1992ApJ. . .384. .508R intended toestablishthefirst consistentsetofglobalabun- Paper II),wereportedtheresults ofaprogramobservations (H iiregions,supernovaremnants[SNRs],andplanetary close enoughfortheirpopulationofgaseousemissionobjects from thefactthattheymaybeviewedintheirentiretywith stand asgalacticsystemsthanourownGalaxy.Thisresults The AstrophysicalJournal,384:508-522,1992January10 (1989, hereafterPaperI)and Russell &Dopita(1990,hereafter is beingseverelydisruptedby the gravitationalinteractionwith well mixedbygasmotionsalongtheircentralbars(seePagel et generation ofsensitivedetectors,thecompletepopulations of nebulae) tobestudiedingreatdetail,and,withthepresent nally, andtheirdistancesarewellestablished.Also,they very littledustobscuration,eitherfromtheGalaxyorinter- the Galaxy(seeMathewson& Ford1984),anda“bridge”of quite complex,sinceweobservethematatimewhentheSMC Dynamically, however,itmustberemembered,theClouds are should reflecttheglobalabundancesofwholeCloud. al. 1978),sothatanabundanceanalysisatanyonepoint they havenowell-definedhaloordisk,andarerelatively supergiants ineachCloudareaccessiblefordetailedanalysis. H igasisobservedtoconnect thetwoClouds. Finally, theyappeartobesomewhatsimplersystems,in that © 1992.TheAmericanAstronomicalSociety.Allrightsreserved.PrintedinU.S.A. The MagellanicCloudsareinmanywayseasiertounder- In ourtwopreviouspapersin thisseries,Russell&Bessell modal starformationandgasinfall.Themodelsarediscussedinrelationtotheobservedchemicalabun- mean metallicity0.2dexlowerthanthelocalGalacticISM,andofSMCis0.6lower. work areinvestigatedforanyobvioustrendswithmetallicityordifferencescomparedtheGalaxy.Con- dances oftheCloudsandarecomparedwithourownGalaxy.Thedetailedabundancesderivedfromprevious elemental ratios.Thisismostevidentwhenconsideringthelightestandheaviestofelements.The material thathasfalleninfromfartherout. Clouds thanintheGalaxy.Ther-process,onotherhand,appearstohavebeenmoreeffective. s-process appearstohavebeenlesseffectiveatformingtheheavyneutron-captureelementsinMagellanic However, theinterstellarmediaofbothMagellanicCloudsandGalaxyhavesignificantlynonsolar and theimplicationsforourownGalaxy. sidering allthedata,conclusionsaredrawnonpossiblestarformationhistoriesofMagellanicClouds abundances arecomparedwiththelocalISM,andsuggeststhatpresentISMinvicinityofSunis abundances, relativetotheSun,asatomicnumberdecreases.Thisisseennotbecasewhen alously low.TheoveralldistributionofthelightestelementsinCloudsshowsagradualfallingoff abundance, whichisincontrasttothecarbonabundancesHnregions,appearbeanom- Subject headings:Galaxy:abundances—ISM:MagellanicClouds This paperpresentschemicalandstructuralevolutionarymodelsfortheMagellanicCloudsassumingbi- The followingconclusionswerereachedinthiswork.interstellarmedium(ISM)oftheLMChasa Of thelightelements,carbonappearstobeofnormalabundanceinCloudstarsrelativeiron © American Astronomical Society • Provided by the NASA Astrophysics Data System ABUNDANCES OFTHEHEAVYELEMENTSINMAGELLANICCLOUDS. Mount StromloandSidingSpringObservatories,InstituteofAdvancedStudies,TheAustralianNationalUniversity, 1. INTRODUCTION Dublin InstituteforAdvancedStudies,SchoolofCosmicPhysics,5MerrionSquare,2,Ireland nuclear reactions,nucleosynthesis,abundances Private Bag,WestonCreekP.O.,Canberra,ACT2611,Australia III. INTERPRETATIONOFRESULTS Received 1990June4;accepted1991July19 Stephen C.Russell Michael A.Dopita ABSTRACT AND 508 compare themwiththemodels describedin§3andwith enrichment ineachoftheClouds. In§4wediscusstheelemen- explore thestarformationhistories andassociatedelemental from thatoftheSun.In§3we describethemodelsweuseto tal abundancepatternswe have foundintheCloudsand evolutionary historiesofeachtheCloudsandwithsolar consistent overthewholerangeofatomicmass. were usedtoprovideanoverlapbetweentheotherwisedispa- abundance patternsoftheClouds anddiscusshowtheydiffer neighborhood ofourGalaxy.In§2wedescribetheoverall vious twopapersintermsofthestarformationhistories the abundances wederivedfromallsources,foreachCloud,were regions. Inthiswaywewereabletoensurethatthesets of rate setsofabundancesderivedfromsupergiantsand H n Dopita, &Tuohy1985;EvansDopita1985).TheSNRs the general-purposemodelingcodeMAPPINGS(seeBinette, low-luminosity FsupergiantsinbothMagellanicClouds. analyses ofhigh-dispersionspectrasamplesmedium-to the verylightest(He,C,N,O)uptosomeofheaviest(Nd, dances forbothClouds,coveringtherangeofelementsfrom samples ofbrightHnregionsandSNRsinbothClouds,using Magellanic Clouds,andtherelationshipsbetweenchemical Paper IIdescribedouranalysisoftheemissionspectra Sm) observable.PaperIdealtwiththedetailedabundance In thispaperweattempttointerprettheresultsofpre- 1992ApJ. . .384. .508R be doneinthisrelativelynewfieldofresearch. ings andmakesuggestionsforsomeoftheworkthatneedsto results fromtheGalaxy.Finally,in§5wesummarizeourfind- have chosenasrepresentativeoftheMagellanicClouds,when elements comefromtheworkonGalacticHnregionsby elements heavierthanZ=18andthelighterC,Na, local interstellarmedium(ISM)relativetotheSun,where Cameron (1982b)(exceptwherementioned),onascaleof Column (3)givestheabundancesbynumberforSunfrom the literatureaswellfromourownwork(PapersIandII). due considerationhasbeengiventoallsourcesofdata,from discussion). Columns(5)and(7)givetheabsoluteabundances studied supergiant,ofthesametypeasMagellanicCloud population ofthepresent-dayISM,sinceitisayoung,well- stars studiedinPaperI(seeforamoredetailed Boyarchuk (1982)inpreference,andotherwisefromDesika- Shaver etal.(1983).Canopuswaschosentorepresentstellar chary &Hearnshaw(1982)orthemorerecentpaperbyReyn- Mg, Al,andSiareallderivedfromCanopus(Lyubimkov& olds, Hearnshaw,&Cottrell(1988),whiletheremaining 12 +logiV(M/H).Incolumn(4)wegivetheabundancesof In Table1wesummarizetheoverallabundanceswhich 2. THEOVERALLABUNDANCEPATTERN © American Astronomical Society • Provided by the NASA Astrophysics Data System comparison betweensolarandextrasolar abundancesisnotmeaningfulinthiscase. N .. Al.. Na . Ne . O .. C .. Ti .. Ca . Ar . Cl.. He . Nd. Zr.. Zn . Cu . Ni . Cr . V .. Sc.. S ... Si .. Mg Ce . Y .. Sr .. Fe . Mn Eu . Sm . La . Ba . a d c b TakenfromAnders&Grevesse1989. TakenfromBlackwell, Shallis,&Simmons1982. TakenfromSpiteetal.1989b. Thederivationofthisvaluedepends, atleastpartially,onextrasolarsources(e.g.,Hnregionsandhot stars). Therefore,a Element (1) ABUNDANCES OFHEAVYELEMENTSINMAGELLANICCLOUDS.HI. (2) 40 26 24 23 22 21 20 29 28 25 63 62 60 39 38 30 58 57 56 18 17 16 14 13 12 11 10 Z 2 7 6 8 ab b b d 10.99, 4.68 4.31 6.50 7.60 6.35 7.99 8.84 7.94 8.62 0.55 0.96 6.37 6.60 7.27 7.58 2.26 2.65 2.26 2.93 6.25 7.53 5.68 3.98 5.08 3.07 5.25 5.54 1.47 1.65 1.14 Sun (3) Solar Vicinity minus Sun c -0.03 -0.09 -0.14 -0.37 -0.29 -0.10 -0.10 -0.22 -0.25 -0.18 -0.09 -0.21 -0.11 -0.06 -0.05 -0.15 -0.08 -0.15 -0.01 -0.18 -0.11 -0.14 — 0.13 -0.14 c 0.05 0.20 0.23 0.01 (4) 0.11 0.00 0.00 Adopted Abundances 10.94 LMC 4.76 4.08 4.81 0.93 4.28 2.64 6.29 6.70 7.81: 7.47 7.15: 7.61 8.35 7.14 8.04 2.03 2.26 2.47 6.04 7.23 5.89 5.21 5.47 1.68 1.52 1.07 1.93 (5) TABLE 1 u LMC column (4).Theselatterabundancesshouldbelessaffectedby dances areincludedintheadjacentcolumnsforeachCloud. the Sun,whileerrorsdeducedfromscatterinabun- and SMC,respectively,relativetothelocalISMasgivenin from theFsupergiantsstudiedinPaperI,supplementedby heavy-element abundancesincolumns(5),(7),(9),and(10),for given incolumns(5)and(7),sincewecomparelikeobjects.The systematic errorsintheanalysesthanabsoluteabundances Finally, thelasttwocolumnsgiveabundancesofLMC of theLMCandSMC,respectively,onsamescaleasfor The otherelementsarederivedfromourworkonHnregions Z >18,andtheelementsC,Na,Mg,Al,Si,arederived abundances ofthethreeSMCsupergiantsanalyzedbySpite, 0.08 0.09 0.13 0.05 0.06 0.15 0.18 0.03 ISM, respectively(asgiveninTable1).Similarly,Figures2a previous abundanceestimatesgivenintheliterature. abundances derivedinthereviewbyDufour(1984)fromall and SNRs(PaperII),withdueconsiderationgiventothe 0.58 0.10 0.09 0.14 0.21 0.09 0.16 0.07 0.21 0.16 0.25 (1982) andDufour,Schiffer,&Shields(1984);theopencircles triangle representsthevalueforcarbonderivedfromUV and 2bweplottheresultsforSMC.Ineachcasefilled LMC abundancesrelativetotheSunandlocalGalactic Spite, &François(1989)andBarbuy,Spite(1989a). 0.18 0.19 0.09 0.09 0.24 0.17 0.10 observations ofHnregionsbyDufour,Shields,&Talbot represent elementswithinsufficientinformationforusto (6) In FigureslaandIhweplotagainstatomicnumberthe 10.91 SMC 4.49 4.70 0.28 0.85 2.33 6.59 7.03 6.40: 6.98 7.27 8.03 6.63 7.73 6.84 5.10 3.54 5.69 5.81 5.96 3.73: 5.85 5.03 1.08 1.49 1.33 1.32 1.99 1.65 1.33 (7) 0.13 0.20 0.27 0.15 0.15 0.20 0.08 0.20 0.15 0.18 0.12 0.24 0.20 0.10 0.20 0.13 0.05 0.08 0.39 0.12 0.27 0.29 0.48 0.22 0.23 0.20 0.31 (8) SMC Solar Vicinity LMC minus -0.13 -0.40 -0.36 -0.10 -0.29 -0.35 -0.43 -0.29 -0.06 -0.07 -0.02 -0.23 -0.24 -0.25 -0.31 -0.39 -0.19 -0.19 -0.10 -0.17 -0.21 -0.23 -0.21 0.18: 0.57: (9) 0.11 0.10 0.20 Solar Vicinity SMC minus -0.59 -0.62 -0.67 -0.94 -0.60 -0.09 -0.24 -0.94 -0.51 -0.53 -1.45: -0.40: -0.41 -0.58 -0.37 -0.47 -0.34 -0.49 -0.52 -0.43 -0.61 -0.46 -0.47 -0.60 -0.30: -0.63 -0.09 -0.14 -0.26 (10) 0.26 509 1992ApJ. . .384. .508R 5 (1982) andDufour,Schiffer,&Shields(1984),theopencirclesareelements filled triangleistheCabundancederivedfromDufour,Shields,&Talbot Sun. ThedashedlinerepresentstheapproximatemetallicityofCloud. !" '°- behave alittle likeanaccretiondisk,itis possiblethatthe low comparedwiththoseoftheheavierelements.Helium is leadingly low,itisclearthattherelativeabundancesof the in ourpresentunderstandingofitsformation,ispossibly mis- garding theNabundance,which,consideringuncertainties appears tobeapronouncedunderabundancerelative the define [M/H]*=logiV(M/H)-. case forthepresent-daylocal ISM. SincetheGalacticdiskmay cloud inwhichmorelightelements wereproducedthaninthe the criticalelementsinbothCloudsshowentirelynormalrela- plotted relativetothelocalISM(Figs,lband2b).Allthree of This trend,however,disappearswhentheabundances are not includedinthistrendbecauseofitslargelycosmicorigin. three well-observedelementsC,O,andNeareanomalously that theabundancesarerelativetolocalISMratherthanSun,sowe with insufficientdataforanerrorbartobeestimated,{b)Sameas(a),except the vicinityofSunthan to thatoftheSunitself(seealso similar tothestarformation history ofthepresent-dayISMin star formationhistoriesofthe MagellanicCloudsaremore tive abundances.Thisrepresentssignificantevidencethat the Sun fortheverylightestelements(Figs,laand2a).Disre- imate Feabundance. assign anerrorbar;andthedashedlinerepresentsapprox- 510 Peimbert 1987). 10objectISM Fig. 1.—(a)AdopteddistributionofabundancesintheLMCrelativeto As notedinPaperI,andshownevenmoreclearlyhere,there The implicationofthisresult isthattheSunwasbornina -1.0 -1.5 -2.0 0.0 0.5 1.0 0 5101520253035404550556065 © American Astronomical Society • Provided by the NASA Astrophysics Data System Fig. la Fig. lb z RUSSELL &DOPITA farther out,wherethestarformationhasnotbeensointense, fallen moretowardthecenterofGalaxy,whilecloudsfrom would representnewevidencefortherealityofradialinflow. gaseous materialfromwhichtheSunwasformedhassince clouds, isthemostlikelydrivingforce.Sincestellardisk is diffusion, throughgravitationalscatteringbygiantmolecular detail byDopita(1990).Itwouldappearthatstellarorbital have fallenintothepresentvicinityofSun.Ifcorrect,this (Z >56)withincreasingatomic number.Thiswillbediscussed energy. TheflatGalacticrotationcurveimpliesthattheinflow is energy-conserving,thegaseouscomponentmustbelosing being heatedataconstantrate,andthecloud-starinteraction radial inflowofgasinourGalaxyhavebeendiscussedsome in alatersection. dances isthepronouncedincreaseinveryheavymetals the gaslayerswhichcausesthemtoshedorbitalenergy and tion, wherebyasymmetricdriftoftheoldstarsputsatorque on and thegas.Thisprocessisessentiallyoneofdynamical fric- rate isproportionaltotherelativesurfacedensitiesofstars and theLMC and comparetheresultswith themodelfor spiral towardsthecenter. the SMC. The mechanismsthatcouldberesponsiblefordrivingthe The othermajorfeatureoftheplotsoverallabun- We presentherepossibleevolutionary modelsfortheSMC Fig. 2.—(a)Sameasla,butfortheSMC.(b)lb, 3. STARFORMATIONMODELS 3.1. TheStructuralEvolution Fig. 2a Fig. 2b Vol. 384 1992ApJ. . .384. .508R 9 9 -2 mines thechemicalenrichmentofISM,whilelow-mass (IMF) (thenumberofstarsbornpersquarekiloparsecyear quence ofthisisabimodalformtheinitialmassfunction star formationappliesbelow1Mandservesonlytolockup high-mass starformationappliesabove~1Manddeter- per [log(M/M)]),similartothoseproposedbyGüsten& takes placeintwoquiteseparateenvironments.Theconse- models areconstrainedbytheobservationsofratio into account,butradialflowsarenot.Theendpointsforthe formation rate,andtheoxygen-to-ironabundanceratio.The gas masstothetotalmass,ageofsystem,star chemical yieldswithmetallicityforthemassivestarsistaken formation duringthefollowingtimestep.Thevariationof processed andunprocessedgasfromeachstellarmass.Any form ofwhitedwarfsorneutronstars;andtheenrichment Mezger (1983),Larson(1986),andWyse&Silk(1989).The have chosenspecificallytofitwithourmodels. parameters listedinboldfacetyperepresentsthosethatwe the modelresultsforGalaxyfromDopita(1990).The derived modelparametersaregiveninTable2,aswellthe ejected matterisassumedtobemadeavailableforfurtherstar dilution oftheISMthatoccursthroughejectionboth return totheISM,andremnantstheyleavebehindin (the modelsbeinginsensitivetotheexactmasslimits); within each0.03logmassbin,overaspecifiedrangeofmasses observationally determinedparametersfromtheliteratureand number ofstarsineachmassbinthatdieandthegasthey amount ofinfallengas;thenumberstarsthatareformed Greggio (1986).Thecodecalculates,ateachtimestep,thetotal described byMatteucci&Tornambè(1985)and for thechemicalevolutionarypropertiesofexternalgalaxies. which canaccountforthechemicalevolutionarypropertiesof the localenvironmentshouldbeequallycapableofaccounting there isnoreasontosupposethatthesolarneighborhoodin any waypeculiarinitsproperties,wemighthopethatamodel Galaxy derivedbyDopita(1990)usingthesamecode.Since No. 2,1992 0 0 0 Total massofsystem(10M) Total gasmass(10M) Total masssurfacedensity(Mpc) Distance tosystem(kpc) IMF exponentp Gas depletiontimescale(Gyr) Ratio ofgasmasstototal iV(He/H) Lifetime ofbinaries,i(Gyr) Low-to high-massSFRratio Peak ofstarformation(Gyrfrompresent) Gas infalltimescale(Gyr) Evolution time(Gyr) Radius ofsystem(kpc) [C/H] [N/H] % ofstarsformingbinaries IFe/H] lO/H] 0 0 0 den In thesemodels,weassumeineffectthatstarformation To thisend,amodelingcodewasdevelopedalongthelines Note.—The fittedparametersaregiven inboldtype. © American Astronomical Society • Provided by the NASA Astrophysics Data System Parameter ABUNDANCES OFHEAVYELEMENTSINMAGELLANICCLOUDS.III. (Dopita 1989)LiteratureModel Galaxy 400.0 28.0 70.0 12.9 14.8 12.5 0.29 0.10 0.22 0.30 0.11 4.0 0.07 2.0 3.5 1.0 1.8 1.0 Physical Parameters TABLE 2 -0.65 -0.77 -0.86 2.0-4.0 -1.31 1.7-6.0 70.0 50.0 10.0 0.081 0.65 0.36 2.14 3.1 1.8 the cold-coremolecularclouds,formingstarsatasteadyrate being formedandthatthelow-masscutoffinIMFisof where severalanalysessuggestthatonlyhigh-massstarsare for thismodeofstarformationcomesfromstarburstgalaxies, mechanism forthecloudsandcompressionnecessaryto cloud-cloud collisionsorcoalescencesprovideaheating The associationofthesecloudswithspiralarmssuggeststhat Good 1989). core (~5-15K)cloudsaredistributedsmoothlythroughout molecular cloudsmapouttheGalacticstar-formingregions, (1990). determined bygravitationalfragmentation,andtherepres- star formationinmaturegalaxiesisbelievedtotakeplace shown, forinstance,inthemodelsofDopita1990),bulk very earlieststagesofdevelopmentanormalgalaxy(as trigger massivestarformationwithinthem.Furtherevidence tion dependsstronglyonthegastemperature(Larson1985). the Galaxy(Scoville,Sanders,&Clemens1986;Scoville trace thespiralstructureofGalaxy(Güsten&Mezger peratures. Thewarm-core(~100K)giantmolecularclouds divided intotwopopulationswhichreflecttheirkinetictem- then theirdistributionoffersstrongsupportforstarformation is derivedfromseveralphysicalarguments.IfCO-emitting have retainedthevalueof4.0derivedforGalaxybyDopita Franco &Cox1983).Supportforthisscenariocomesfromthe M) starsastheyform(Norman&Silk1980;Franco1983; surization oftheISMbyTTauriwindsfromlow-mass(<1 Ekman 1984;Augarde&Lequeux1985). order 3M(Riekeetal.1980,1985;Olofsson,Bergvall,& stars formpreferentially,sincethecriticalmassforfragmenta- occurring attwoquitedifferentsites.TheCOcloudsareclearly tionary modelsarerelativelyinsensitivetothisparameter,we tion isdetailedasafreeparameter;however,sincetheevolu- gas inlong-livedstars.Theratioofthetworatesstarforma- 1983) andareassociatedwithHnregions,whereasthecold- 0 0 SMC Although high-massstarformationmaydominateinthe The logicinfavorofadoptingabimodalstarformationrate In thewarm-corecloudsitisbelievedthathighermass -0.64 -0.76 -0.83 -0.62 50.0 0.082 4.0 3.6 0.36 2.9 2.32 2.5 8.0 3.0 1.5 130.0 -0.30 0.6-1.5 -0.44 -0.80 -0.58 4.0-7.0 52.0 10.0 0.089 0.70 0.11 6.1 2.29 3.8 LMC 130.0 -0.30 -0.44 -0.24 -0.32 0.091 4.0 5.1 0.11 2.0 2.20 3.0 8.0 1.1 1.85 This work This work This work This work This work Dopita 1987andreferencestherein Model Dopita 1987andreferencestherein Derived Derived Lequeux 1984 Lequeux 1984 Humphreys &McElroy1984 Model Model Frantsman 1988 Lequeux 1984 Mathewson &Ford1984 Model Reference 511 1992ApJ. . .384. .508R s -2 1/2 2 1 mass slopeoftheIMFisfairlywelldeterminedbyobservation collapse timescaleofthegalaxy.Thisscale,%,isgiven by low intheClouds(seeIsrael1984forafulldiscussion). probably lowerbounds,sincethereisanunknowndegree of from Lequeux1979,1984)andTisthegasinfalltimescale. where Misthetotalsurfacemassofsystem(determined to lieintherange1.5star formation,andcistheratiooflow-masstohigh- the instantaneousgasmassdividedbyrateofhigh-mass assumes asimplepowerlaw,anditistheslopepthatfunda- tion, sincethelowestmassstarshaveverylittleinfluenceon for thehigh-masscomponentis it hasnoeffectonthechemicalevolution.Theformadopted of thistruncatingfunctionisunimportantforchemicalevolu- expected ifthecloudswereclosetogravitationalinstabilityin where Ro^theprotogalactic radiusinunitsof100kpc,and where Misthesurfacemassofgas(inpc), a constant-pressureenvironment. Chièze (1987)hasshownthatthisisexactlywhatwouldbe turbulent velocityandtheradius,AvocR(Larson1981). the virialmassandradius,MocR,between 512 total surfacemassofthematerialthathasalreadyfallenin,and scaling relationshipsobservedforthecold-corecloudsbetween density occursatthetime of greateststarformation.This Gyr. However,thereistheadded constraintthatthepeakgas M isthegalaxianmassin units of10M(Dopita1987). the constantbisgiven For theCloudsthisvalueisestimated tobebetween~2and6 ing, zero-metallicitygasaccordingto to beinitiallyzero,andhaveincreasedasaresultofinfall- 1990), sothat,atanyparticulartimet, 1984). 1 MandtotruncatetheIMFbelow0.5.Theexactform inf r depl low i0 gas0tot vir l 0 0 dN/d (logM)=a{l-exp[-(MM/2)/2M]}M^, low The gasinfalltimescaleislooselyconstrainedbythefree-fall The high-massstarformationrate(SFR)isdeterminedby The totalsurfacemassofthestar-formingregionisassumed The formofthelow-masscomponentisunimportant,since © American Astronomical Society • Provided by the NASA Astrophysics Data System 1/2 Mtot =M[l-exp(-i/T)], rinf r =1.65(R/M)Gyr, ff10 31/ 1 SFR(i) =hM^M, st0 b =[tdeplO+C)]“, RUSSELL &DOPITA 4121314 lar andspiralgalaxiesobservedbyDonas&Deharveng(1984). massive starsandthegasfraction,inlargesampleofirregu- Nomoto, &Yokoi1986),inwhich casetheywouldexplodeby (Dopita &Meatheringham1991a, b). mass-loss parameterofrj=0.33andamixing-lengthparam- for starsincapableofignitingcarboninanelectron-degenerate lifetimes suggestedbyI.Iben(1987,privatecommunication): Dopita (1985),betweenthespecificrateofstarformation results fromthestrongobservationalcorrelation,notedby limit (seeNomoto,Thielemann, &Yoloi1984;Thielemann, production ofHe,C,N,andthes-processelements contribute totheenrichmentofISMmainlythrough the eter a=1.5,hasbeenusedinthismassrange.Thesestars nucleosynthesis prescriptionofRenzini&Voli(1981),with a to formplanetarynebulae,andleavebehindawhitedwarf. The core. Thesestarsendtheirlivesbyejectionofoutershells mann &Arnett1985).Theleastmassivestarsareassumed to ments (Arnett1978;Woosley&Weaver1982,1986;Thiele- elements suchasO,Ne,thea-elements,andr-processele- computing themodels,weassumethatstarsdieinstantane- the gasdepletiontimescale.Wethereforeadoptstellar since thisisinaccurateforstarswithlifetimesoftheorder degenerate C-Ocoreshaving masses uptotheChandrasekhar the starsaretheoretically capable ofbuildingelectron- have anupperlimitof5M(see,forinstance,Tosi&Diaz die asTypeIIsupernovae,andarethemainsourceofheavy Gyr, theageofoldestknownobjectsineachsystem(see time foreachoftheClouds,whichwasinitiallytakentobe10 have chosenspecificallytofitwithourmodels. The parameterslistedinboldfacetyperepresentthosethatwe ature andthemodelresultsforGalaxyfromDopita(1990). as theobservationallydeterminedparametersfromliter- the ISM. ously andreturntheappropriateamountofheavyelementsto If thestellarlifetimeislessthanonetimestep(0.15Gyr)in stars isstillsomewhatcontroversial. Uptoamassof~8M, stellar evolutiontheory.Themostmassivestars(M>12M) according totheirultimatefateasdeterminedbycurrent modeling shows,however,thattheabsoluteageofsystem each Galaxy.Ourmodelsarethereforeincapableofpredicting which probablybetterreflectsthebeginningofdiskcollapsein were obtainedbyassuminganageof8GyrfortheClouds, references quotedinTable2).However,moreaccuratemodels (within plausiblebounds)doesnotseriouslyaffectthemajor especially intheperiodofslowcollapsehalo.Further likely, thisistellingusthatthestarformationlawfor possibly inaccurate(since,asshowninTable2,theagesfor anything olderthan8Gyr,andonthosegroundstheyare results derivedfromthemodels. Clouds issomewhatmorecomplexthanwehaveassumed, some starsareestimatedtobeoftheorder10Gyr).Most 1985), whichis“classically”thoughttobetheuppermasslimit 0 0 0 We avoidusingtheinstantaneousrecyclingapproximation, The derivedmodelparametersaregiveninTable2,aswell In themassrangefrom5to12 M,theultimatefateof The modelswerecalculatedoveranassumedevolutionary Stars canbedividedupintothreegroupsbymass,defined 0 3.2. TheNucleosynthesisPrescriptions 9-2 16 10-3,5 2.64 x10myr,>2.3M, 1.1 x10myr;<2.3M. 0 0 Vol. 384 1992ApJ. . .384. .508R No. 2,1992 less likelybecauseofthelargeoverabundanceFeexpectedif in totaldisruptionofthestar;however,formerisrendered detonation orcarbondeflagration.Bothscenarioswouldresult produce arangeofpossiblenucleosyntheticprescriptions Woosley 1986).Thecarbondeflagrationsupernovaewould white dwarfs(discussedfurtheron).Withtheinclusionofcon- by andlargemimictheeffectsofdeflagrationbinaryC-O depending onthevelocityofdeflagrationwave,andwould they occurinappreciablenumbers(seeAudouze,Chiosi,& mass rangewheresingle-stardeflagrationispossiblemaybe vective overshootinginstellarmodelcalculations,however,the lan! etal.1985;Renzini1985),anditmaybethatstarsof reduced toinsignificantlevels(Matteucci&Tosi1985;Castel- Tornambè &Matteucci(1987). result ofelectroncapturesinthecore,andifexplosions reviewed byNomoto(1984).Theyarethoughttocollapseasa 6-8 MactuallyproduceTypeIIsupernovaeassuggestedby enriched matteroverlyingthedegenerateO/Ne/Mgcore. probably makenosignificantcontributiontotheenrichment resulting fromthisdonottotallydisruptthestar,ejecta whole massrangefrom5to12M,weassumeforourmodels of theISM,owingtosmallamountheavy-element- contributions fromthemassrange8supernova is,however,relativelyrare,formingjust10%of all they wouldcoalesce.Wesimplify theproblembyassuming that somefraction,takento beafreevariableintherange and thatanexponentially decayingfraction of thesedeflagrate ~ 1-4%,ofallwhitedwarfsform TypeIsupernovaprecursors, The fateofstarsinthemassrange8-12Mhasbeen Owing tothepresentuncertaintiesinvolvedinmodeling The mass(insolarmasses)oftheremnantsm,assumedto 0 rem All starsinthemassrange312M. 0 0 ABUNDANCES OFHEAVYELEMENTSINMAGELLANICCLOUDS.III. may besomewhatlessaccurateinprinciplethantheapproach Galaxy wereproducedbyDopita(1990)adoptingtheprecur- factor of2)theexpectednumberwhitedwarfs(ifwe sor fractiontobe2%.Althoughthisfigureisnotwellcon- of Matteucci&Greggio(1986).Satisfactorymodelsforthe supernovae fortheGalaxy(vandenBergh,McClure,&Evans strained, itisinagreementwiththeexpectednumberofTypeI assume eachwhitedwarftobe1.4M). with theGalacticmodelderivedbyDopita(1990),whichis models ofboththeSMCandLMC.Thismaybecompared density andofthestellarsurfacemassdensity,relativeto total surfacemassdensityofthecompletesystem,forour as timepasses.Thisapproachinvolvesfewervariables,butit 1987), whichalsoresultsin2%(withanestimatederrorofa 0 plotted againsttime, (b)Sameas(a),butfortheGalaxy. relative tothetotalsystemicmass for theClouds(dashedlinesasmarked), the SMCandLMC(solidlines as marked)andstarsurfacemassdensity In Figure3aweshowtheevolutionofgassurfacemass Fig. 3.—(a)Gassurfacemassdensity relativetothetotalsystemicmassfor 3.3. StarFormation Fig. 3a Fig. 3b Time(Gyr) 513 1992ApJ. . .384. .508R mass), thedataarenotinconsistentwithourconjecture. the largeobservationalerrorinherentinestimationof although thescatterissubstantial(owing,atleastinpart,to irregular galaxiesstudiedbyHunter,Gallagher,&Rauten- second-order fithasbeendrawnthroughthedata,and, to thetotalmassagainstmass.Aleast-squares Clouds. InFigure4weplottheobservedratioofstarmass kranz (1982),aswellfortheGalaxyandMagellanic increases. Thisconjecturewastestedusingthesampleof expected todecreaseprogressively,whilethegascontent Thus, forlessmassivegalaxiesthestarformationefficiencyis whereby thediskofanygalaxyisequallyefficientatforming in themoremassiveLMC.Thissuggestspossiblescenario stars, aslongitexceedssomecriticalsurfacemassdensity. cantly lessefficientatproducingstars,andagreaterspecific efficiencies at8Gyr.TheSMC,ontheotherhand,issignifi- order fittothedata. marked) andforthesampleofirregular galaxiesfromHunter,Gallagher,& surface densityofgasisallowedtobuildupinthisgalaxythan total systemicmass,fortheGalaxyand theMagellanicClouds{opencirclesas the Galaxyappeartohaveverysimilarspecificstar-forming systemic mass,donotshowamonotonietrend.TheLMCand the massofstarsformedatagiventimedividedbytotal Rautenkranz (1982)[filledcircles).The linerepresentsaleast-squaressecond- peak withincreasinggalacticdisksurfacedensity. but theysuccessfullypredicttheincreasingdefinitionof man 1988).Ifthereisone,itprobablyatalook-backtimeof be observedatall(seeBica,Dottori,&Pastoriza1986;Frants- the starformationratewerespecificallyfittedinourmodels, around 3Gyr(seeHawkins&Brück1982,1984).Thepeaksin and fortheSMCthereissomequestionwhetheranypeakcan peak atalook-backtimeofaround5Gyr(seeButcher1977; hand, theLMCisobservedtohaveabroaderstarformation very earlyinthehistoryofGalaxy.Suchapeakispromi- nent featureofthemodelshowninFigure3b.Onother basis ofapromptinitialenrichment(see,forinstance,Pagel with theobservationsforallthreegalaxies.Theof broadens asthesurfacedensityofgalaxydecreases,well in gasmass,andhencethestarformationrate,monotonically Stryker 1981;&ButcherFrogelBlanco1983), our ownGalacticneighborhoodcanbestbemodeledonthe as beingprogressivelyfurtherdelayedintime.Thisfitswell given hereinFigure3b.Wecanimmediatelyseethatthepeak 514 1988), whichrequiresanarrowintensepeakinstarformation The specificstar-formingefficiencies,definedhereasbeing Fig. 4.—Stellarmassfractionofthe total systemicmassplottedagainstthe © American Astronomical Society • Provided by the NASA Astrophysics Data System RUSSELL &DOPITA Truran 1989). the [a-elements/Fe]ratioplottedagainst [Fe/H](fromWheeler,Sneden,& different deflagrationtimescalesT (inGyr)tothecollectedGalacticdataon and itdependscritically(tothefourthpower)onsemimajor deflagration timescale(T)isthecharacteristicbetween for theinfallofhalogasanddeflagrationtimescale.The the formationofawhitedwarfanditssubsequentdeflagration, by Dopita(1990)indicatethattheobservedabundanceratios slope oftheIMFandbyrelevanttimescales;scale dominated thenucelosynthesisduringcollapsephaseof observed inextremePopulationIIhalostarstheGalaxy, the valueofthisratio.Thissupernovaproducedaratio the [a/Fe]ratiobecomestoosteep.Attimeslongcomparedto supernovae startstotakeeffectandtheslopeofincreasein scale givestoolongadelaybeforeFeenrichmentfromTypeI order of1.0ÍJ;5Gyr.Anoverestimateinthedeflagrationtime of thea-elementsrelativetoFeasafunction[Fe/H](see axis oftheprecursorbinarysystem(Iben&Tutukov1984; our Galaxy. Dopita (1990),theexplosionofSN1987Aallowsustoestimate supernovae havenothadachancetoevolve).Asdiscussedby Type IIsupernovae(sincethelowermassprecursorsofla puts strongconstraintsontheproductionofironinnormal Fig. 5),arebestfittedassumingadeflagrationtimescaleofthe where weusetheusualnotation Matteucci &Greggio1986).Modelsofthesolarneighborhood [O/Fe] =0.5±0.2(Sneden1985;Wheeler,Sneden,&Truran [O/Fe] =0.65±0.25,whichisverysimilartothevalue extent Zn),whereasenrichmentofthea-elements,Ne,Mg,S, Type IIsupernovae.IncludedintheFe-peakelementsand Si, Ca,andTi,wouldbeexpectedtofollowtheevolutionofO. sharing theevolutionofFewouldbeCrandNi(andtosome supernovae, butwithasignificantcontribution(~i)from stars (>12M),andFeisproducedmainlyfromTypela low- tointermediate-massrange,Oisproducedinmassive defl 1989), suggestingthatsuchTypeIIsupernovaemayhave sources ofchemicalenrichment:CandNareproducedinthe He, C,N,O,andFe.Theseelementsencompassthethreemain defl 0 Fig. 5.—Illustration(fromDopita1990) ofthefitsseveralmodelswith The evolutionof[O/Fe]withmetallicityisdrivenbythe The low-abundanceasymptoticvalueforthe[O/Fe]ratio, We havespecificallymodeledtheabundancevariationsof [M/X] =logN(M/X)-AT(M/X), 10object 3.4. ChemicalEvolution Vol. 384 1992ApJ. . .384. .508R T incomplete coverageofthefull rangeofelementsobservablein any oneobject, differentobjectsarerepresented ineachfigure. from theliterature.Thefilled circlesrepresentdatafromthe for the[C/Fe],[N/Fe],and [O/Fe]ratios,respectively.We have includedinthesediagrams dataderivedfromPapersI SMC, andtheopencirclesare fromtheLMC.Owingto and II(relativetothelocalGalactic ISM),andseveralsources common slopeforallvaluesofthetimescale. defb however,the[a/Fe]ratioasymptoticallyapproaches a (b) Sameas(a),butforthe[N/Fe]ratio,(c)[O/Fe] (dashed line)andtheMagellanicCloudmodels(solidlinesasmarked).Thedata ratio. the readerofscaleandtypicalsorterrortoexpectin data. derived fromPapersIandIIareplottedhereasopencirclesfortheLMC as closedcirclesfortheSMC.Anerrorbarof±0.2dexisincludedtoremind No. 2,1992 CJ o In Figures6a-6cweplotagainst[Fe/H]themodelresults Fig. 6.—(a)Plotofthe[C/Fe]ratioagainst[Fe/H]forGalacticmodel 4. THERELATIVEABUNDANCEDISTRIBUTIONOF -1.4 -1.2 -1.0 -0.6 -0.8 -0.4 -0.2 0.0 0.2 0.4 -1.0 -0.9-0.8-0.7-0.6-0.5-0.4-0.3-0.2-0.10.0 © American Astronomical Society • Provided by the NASA Astrophysics Data System _ Galaxy 4.1. Carbon,Nitrogen,andOxygen TmcT -i—.—i :i■. SMC ABUNDANCES OFHEAVYELEMENTSINMAGELLANICCLOUDS.III. ELEMENTS Fig. 6c Fig. 6a # • A3 [Fe/H] [Fe/H] °A12 J N49 spectra ofHnregionsbyDufour etal.1982andDufour Clouds areapparentlysovery low(fromtheanalysisofUV the questionofwhyC abundances intheMagellanic important, however,webelieve thatwehaveatlastanswered dances (notspecificallyfitted inourderivationofthemodels) of theFsupergiantsexpresses supportforourmodels;more high Oabundances. analysis ofthesestars,andotherslikethem,substantiates the indicates thatitistheOabundanceinthesestarswhich is anomalously high.Itwillbeinterestingtoseewhetherfurther 1984). Theanswer seemstobethattheC abundancesare underabundance ofCrelativetoOinFigurela.Thisclearly normal CabundancerelativetoFeinFigure6a,butalarge stars studiedbyReitermannetal.(1990)areobservedtohave a rather thanbothFeandOthatarehigh.Conversely,thetwo B to O),whichsuggeststhatitisindeedthecarbon low, nova remnantN49isobservedtobeoverdeficientinC(relative our modelsprovideanexcellentfittothesedata.Thesuper- from Figurela,whichshowsthemodelresultsfor[C/O] these quantitiesintheMagellanicClouds.Againweseethat versus [O/H],andalltheavailableobservationaldatafor survival ofgraphitegrainsintherecombinationregion what overdeficientinC(relativetoFe).Thismaywellbedue SNR. FurtherinformationontheCabundancesmaybegained included inthisfigure(N49asmarked)appearstobesome- within theexperimentalerrors,althoughonlySNRthatis included anerrorbarof±0.2dextogiveindicationthe scale ofthemostprobableexperimentalerror. (1990). Forthesediagrams,andthosethatfollow,wehave al. (1989a),andtheBstars(A3A12)fromReitermannet supergiants westudiedinPaperI,theFofSpiteet the Clouds. sequence becauseoftherelativestarformationefficiencies bination theymaybeconsideredinsomesenseasasingleage reflect thepresent-dayabundancesoflocalISMs,incom- form acontinuousabundanceserieswithoutintroducingmuch additional error.Therefore,eventhoughtheobjectsstudied accuracy, wemayconsidertheobjectsinbothofCloudsto It maybeseenfromthesegraphsthat,atthepresentlevelsof In conclusion,theexcellentfitofourmodelstoCabun- The modelsareinexcellentagreementwiththedatatowell In Figure6awehaveplottedthe[C/Fe]ratiosforF Fig. 6b [Fe/H] 515 1992ApJ. . .384. .508R (1983), forinstance, suggestedthatthespread inNandO explain theobservedspread in[N/O].Matteucci&Chiosi most notablyPuppisA,and theobservationofhighly from observationsofnitrogen-rich knotsinseveralSNRs, nitrogen-enriched ejectaaround SN1987A(Kirshner1988; ejected asastellarwind,thismaterialcouldpollutethe sur- massive starsiswidespread(Luck&Lambert1981),and, if Fransson etal.1989). rounding ISM.Evidencethatthismayindeedhappencomes enhanced. EvidenceforNenhancementsintheatmospheres of types ofobjects.EithersomeprocessisdepletingtheNabun- found in[N/O]fordwarfirregulargalaxies(Dufour1986; dances observedinHnregions,ortheSNRabundances are seeing agenuinedifferencebetweentheNabundancesinthese so, thecauseremainsobscure.Moreinterestingly,wemay be the Hiiregions.Thismaywellbeamodelingproblem,but, if that theSNRsaresystematicallymoreenhancedinNthan are regions (Pagel1985).However,itisstillevidentfromFigure lb except SNR0104—723,surprisinglywell.Thescatterincreases found inmostHnregions,ourmodelsfitalltheSNRdata, models, andthedataderivedsolelyfromSNRswestudied ratio against[O/H]forboththeSNRs(asmarked)and in PaperII.ConsideringthesevereunderabundanceofN weakness ofthe2200ÂabsorptionfeatureobservedinISM Edmunds 1981andreferencestherein),theGalactic H n H iiregionsfromPaperII.Thisisreminiscentofthescatter somewhat whenweturntoFigurelb,whereplotthe[N/O] is commoninourownGalaxyordertoaccountforthe grains wouldhavetoparticlesizessomewhatlargerthan processes asthelockupofcarbonontographitegrains.These Matteucci &Tosi1985),bluecompactgalaxies(seePagel Bromage &Nandy1983;FitzpatrickSavage1984;Nandy, of theMagellanicClouds(seeKoornneef&Code1981; and anexplanationforthismustbesoughtintermsofsuch These underabundances,then,areintrinsictotheHnregions, entirely normalintheMagellanicClouds,anditisonly Morgan, &Houziaux1984). H iiregionsthatarereflectingsevereunderabundancesofC. 516 Many suggestionshavebeen put forwardinrecentyearsto In Figure6bweplot[N/Fe]againstthe[Fe/H]ratioforour © American Astronomical Society • Provided by the NASA Astrophysics Data System Fig. 7.—(a)Sameas6a,butfor[C/O]plottedagainst[O/H],(b)[N/O][O/H]. Fig. la [O/H] RUSSELL &DOPITA 1216 studied inPaperII;thethree F supergiantsandoneredsuper- available tous.Thesedataare derivedfromalloftheSNRswe plotted against[Fe/H],superposed ontheobservationaldata of intermediate-massstars. abundance istellingusalotabout theformationandevolution star formationhistoriesofgalaxies, butclearlythenitrogen diagnostic forthedeterminationofgeneralfeatures the cally. Untiltheyaredetermined,nitrogenwillremainapoor still highlycontroversial,bothobservationallyandtheoreti- (see Matteucci1986foradiscussionoftheconsequences of N abundancemightvaryfromoneenvironmenttoanother adopting thehigherrateputforwardbyKettneretal.1982). throughout), couldalsohaveasignificantinfluenceonhow the the C(a,y)0reaction(wehaveassumedstandard rate be regardedasprimary.Otheruncertainties,suchtherate of cesses areresponsibleforthis,thenitrogeninthesecasesmay (Meatheringham &Dopita1991a,b).Sincedredge-up pro- mass rangeofstarsexperiencingenvelopeburning(seeGreggio Type Iplanetarynebulaeareobservedtoberichinnitrogen envelopes asthecoresizeisincreasedandtodecreasein shooting areincluded,owingtothereductioninstellar primary nitrogen.Inaddition,Tosi(1988)showedthatNis thereby affectingthemassrangeofstarscapableproducing required foraccumulatingdegeneratecarboncoresand endeavored toexplainthespreadin[N/O]ratioterms & Tosi1986).IntheLMCandSMC,however,massive strongly reducedinabundancewhentheeffectsofover- the ambientmetallicityaffectinglimitingmass,M, of differingcontributionsprimaryandsecondarynitrogen. variation inthegalacticwind/SFRratiofromgalaxytogalaxy. due tovariationsfromgalaxyin(1)theIMFand/or abundances withthefractionalgasmass,M/M,mightbe other hand,suggestthatthespreadmaybeexplainedbya of massoutflow/SFRratio;Matteucci&Tosi(1985),onthe the chemicalyields,(2)infallrate/SFRratio,and(3)rate Matteucci &Tosi(1985)suggeststhatitisduetovariationsin Pagel &Edmunds1981;SerranoPeimbert1983)have up gastot $ -0.2 Z In Figure6cweshowourmodel resultsforthe[O/Fe]ratio In conclusion,theproductionmechanismsfornitrogenare Similarly, manyauthors(forexample,Alloinetal.1979; -1.2 -0.8 -1.0 -0.6 -0.4 0.6 0.8 0.4 0.2 0.0 -1.4 -1.2-1.0-0.8-0.6-0.4-0.20.00.2 Fig. 1b [O/H] Vol. 384 1992ApJ. . .384. .508R 44 well-observed a-elementsinourdataaccordingtotheformula esting constraintsonthepasthistoryoflow-masstohigh-mass massive stars,whilethebulkofFeisderivedfromdeflagration star formation. the a-elementabundancetoFecomparedwithsolarputsinter- the a-processelements.(SeealsoreviewbyWheeleretal. understood asaconsequenceofa-richfreezeoutinTypeII from theequilibriumprocessmustbequitesmall.Woosley& with Feabundance(seeMagain1989),sothecontribution No. 2,1992 of C-Owhitedwarfsonamuchlongertimescale,theratio supernovae. Onthisbasis,Tiisincludedhereasamemberof Hoffman (1991)haveshownthattheabundanceofTicanbe equilibrium burningtotheiron-peakelements.However, where atleasttwoelementshavebeenusedindeterminingthe excluded. CertainlyapartoftheTiabundanceiscontributed isotope. MostworkersagreethattheelementsO,Ne,Mg,Si,S, within theexperimentalerrors,althoughtheremaybeanindi- ratio, which,asdiscussedpreviously,followsthetrendsof ences betweenthatelementandtheothertwoissubstituted.In through thea-process,whilerestprobablyresultsfrom number ofprotonsandneutronsinthenucleitheirmajor a-particle additiontoseednuclei,andthereforehaveaneven other a-elementsclosely.Weseethatthemodelsfitdata addition, wehaveplottedourmodelresultsforthe[O/Fe] average; whenoneismissing,themeanofaveragediffer- and Cabelongtothisgroup,butTihassometimesbeen could notbedisplayedinthisfigure). having beenforcedtofittheHnregionOabundances,which (1990). AgaintheBstarsarerelativelyoverabundantin [Ti/Fe] ratioappearstoactlikeana-elementinitsvariation thus addingsupporttothevalidityofmodels(the 1989.) oxygen, butourmodelsfitalltheotherdataremarkablywell, respectively; andthetwoBstarsstudiedbyReitermannetal. giant studiedbySpiteetal.(1989b)and(1986), a-elements aredefined inthetext. In Figure8weplotthecombinedcontributionsofthree Since thea-elementsareproducedbyTypeIIsupernovaeof The a-elementsaredefinedasthosewhichhavebeenbuiltby Fig. 8.—Sameas6a,butfor the [a-elements/Fe]ratio,where [a/Fe] =l/3([Mg/Fe]+[Ca/Fe][Ti/Fe]), © American Astronomical Society • Provided by the NASA Astrophysics Data System 4.2. Thea-Elements ABUNDANCES OFHEAVYELEMENTSINMAGELLANICCLOUDS.III. [Fe/H] lanic Clouds(daCosta1990).However,thefieldstarsdo not apparent absenceofintermediate-ageclustersintheMagel- is thesystematicallylower[a/Fe]ratioobservedinClouds proposed byMatteucci(1990)andindependentlyGilmore mass starsand,therefore,asteeperIMFonaverage. will experiencefewerhigh-velocitycollisionsandbemore numbers ofmassivestars. reducing the[a/Fe]ratio,throughrelativedecreasein in theoppositesensetooneobserved.Itappears,therefore, infall timescale,andthedeflagrationscale.Asmaybe relative totheGalaxy.Asdiscussedin§2,evolutionofthis individual element.Indeed,variationsofthisorderbetween formation historyoftheLMCbeforethispointcanbedefini- interesting, wemustawaitmoredetailedstudiesofthe star Thus, althoughthehypothesisofepisodicstarformation is Galaxy andtheotherspiralgalaxies,resultinginfewerhigh- ditions thaninthemoreviolentconditionsexperienced prone tolow-velocitycoalescence.Undersuchconditions,we their rotationvelocitiesarelowandtheybothhavelargeareas upper IMFintheCloudsbyHumphreys&McElroy(1984). that thesteeperslopeofIMFiscriticalfactorin which wouldgivealowerFeabundanceandthushigher Clouds, sinceitislongerintheCloudsthanGalaxy, time scalethatcouldberesponsibleforthelowerratioin ratio withmetallicityisdrivenbytheslopeofIMF, this metallicityrange(seeAndersenetal.1988). different a-elementsisafeatureoftheGalacticabundancesin different stellarmassrangesresponsibleforthebulkofeach relative toOintheClouds,whichisperhapsresultof cation thattheyunderestimatethea-abundancesby0.1-0.2 tively addressed. appeared tobecontinuouslyenrichingitselfoverthisperiod. appear toshowacorrespondingdeficit,andtheLMC bursts. Someevidenceinsupportofthisviewpointis the tonically decreasingwithtimebutcanincreaseagainnear gration supernovae,the[O/Fe]rationeednolongerbemono- quiescence. Then,withasomewhatlargerfractionofdefla- suffer burstsofstarformationseparatedbylongperiods & Wyse(1991).Theseauthorsproposethatdwarfgalaxies speculate thatfragmentationwilltakeplaceundercoolercon- of solid-bodyrotation.Itfollowsthattheirmolecularclouds the disksofbothLMCandSMCmustbelow,since structures oftheCloudsandGalaxy.Shearingmotionsin that oftheGalaxyistoappealdifferencesinglobal substantially longerthanfortheGalaxy,againhavingaresult seen fromTable2,ourresultsindicatethatitisnotthelatter this wouldindicatethattheothera-elementsareoverabundant dex. SincethemodelshavebeenfittedtoOabundances, One possiblewaytoexplainsuchasteepIMFcomparedwith [a/Fe] ratio.TheinfalltimescaleforeachoftheCloudsisalso solar vicinity,withintheexperimental errors.ForAlthesitu- abundances relativetoFeare nodifferentfromthatofthe These liebelowthoseofthe Fe-peakgroupinatomicmass, on atotalofsixstars,whichis onlysufficienttoshowthattheir and wehavelittleinformation ontheirabundancesatpresent. Spite etal.(1986),and al.(1989b)giveusinformation For Na,thecombinedanalyses ofourownwork(PaperI), This agreeswiththeobservationalresultsonslopeof The mostobviousfeatureapparentinbothFigures6cand8 An alternativeexplanationforthelow[a/Fe]ratiohasbeen We includeinthissectiontheoddelementsNaandAl. 4.3. TheOddElements 517 1992ApJ. . .384. .508R since theseelements,together withFe,areformedinthe literature agreesthatthisisalso thecaseforGalaxy,and ments areinsubstantialagreementwiththeGalacticresults, nuclear statisticalequilibrium processduringsupernovaexplo- proportional productionwith iron.Mostoftheevidencein these elementsareplottedagainst [Fe/H]inFigures10aand besides Feitself,areCrand Ni.Therelativeabundancesof field stars. except thatVmaybesomewhatoverproducedrelativeto the rule outasimilardropintherelativeMnabundancesfor the our Galaxyshowssomesignofdecreasingfor[Fe/H] < tance intheMagellanicClouds. abundance intheGalacticglobularclustersisalsoofimpor- be anindicationthattheprocessactingtoenhanceV hand, displayasystematicoverabundanceof~0.25dexin weakens theseconclusions.Theglobularclusters,ontheother invariant relativetoFewithdecreasingmetallicity(Wheeleret even elements,toseewhetherthereisanysignofanodd-even Magellanic Clouds. al. 1989);however,thelackofaccurateHFScorrections effect. WeseeimmediatelythatthereisnoobviouseffectforSc (often notavailableforthetransitionofinterestanyway),most et al.1989b). or Mn,whileVisapparentlyoverabundant. correction shouldcanelout. differentially withrespecttoCanopus,anyerrorsintheHFS workers havereliedonempiricalcorrectionstotheturbulent in ourownGalaxy.Insteadofusingpreciseoscillatorstrengths [V/Fe] (Wheeleretal.1989andreferencestherein).Thismay the ratiosofoddelements,withtheirnearestneighboring respectively, againsttheFeabundance[Fe/H].Thatis,weplot HFS, basedonsomecarefulworkbyGratton(1982)andthe technique toestimatetheextra£requiredcorrectfor Wheeler etal.1989).InPaperIwealsousedanempirical derived fromstarsdifferentsources(Lambert1989; unsatisfactory, asshownbythelargescatterinabundances velocity £inderivingtheabundances.Thisisclearly into correctingfortheHFSinpast(exceptcaseof interiors. Unfortunately,thefactthatalltheseelementshave ation isworse,sincetherearedataforonlyonestar(fromSpite Liège solaratlas.However,sincewemeasuredourabundances Gratton 1989),soitisstilluncertainhowtheabundancesvary ence ofanuclearmagneticmoment.Littleefforthasbeenput lOfi, whereitisevidentthat both elementsdeviatelittlefrom splitting (HFS)oftheirenergylevelsresultingfromthepres- unpaired protonsmeansthattheyallexperiencehyperfine tial foractingassensitiveprobesofthetemperaturesstellar dances arethereforehighlydependentontheneutronfluxand of theseednuclei.Accordingly,theseelementshavepoten- the interiortemperaturesofstars,aswellonabundances seed nucleiduringthemajorstellarburningphases.Theabun- and Cu,theyowetheirexistencetoneutronirradiationof Mn, whichhasbeenstudiedcarefullybyBeynon1978a,band 518 ~ —0.4.Consideringthescatterinourabundances,wecannot turb turb The onlytwoelementsthatweconsiderinthiscategory, We mayconcludethatourobservationsforallthreeele- The relativeMnabundancesinthesolarneighborhoodof The oddelementsoftheFepeakcompriseSc,V,Mn,Co, In thefieldstarsofGalaxy,ScandVarebelievedtobe In Figures9a-9cweplot[Sc/Ti],[V/Cr],and[Mn/Fe], © American Astronomical Society • Provided by the NASA Astrophysics Data System 4.4. TheEvenFe-PeakElements RUSSELL &DOPITA ponent withA > 89isproducedmainlyby intermediate-mass two differentmassrangesof stars.Theso-calledmaincom- capable ofbuildingheavierelements beyondtheFepeakare tions. Thes-processelements arethoughttobeproducedby the slow(s-process)orrapid (r-process) neutroncapturereac- if theirabundancesdidnotcorrelateaswelltheydo. scale, (b)Sameas(a),butfor[V/Cr].(c)[Mn/Fe]. the LMC(opencircles)andSMC(closedcircles).Asforpreviousfigures, sions ofbothTypeIandII,itwouldindeedbesurprising a 0.2dexerrorbarhasbeenincludedasanindicationoftheerrorsand the Except forZnandcertainrare isotopes,theonlyprocesses Fig. 9.—(a)Odd-evenclosestneighbors[Sc/Ti]plottedagainst[Fe/H],for ^ 0.0 S -0-2 “ -0.4 o’ 0.2 -0.6 -0.8 0.4 0.6 0.8 it 14 4.5. TheNeutronCaptureElements - -1.2-1.0-0.8-0.6-0.4-0.20.00.2 Fig. 9c [Fe/H] Vol. 384 1992ApJ. . .384. .508R 2225 1316 2225 No. 2,1992 contribution fromtheNe(a,n)Mgreaction(seeKäppeleret up theasymptoticgiantbranch(AGB).Theneutronsourceis stars duringthethermallypulsing(TP)phaseoftheirevolution (M/Mq >10)neartheendofcoreheliumburning(see thought tobemainlytheC(a,n)0reaction,withasmall al. 1990).Forthe“weakcomponent”withA<89,neutron The LMC(opencircles)andtheSMC(closedareplotted,and,asfor halo stars([Fe/H]<—2.0),onlyther-processreactionshave source istheNe(a,n)Mgreactionaloneinmassivestars errors andthescale,(b)Sameas(a),butfor[Ni/Fe]. previous figures,a0.2dexerrorbarhasbeenincludedasanindicationofthe ments. Thishasreceivedsomesupportinrecentyears(Gilroy contributed totheabundancesofneutroncapture ele- et al.1988;Wheeler1989),throughcomparing the Prantzos, Hashimoto,&Nomoto1990). with thetheoreticalabundancepatternsproducedsolelyby the r-processes. observed abundancepatternsfortheneutroncaptureelements (1989), andsuperposedonthesewehaveaddedther- and Z =37.Includedonthisdiagramaretheaverageofhalo abundance patternsfortheobservableelementsheavierthan s-process contributionstothe solarabundancesderivedby star abundancesusedbyGilroyetal.(1988)andWheeler al. (Z =39)waschosenbecause thiselementseemstobeinrela- Cameron (1982a).Wehavealso computedamodelwhereby been normalizedtozeroat Y. ThenormalizingpointatY the fullr-processcontribution tothesolarsystemhasbeen added tohalfofthes-process contribution,andtheresulthas tively constantproportionto Fe. Fig. 10.—(a)Eveniron-peakelementratio[Cr/Fe]plottedagainst[Fe/H]. In 1981,Truranproposedthat,forextremelymetal-weak We haveplottedinFigure11atheMagellanicCloud[M/Y] © American Astronomical Society • Provided by the NASA Astrophysics Data System ABUNDANCES OFHEAVYELEMENTSINMAGELLANICCLOUDS.III. Fig. 10a Fig. 10b long-dashed linesrepresentthecontributionsfroms-processandr- equal tozero.Theshortdashedlinerepresentsthesumofr-processand process, respectively,tothesolarabundances,normalizedsothattheirsumis triangles), andtheLMC(opencircles)plottedagainstZ.Thesolid Magellanic Clouds.Fromthiswemayconcludethatthe (a), exceptthatthesolidlineissolarr-processcontribution,longdashes (see PaperI),areincludedinthisdiagramforillustrativepurposes,(b)Sameas error barsfortheLMC,dueonlytoobservedscatterinabundances half thes-processcontributionstosolarabundances,normalizedatY.The error bars. neutron processinghistoriesoftheCloudsandGalactic are duetother-processmodelofGilroyetal.(1988).See(a)forillustrative are duetother-processmodelofWheeleretal.(1989),whileshortdashes halo havebeenmarkedlydifferentfromthatoftheSun.Even a models derivedfromGilroyetal.(1988)andWheeler al. has anentirelydifferentdistributionoftheseelements. neutron captureelements(Sr,Y,Zr),whereastheGalactichalo bution wouldfittheMagellanicClouddataforlight r-processes thatincludesatleasthalfthesolars-processcontri- However, itisalsoapparentthatanycombinationofs- and the s-processisinsufficienttoaccountforobservations. contributions andthemodel due toGilroyetal.(1988)fitthe (1989), respectively.Weseefromthisthatthesolarr-process contributions fromCameron(1982a)andthetwor-process Galactic haloandtheMagellanic Clouds,atleastqualitatively. 50% increaseinthecontributionbyr-processrelative to except perhapsahighBaabundance intheLMC,whichsug- tion oftheCloudsand halo inthisrangeofelements, Indeed, thereislittleobvious differencebetweenthedistribu- observed abundancedistribution beyondZ=55,forboththe gests anormalcontributionfrom thes-processforthiselement. None ofthesecurvesfiteithertheGalactichaloor Fig. 11.—(a)[M/Y]fromtheGalactichalo(closedcircles),SMC(open In Figurellhwesuperposeonthedatasolarr-process ' 3540455055606570 Fig. Ub z 519 1992ApJ. . .384. .508R licity 0.2dexlower thanthelocalISM,and themetallicityof in theCloudsareproducedbymassivestars(andhence the stars. r-process), withlittlecontribution fromtheintermediate-mass is incontrasttothestrictproportionalitywithFefound in the Galaxyatthesemetallicities(Luck&Bond1985; decreasing relativeabundancewithincreasing[Fe/H].This except fortheapparentlydiscrepantpointfromAV198 (see ments shownoobviousdeviationfromstrictproportionality Those forwhichwehadnodatawerereplacedwithvalues where atleasttwoelementsweremeasuredfromanystar. Magain 1989),andsuggestsagainthattheheavyelements to [Fe/H].Fortheheavyneutroncaptureelements,however, between thatelementandCe.Thelightneutroncapture ele- maintaining theaverageabundancedifferenceobserved Paper I),thepointsdefinearemarkablytightrelationship of capture andheavyneutronabundances,respectively, neutron captureabundances,[M/Fe],aredefinedheretobe here tobehalfthesumofYandZrabundancesrelative against [Fe/H].Thelightneutroncaptureelementsaredefined Fe forthosestarswherebothweremeasured,whiletheheavy heavy elementsintheClouds. apparent r-process-dominatedabundancepatternforthe been lowerthaninourownGalaxy.Thismaycontributetothe diation intheintermediate-massstarsCloudsmayhave for itsformation.Theseresultsindicatethattheneutronirra- the fallisthatBarequiresahigherneutronirradiationthanY indication oftheerrorsandscale. derived bySpite&(1978).A0.2dexerrorbarhasbeenincludedasan that the[Ba/Y]ratiomightfallmoresteeplywithdecreasing best fitstheGalacticdata(fromSpite&1978).Itappears and theSMC{closedcircles).ThedashedlineisresultforGalaxy Ba abundanceintheCloudsthanGalaxy.Thereasonfor the Clouds,togetherwithaleast-squaresfitandlinethat the a-elements. our Galaxy,oritwouldhaveresultedinanoverproduction due toalargernumberofmassivestarsintheCloudsthan in theMagellanicClouds.However,thiscouldnothavebeen hn additional r-processcomponenttotheelementalabundances 520 [M/Fe] =i([La/Fe]+[Ce/Fe][Nd/Fe][Sm/Fe]), hn 1. Theinterstellar mediumoftheLMChas ameanmetal- In thisworkwehaveestablished fourmajorresults: In Figures13aand13bweplottheaveragelightneutron Fig. 12.—[Ba/Y]ratioplottedagainst[Ba/Fe]fortheLMC{opencircles) In Figure12the[Ba/Y]ratioisplottedagainst[Ba/Fe]for It seems,therefore,thattherehasperhapsbeenasignificant © American Astronomical Society • Provided by the NASA Astrophysics Data System 5. SUMMARYANDFUTUREWORK RUSSELL &DOPITA were observed in theSMC).Thepossibility of observingsome of thelighterelements bystudyingnear-main-sequence Bstars sometimes embarrassinglysmall (forexample,onlytwoSNRs fully (Na,Si,S,Sr,Ba,and Eu), whileothershavenotbeen observed atall(Al,Gd,and Dy).Thesampleofobjectsis Clouds. Manycriticalelements needtobestudiedmorecare- analysis ofthedetailedglobal abundancesintheMagellanic ing heavyneutroncaptureelements,whencomparedwith the Galaxy. r-process moreeffective,intheMagellanicCloudsatproduc- respect totheSunthroughstellarorbitaldiffusion. for thisisthattherehasbeenaradialinflowoftheISMwith that theSunisalready~4Gyrold.Onepossibleexplanation are systematicallylessthanthoseoftheSun,despite fact entirely normal;itisthereforetheHnregionsthatmust be overdeficient inthiselement. for theLMC{closedcircles)andSMC{opencircles).A0.2dexerrorbarhas nonsolar elementalratios. but forthe[heavyneutroncaptureelements/Fe]. been includedasanindicationoftheerrorsandscale,{b)SameFig.11a, both theMagellanicCloudsandGalaxyhavesignificantly the SMCis0.6dexlower.However,interstellarmediaof There is,ofcourse,muchroom forimprovementinthisfirst 4. Thes-processappearstohavebeenlesseffective,andthe 3. Thelight-elementabundancesofthelocalGalacticISM 2. ThestellarcarbonabundancesintheCloudsappeartobe Fig. 13.—{a)[Lightneutroncaptureelements/Fe]plottedagainst[Fe/H] -1.0 -0.8-0.6-0.4-0.2-0.00.20.4 Fig. 13b Fig. 13a [ Fe/H] [ Fe/H] Vol. 384 1992ApJ. . .384. .508R .1989,A&A,208,171 Güsten, R.,&Mezger,P.G.1983,Vistas Astron.,26,159 Greggio, L,&Tosí,M.1986,A&A,156, LI Gilroy, K.K.,Sneden,C,Pilachowski, C.A.,&Cowan,J.1988,ApJ,327, Hawkins, M.R.S., &Brück,M.T.1982,MNRAS,198,935 Gratton, R.G.1982,ApJ,257,640 Gilmore, G.,&Wyse,R.F.G.1991,ApJ, 367,L55 Gaskeil, C.M.,Cappellaro,E.,Dinerstein,H.L.,Garnett,D.R.,Harkness, Evans, I.N.,&Dopita,M.A.1985,ApJS,58,125 Dufour, R.J.,Shields,G.A.,&Talbot,Jr.1982,ApJ,252,461 Frantsman, J.L.1988,Ap.SpaceSei.,145,287 Fransson, C,Cassatella,A.,Gilmozzi,R.,Kirshner,R.P.,Panagia,N.,Sonne- Franco, J.,&Cox,D.P.1983,ApJ,273,243 Franco, J.1983,ApJ,264,508 Fitzpatrick, E.L.,&Savage,B.D.1984,ApJ,279,578 Filippenko, A.V,&Sargent,W.L.1986,AJ,91,691 Dufour, R.J.,Schiffer,F.H.,&Shields,G.A.1984,inTheFutureofUltravio- Frogel, J.A.,&Blanco,V.M.1983,ApJ,274,L57 .1986,PASP,98,1025 .1991b,ApJ,377,480 .1990,inTheInterstellarMediumGalaxies,ed.H.A.Thronson& .1987,inIAUSymp.115,StarFormingRegions,ed.M.Peimbert& .1982b,inEssaysNuclearAstrophysics,ed.C.A.Barnes,D. 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