1954Apj. . .120 . . .48L PHOTOELECTRIC

1954ApJ. . .120 . . .48L 3 43 3 34 scope. Theabsolutesurfacebrightnessesareingoodaccordwithresultsobtainedbythefilterphoto- photometric measuresmadesomeyearsagobyoneofus.Theelectrondensitiesrangefrom10toabout with theZanstratemperatureofcentralstar.Thislackcorrelationisinaccordancerecent excellent agreementwiththosefoundpreviouslybyMacRaeandStocksubstantiatethephotographic planetary nebulaehavebeenmadewiththeaidof10°objectiveprismonCurtisSchmidttele- and oneofus.Theelectrontemperaturesfallbetween8000°20,000°Kshownostrongcorrelation 2 X10electrons/cm;theyareofthesameordermagnitudeasthosefoundsomeyearsagobyMenzel electric method.ThemeanN1/N2intensityratiois3.03,whilethe(Ni+N2)/Hßratiosarein Lick Observatoryandhavegiventherelativetotalbrightnessesofmonochromatic White (1952),andothers,includingoneofthepresentwriters(Aller1941,1951).Ber- man andoneofusemployedtheslitlessspectrographsonCrossleyreflectorat Berman (1930),BowenandWyse(1939;Wyse,1942),R.Minkowski(1942),M.L. ures orestimatesoflineintensitieshavebeenpublishedbyH.Plaskett(1929),Louis images. Others,usingslitspectrographs,measuredtherelativeintensitiesreferredto theoretical expectations. planetarios, havebeenutilizedinconjunctionwiththeoriesofphysicalprocessestoevalu- tions oftheplanetarynebulaehavebeenmeasuredbyanumberinvestigators.Meas- ate thedensities,temperatures,andchemicalcompositionsofnebulae.WithHebb some fixedpointinthenebulaoraveragedroughfashionoverentirenebula. these observationsgaveelectrontemperaturesintheneighborhoodof8000-10,000°K and MenzeltargetareasforthecollisionalexcitationofmetastablelevelsinOm, lines areoftententotwentytimesasstrongHß andfallinaspectralregionwherethe The 7(4363)/I{Hy)ratiocanbemeasuredreasonably wellbyphotographicphotometry; ratio ofthegreennebularlineintensitytothatX4363,I(Ni+A2)//(X4363),can with theaidofHebbandMenzeltargetareas.Hisresultsrequireintensityratios electron temperaturesthatareinsharpdisagreementwiththosefoundbyothers.He electrons/cm. The photoelectriccellcanreachthestrongerlines easily,butthephotometryof sensitivity ofthephotographicemulsionchanges rapidly withwavelength. be measuredmostconvenientlybycomparing4363withHyandAi+Y2Hß. has notpublishedtheobservationaldatausedinhiscalculationsofelectrontemperatures fainter lineswouldbemuchmoredifficult.Atleast for thepresent,spectrophotometry the greennebularlinesto4363markedlydifferentfromthosepreviouslyobtained.The of thefainterlineswouldappeartorequireuse ofthephotographicplate.According- the I{Ni+ÍV2)/I(Hß)ratioismuchmoredifficult toestablish,becausethegreennebular (Menzel etal.1941).Theelectrondensitiesappearedtolieintheneighborhoodof10-10 © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem Photoelectric spectrophotometricmeasuresoftheN\,N2,andHßlineintensitiesintwelvebright In thelastthirtyyearsrelativeintensitiesofprincipalmonochromaticradia- These data,combinedwithestimatesofthedistancesandsurfacebrightnesses More recentlyHenriAndrillat(1950,1952)haspublished,foranumberofplanetaries, Hence acheckonthephotographicspectrophotometry isverymuchtobedesired. The Observatory,UniversityofMichigan,AnnArbor,Michigan PHOTOELECTRIC SPECTROPHOTOMETRY William LillerandLawrenceH.Aller OF PLANETARYNEBULAE Received January22,1954 I. INTRODUCTION ABSTRACT 48 1954ApJ. . .120 . . .48L ly, wedecidedtomeasuretheintensityratiosofX5007(iVi),4959(7^2),andHßinas by DonaldMacRaeandJurgensStock(1954).Theirresultsareinexcellentagreement season. SectionIIofthispaperdescribestheobservationalproceduresemployed; nebulae. PhotoelectricmeasuresoftheI(N\+N^)//(Hß)ratiohavebeenmadepreviously observers; SectionIVtreatsoftheintensityratiogreennebularlines;whilein III givesthemethodsofreduction,results,andcomparisonwiththoseother many nebulaeascouldbereachedwithourequipmentinthelate-summerobserving Section Vwediscusstheelectrondensitiesandtemperaturesobtainedforplanetary with ourown,aswillbeseeninSectionIIIofthispaper. high-excitation object. focal lengthofthetelescope,pair[0in]imagesingreenwillbecompletely measure photoelectricallytheintegratedintensitiesofTVi,7^2,andHßimages separated onlyforplanetarieswithdiameterslessthan29";theoxygenimageatX4959 monochromatic nebularimagesslowlypassedover aslit0.17mmwide,correspondingto will overlapHßonlyifthenebulaismorethan62"indiameter.Thusitpossibleto planetaries bytwodifferentmethods.Thefirstconsisted ofsecuringspectraltracesall majority ofplanetarynebulaewiththisequipment. telescope inhourangleataspeedslightlydifferent fromthesiderealrate.Then the nebulaebyorientingdispersionineast-west directionandbydrivingthe 10° combination,giveadispersionof160A/mmatHß.Becausetherelativelyshort telescope motioninhourangle.Thismethodissimilar tothatemployedbyMacRaeand Hulbert Observatoryprimarilyforthewideningof photographicspectra,providedthe tral starcontinuatothe violet ofHßandtotheredX5007image [Om].Be- individual deflectionsofthe monochromaticnebularimagesandofthe andcen- Stock (1954). 27 AatHß.Frequency-controlequipment,designed bystaffmembersoftheMcMath- cause thefaintnessof nebulaeoftenmadeitdifficulttocentertheimages accurately © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem Fig. 1.—Photoelectrictracingofthespectrumaplanetarynebula.NGC7027representstypical The 4°and6°objectiveprismsoftheCurtisSchmidttelescope,whenusedintheir On thenightsofSeptember13,14,15,and16,1953, weobservedatotalofthirteen In addition,wemeasuredthespectraofallbuttwo oftheplanetarynebulaebytaking II. THEOBSERVATIONALPROCEDURE PLANETARY NEBULAE49 1954ApJ. . .120 . . .48L 'in goodagreement.Weregardedtheslittracings asmorereliablethanthediaphragm was toofainttomeasure;aboutallthatcanbesaid isthatitseveraltimesfainterthan optical designofthephotometer,however,toaccommodatethe//3.5beamCurtis with thevaluesofthis ratio foundphotoelectricallybyMacRaeand Stock (1954) Ah- deflections andassignedthemgreaterweight.The finalmeanvaluesarelistedincolumn extinction, andthecontributionmadetodeflectionsbycontinuousspectraof took intoaccounttheeffectsofspectralresponsephotometer,atmospheric the photometerplusopticalsystemoftelescope.Inthesereductionswehave continuum canbemeasureddirectlyfromtheslit tracings. derived fromthetracingsofVegaandDeneb.The intensityofthestellarandnebular NGC 1535. NGC 7027 NGC 7009 and D.Chalonge(1940). enable thedeterminationofatmosphericextinctionandspectralresponse Schmidt. this fashion.Theelectronicequipmentusedininvestigationisidenticalwiththat in thediaphragm,wemadenoattempttomeasureAhandimagesseparately 4 ofTable1,andcolumn5liststhenumberobservations securedoneachobject.The nebulae andthecentralstars.Asmentionedabove, thefirsttwocorrectionsareeasily NGC 7662. NGC 6826 NGC 6818. NGC 6803. NGC 6572 NGC 6543. NGC 6210 NGC 7027.Itisarathersmallobject;hencetheseparationofAhandimages 5Ö WILLIAMLILLERANDLAWRENCEH.ALLER adopted theenergydistributionspublishedbyR.C.Williams(1939)andD.Barbier average probableerrorofasingleresultis+2.3per cent.TheHßimageofNGC6803 clear. TheRingNebulainLyra,NGC6720,wastheonlynebulaforwhichdiameter continua requiredseparatemeasureswhenthediaphragms wereused.Thisbackground described inaseparatepaperbyoneofus(Liller1954).Itwasnecessarytoalterthe (70") wastoolargetopermitthemeasurementofrelativeintensitiesgreen [O m]pair. {circles) andphotographically byoneofthepresentwriters(Aller1941,1951) {crosses). IC 418. IC 2149. © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem Tracings ofthespectraaLyraeandCygnisecuredatdifferentzenithdistances Reproduced inFigure1isthespectraltraceofhigh-excitationplanetary, The diaphragmdeflectionsandslittracingsgave /(Ah +Ah)/I(H0)ratiosthatwere In Figure2wecomparethe valuesof/(Ah+Ah)//C0ß)derivedinthisinvestigation To converttherawdeflectionsofAh,andHßlinesintorelativeintensities,we Nebula (1) bI 21 01.4 23 23.5 21 05.2 19 43.4 19 40.0 19 29 18 09.6 17 58.6 1642?5 a(1950) 4 11.7 5 25.2 5 43.5 (2) m. THEREDUCTIONOEOBSERVATIONS Observed DataonPlanetaryNebulae 5(1950) +42.0 +46.1 +42.3 +50.4 + 6.8 +10.0 +66.6 +23?9 -12.9 -11.6 -14.3 -12.7 (3) TABLE 1 /(A1+A2)/ IW) 20.2 15.4 16.1 11.6 18.2 15.6 15.1 9.32 8.86 5.4 (4) 1.78 (5) n /(Ai)/ /(A*) 3.00 3.11 3.00 3.00 3.18 3.04 3.40 3.02 2.72 2.95 2.90 (6) 13 10 A” (7) 9 9 8 9 6 5 2.5 4" 6 7 log (5i+52) + .36 + .22 + .38 + .15 +0.55 - .45 - .35 - .16 - .05 -0.18 - .68 - .57 (8) 1954ApJ. . .120 . . .48L believe thatthefluctuations areduetotheseerrors. from theuncertaintyindrawing inthebackgroundcontinuum,andweare inclinedto terminations ofoneus.Colonsfollowingtwothecrosses meanthatthephotographicvalueisbased circles showthephotoelectricresultsofMacRaeandStock; thecrossesindicatephotographicde- Ai +NtracingthengavethesizeofNideflection. Asafurthercheck,thespectra A subtractionoftheareaunderneathprofile fromthetotalareaunderneath on asingleobservation. real orwhetheritisduetoobservationalerror.The largestsourceoferrorseemstoarise for boththe[0m]lines. twice; thespectraofremainingplanetarieswere tracedonlyoncewithaslit.As of anumberthenebulaeweremeasuredbyconstructing profilesintheabovemanner redward wingoftheAlinegeometricallysimilartoN\line. The twophotoelectricinvestigationsareincloseagreement,reflectingtheaccuracyof are thephotographicresults.Theredoesnotappeartobeanysystematicerrorin photoelectric measures.Alsoingoodagreement,butshowingasomewhatlargerscatter, result, itisdifficulttotellwhetherthevariation in thisratiofromnebulatois single photographicresultdropsto±12percent. latter results,onlytherandomscatterthatwouldnaturallyarisefromuncertainties nebulae weresphericallysymmetric.EachprofilewasconstructedbyassumingtheN\ based ononeobservation,andifthesetwonebulaeareomitted,themeanerrorofa and Aimagestobeidenticalinshapeintensitydistributionbymakingthe files” ofatleastonethetwolines,sinceitcouldnotbesafelyassumedthat NGC 1535and6818showthelargesterrors.Bothphotographicdeterminationswere ones, wefindthatthemeanerrorofasinglephotographicresultis+18percent. in photographicphotometry.Ifweassumethatthephotoelectricvaluesaretrue were theN\andAlinescompletelyseparated,itwasnecessarytoconstruct“pro- 2 2 2 2 2 © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem Fig. 2.—AcomparisonofthevaluesI(Ni+N2)/I(Bß)foundbyseveralobservers.Theopen The 7(Ai)//(A)ratiosarelistedincolumn6of Table1.NGC1535wasobserved From theslittracingswealsodeterminedquantity/(Ai)//(A).Fornoplanetary 2 2 PLANETARY NEBULAE51 1954ApJ. . .120 . . .48L 10 2 42 24 52 WILLIAMLILLERANDLAWRENCEH.ALLER like tofindtherelativeintensitiesofsunandVegaatwavelengthsgreen Deneb, wecanderivemonochromaticsurfacebrightnessesforthenebulae.Ambarzumi- Johnson andMorgan(1953)thatofthesuntobe—26.87,meanvalues nebular lines.IfwetakethephotovisualmagnitudeofVegatobe+0.03,asgivenby length rangefrom4860to5440,weshouldbeablecalculatetheratioofintensitiesat mean oftheeffectivewavelengthsis5433A,accordingtoScaresandJoyner(1943). intensities ofthenebulaewithspectrumasolar-typestar.Therefore,weshould an (1933)hasdescribedamethodofderivingthesesurfacebrightnessesbycomparingthe If weassumethatbothVegaandthesunradiateasblackbodiesoverlimitedwave- two objectsattheeffectivewavelengthofInternationalPhotovisualSystem.The photovisual magnitudes(26.90)shouldcorrespondcloselytotheratioofintensities published byKuiper(1938)andDeVaucouleurs(1949),thenthedifferencein the meanwavelengthofNiandN2lines,X4995.Herewetookcolortempera- tures ofthetwoobjectsinthiswave-lengthregiontobe16,500°and5700°.Theresulting 4.79 X10.Henceweareabletocalculatewhatthedeflectionwouldbefrom1Aof illuminated diskofradiusA.ThevaluestheradiiarethosegivenbyCurtis(1918)and the solarspectrumatmeanwavelengthofgreenforbiddenoxygenlines. brightnesses derivedphotoelectricallywiththeuseofnarrow-bandinterferencefilters. ing fromtheuseofeffectivewavelengthsInternationalPhotovisualSystem, the logarithmsoffluxesexpressedinergs/cm/sec.Becauseuncertaintiesaris- values. fluxes oftheplanetarynebulaeinlightgreen[0in]radiations.Theunitsare are listedincolumn7ofTable1.Thelastthistableliststheresultingsurface difference inmagnitudeatX4995is26.70,whichequivalenttoanintensityratioof The twosetsofdeterminationshavealreadybeencomparedintheotherpaper,andwe feel thatthesurfacefluxesderivedhereshouldbegivenhalfweightofother one ofus.However,theseresultsgiveusanexcellentindependentcheckthesurface these valuesshouldnotbeexpectedtoofashighprecisionthosegivenelsewhereby tainty indrawingthebackgroundcontinuum. have foundnodependenceofthevalueuponsizenebula.Themeanratiofrom Minkowski foundaratioof2.99±0.15fortheOrionnebula(1934). later, H.Plaskettpublishedaratioof2.73(1929),whileL.Bermanfound2.63(1930)^ ratios intheS—Dtransitionof[On].Thediscrepancy wasremovedbylaterinvestiga- electron’s spinwithitsownorbit,givesresultsthat contradicttheobservedintensity mediate coupling.Theyyieldedanintensityratio of2.83.Thistheory,whichassumes tors, whotookintoaccounttheeffectsof(spin)— (spin) interactionsandof(spinone that theperturbationfromLS-couplingsituation arisesfromtheinteractionofan electron) —(orbitofanother)interactions(Alleret al.1949).R.H.Garstang(1951)has independently byShortleyandhisassociates(1941) werebasedonthetheoryofinter- abilities inthepandconfigurationsofC1,An, Om,Fiv,andNeVof01,n, applied thisimprovedtheorytothecalculationof the energylevelsandtransitionprob- data favorGarstang’sresult,butthescatterisstill comparablewiththedifferencesbe- and Nem,respectively.Hepredictsanintensity ratio of2.93.Thusourobservational tween thetwotheoretical predictions. 12 planetariesis3.03±0.11(meanerror).Thechieferrorseemstoarisefromtheuncer- 3 3 © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem Finally, bycomparingthenebulartracingswithspectralofVegaand In calculatingthemeansurfacebrightnesses,wetreatedeachnebulaasanevenly The smallerplanetariesshouldgivethebestdeterminationofN1/N2ratio,butwe Theoretical calculationsofthe7(^2—P2)7/(^2— Pi)ratiobyPasternack(1940)and Half acenturyago,WilsingandSchemer(1902)foundthisratiotobe2.5;manyyears IV. THEINTENSITYRATIOOEGREENNEBULARLINES 1954ApJ. . .120 . . .48L 2 replacing ^4pdbyOpd,however.Ifweuse where AdenotestheEinsteincoefficientofspontaneous emissionandvthefrequency. ^2 levelsby 7(4363). from thevalueappropriatetothermodynamicequilibriumattemperatureT density by of thetheoreticalb’sgivenbyMenzelandBaker. planetaries (Aller1951)indicatethatwhencorrectionsaremadefortheeffectsofinter- N maybefoundfromEß,ThefluxinHßergs/cm/secisrelatedtotheelectron as ashellofthicknessdandouterradiusA,wedefine rather easilytheelectrontemperatureintermsofobservedratioI(Ni+Nz)/ Menzel’s calculations(1940).Morerecently,M.J.Seaton(1953)hascalculatedtarget ultraviolet Lymanquantumabsorbedinthenebulaisultimatelydegradedintoa ment. InthismodelthecentralstarradiatesnoenergyinLymanlines,andeach stellar absorption,MenzelandBaker’s“modelB”(1938)fitstheobservedBalmerdecre- Here b^(T^)isafactorthatmeasuresthedeparturesofpopulationinfourthlevel If observationsofSßacarenotavailableorsufficientaccuracy,anestimate Further, ifNi=N„theelectrondensitymaybecalculatedfrom(MenzelandAller the Balmercontinuumnearitslimit(3630-3650A).Ifnebulacanbeapproximated to correctthelasttermindenominatoroftheir equationforatypographicalerrorby Hebb (1941)givethenecessaryrelationshipasequation (9)oftheirpaper.Itisnecessary area parameters,Ops,Opd,andOds,theEinstein Acoefficients.Menzel,Aller,and areas forandotherionsbyanimprovedtheory.Usinghisresults,wemayderive quantum ofLyman-a.Theagreementbetweentheoryandobservationjustifiesouruse are known.EarlierdeterminationsoftheelectrontemperaturewerebasedonHebband 1941) and greennebularlinesof[0m],providedthatthetargetareasforcollisionalexcitation (Menzel 1937,eq.[4]).ExtensivemeasuresoftheBalmerlinesinalargenumber t n e 2 r1 © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem Let Sßacdenotethefluxinergs/cm/secradiatedbynebulaa20Aintervalof The electrontemperaturemaybecomputedfromtherelativeintensitiesof4363 The ratioA^So)/A(D2)dependsontheelectrondensity andtemperature,thetarget- The intensityratio7(X4363)//(iVi+N^)isrelatedtothepopulationsofand V. THEELECTRONDENSITIESANDTEMPERATURESINPLANETARYNEBULAE 3 3 3 ¿OS, P)=0.23¿0D, Pi)=0.0071] ¿OS, ^=1.6¿CD,P)0.021] 2 Ops =0.195,OpD=1.73,Ods0.61(Seaton 1953), l3 7 (Ni+Nt)NODs)A(D,P)hv(T),P)’ 1 7(4363) _NOSo)AOS,D)hv -20Xn Sß= 7.37X10^¡TTMF.)De. lo12 N =1.68X105/.(2) £ -DOC PLANETARY NEBULAE Z> =3d[l \ (Garstang1951), 53 (3) (i) 1954ApJ. . .120 . . .48L 2 was tocomparetheintensity intheBaimercontinuumwithnear-byBahner lineswhose grams obtainedattheMcDonald Observatoryhavealsobeenused.Theprocedure then graph attachedtotheCrossley reflectorattheLickObservatory,although slitspectro- I(Ni +N2)/I(Hß)madebyoneofus(Aller1941, 1951).Column8giveslog(Sboc/S¿), planetaries (Ap.J.,inpress). The 7(4363)valuesthatarenotfollowedbyanumberinparentheseswere adoptedfromalldataavailabletous,includingiso- the data.Mostofratiosarebasedonplatesobtained with.thequartzslitlessspectro- determined byphotographicphotometry.Detailed referencesaregiventothesourcesof photic contourmeasuresof4363and4340onMountWilsondata,, MinkowskiandAller’sspectrophotometryofselected 5. L.H.Aller,Ap.J.,118,547,1953. 2. L.H.Aller,Ap.J.,113,125,1951. 4. Slit-spectraobservationsobtainedattheMcDonaldObservatoryin1945 byoneofus. 3. HeretoforeunpublishedvaluesobtainedattheLickObservatory,1943-1945, byoneofus. the ratiooffluxin20AintervalBaimer continuumtothefluxinHß,as NGC 7027. 1. D.H.MenzelandL.Aller,Ap.J.,93,195,1941. NGC 7662. NGC 7009. NGC 6826. NGC 6818. NGC 6803. NGC 6572. NGC 6210. NGC 3242. NGC 2392. NGC 6543. NGC 1535. nebulae notincludedinTable1wehaveusedthephotographicdeterminationsof Hß. Allthedataofcolumn6areobtainedfromphotoelectricwork,butforthose of thenebula,andincolumn7ratiointensitiesgreennebularlines relative thicknessoftheshell,d/A,accordingtodataCurtis(1918),distance KNx+Ni) flux inergs/cm/secforthegreennebularlinescorrespondingtoassumedangularsize are giveninTable2.Foreachnebulatheaverageangularradius,A",and for suchlowdensitiesasprevailinplanetarynebulae. in parsecsfromtheworkofBerman(1937),quantityD,logarithmmean we obtainthefollowingexpression: 54 WILLIAMLILLERANDLAWRENCEH.ALLER 7(4363) © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem IC 4593. IC 418. IC 2149. * Thenumbersinparenthesescols.8and9refertothesourceofnumericaldataasfollows: The basicdataneededforthecalculationsofelectrondensitiesandtemperatures Nebula (1) A" 13 23 (2) 10 10 8 5 9 9 4 2. 5 7 6 6 0.2 0.3 0.3 0.2 0.2 0.2 0.2 1.0 1.0 0.3: 1.0 1.0 1.0 1.0 1.0 d/A (3) 5u i.oi +2.70xioVr/wJ e 1.012 +2300vryw.» Basic DataonPlanetaryNebulae 2130 2380 2640 1200 2000 1050 1230 1080 2250 1720 1640 1800 1720 (Par- secs) 930 860 (4) f 7 D XlO-1 0.70 0.82 0.98 2.10 0.79 2.04 1.59 0.79 1.29 2.02 1.03 1.20 1.50 1.44 1.52 (5) TABLE 2* +0.08 +0.15 +0.40 (Si+SO +0.34 +0.56 +0.10 -0.55 -0.35 -0.16 -0.44 -1.31 -0.18, -0.68 -0.45 0.00 Ipg (6) [/(Vi+W)/ 2 IW)\ 0.97 0.95 0.88 1.26 1.30 0.25 1.21 1.06 1.16 0.74 1.19 1.18 1.29 1.16 1.19 log (7) —14,300/r _14300/7T log (S/Sß) ^0.113 X10’€ -1.62: (3) Bac ■1.2 •1.57 ■1.06 -1.12 ■1.0 ■1.7 ■0.75 ■1.4 ■1.4 ■1.04 -0.80 0.6: 0.82 0.72 1.07 1.10 (8) £ (2) (1) (2) (2) (1) (1) (1) (1) (1) (4) (2) (4) (3) (2) (1) (1) 0.55 (5) 3.15 0.8 (5) 0.12 (5) 0.32 (5) 2.25 3.0 0.33 0.60(2) 2.1 (3) 1.35 1.6 1.07 (2) 1.5 1.75 [5/3 =10] 7(4363) (9) 15,000 16,800 13,900 11.200 15.700 12,800 13,500 10,800 17.700 19,600 14.200 18.300 14.300 9,500 7,700 (° K) (10) (5) 1954ApJ. . .120 . . .48L crease inT. ment asgivenbyMenzel and Baker.ThisopiniondiffersfromthatofT,L, Page(1942), because ofthenewcross-sectionsadoptedandpartly becausenoattempthadbeenmade that thèHBac/Hßratiois insatisfactoryagreementwiththetheoryofBalmer decre- effect oftheloweringadoptedsurfacebrightness iscompensatedforbythein- electron densitiesareofthesameordermagnitude asthosepublishedin1945.The to correcttheolderobservationsof7(4363)//(Ah + Ah)forspaceabsorption.Thenew values derivedfromtheolderdata. (see Fig.3)showsnosystematicdifferencesbetween thetwosetsofresults.Weconclude the Hßmeasurements.Forcomparisonwegivefor thesesamenebulaetheTandlogN with theaidoflogSßacbymeansequation(2).Sincevaluesaresubjectto rather largeuncertainties,thefinallyadoptedvalues oflogNarebasedprimarilyupon equation (3)aregivenincolumn4,while5givesthevaluesoflogNdeduced umn 3givesthecorrespondinglogZ>.ThevaluesofNfoundfromSwithaid NGC 7009. c NGC 7662. NGC 7027. NGC 6826. NGC 6818. NGC 6803. NGC 6572. NGC 6543. NGC 6210. Column 2againgivestheelectrontemperatureforeachnebulalistedincolumn1.Col- NGC 3242. NGC 2392. NGC 1535. grams securedattheMcDonaldObservatorybyoneofus.Column10givesvalue both fromSßacandthephotoelectricdeterminationsofS.Thechiefuncertainty slitless plates,itisverydifficulttodetermineSßac-Accordingly,wehaveestimatedN eé of theelectrontemperaturesascomputedwithaidSeaton’scross-sections. tances, wenowcomputeNbyequations(2)and(3).TheresultsaregiveninTable3. taken fromslitlessspectrogramsobtainedattheLickObservatoryandslitspectro- in thecalculationofNthenarisesfromerrordistanceplanetary. e of theunderlyingcontinuumcannotbefixedwithverygreataccuracy,eitherfromslitor e 4eß Column 9givestheintensityofX4363[0m],referredtoas10.Thesevaluesare relative intensitiesareassumedtofollowMenzelandBaker’smodelB.Sincetheposition ß e e e © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem A comparisonofthelogNvaluesobtainedfromHß andfromtheBahnercontinuum The newelectrontemperaturesareallhigherthan thosepreviouslyassigned,partly IC 4593. IC 2149. IC 418. e With theaidoftheseelectrontemperatures,N^ac’s,Sß’s,andassumednebulardis- Nebula (1) Calculation ofElectronDensitiesinPlanetaryNebulae 15,000 16,800 13,900 11.200 15.700 12.500 13.500 10,800 17.700 19,600 18.300 14.200 14.300 (° K) 9,500 7,700 (2) T e PLANETARY NEBULAE55 LOG bi -0.50 -0.58 -0.82 -0.72 -1.02 -0.44 -0.40 -0.43 -0.56 -0.55 (3) 0.52 0.46 0.57 0.70 0.62 TABLE 3 3.81 3.69 3.81 3.35 3.46 4.01 3.63 3.80 3.93 3.46 3.72 3.05 4.20 3.45 3.40 Hß (4) LOG Ne 3.75 3.80 3.50 3.46 3.30 3.62 3.75 3.74 4.05 3.33 3.72 4.19 3.45 3.55 2.82 Bac (5) Adopted 3.80 3.69 3.80 3.34 3.47 3.62 4.01 3.80 3.94 3.72 4.20 3.46 3.04 3.45 3.44 (6) Ap. 102,239,1945 log N ( 4.12 3.77 4.31 3.65 3.50 4.02 4.07 4.04 3.63 3.60 3.98 (7) Te (°K) 10,000 11,600 9,600 8,900 8,200 9,400 9,000 6,500 9,500 9j7ÓÓ 7,000 (8) 1954ApJ. . .120 . . .48L less thanthevaluepredictedbytheoryofapurerecombinationspectrum. more easilythantheI{Ni+N2)/I(Hß)ratio,onesuspectsthatdiscrepancymaylie would bemuchhigher.Sincethe7(4363)//(i?ß)ratiocanmeasuredphotographically given inTable4.IfSeaton’scross-sectionsareused,Andrillat’selectrontemperatures who foundthattheintensityofBahnercontinuumascomparedwithHhwas pute TfromourdatawiththeaidofHebb-Menzeltargetareas.Theresultsare 56 WILLIAMLILLERANDLAWRENCEH.ALLER in thelatterratio. ments ofHßanda20AintervaltheBalmercontinuum. for anumberofplanetaries. Recently,OlinC.Wilsonhassecuredslitless spectrograms higher electrontemperaturesthandolow-excitation nebulae.Forexample,IC418,one NGC 6803. NGC 6572. NGC 6543. accurate valueswhenuse ismadeoftheisophoticcontourscorresponding mono- t of thelow-excitationplanetarios,almostcertainlyhas ahigherelectrontemperaturethan electron temperature,althoughhigh-excitationnebulae tend,ontheaverage,tohave chromatic imagesofthese nebulae.ManyyearsagoBermanderivedsuchcontours (1930) does NGC7009.Thisresultisinaccordancewith theoreticalexpectations(Aller1953). © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem In ordertocompareAndrillat’selectrontemperatureswithourresults,wemustcom- Fig. 3.—Acomparisonofthelogarithmselectrondensitiesderivedfromintensitymeasure- We findnostrongcorrelationbetweenthedegree ofexcitationthenebulaandits The meanvaluesofSorSni+Niemployedinthis papermaybereplacedbymore ß Nebula Data) (°K) T (Present e 10,000 9,800 7,850 Electron TemperaturesDerivedfrom Hebe andMenzelCross-Sections T (Andrillat) e 16,500 15,000 9,500 TABLE 4 NGC 7009 NGC 7662 NGC 7027 Nebula Data) (°K) T (Present e 10,600 11,200 12,300 Te (Andrillat) 25.500 21.500 15,000 PLANETARY NEBULAE 57 with the 100-inch coudé spectrograph, and R. Minkowski has obtained direct photo- graphs with the 200-inch reflector that permit much more satisfactory isophotic contours to be derived. Many of these plates now are being analyzed with the isophotometer at the University of Michigan. Thus the intensity of a single line can ultimately be given in terms of an isophotic contour diagram, each closed curve of which corresponds to a known intensity expressed in ergs/cm2/sec/unit solid angle. Finally, the surface brightnesses we have tabulated will also have to be corrected for space absorption. For some nebulae, such corrections may turn out to be large. In NGC 7027, for example, inter- stellar absorption appears to cut down the surface brightness by a factor of about 10! Estimates of the amount of absorption may be made by comparing the Balmer decrement with the Menzel and Baker (1938) “case B” decrement as has been done by Aller (1951). We defer this calculation to a later paper.

We are indebted to Donald MacRae and Jurgens Stock and to M. J. Seaton for communicating their results to us in advance of publication.

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