent, althoughmuchweakerthaninother and thehigher-excitationobjectsNGC2165, 2440,andIC351.InNGC2165 sures onthisobjectseemunprofitableunlessmuch fasteropticscanbeemployed. © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem This beautifuldouble-ringplanetaryhasamorenearlyregularstructurethanmost In spiteofthesmallscaleresultingspectrograms,wemeasuredintensities We havealsosecuredobservationsofthelow-excitation objectsIC1747andJ320 Comparison ofthemeasuresforbrightringwith valuespreviouslypublishedby 4471 4541 4685 4740 4861 4959 0 3835 4 3868 7 3888 9 3969 4101 7 4340 4 4363 4640 4712 0 6562 5875 * ApJ,113,134,Table3,1951 Line IntensitiesinNGC1535asMeasuredwith B. m H nr He I He II He II H/3 He I Ha Ne III N III [Ne III],He [A IV] [O III] [O III] [A IV] v t PLANETARY NEBULAE105 Ident the 60-InchReflector TABLE 10 NGC 1535 363 0 361 0 Bright 100 0 Ring 40 0 89 0 21 8 28 8 51 20 0 16 19 8 4 1 4 6 7 5 2 7 7 Outer 48 0 22 0 27 6 11 0 13 Ring 4 8 6 6 9 7 Bright Ring Previous Values* 100 0 49 0 30 0 21 0 21 0 24 0 74 0 19 0 6 0 1956ApJ. . .124 . . .93M 4861 4959 6563 hand, Hen,X5411,and4686,aswell4640Nm,4740[Aiv]arestronger low- andhigh-excitationfeatures.Forexample,thelow-ionizationlinesof[Oi]X5755; higher dispersionwouldhavebeenappropriate,inordertominimizetheeffectsof [O n]X3727,andalso5200arestrongerinNGC2440than1535.Ontheother intensity. Exposureswerenotobtainedforthestrongerlines;weakerlinesa continuum. some yearsagobyoneofus.TheMountWilsonplatescoveronlylinesintermediate 7027 inanumberoffeatures.Inparticular,NGC2440seemstoshowcombination 106 R.MINKOWSKIANDL.H.ALLER © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem NGC 2165appearsasasmall,nonuniformdisk,whereas2440resembles In Table12wecomparetheresultsforNGC2165and2440withthoseobtained IC 351 6713+6730 6678 6364 6300 6085 5875 4861 4740 4640 4541 4200 5755 5411 5192-99 4712 4471 4363 4340 4101 4076 4026 3967 3889 3835 3797 300: 350 100 J 320 310 280 100 Intensities forNGC2165and2440 Line IntensitiesinIC351andJ320 4340 4686 4363 Mt W. 0 93 0 64 0 30 0 74 0 18 0 72 0 56 0 66 0 35 0 21 0 44 1 74 0 10 0 21 Ó 22 0 55 0 50 Í 58 1 95 1 41 / NGC 2165 TABLE 12 TABLE 11 IC 351 10 00 49 52 0 65 0 72 0 47 0 18 15 0 34 0 20 0 53 0 48 Lick I 60 60 30 80 10 12 18 J 320 44 20 Mt. W. 0 87 0 22 0 42 Ó 30 4 60 1 95 1 45 1 60 2 04 1 01 / 8 27 2 6 8 1 1 NGC 2440 4101 3868 3969 ÍÓ 0Ó 0 30 0 27 2 90 5 20 2 60 1 60 1 00 3 70 Lick I IC 351 31 24 78 J 320 46 93 27 1956ApJ. . .124 . . .93M than inNGC1535.TheX4640linemayarisefromanuclearstar,sinceitisshorter mixed regions. effects; thelow-andhigh-excitationradiationsappeartoariseincontiguousorwell- than anyoftheotherlinesonplate.Thereislittleevidenceforstratification a simpleprocess;onecomparesthenebulawithstandardstar.Inpractice,itisdiffi- protons andelectrons;someofitmaycomefromdoublephotonemission,amechanism which thecontributionsofweaklinesareaddedtotruecontinuum.Evenatbest, this troublecanbepartlyovercome,butatthepriceofaspectrumlowerpurity,in the calibration-curvesleadtoappreciableerrorsinintensities.Ifawideslitisused, the blueregionarederivedfromverylowphotographicdensitieswhereslighterrorsin suggested byBreitandTeller(1940)favoredGreensteinSpitzer(1951). show anunderlyingcontinuum.Someofitmustoriginatefromtherecombination served, andthisdifficultydisappears,ofcourse. gated exceptNGC7027.Withpracticableexposuretimes,themeasuredintensitiesin cult primarilybecauseoftheextremefaintnesscontinuuminallobjectsinvesti- measure oftheratioI{n)/I\{s)T,whereI\(s)denotesenergydistributionin to findthecontinuuminanabsorption-linestar.Becauseofverysmalldispersion the presenceofweakerlinesmakesitdifficulttoestimatepositioncon- gest stronglythatthemeasuredenergydistributionincontinuumisaffectedby which haslowsurfacebrightnessandaverybrightcentralstar,theresultsobtainedsug- tinuum inmuchthesamewayaspresenceofstellarabsorptionlinesmakesithard stellar continuumandTthetransmissionofquartzdiffuser. investigated. Ontheotherhand,inNGC7027centralstaristoofainttobeob- stellar light;themeasuredintensityincreasesbyafactorofmorethan6fromX8000to supporting thesecondarymirrormaycausetrouble.Themeasurednebularemission, better todisregardituntilthepossibleeffectofscatteredlightinthiscasehasbeen tion bandswhichmaybeallowedforonlyapproximately. servatory, whereastheMcDonaldplatecovers theintervalXX3200-5000.In erately exposedplates.Thelinespectrum,however,isexceedinglyrich,sothattheloca- fit ona1/Xplot,thedeviationsinlogIarewithin +0.075.Whenweconsiderthediffi- plates, althoughthefitisnotmuchpoorerthanone mightexpect.Usingthebestaverage tion ofthecontinuumisdifficult.Observationscovering thevisualandinfraredregions nebula hasarelativelystrongcontinuumwhichisprominentattheredendofevenmod- X 4000.Inthelightofpresentknowledge,thisresultappearsimpossible,anditseems therefore, maybepartlyaffectedbydiffractedlightfromthecentralstar.InNGC2392, the infrared,itisalsodifficulttotakeintoaccounteffectsofatmospheric-absorp- used ascomparisonstars.TheBalmercontinuum ismuchstrongerthanthegeneral range isnotsurprising. which operatessodisadvantageously inotherpartsofthespectrum,should playnorole. culty indrawingthecontinuum,abouta10per centdiscrepancyoverthecommon XX 4000-4700regiontheTexasplateshowsasteeper gradientthandotheMountWilson (XX 4000-8500)wereobtainedon103a-FandI-N emulsionsattheMountWilsonOb- continuum, andtheinfluenceofnumerousweak moderatelystrongemissionlines, On theassumptionthat the Balmercontinuumisapurerecombinationspectrum and c © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem In principle,thedeterminationofenergydistributionincontinuumshouldbe In additiontotheircharacteristicbrightemission-linespectra,theplanetariesalso The energydistributioninthecontinuumofnebula,/(^),followsfromadirect If thecentralstarisbright,scatteringanddiffractioneffectsproducedbyweb NGC 7027istheonlyobjectforwhichresultsofsomevaluecouldbeobtained.This The measuredenergydistributionisgiveninTable 13.BothVegaandaPerseiwere c V. THECONTINUOUSSPECTRAOEPLANETARIES PLANETARY NEBULAE107 108 R. MINKOWSKI AND L. H. ALLER that there is no self-reversal, the electron temperature Te may be obtained from the emission intensity E\ at two points Ai and A2 from

0.4343/zc log Ei — log E = (2) 2 ~ kTe

The observed intensities are, of course, to be corrected for interstellar reddening. With the differential correction of 1.43 mag. between A 3750 and A 8750 found from the ratio of the Paschen and Balmer lines in the following paper (Aller and Minkowski 1956), the measured intensities in the Balmer continuum vary reasonably well as 1/A and lead to Te = 12300 ± 3600, in satisfactory agreement with the value 14300 derived from line intensities. The intensities immediately to the red of the Balmer limit are low and could therefore not be determined with great accuracy. They contradict, however, the result by Page

TABLE 13 Energy Distribution Measured in the Spectrum of NGC 7027

LOG I LOG / McDonald Mt Wilson Qf/2 82 70 Mt Wilson 3300 1 85 4800 74 3400 1 91 5000 77 3500 1 97 5200 1 85 3600 2 02 5400 1 89 3700 1 16 5600 1 92 3800 1 19 5800 95 3900 1 25 6000 98 4000 1 34 1 39 6200 00 4100 1 40 1 44 6400 03 4200 1 45 49 6600 05 4300 1 53 53 6800 07 4400 1 59 58 7000 07 4500 1 65 62 7500 04 4600 1 70 1 67 8000 1 98 4700 1 74 1 70

(1942), who found constant intensity in this region. The McDonald results lead to a value of A log I of about 0.9 for the Balmer discontinuity; the Mount Wilson results above A 4000 carry some suggestion that the McDonald intensities below A 4300 are too low and that A log I may be slightly smaller. The observed value would lead to an elec- tron temperature of only 6600° if the two-quantum emission contributed in the amount computed by Spitzer and Greenstein (1951). An electron temperature as low as this is contradicted by the gradient of the continuous spectrum and by the values of around 14000° found from line intensities. It seems necessary to assume that the two-quantum emission is weak in NGC 7027. Actually, the two-quantum emission may be squelched at high densities. Seaton (1954) has made quantitative estimates of the degree of sup- pression of two-quantum emission as a function of density. Its weakness in NGC 7027 agrees with the suggestion by Seaton and with the result of Aller (1954), who found for the average electron density the value of 1.7 X 104 cm“3 and concluded that the local densities in condensations must be much higher, to explain the [S 11] transauroral- and

© American Astronomical Society • Provided by the NASA Astrophysics Data System 1956ApJ. . .124 . . .93M justifiable todiscussthediscordance,whichcannotbeconsideredashavinganyphysical Williams, R.,andHall,J1942,Ap.,95,225 .1939,ibid,pp147and159. Filler, W.,andAller,LH1954,Ap/.,120,48 Barbier, D,andChalonge,D.1940,Annd’ap3,2. .1956,GaseousNebulae(NewYork:JohnWiley&Sons,Inc),p256 .1951,ibid,113,125. Abbott, CG.\90&,SmithsonianAnn,\o\2 significance. limit. Wyse, AB1942,^^/,95,359 Wilson, OC,andAller,LH1954,ApJ.,119,243. Williams, R1938,PubObsMichigan,7,93 1942,ApJ,96,78 Filler, W1955,ApJ,122,235. Breit, G,andTeller,E1940,Ap/.,91,215. Berman, L1930,LickObsBull,15,86 Aller, L.H,Bowen,ISandMinkowski,R1955,ApJ122,62 interpretation. Thisdiscordanceismostprobablyduetothedifficultyindetermining of theintensitydistributionbetweenBalmerlimitandX8000defiesanytheoretical This contradictstheresultofPage,whofoundnearlyconstantintensity.Butshape would leadtoanelectrontemperatureof20000°.Thisisbeconsideredasupper emission ispresent.Ifthiswereentirelyabsent,theobserveddiscontinuity nebular-line intensities.Itisimpossibletofindoutexactlyhowmuchofthetwo-quantum the truelevelofcontinuousspectrum.Inviewthisdifficulty,itwouldnotbe White, ML1952,Ap/.,115,71. Stebbins, J,andWhitford,AE.1945,Ap102,273 Spitzer, L,andGreenstein,J1951,Ap114,407 Popper, DM1937,thesis,UniversityofCalifornia MacRae, D.,andStock,J1954,Nature,173,589. Code, A1956,privatecommunication Aller, LH.1941,Ap/,93,236 Seaton, M.J1954,Annd’ap,17,296 Plaskett, H.H1931,PubDornApObs.Victoria,4,187. Page, TL.1936,MN,96,627. Kienle, H.,Strassl,H,andWempe,J.1938,ZsfAp16,201. Fowle, FE.1933,SmithsonianMiseColl,88,611 © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem Redward fromtheBalmerdiscontinuityintensityincreases,aswouldbeexpected. PLANETARY NEBULAE109 REFERENCES