1981MNRAS.195..939K * Presentaddress:HopkinsObservatory, WilliamsCollege,Williamstown,Massachusetts 01267, USA. individual stars(vandenBergh1976).Gordon(1969) hassummarizedtheearlyhistoryof units (mfu).Forcomparison,ifSgrAwerethere, it wouldbeapointsourcewithfluxof and Sell—III(vandenBergh1960).Ithaswelldevelopedspiralarmsalthoughthenucleus may comefromsurroundinggas.Althoughthenucleus ofourGalaxyhasbeenlikenedto & Aller(1942)foundanabsorptionspectrumresembling A7;the[On]X3727emission NGC 598isveryinconspicuousand,asgalacticnucleigo,quiet.Israel&vanderKruit Introduction an accumulationofstarclusters,thatagalaxy suchasM33maybedominatedby (1974) foundnoradio-frequencysourceattheoptical centretotheirEmitof1.2milliflux work onM33. The TriangulumgalaxyisthenearestScspiral.Ithasbeenclassifiedas(Hubble1936) 80mfu. Walker(1964)foundadistinctnucleuswith adiameterof1to1.4arcsec.Mayall Early listsofemissionnebulosities werepreparedfromexaminationofdirect photographs H iiregions (see, e.g.Mayall&Aller 1939,1942) butwiththeadventoffastred-sensitive emulsionsand © Royal Astronomical Society • Provided by theNASA Astrophysics Data System not onlyofnitrogen,butalsoneon,sulphurandargon,withrespectto Wampler ImageTubeScannerattheShaneTelescope,lickObservatory are found:N=-1.25,Ne0.77,S-1.5,Q3.6andAr2.4. increasing radialdistancewithcloselysimilarrates.Consequently,theratios are usedasinterpolationdevicestoestablishionizationcorrectionfactorsfor the nucleusrangefrom1to6kpc.Thesedataareanalysedestablish cover thespectralrangeXX3700—7600.Thedistancesoftheseregionsfrom Summary. Measurementsof12HuregionssecuredwiththeRobinson— previously employedforstudiesoftheMagellanicClouds,theoreticalmodels plasma diagnosticsandchemicalcompositions.Inamannersimilartothat oxygen, remainessentiallyconstant.ThefollowinglogTV(element)/7V(oxygen) S, ClandAr.Exceptforhelium,the7V(element)/7V(H)ratiosfalloffwith Mon, Not.R.astr.Soc.(1981)195,939-957 Received 1980October21;inoriginalformJuly30 Astronomy Department,UniversityofCalifornia,LosAngeles,California90024,USA Triangulum spiral,M33 Chemical compositionsofHnregionsinthe Karen B,Kwitter*andLawrenceH.Aller 1981MNRAS.195..939K 940 relatively narrowband-passfiltersitwaspossibletofindandmeasuremanymoreobjects with afocalreducerandnarrowbandpassfilter(~25ÂatHa).InadditiontodiscreteHu the developmentoffastopticalsystemsandinterferencefiltersitwaspossibletodetectvery regions, theynoteanextensivebackgroundemissionthatconnectsHnregionsinthearms. faint Huregions,interarmgas,etc.(see,e.g.Courtes&Cruvellier1965).Sofar,themost (Haro 1950;Aller1950,1954,1956;Shajn1954;Sandage1962;Hodge1969).Finally,with most intenseHuregionssuchasNGC588,592,595,604andIC131arefoundoutsidethe M33 appearnotinthedensestpartsofHiclouds,butonedgesthereof.Infact, complete catalogueisthatgivenbyBoulesteixetal(1974),basedonobservationsobtained spiral arms.Presumably,starformationoccursnotwherethedensityisgreatest,butrather where thedensitygradientisgreatest. Although Hiandnspiralstructurearestronglycorrelated,mostoftheregionsin At thedistanceofM33,Orionwouldhaveaflux0.25mfu.Hence,accordingtoMezger’s down to1.2mfuat1415MHz.Theyobtainedaresolutionof23x45arcsec(78158pc). bounded, asdidComte&Monnet(1974)whoargued thattheinterarmregionisionizedby (1970) definition,allHnregionssofarobservedinMessier33aregiantregions. exposure time,Boulesteixetal(1974)concludedthatmostHuregionsaredensity radiation fromhotstarsinspiralarms.Fromtheir radiodata,Israel&vanderKruit(1974) limited. maximum UVphotonproductionrateexists,irrespective ofotherfactorssuchasdiameter above theexcitationparameterw=115whichcorresponds tothedetectionlimitof system; butw=210definesanupperlimittothe observedexcitationparameter.Thus,a drew adifferentconclusion.Ifyouplotelectron densityagainstdiameter,allnebulaefall or (JSfç)andtheyconcludethatHuregionsin M33areionizationboundedorphoton Spectroscopic andabundance studies which wasthen interpretedasanexcitation ratherthananabundance effect.Searle(1971) An earlyspectroscopicstudy of19emissionregionsinM33(Aller1942)was restrictedto the XX5007—3727region. Calibratedeyeestimatesofintensitiesgave of[Oin]/Hj3, [O ii]/H0andrevealedadependence ofthe[Oiii]/[0n]ratioondistancefrom thenucleus, © Royal Astronomical Society • Provided by theNASA Astrophysics Data System With theWesterborksynthesistelescope,Israel&vandenKruit(1974)measuredsources Since theapparentdiameterofindividualHuregionstheyobservedincreasedwith K. B.KwitterandL.KAller 10 Some basicdataforM33 Position angleofmajoraxis Distance Mass-to-light ratio MassüO^©) Inclination Systemic radialvelocity Neutral hydrogen [1] MayallÄAller(1942). [2] Warner,Wright&Baldwin(1973). [3] Sandage(1962),vanderKruit(1973),deVaucouleurs(1978). [4] Lohmann(1974). [5] Wright(1972). 1 o 3 -3 4.9 ±0.3[2] Pattern similartoopticalwith 57° [1];54°±3°[2] 720 kpc[3] -172 kms"[1];-180[2] 20° [1];22°±I[2] density decreasingfrom1cm" to 0.4cmat5kpc[5] 1.29 [4];0.92[2] 1981MNRAS.195..939K + made photoelectricmeasurementsoftheabsolutebrightnessinHa,Hß,[On],[Om]and to acompositiongradient.Smith(1975)investigatedseveralemissionregionsandde- variation, andconstantNe/OS/Oratios. as inMl01thereisastrongradialgradienttheO/Hratio,apparentlyweakN/Oratio termined theelectrontemperatureinouterregionsofspiral.HeconcludedthatM33 He favouredthehypothesisthatchangeinappearanceofspectrumwaslargelydue although [Om]photographsshowedmanyHuregions.Thus,the[Oi]linesrecordedon from thecentreofM33. spectra ofHnregionsmayoriginateinstratawellremovedfromregions. Ha ratioincreasessignificantlytowardthecentresofgalaxies(Peimbert1971;Warner1973; [N n]foreightbrightHnregionsandmeasuredtheHß/[0il]ratioin24additionalobjects. models (truncatedStrömgrenspheres).Compositeorsegmented(leaky for ionization-limitedmodels,i.e.fullStrömgrenspheres,butnotmaterial-limited the behaviourof[Sn]linesinM33andsimilarHnregionsothergalaxies. spheres) canyield[Su]linesofmoderateintensityandprovideradiationtoionizethe surrounding medium.Itseemspossiblethatsuchinhomogeneousstructurescanreconcile Rubin &Ford1971).InM33itisnotedthatthefainterHuregion,larger 1974) havestudiedintensityvariationsofHa,[Nn]and[Su]asafunctiondistance the seeminglyconflictingdataofBoulesteixetal.(1974)ononehandandIsrael&van identified inM33byDanzigeretal.(1979).Dopita,D’Odorico&Benvenuti(1980)have radiation isproducedbehindtheshockfront.Threesupernovaremnantshavebeen must comefromtheN,0Hzone,i.e.atleastlow-excitationnzones. originate therebecausetheelectrondensitymustbeverylow.Mostof[Su]radiation der Kruit(1974)ontheother. ordinary Hnregionsaretobeattributedshockphenomena. program tofindabsoluteabundancegradients.Itseemsunlikelythat[Sn]enhancementsin compared spectroscopicallyanumberofsupernovaremnantsandHnregionsinM33 stars although,perunitmass,therearepossibly fewer suchobjectsthaninourgalactic system (Israel&vanderKruit1974).Huregions are fundamentaltoassessingthechemical composition oftheinterstellarmediumandestablishing cluestotheearlyhistoryof [Sn]/Ha ratio. active domainsofrecentstarformation.Theyare excitatedbyseveralearlyO-typestars. chemical processing.TheHuregionsinvolvedhere areallgiantstructuresindicativeof and distributionofH°willallassistinworking out thehistoryofthisgalacticchemical evolution. Chemical compositionoftheinterstellarmedium, data onstellarcontent,massdistribution, Observational programme nucleus out to nearly30arcminor about 6kpcaway;(b)observe thediagnosticratios thoroughly aspractical,choosing objectsdistributedindistancefromabout 1kpcfromthe Our objectiveswereto(a) coverthespectraofHuregionsin3700—8000Âinterval as + © Royal Astronomical Society • Provided by theNASA Astrophysics Data System Other observers(Benvenuti,D’Odorico&Peimbert1973;Comte1975;&Monnet From monochromaticphotographsin[Oi]radiation,ComtefoundnoHuregions, Peimbert, Rodriguez&Torres-Peimbert(1974)havediscussedthedifficultiesposedby The greatstrengthof[Sn]presentssomedifficulties.Itisoftenobservedthatthen]/ Our owntheoreticalmodelscanpredict[So]intensitiesagreeingwiththoseobserved Although ScanexistinH°regions,itseemsunlikelythatmuch[Su]radiation Strong [Sn]radiationisapopulardiscriminantforsupernovaremnants,wherethe Thus M33isaspiralsystemwithrichgaseous component andnumerousearly-type Chemical compositionsofHuregionsintheTriangulumspiral,M33 941 1981MNRAS.195..939K and gratingscouldbechangedeasily.Oneobservestheobjectalternatelyinleftright scales oftheresultantscansbystandardtechniques.Appropriateobservationsa slot usuaUywithadweUtimeof4minineachposition. and alsoSAr(plusQinMA3NGC604). NGC604 (thechlorineUnes5517/5537aretooweaktosupplyasatisfactoryratiofor flat-field corrections,correctedforatmosphericextinction,andlinearizedthewavelength diagnostic needs);(c)improvetheabundancesandabundancegradientsofHe,N,0Ne star chosenfromaUstbyStone(1974)enablesustoderivetheresponsefunctionand reduce thescanstofluxesexpressedincgsunits. Scanner. Inthenewermoreversatileversionofinstrument,slotsizes,wavelengthsettings this continuumcanbedifficultandleadtosystematicerrorsthatareespeciaUysevere background continuumintroduceserrorsthroughrandomphotonstatistics.Placementof from theplots.Weoftenpreferrredlatterprocedurebecauseofnoiseinscans.In 942 the theoretical‘caseB’decrement(Clarke1965;Brocklehurst1971).Weassumed factor of2. to 20—30percent.InIC131,theintensityofX4363mayhavebeenoverestimatedbya for weaklines.Exceptwhereotherwisenoted,webehevetheerrorin5007/4363toamount straightforward procedure—sincetheinstrumentalprofileisknown. cent. Wecombinelineintensitiesfromdifferentgratingsettingsbyusingstronglinesinthe error mayexceed50percent.Areasofstronglineswerefoundtoberepeatable7 each methodthecontinuumpositionisjudgedbyeye.ForweakUnesinanoisyscan arise frominternaldust(cfMathis1970). Whitford (1958)reddeningfunctionandobtainedthelogarithmiccorrectionC(Hj3). overlap region. and 6twoormorescansareshownwithdifferentmagnifications.SomeHuregionsshow found Cvaluesgreaterby0.4to0.8thanthosereportedSearle.Thediscordancemay [Om]4959+ 5007/4363,[Su]6717/6731,[On]3727/7325and[Nn]6584/5755for We employeda2xarcsecslotsize.summedthescans,subtractedsky,appHed AU oftheobservationsweresecuredwithRobinson—Wampler(1972)ImageDissector Intensities ofindividuallinesmaybemeasuredbyamachineprogramordirectlyeye Interstellar extinctioncanbeassessedbycomparingobservedBalmerlineintensitieswith Overlapping profilessuchasXX6548,6584[Nn]andHacanbedeconvolvedbya © Royal Astronomical Society • Provided by theNASA Astrophysics Data System In Figs1—7wereproducesomerepresentativescansforMA3andNGC604.2,3 Sources oferrormaybesummarizedbrieflyasfoUows:thestrong,essentiaUysteUar By comparingSearle’sHj3fluxeswiththeirown21-cmdata,Israel&vanderKruit(1974) K. B.KwitterandL.H.Aller Figure 1.Scan of spectrumMA3(\\3700-5100 Â). 1981MNRAS.195..939K with higherdispersion. Figure 2.Higherdispersionscanofregion\\4300-4600ÂinMA3.Here\4363,[OUI],HeI4388and He I4471areeasilymeasurable.Twomagnifications,IXand8X,employed.Thisscanwassecured © Royal Astronomical Society • Provided by theNASA Astrophysics Data System Figure 3.Theregion\\5400-6900AinMA3.Magnifications of1X,4Xand10aredisplayed. 1981MNRAS.195..939K 944 and HeI,\\6678, 7065,[Arm]\\7135,7751. Figure 7.ThespectrumofNGC604, W6600—7900A.Noticetheprominenceof[Oll] W7319,7330 © Royal Astronomical Society • Provided by theNASA Astrophysics Data System Figure 6.ThespectrumofNGC604,AA4820-6750Â.Magnifications1Xand4Xareemployed. K. B.KwitterandL.KAller Figure 5.ThespectrumofNGC604intheregionXk3600-5100A. 7000 7500 1981MNRAS.195..939K Wolf—Rayet ‘bands’nearX4640,whichisnotsurprisinginviewofthediscoveryseveral gives thedesignationincataloguebyBoulesteixetal(1974).Thethirdandfourth Wolf—Rayet starsinM33byWray&Corso(1972). nucleus, Fig.8showsthebehaviourof[Nii]/[Sii]. Thedistinctdeclineistobeattributed columns give(a,5)(1950)andthelasttwo thedistancesofnebulositiesfrom the nucleus.AsasampleofvariationsUne ratioswithrespecttodistancefromthe object; MAdenotesthenumberinlistbyMayall&Aller(1942).Thesecondcolumn to asubstantialweakeningof[Nn]withincreasing distancefromthecentreofgalaxy. affected bytheOHX6577airglowline. The effectismuchmoremarkedthaninFig.2of Comte(1975)where7(6584)isstrongly corrected forintersteUarextinctionbytheWhitford (1958)curve.ThelastUneofthetable Umited data are tobehadforHuregions inM33.[7(4959)+/(5007)]//(4363) ratioshave gives C(Hj3). Analysis ofthedata In comparisonwithrich diagnostic dataoftenavailableforplanetarynebulae, onlyrather Table 1liststheHuregionsobserved.Thefirstcolumngivesdesignationof © Royal Astronomical Society • Provided by theNASA Astrophysics Data System Table 2givesthefinallyadoptedlogarithmsof UneintensitiesreferredtoH0and hm Chemical compositionsofHuregionsintheTriangulumspiral,MSS major axispositionof20°.Thedistancesarecalculatedforanassumeddistance ^In theplaneofM33,assuminganinclination57°tolinesightanda centre a=0l'31OR2;6+30°24'27"(1950),Warneretal(1973). of 720kpc.D(o)=50'(deVaucouleurs&de1964).Coordinates Table 1.DataforobservedHIIregions. MAI MA9a NGC588 280 NGC604 680 MA3 IC133 MA2 IC132 MA11 IC131 NGC595 Region IC142 423 638 290A 650 301 49 88 85 1:30 12 1:29:55 1:30:25 1:30:26 1:30 :21 1:31 :42 1:31:43 1:31:12 1:31:25 1:30:45 1:31:12 1:31:12 : 30:56 :36 30:23 :48 30:38:00 30:41:36 30:45 :12 30:30:12 30:32 :05 30:26:30 30:36:43 30:23:45 30:22:15 30:30:07 (1950) r(arc min)* 29.8 27.1 24.0 23.4 18.3 22.6 10.6 14.6 13.5 4.7 6.1 7.5 r(kpc) * 6.24 5.68 5.03 4.90 4.73 0.98 3.83 1.28 3.06 2.83 2.22 1.57 945 1981MNRAS.195..939K -34 Figure 8.Dependenceofthe[NII]/[Sratioondistancefromcentrenebula.AA6548,6584 investigation. appear toweakenmoresteeplywithdistancethandothe[SII]lines. use anindirectargument.Shields&Searle(1978) proposedatheoreticallysoundprocedure have usedcross-sectiondatabyKrueger&Czyzak(1970).TheA-valuesarebasicallythose in aplasmaexposedto a fixedfieldofradiation,theelectrontemperature islowered much lowerthaninHnregionswherethediagnostic Unesarenormallyobserved. ment ofTviathe[TVj+Af]/4363ratioorestimate from3727/7325isathand,wehaveto M101, wherethemetal/Hratioexceedssolar value, theywereabletoshowthatTwas Unfortunately, itsapplicationrequiresdatathatwe donothave.Intheinteriorregionsof compiled byGarstang(1968),updatedwhennecessarymorerecentdata(Nussbaumer and Tagainst thisparameter.They findanincreaseof[On] +[Oin]/Hj3from2.5 to for calculatingthestellarfluxandnebularionization structureand,thus,estimatingT. expressions fortheappropriateforbiddenlines.Weuse[Nn],[Om]and[Nem]datafrom from considerationsoftheoreticalmodels. cm. Columns(2)and(3)ofTable3listtheadoptedvalueslogxtwhere=10“ sensitive 6730/6717[Su]ratioisavailableforallourobjects.FormostobjectsN<200 and henceisinferiorto5007/4363asatemperaturediscriminant.Theelectrondensity 946 fundamental parameter,combined dataforseveralgalaxies,andgiveplotsof log«(0)/h(H) objects whereX4363is not observed.Pageletal(1979)use([On]+ [Oin])/H0 asa and byAUoinetal(1979) toderiveelectrontemperaturesandelemental abundancesin (Menzel &AUer1941). This basicrelationshiphasbeenemployedbyPagel etal.(1979) Seaton (1975),and[Su]datafromPradhan(1978).For[Sm],[Clm][Arm]we nebulosities, NGC595,MA3andIC133,wehavecorroborativeevidencefromthe3727/ T/\/N andt=[r1000].Valuesofthetemperatureindicatedinparenthesesareadopted been measuredonlyforNGC604,IC131,NGC588andIC132.Forseveraladditional 1971; Osterbrock1974).Accurate,modernexpressionsforderivingTfrom[Om]and 7325 ratio.ThisratiodependsonbothNanduncertaintiesintheinterstellarextinction, e2 e [N n]aregivenbySeaton(1975).Weappendnotesforindividualobjectsdiscussedinthis e e e e e e It haslongbeenknownthatwhentheconcentration ofatomssuchasoxygenisincreased In ourderivationofionicconcentrationsfornebulosities forwhichnodirectmeasure- The remainingcolumnsofTable3giveionicconcentrationscalculatedbyconventional © Royal Astronomical Society • Provided by theNASA Astrophysics Data System K. B.KwitterandL.H.Aller t02 I J* +0.1 Ü - +0.3 +0.4 -0.1 0 500 10001500 r" CT) CT) 00 CT) LO «

Table 2. Line intensities [log ///(H0) ]. © Royal Astronomical Society Chemical compositionsofHnregionsintheTriangulumspiralM33 v cMr^cocM—II—lOCNOOCMCMrHr—ICMCMrHOOrH Or—IT—I •<ícMincoooor^sosûsos£)sosos£)sor^.r^i— r^o r^r^-n'OOooooooooOiHco coooososoc^f^oocococoininmsof 'r^o>Hcoi^>—' r^On-mOsasr^sOn-rHOCOrHrHrHOSr^COmsOOCMCOOOCO-ví-OOr^rHininrHrHCÚ o osoom m oooos m coooos I II O vO>H rH o CN o o oor^-i i o L—I z H -Hoa Pd PCHpc I I I ro o^oom kj- so -H rHOCM o pc M ai rH O I 53 Ocji5) CU MHOÍ2:aio Provided bythe NASA Astrophysics Data System o 0 o O CN rH 00 o 1 I I m o vooo o oo OsO'CfOSCMinOOOsOO'd- iH00CMCOr^-oovo m <1-ocmro «-HOOrHrHi—IrHrHi—Ir—IO r—I O>—íT—Ii—irH o er» Galaxy (Churchwelletal1978;Wilson1979;Mezger etal1979). consequent riseinTmorethancompensatesforthe fewerionspresent.Overthesamerange T risesfromabout6000to12000K.A similar effectseemstoexistinourown 1979; Sarazin1976;Aller etal1979).Themodelsemployedherearebased onaformula- 10.0 tocorrespondadecreaseintheoxygenabundance«(O)/«(H)byanorderof e 948 e © Royal Astronomical Society • Provided by theNASA Astrophysics Data System Extrapolation fromionicconcentrationstoabundances maybemadebyformulaesuch Another approachistouse theoreticalmodels(Balick&Sneden1976;Stasinska 1978, For theinnerHuregionsinM33,weadoptedT -8000Kasareasonableupperlimit, ç MAI (-2) MA9a -1.65 IC132 -1.6 NGC588 -1.5 MA3 -1.7 MA2 -2.5 IC133 -2.5 NGC595 -2.0 IC142 -2.0 MA11 -2.5 NGC604 -2.3 IC131 -2.0 Table 3.Ionieconcentrationslog7V(i)/yV(H). K. B.KwitterandL.H.Aller + Log XtHe (1.0) 11.01 (0.8) 10.76 (0.8) 10.95 (0.8) 10.97 1.0 1.23 11.00 1.1 10.88 1.28 10.96 1.15 10.89 1.08 10.89 0.9 10.81 0.93 10.88 5.95 7.10 5.91 7.24 6.23 7.56 6.34 7.63 6.87 8.22 6.98 8.17 6.81 8.11 6.59 7.95 6.70 7.97 7.28 8.47 7.18 8.53 7.27 7.99 7.33 8.10 7.34 7.94 7.24 7.75 7.21 7.65 6.60 7.87 6.72 8.26 7.41 7.73 6.82 7.81 7.16 8.17 7.73 5.15 5.07 5.25 5.92 5.41 5.82 6.58 5.51 <6.6 5.63 5.74 6.82 6.08 6.59 6.13 5.55 5.42 5.80 5.66 5.71 5.90 5.90 5.85 Ar 1981MNRAS.195..939K and argon.Chargeexchangeeffectsappeartobeimportant.Uncertaintiesintherelevant exchange reactions(Butler,Bender&Dalgarno1979).Weemployedthemodelsasinterpola- tion devicesforestimatingionizationcorrectionfactors(ICF)helium,sulphur,chlorine cross-sections canproducelargeeffectsontheionizationequilibrium. N fromthe[Su]ratioand3727/7325(ifavailable),weuseobserved[A^+N]I3721 nately, atmosphericmodelsfordifferentmetal/Hwithdesirablerfandloggvaluesarenot atmospheres forr=30000,35000,40000,45000and50K(log#4.5).Unfortu- available. ThepresentdiscussionisbasedonthegridsummarizedinTable4.assumed for NGC604,IC132andIC142.Table4listspredictedvaluesof7(5876),7(4959+5007)/ calculated moreUmitedgridsformodelscorrespondingtocompositionsderivedinTable5 chemical compositionofthenebulargasisUstedatbottomtable.Wehavealso sphere expressedinunitsofthesize(N=100,T45000)model.Givenavalue to thelastUneandreflectsinterplaybetweenenergydistributionscoolantswith 3727,7(3727) and7(5007)onthescale7(Hj3)=100.Theaverager,isgiveninnext ratio toselect(orinterpolate)amodelofappropriateexcitationwherebyionization change ofradiationfieldanddensity.ThelastrowgivestherelativesizeStrömgren €2 ef e He e © Royal Astronomical Society • Provided by theNASA Astrophysics Data System We calculatedanextendednetworkbasedonKurucz’s(1979)solarcompositionmodel Chemical compositionsofHuregionsintheTriangulumspiral,M33 1^=100 N+N/372 1^=30 N+N/372 Table 4.Synopsisofsomemodelcalculations. N=300 N-+N/372 12 12 rl2 <> 5007 5876 6584 T 5007 5876 He 5876 3727 3727 6584 N 6584 5007 0 C 3727 e £ £ 0.68 5.4(-5) 0.306 0.68 8439 115 113 30 000 118 8.3(-5) 320 8373 3.5(-5) 311 Assumed CompositionforNetwork ofModels 8546 325 .013 .008 .137 .67 .020 .066 0.098 4.17 (-5) 3.47 (-4) 4.27 (-4) .815 9356 35 00040 8.9 520 3.66 9316 149 149 3.65 149 5.9 520 3.7 9440 517 12.8 .033 ,023 .0152 .361 .175 I (Hß)=100 1,445 0.89 13.4 13.3 13.4 8523 371 376 61 1.32 47 452 1.98 10600 300 78.1 Ar Ne 8743 Cl S 453 301 .645 .308 9503 13.5 5.5 2,24 203 13.4 1.00 836 24 13.5 3.6 45 000 17.7 8.23 281 35 1.15 (-7) 923 9321 750 1.82 (-6) 5.01 (-6) 9776 150 8.71 (-5) .480 10134 50 000 164 21.6 9.1 225 13.4 121 16.1 13.2 1110 13.3 1026 30 6.1 13.3 2.261 9922 1193 10429 1.00 .482 949 1981MNRAS.195..939K 2+ +2 +2 2+ + + 2 problem that [Six]isprobablyproduced inlow-ionizationenclaves whichareinadequately accuracy ofr(S).When only[Su]isobservedwehavetorelyonmodel interpolation We havealsoestimatedICFsforneonfromtheoretical modelsandhavenotedthatthey and theionizationcorrection factorcanbecomelargeanduncertain.We have thefurther chief sourceofuncertainty accruesfromthemeasuredintensityofX6312 Uneandthe sulphur existsmostlyas S and.Whenboth[Su]X6312[Sin]are observed,the often exceedsimpleextrapolations.Theexcitation levelformostHnregionsissuchthat i.e. ionization-limitedstructures.Material-limitedmodelstendtopredict[Su]intensities from theaforementionedrelations, 7V(0)/iV(H)= [jV(0)+7V(0]V(H.Thenitrogenandneonabundancesarecalculated that aremuchtoolow. correction factorscanbeestimated.NotethatwehavealsousedfullStrömgrenspheres, 7V(Ne)_7V(Ne) N(0) 7V(0) 7V(N)_7V(N) 950 e and N(0)-N(0+) © Royal Astronomical Society • Provided by theNASA Astrophysics Data System Table 5givesthefinallycalculatedabundances.Themodelssupportourconclusionthat MA2 NGC595 MA11 Table 5.DerivedabundancesforMessier33. NGC588 MA 3 Object tHe MA9a IC142 IC132 NGC604 IC133 IC131 (*) K. B.KwitterandL.H.Aller For NGC604,weassumedt (1.0) 11.16 (1.0) 10.96 0.8 10.98 0.8 11.01 0.9 10.95 0.8 11.00 0.93 10.95 1.23 11.01 1.0 10.89 1.15 10.95 1.28 10.97 1.1 10.89 (*) 11.00 = 1.08inbotterzones. 7.11 8.327.75 7.39 8.577.42 7.25 8.60 6.97 8.307.49 7.11 8.38 6.80 8.167.12 6.65 7.947.07: 6.72 8.067.39 6.84 8.007.30 6.88 8.217.45 6.97 8.287.33 7.10 8.457.63 6.96 8.367.65 N 0Ne 0.89 incoder. 6.78 6.78 7.01 6.05 6.75 5.025.94 7.11 6.08 6.70 4.665.75 6.67 5.83 6.52 5.70 6.8: 5.88 6.70 5.70 6.9: 5.97 6.68 5.58 7.10 S Ci,Ar 1981MNRAS.195..939K handled byourmodels.TheimportanceofobservingX9069[Sin]cannotbeover- in NGC604whereweappeartohaveobservedanareaofmuchlowerexcitationthanthat excitation differencesbetweenvariousareasinanebulosity.Astrikingillustrationisfound emphasized (Hawley&Grandi1977).Aserioussourceofdifficultyisthattherecanbe measured byHawley&Grandi(1977)(compareourTable2andFigs67withtheir models suggestIGFfactorsrangingfrom1.1to1.4formostobjects.Wethatwhen Table 1). states ofastar’sevolution,wechooseArratherthanS.Inobjectsmoderatetohigh processing inthecarboncycle,andheavyelementswhichareproducedonlyadvanced comparisons aremadebetweengradientsofelementssuchasNor0,whichaffectedby excitation therelevantUnesallfallinobservablespectralregions.Chlorineisobservedonly as [Clin].Fromthemodelsweinferanionizationcorrectionfactorofabour1.4forboth than thoseindicatedbythecalculationsPeimbertetal(1974).Indeed,Stasinska’s NGC 604andMA3. abundance determinations. (1979) investigationsillustratethatinhomogeneitiesinflictlargeuncertaintiesonhelium Discussion the nucleus.Notesystematicdeclineinoxygenabundancewithdistancefrom tions areshowninFigs9and10.InFig.9,weplotlogA(0)/A(H)+12versusdistancefrom The principalresultsoftheinvestigationaresummarizedinTable5;somefurthercorrela- centre. CompareSearle(1971),Smith(1975)andPageletal.(1979).Thecorresponding Figure 9.TheA(0)/A(H)gradient inM33.Notethesteadydeclineabundance with distancefrom nucleus inagreementwithresults foundbyvariousinvestigators(seetext).Thelowpoint correspondsto et al{1919). removed. A:Tisassumed to be8000KforMA11,1042,MA2and10000K MA9a. □:Tfrom IC 131withanassumedtemperature of11500K.Ifweassume7^=10000K,thediscordance islargely 3727/7325 [OII]ratioandlimits onx.©:Tfrom[7(4959)+(5007)]//(4363)ratio.Compare withPagel e e e 2+ In thenebulaeinwhich[Arm]isobserved,mostofatomsappearasArions; © Royal Astronomical Society • Provided by theNASA Astrophysics Data System Helium presentsadifficultproblem.Ourmodelssuggestmuchsmallercorrectionfactors Chemical compositionsofHnregionsintheTriangulumspiral,M33 0 300600900120015001800 DISTANCE r" 951 1981MNRAS.195..939K r yVXN)/7\f(H) showsthesamefeaturesas7V(0)/A(H)plot—asteadydecreasein 952 regions areevenlower.AplotoflogAf(0)/7V(H)versus([On]+[Oin])/H/3(Fig.10)shows electron temperaturesarefixedbythe(4959+5007)/4363ratio.Insomenebulosities, gradient, A,in[logO/H]is0.13perkpc.Thepointsofhighestweightarethosewhere removed. mean O/Sratioappearstobeabout36. uncertainties intheIGFandlackofobservations ofthenebulartransitionin[Sm].The with rwhilelogN(0)¡N(Ar)indicatesnodependence ondistanceandascatterwithin scatter within±0.14aboutameanvalue+1.25.AplotoflogN(Ai)¡N(H)showsdecrease nitrogen abundancewithdistancefromthenucleus.AplotoflogO/Nagainstrshowsa a tendencyfordeclinewithdistance.Again,IC131appearstobediscordant.Aplotof from workofShields&Searle(1978)onM101,byPrometal(1979)andPagel 3727/7325 lineratioishelpfulalthoughitcannotgiveasaccurateavaluecanthe denotes thesumofintensities3121,4959and5007.NotesteadydeclinelogA(0)/A^(H)as Figure 10.TherelationshipbetweenlogN(0)/N(H)+12and([OII]III])/Hß.Here(OHI] with notrenddependingondistance.Weconclude thatscatterariseslargelyfrom (1979). Thegradientcanbesteepenedifoneassumesthatthetemperaturesininterior A (elementyTV(O)ratioseemstoremainfairlyconstant. InTable6,wecomparethelogarithm the Hiiregiondata(Pageletal1979). For fournebulositiesadhoctemperatureswereassigned,primarilyonbasisofevidence distance asfoundbyDopitaetal.(1980)fromsupernova remnantsareingoodaccordwith ±0.16 aboutavalueof2.4.Plotslog7V(0)/A(S) orlogN(N)/N(S)showalargescatter T= 11500K;ifweassumethat7(4363)istoostrong,andadopt10000K,thediscordance [O II]+III]increases(c/.Fig.7ofPageletal.1979).ThediscordantpointappliestoIC131ifwetake [Om] data,partlybecausethisratiodependsonelectrondensityaswelltemperature. of theA(element)/A(0) ratio observedinM33withthosefoundtheOrion nebula,inthe made thatthe A(C1)/A(O)andA(Ar)/A (O)ratiosdonotvary by morethanafactorof 2 or respect tohydrogen.The solarvaluesforArandClareuncertainbutanargument canbe Sun, inplanetariesand the LargeandSmallMagellanicClouds. e © Royal Astronomical Society • Provided by theNASA Astrophysics Data System Pagel &Edmunds(1981)havenotedthatthedependence oftheN/HandO/Hratioson Within afactorofabout 2, theOrionNebulaandM33havesimilarabundance ratioswith Although theN(elementj/TV(H)ratiodeclineswith rforallelementsexceptHe,the K. B.KwitterandL.H.Aller ^ 8.2 CM O a» 8.6 8.4 8.0 2.0 5.08.0 [OH] +[oni] H/j 1981MNRAS.195..939K excitation effects(suchastemperatureoftheilluminatingstar)anddustmayplaysomerole but alsowithrespecttooxygenifoneusestheSunorOrionnebulaasastandard. abundance gradient(see,e.g.Collin-Souffrin&Joly 1976). as notedbySearle(1971),thereappearstobe no doubtastotherealityofanactual there isastrongO/Hgradient,althoughprobablynoN/Ogradient.Althoughvariationsin chemical compositiononpositionintheseobjects.YetspiralgalaxiessuchasM33,Ml01 elements proceedatnotgreatlydissimilarratesinM33,ourGalaxyandthe.Magellanic and theoriginofnitrogen.Itappearstobemostly primaryratherthansecondaryorigin. (Smith 1975;Hawley1978;Shields&Searle1978),orinNGC300(Pageletal1979) that showanenrichmentofnitrogen;themeanvaluereflectsthiseffect.InMagellanic 3 fromonetypeofobjecttoanother.Theplanetarynebulaecompilationincludesobjects important role.Plausiblescenarioshavebeenproposed byanumberofinvestigators,e.g. nitrogen iscontributedprimarilybylow-massstars andthatinthepastdifferingratesof function forthestars,gasdensity,andfraction ofthetotalmasswhichisinform Clouds. Noneoftheaforementionedobservingteamshavefoundanydependence of gas.Varioustheoreticalscenarioscanbedevised. Edmunds&Pagel(1978)suggestedthat A largenumberoffreeparametersareavailable— the rateofstarformation,initialmass Clouds. TheabsoluterateapparentlydoesnotchangefrompointtointheMagellanic Clouds, allauthorshaveagreedthatnotonlyisnitrogendepletedwithrespecttohydrogen, resultant nitrogenabundance (forafixedgasfraction).Theyinterpret thenitrogen star formationmayhaveoccurred.Starsofintermediate mass(4—10Af)mayalsoplayan and theinitialmassfunction. Therateofstarformationaffectsthenitrogen abundance,in deficiency inM33assuggesting thattheinitialmassfunctionmayhave been flatter,i.e. the sensethatgreater therateearlyinhistoryofagalaxy,greater willbethe Alloin etal(1979).Formation ofC,NandOdependsonthegasmassfraction ofthegalaxy o © Royal Astronomical Society • Provided by theNASA Astrophysics Data System A numberofhypotheseshavebeenproposedto account forthevaryingratiosofN/O Thus itwouldseemasthoughtherelativerateofmanufactureoxygenandheavier Table 6.ComparisonofelementalabundanceratiosinM33withrespecttooxygen. Magellanic Clouds:valuesquotedarefromamodelanalysis(Alleretal.1979), Planetaries: Aller(1978)exceptfornewsulphurdatabyDinerstein(1980)and Orion Nebula:Peimbert&Torres-Peimbert(1977). Sun: Ross&Aller(1976)updatedfornewdeterminationsofC,NandOabundances Ne N Dufour (1975)and&Harlow(1977). be made,andinharmonywithresultsbyPeimbert&Torres-Peimbert(1974,1976), Ar Cl except forClwhichistakenfromPageletal.(1978).Thederivedcompositionswere by Lambert(1978)andsulphurabundance&Luck(1978). in goodagreementwiththosefoundbyPageletal.(1978)wherecomparisonscould Beck etal.(1981). Sources ofdataareasfollows: S Chemical compositionsofHuregionsintheTriangulumspiralM33 y M3 3OrionSunPlanetaryLMCSMC -1.25 -1.55 -0.77 -2.40 -3.56 -0.99 -3.60 -0.85 -2.05 -1.34 log N{q\)IN{0) -0.82 -0.9 -2.30 -3.34 -1.6 nebulae -0.69 -3.40 -1.53 -0.66 -2.28 -3.5 -1.41 -2.08 -1.53 -0.66 -1.81 -0.62 -1.55 -2.24 -3.5 953 1981MNRAS.195..939K 954 would bedifficulttoestablish.Dopitaetal(1980)concludethatnogalacticenrichment below solarmass. had declined—itisproducedasasecondaryelement. mediate massthatmakeN.Theyieldisthenadjustedbyincreasingtheproportionsofstars as aprimaryproductofnuclearreactions,later—atstagewhentheratiogas/starmass theory issuccessfulinexplainingtheirobservationsofM33.Althoughnitrogenproduced with moreverymassivestarsthatmakeoxygen.Therewererelativelyfewerofinter- massive starsmightgiveusadditionalconstraintsthatwouldhelpinnarrowingtherangeof plausible scenarios. We acknowledgewiththanksservicesrenderedbytheOfficeofAcademicComputing. to theUniversityofCalifornia,LosAngeles. the observationalprogramme.Wearedeeplyindebtedtoourassociates,ProfessorsHolland We aregratefultothestaffoflickObservatorywhocontributedsomuchsuccess services. ProfessorB.E.J.Pagelreadthemanuscriptandmademanyvaluablesuggestions. George JacobyandMrsZdenkaPlavcovasuppliedvaluablecomputerprogramsasdidalso Acknowledgments This programmewassupportedinpartbyNationalScienceFoundationGrantAST77-21-22 C. D.Keyes,whodevisedthetheoreticalmodellingprogramandperformedmanyother Ford andHarlandEppswhohelpedussecuresomeoftheobservationsattelescope. References Brocklehurst, M.,1971.Mow.Afoi.Æ.¿«fr*.Sbc.,153,471. Benvenuti, P.,D’Odorico,S.&Peimbert,M.,1913.Astr.Astrophys., 28,447. AUer, L.H.,1942.95,52. Churchwell, E.,Smith,L.F.,Mathis,J.,Mezger,P.G.&Huchtmeier, W.,1978.Astr.Astrophys.,70,719. Butler, S.E.,Bender,C.T.&Dalgarno,A.,1979.Astrophys. 230,L59. 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Robinson, L.&Wampler,E.J.,1972.Pubis,astr.Soc.Pacif, 84,161. Sandage, A.,1962.IAUSymp.No.15,ProblemsofExtragalactic Research,p.354,ed.McVittie,G., van denBergh,S.,1960.Astrophys. 7.,131,215. Sarazin, C.L.,1916.Astrophys.J.,208,323. van denBergh, S.,1976.Astrophys.J.,203, 764. Shajn, G.A.,1954.Izv.Krymsk.Astrofis.Obs.,2,3. Seaton, M.J.,1975.Mon.Not.R.astr.Soc.,170,476. Searle, L.,1971..dsfrop/iys.168,327. r Shields, G.&Searle,L.,1978.Astrophys.J.,222,321. Smith, H.E.,1975. 199, 591. Stasinka, G.,1979.Astr.Astrophys. Suppl,inpress. Stasinka, G.,1978.Astr.Astrophys. Suppl,32,429. Stone, R.,1974.ydsirop/rys.193, 135. © Royal Astronomical Society • Provided by theNASA Astrophysics Data System C. R.,Reidel,Dordrecht. MacMillan, NewYork. 189,95. 569. Chemical compositionsofHnregionsintheTriangulumspiral,M33 955 1981MNRAS.195..939K 5 6 2+7 6 6 3 3 MAIL Thislowexcitationnebulosityappearstobeilluminatedbystarswhichdonothave van derKruit,P.C.,1973.Astr.Astrophys.,29,231. Walker, M.F.,1964.Astr.69,744. ICI42. Fromrfmeasurements,Israel&vanderKruit(1974)findatotalmassof 956 Appendix: notestoTables2,3and5—dataforindividualnebulosities NGC595. Thisisoneofthe‘supergiant’Huregionswithanionizedgasmassorder lower value.NoreliablemeasurementofX4363couldbemade. presented inthetextwetakeT=8000K.Argumentscanbefavourofaneven a strongultravioletradiationfield.IfKuruczmodelsareused,T<37500K.Forreasons Whitford, A.E.,1958.Astr.J.,63,201. Warner, P.J.,Wright,M.C.H.&Baldwin,J.E.,1973.Mon.Not.R.astr.Soc.,163,16. Warner, J.W.,1973.Astrophys.186,21. Wray, J.E.&Corso,G.J.,1972.Astrophys.172,577. Wilson, T.L.,1979.Astr.Astrophys.,77,L3. Wright, M.C.H.,Warner,P.J.&Baldwin,E.,1972.Mon.Not.R.astr.Soc.,155,337. MA2. Thisisamediumexcitationobject.WeadoptedT=8000K.No[On]X7323data estimate of0.08(Israel&vanderKruit1974).Theyobtainamass1.2xlO3f. extinction C(H|3)=0.70istobecomparedwithrfvaluesofC1.2and2.2anoptical Wright, M.,\91\.Astrophys.Lett.,7,209. MA3. Fromthe6717/6730,(4959+5007/4363,and3727/7325ratiosweadoptedT= in fairlygoodagreementwiththoseofSmith(1975).Comte&Monnet(1974)obtained the giantgalacticsourceW51alsoshowsacore—halostructure.Ourmeasuredintensitiesare core andafaintextendedcomponent1.7x1arcmin.Observedwithsimilarlowresolution of 103/(Israel&vanderKruit1874).Theyfindittobematerialboundedwithadense NGC604. Thisbrightnebulosityappearstobe comparable with30Doradus,although T =8000onthebasisoftheoreticalmodelsandabsenceX7325[On].Thederived which issuggestedbythe3727/7325ratio,butmaybeatriflehigh. e e are available. 125 0003/©.Theobservedionizationrequiresfive05starsor1706stars.Weadopted with theaidofmodelinterpolationwefindlogn(Cl) +12=5.02. 9300 Kandlogx=—1.70.From[Clm]weobtain h(C1)/«(H=1.05x10"andthen e 0 neutral componentisapproximately5xl0Af©and theionizedgasisabout1.6xl0Af Wright (1971)findsittobedynamicallyunstable. Israel&vanderKruit(1974)suggesta € N =70—100cm",while Israel&vanderKruitgotN=47cm".Withour presentdata, requiring theequivalentof3305starsor11306 toproducetheionization.Thisbright dense corewithadiameterofabout43pcand surrounding haloof2.3x1.2arcmin.The o nebulosity hasbeenthesubjectofmanyrecentinvestigations: Mathis(1962),Aller,Walker [No] and[Sn]intensitieslowerthanthosewehavefound.Weadopted!F=9000K e & Czyzak(1968),PeimbertSpinrad(1970), (1970),Smith(1975),Hawley& the integratedlightof nebulosity.Fromthe3726/3729[On]ratio,Aller etalfound of Smith,althoughthere appear tobesomedifferenceswithHawley&Grandi whoobserved Grandi (1977)andDopita etal(1980).Ourmeasuredintensitiesagreefairly wellwiththose @ diagnostics are: 7(5007+4959)//(4363)-> r=10800K[Oui]; 6584/5755r=8900 e e e e e © Royal Astronomical Society • Provided by theNASA Astrophysics Data System K. B.KwitterandL.H.Aller 1981MNRAS.195..939K 3 2+-7 -3 7 2+ MAI. Isaveryfaintnebulosityforwhichourdata areverylimited. ICI32. Thisobjecthasfairlyhighexcitationfor which Israel&vanderKruit(1974)finda NGC588. OurmeasuredlineintensitiesareingoodagreementwiththoseofSmith.The ICI33. WecarriedoutcalculationsforbothT=11000and10000K,whicharevalues MA9a. Wechosei=1.0fortheanalysisofthisnebulositywhichischaracterizedby ment withmeasurementsbySmith(1975),although wefindalowerelectrontemperature. mass of14000047©.Tandlog*appeartobe well determinedfromN+7V/4363and may havebeenestimatedtoostrongbyafactorofabout1.5.Israel&vanderKruit(1974) 6717/6730, respectively,as12300Kand—1.65. The intensitiesappeartobeingoodagree- stars wouldberequiredtoproducetheobservedionization. find atotalmassof19500047fortheregionandsuggestthatthree05or11equivalent06 ICI31. Fromthe(4959+5007)/4363ratiowegetr=11500K,while6717/6730 The abundancesofO,NeandNarelowerthanfound intheinteriorregionsofM33. oxygen andotherabundancesarelow.WefindT=12800Kand7V-350cm"but7(4363) consistent (withintheobservationalerrors)with3727/7325and6717/6730ratiosfor moderate excitation.The[Su]ratiosuggests7^~200.ICFforneonisuncertain. values oftheabundancesareobtained.WefindC(Hß)=0.47,whichisclosetomean ionic concentrations.Wefindw(Cl)/«(H=5.13x10andestimateatotalchlorine (See Figs6and7). two estimatesbyIsrael&vanderKruit. pared withothernebulositiesatthesamedistancefromnucleusand([On]+ ratio indicates,<100cm.Unfortunately,the4363intensityissuspect.WithT= hence N/S=0.89. smaller thanthosefoundbyHawley&Grandi,whogetlogft(N)=7.23and«(S)7.28; abundance of5.7x10".Fornitrogenandsulphur,bothourionictotalabundancesare e a 2xarcsecslotatfixedpointnearanedgeofthebrightpartnebulosity: e x2 differences inthemeasuredintensities.Theyusedanapertureof14arcsec;weemployed 0 e T =10800for0,NeArandCl8900NSincalculating e 11 500K,theabundancesofnitrogen,oxygenandneonappeartobeabnormallylowcom- e [Oiii])/Hjß (comparePageletal1979).IfweselectT=10000Kmuchmoreplausible [Nn] (assuming7V=100);3727/7325^T9000K(N100)[On].Hencewehaveused e e eç = 30and100,respectively.Thenebularexcitationismoderatelyhigh. © Royal Astronomical Society • Provided by theNASA Astrophysics Data System The differencesbetweenHawley&Grandi’sresultsandoursarisealmostentirelyfrom Chemical compositionsofHnregionsintheTriangulumspiral,M33 2X2 arcsecslot(K&A)0.982810.810 14 arcsecaperture(H&G)4.04111.315 X 63126584\6717X6730 957