1987ApJ. . .317. .190M 2-3 4 62-3 6 6 4 Jeffrey (1983).Meanwhile,the evidenceforsuchshellsinthe (1980), Tomisaka,Habe,and Ikeuchi(1980),andCowie expanding shellinthedisk gas andmentionedavarietyof with agalacticscaleheightconsiderablygreaterthanthat of shells. Thatideahasbeen developed byBruhweileretal. observations thatmightbe interpreted asevidenceforsuch stellar associationwouldproduceahuge(radius>100 pc) the galacticHi,sothattheirimpactonthisgasisdiminished.) the TypeIsupernovaefromPopulationIIstarsand ations oftensorhundreds.(Itistruethatmanysupernovae, disk probablyresultfromthecollapseoffairlymassive ( >7 case, becausethe(TypeII)supernovaethatareconfinedto the However, intheMilkyWaythesesupernovaearedistributed II supernovaefromrunawaystars,arenothighlycorrelated. M) PopulationIstars,whicharetypicallyformedinassoci- are highlycorrelatedinspaceandtime.Thatshouldbe the predicted byMcKeeandOstrikerifthesupernovaein disk which cool(T<10K),dense(n~cm)“clouds”sur- ive theoryfora“three-phase”interstellarmedium(ISM),in Milky Wayandinothergalaxies hascontinuedtoaccumulate. structure ofthediskgasmightdiffersignificantlyfromthat theory isthatthesupernovaeoccuratrandomindisk. ded inthecoronalgas.Afundamentalassumptionofthis rounded bywarm(T~10K)intercloudmediumareembed- was developedbyMcKeeandOstriker(1977)intoaquantitat- (T >10K)cavityoflow-density(n<10“cm)coronalgas persist fortimesmorethanabout10yrandthereforemight Cox andSmith(1974)recognizedthatthesecavitieswould occupy asignificantfractionofthediskvolume,anideathat that maypersistfor>10yr,muchlongerthanthetimescale 0 ~3 x10yrforwhichtheradiooropticalremnantisvisible. The AstrophysicalJournal,317:190-196,1987June1 © 1987.TheAmericanAstronomicalSociety.Allrightsreserved.PrintedinU.S.A. However, asMcCrayandSnow(1979)pointedout,the McCray andSnownotedthatrepeatedsupernovaefrom a A supernovaexplosioninthegalacticdiskcreatesahot 7 7 mation andmayaccountforburstsofstarformation,especiallyinirregulargalaxies. irregular galaxiesfrom21cmemission-linesurveys,opticalandstudiesofsupernova Subject headings:interstellar:matter—stars:formationsupernovae remnants. Thegravitationalinstabilityofthesupershellsprovidesaphysicalmechanismforinducedstarfor- new starformation.ThereiswidespreadevidenceforsupershellsintheMilkyWayandotherspiral gas diskofagalaxy,typicallyafterfewtimes10yrandwithradius~100-300pc.Atapproximatelythe enters thesnowplowphasewhenradiativecoolingbecomesimportantorshellburststhrough into thesupershellatanearlyconstantrateforabout5x10yr.Thelosesitsinteriorpressureand same time,thesupershellbecomesgravitationallyunstable,forminggiantmolecularcloudswhicharesitesfor interstellar medium,withradiusgreaterthan100pc,surroundedbyadense,expandingshellofcoolinter- stellar gas.Iftheassociationhasatypicalinitialmassfunction,itssupernovaeexplosionswillinjectenergy © American Astronomical Society • Provided by the NASA Astrophysics Data System Stellar windsandrepeatedsupernovaefromanOBassociationwillcreateacavityofcoronalgasinthe I. introduction Joint InstituteforLaboratoryAstrophysics,UniversityofColoradoandNationalBureauStandards SUPERSHELLS ANDPROPAGATINGSTARFORMATION Received 1985January17;accepted1986November17 Department ofPhysics,GeorgeMasonUniversity Richard McCray Minas Kafatos ABSTRACT 190 AND 7 51 51 1 6 7 main-sequence spectraltype B3. Themain-sequencelifetimes massive starthatisexpected to terminateasaTypeIIsuper- of massivestarsaregivenapproximately byi~3x10yr nova hasinitialmass~1M (Trimble1982),correspondingto explosion, whichweestimate tobeE~10ergs.Theleast ISM isprobablydominated by thatofitsterminalsupernova also haveasignificantstellarwind,butitsenergyinputto the energy ~10ergs,comparabletothethermalof its H iiregion(Abbott1982).Aninitiallylessmassivestar may terminal velocity~2500kms“,impartinganetmechanical Wolf-Rayet phase,amassive(>30-40M)starwilllose a substantial fractionofitsmassinastrongstellarwindwith than ~5x10yr,duringitsmain-sequenceorasubsequent than ~30Mandlifetimesless~10yr.Inatime less provided almostentirelybytheOstars,withmassesgreater photons, stellarwinds,andsupernovaeexplosionsisdomi- nated byOBstars(Abbott1982).Theionizingradiation is mogeneous andstratifiedISM. (MacLow andMcCray1987,hereafterPaperII)weshallcon- formation intheMilkyWayandothergalaxies,wediscuss sider inmoredetailthedevelopmentofsupershellsaninho- instability intheexpandingshellandshowthatsupershellscan § IVweconsiderthecriterionforonsetofgravitational the limitationsofpresentmodel.Inasubsequentpaper trigger burstsofstarformation.Finally,in§V,wereviewthe observational evidenceforsupershellsandpropagatingstar and evolutionofasupershellcausedbysuchanassociation.In we discusstheenergyinputtoISMbyyoungstellarassoci- interstellar Hiisassumedtohavefairlyuniformdensity.In§II MS ö SN ations. In§IIIwepresentanidealizedmodelforthedynamics quences inthecontextofanidealizedmodelwhich In thispaperwedevelopideafurtherandexploreitsconse- 0 0 The mechanicalpowerimpartedtotheISMviaionizing II. evolutionOFOBassociations 1987ApJ. . .317. .190M 7 y 5 x6 7 235-1 481 6 50 51 50 -1 ß 605 a N* >10).Ifso,thecombined actionofthewindswillcreatea association containsstarswith massgreaterthan30M©(i.e.,if imparted totheISMisdominated bystellarwindsifthe years ofanOBassociation’s lifetime,themechanicalenergy density, n.Aswediscussed in §II,forthefirstfewmillion which theambientISMconsists ofgasuniformatomic stellar windsandsupernovae,wefirstconsideramodel in and theymayhavemigrated—50pcfromtheiroriginalsites. ation maybehardtorecognizebecausetheBstarsarefainter first 10yrofitslifetime;mosttheenergyisdelivered after energy availablefromtheassociationisdeliveredduring the the ionizingOstarshaveperished.Bythattime,associ- mass greaterthan7M©.Notethatonly-20%ofthetotal where N*isthenumberofstarsformedinassociation with delivered bysupernovaexplosionsfromanOBassociationas Thus, ifeachsupernovaexplosionproducesanenergyE= energy input),therateofsupernovaexplosionswillremain approximately constant:r~t,wherey=([>//?]-1)»0. function ofthe7-30M©stars(whichdominatesupernova expressions forthemain-sequencelifetimesandinitialmass main-sequence lifetime,is~10yr.Thestarmayrelease massive (~7M©)starthatcanexplode.Accordingtoour will continueuntili~5x10yr,thelifetimeofleast wind powerL=MV/2~6x10ergss(Abbott say, a35M©type07Vstar.Suchstarwillproduceionizing photons atarate~7x10s“(Panagia1973)andstellar 0 power fromtheassociationthendecreaserapidly,vanishingby ionizing radiationandstellarwindofthemostmassivestar, nova orblackhole.Theionizingradiationandstellarwind modest OBassociationformedwith,say,20typeB0-B3stars i ~5x10yr,thelifetimeoflastOstar. subsequent Wolf-Rayetphasebeforeitterminatesasasuper- another fewtimes10ergsinastrongstellarwindduring and threetypeOstars.Initially,thepowerisdominatedby 10E ergs,wemaywriteanexpressionforthemeanpower than 30M©(MStypeO). 1982), givingatotalwindenergyE~10ergsduringits sequence spectraltypeB3-B0)asstarswithmassesgreater times asmanystarswithmassesintherange7-30M©(main- estimate thatanOBassociationshouldproduceroughly9 simplicity (ourresultsarenotverysensitivetothischoice),we with velocities~5kms.Theinitialmassfunctionofsuch SN stars canbewrittendNJd(\ogMJ~M*~,where/?1.0- densed, becausethestarsareunboundandmaydriftapart SN M stars. Infact,newbornassociationsmaybeevenmorecon- w of theSunthatcontaintensOstarsandhundredsB0-B3 in aregionofdiameterlessthan~100pc(Blaauw1964; Humphreys 1978;Garmany,Conti,andChiosi1982;Heiles cally contain~20-40starswithspectraltypeearlierthanB3 1.7 (Garmany,Conti,andChiosi1982).Adopting/?=1.6for associations (cf.MillerandScalo1978).OBtypi- (Stothers 1972)andbyt~9x10yrM])“-for (MJ110 M])",wherea«1.6for7M*<30 5l 30 <80M©(Chiosi,Nasi,andSreenivasan1978). 1987), andthereareseverallargeOBassociationswithin3kpc w MS0 0 In ordertodiscussthedynamicsofasupershellcausedby By thistime,afewsupernovaexplosionshaveoccurred;they Consider theenergydeliveredtoISMbyatypical We presumethatmostOBstarsareformedinclustersor © American Astronomical Society • Provided by the NASA Astrophysics Data System 35_1 P «6.3x10ergss(N^EsJ,(1) SN III. EVOLUTIONOFSUPERSHELLS SUPERSHELLS ANDSTARFORMATION191 - x 21/ 1/2 -1 2 -3 pagate throughtheinterior. 7 381 However, n,andT¡fluctuate considerably asblastwavespro- and imately by(hereafter,exponentsareroundedofftothenearest from thermalevaporationtheshellaregivenapprox- tenth): averaged interioratomicdensityandtemperatureresulting and Ikeuchi(1981). results ofhydrodynamicalsimulationsbyTomisaka,Habe, interior. However,ourexpressionsagreefairlywellwith the nova phasebecausetheyneglectedthepressureof hot equations (3)and(4)ofBruhweileretal(1980).Thoseauthors underestimated thesizeofsupershellinrepeatedsuper- supershell exceedsubstantiallythevaluesthatfollowfrom -3025 6A (km s)intheshell.Thekineticenergy,E,ofshellisequal to 20%ofthenetsupernovaenergy(Weaveretal1977): where a=(kT/fi+B/4np)isthemagnetosonicspeed will continuetooccurinsidethesupershellfort< Note thattheshellexpandsmorerapidlythanstarsof 5(N*E/n). Thedensityintheshellisgivenby= association driftapart(at—5kms),sothatthesupernovae n(Vs/as), or and L =P,whereisgivenbyequation(1).Theresultsare of theoutershellfollowfromequation(2)withreplacement the netsupernovaenergy)isconserved,radiusandvelocity have vanished.Then,iftheenergyofhotinterior(45% tinue tohammerattheshelluntilt»5,longafterOstars s supershell willbecausedbysupernovaexplosions,whichcon- be lessthan—100pc.Inthatcase,themaingrowthof s 7 the radiusofshellatendwind-drivenphasewill and L=5.3. from CygOB2,anunusuallyrichassociationwithN*—200 510s 0 a timescalet=0.2(forn0.35cm)bythestellarwinds with R=225pc(Cashetal1980),couldhavebeencreatedin and Churchwell(1981)havearguedthattheCygnussupershell, first supernovahasoccurred.Forexample,Abbott,Bieging, alone cancreateasupershelloflargeradiusevenbeforethe wSN cal luminosityofallthestellarwindsinassociation,and t =i/(10yr).Thus,forsuchanassociationthestellarwinds where L=/(10ergss"),isthecombinedmechani- (1977), whichmaybewritten 7 supershell withradiusgivenbyequation(21)ofWeaveretal 38 70 s 7 38w n¿ —1.5x10cm(N*E)'n°’t~(lr/R)~ , 5107s According tothetheoryofWeaveretal(1977),time- Note thattheradius,velocity,andkineticenergyof However, formostOBassociations,whicharenotsorich, 602 012 7¡ ~1-1x10K(N*E)-n t~°'(l -r/R)°*.(8) 51 01s 3 n =32cm',(5) s 1/52 Vs =5.7kms'(JV*£/no)i7-•(4) 51 49 E =4.0x10ergs(A/*)t.(6) 135 s517 R =97pc(NE/n)l%i,(3) sill510 l531 = 269pc(L/n)t,(2) 3807 (7) 198 7ApJ. . .317. -1 1 2/5 4 6 expansion willbegintoaccelerate.Ifitisexpandingslowly, the becomes distortedandequation(3)isnolongervalid.If,atthis the densityscaleheight,z,ofgalacticHilayer,shell time, theshellisexpandingrapidly(comparedwith~10 km waves withinasupershellisgivenby uncertain) probabilityforcatchingoneormoresuchblast to reachtheshellisgivenbyAi~0.4R/l^,and(very E =1,i2.5,andn1.Thetimescalefortheblastwave or F~44kmsfora“typical”supershell,withA/*=20, (H andCO)layerveryearlyduringitsevolution.Using the internal pressureintothegalacticcorona(Tomisaka and sion (Bruhweileretal1980).Ineithercasethepolarcapsof the gravity ofthegalacticdiskwilldecelerateverticalexpan- s", thetypicalRMSvelocityofdiskgas),vertical (1976) fortheformationof Hongrainsanditsphoto- supershell to“burst”throughtheHilayeranddischarge its supershells willbecomeRayleigh-Taylorunstable,causing the the zero-pressuresnowplowlaw, masses ofhotinteriorgasareencounteredatr~30pc.Then dissociation bystarlight,weestimate thatsuchalayerislikely theory ofJura(1975)andHollenbach,Chu,McCray should increaseaccordingtoequation(13). Ikeuchi 1986;PaperII).Thereafter,theradiusinplane rapidly whenitencountersthehigherdensitygasnear according totheSedovlaw,roci,butthendeceleratesmore an adiabaticblastwaveisestablished,whichatfirstexpands shell, itsejectaexpandfreely,fort~10yr,untilafewsolar to developwithint~10yr. Ofcourse,thesupershellwill wave willhavevelocity supershell. Theblastwavethanmergeswiththeshell,losingits persist, aninnerskinofHn aswell.However,mostofthe always containalayerofHi and, aslongtheionizingstars al. 1980).Weestimatethatbeforeitstrikestheshellblast remaining kineticenergy(~0.28£)toradiation(Kafatoset swept-up massintheshellwill probablybemolecular. and radius cooling becomesimportantinthehotinterior,atatime or 0 (In ordertoderiveeqs.[11]and[12],wehaveusedtheradi- where Çisthemetallicity(C=1forsolarsystemabundances). i.e., P(r The adiabaticphaseofthesupershellpersistsuntilradiative SN s 6150 3 © American Astronomical Society • Provided by the NASA Astrophysics Data System -1046 i ~4x10yrC~'(N^E)n,(11) c510 V ~240kmsN^~’‘Et,(9) 90A06 51 R ~50pcC~°'(N*E)n,(12) c510 P(r (22) Bso s0 KTB t0s K0s 0H 510s 4315 M ä5x10(N^E^y^no-^W^it/t,)-,(20) The ideathatsupernovaemightinitiatestarformationhas An approximateanalyticmodelforthisinstabilitywaspro- r0 1/54 /1/571 R «100pc(JV*£)n“ V.Rioo ,(23) 150 1 15 7i8l251 M*9 xlO^Me^E,!)- ' r tj »3.2x10yr(N*E)~-na(17) 510s l81/23/ 1/456 R «200pc(N*E)n~a,(18) 9 «0A4(N*E)~a'(t/t,(19) 150s r5lsi l519 1 IV. GRAVITATIONALINSTABILITY x n~aRo(t/ti)~ * »(25) 0s1 2 0.67GpR/(Va)>l .(14) os 2 e*9a/(4GpR) (15) rs0 t ~3a/(nGpR).(16) 315 gs0 *tMtr•(2i) tg Vol. 317 1987ApJ. . .317. .190M n7 1 -1 2 _ 31/2 1 -1 1/2 23l2 a 1/27 5 and otherspiralirregular galaxiesintheLocalGroup. gas intheshellbecomesmolecularwithionizedfraction magnetic fieldtoleakoutoftheshellinatimescale H For example,ifVisgivenbyequation(4),theshellwillbecome H2 0 s bilities intheundisturbedISM.ForRoctconditionthat s 1 To seewhetherthin-shellformationcanactuallyaccelerate © American Astronomical Society • Provided by the NASA Astrophysics Data System M/Mo «O.Sias/aofias/VsY^tJt).(28) r a =\—-VsinOJ,(29) s0 1/2 Í3n \ a) Supershells V. DISCUSSION 2 ^ 19a.(27) (26) 6 x 7 6 1 7 503 and wouldhavedispersedsignificantly. recognizable cluster,because the remainingBstarsarefainter energies oftheexpandingH ishellsareconsistentwiththe theory iftheyarecreatedby OB associationswith10100pc. contain theirinternalpressurefori>1.Thisinterpretation the moreenergeticshellsfoundbyHeilescouldbeproduced pressure ofthehotinteriorsupershellisneglected.Ifa great. Wedisagreewiththatargument,becauseitisbasedon ergs) requiredtoproducethelargerexpandingshellsaretoo shells inferredfromtheirHimassesandexpansionvelocities these holes.Notealsothatthekineticenergiesofsuper- that expandingringsofCOemissionshouldbeseenaround shells werepredominantlyH;thishypothesiswouldimply ative coolingonmetallicity,C>andambientdensity,n . As the outerpartsofspiralgalaxiesisdependenceradi- through onlyonesideofthegalacticdisk. arcs mightbeinterpretedassupershellsthathaveburst spiral galaxies,asobserved(Kafatosetal1980).Thepartial solar vicinity(ShullandVanSteenberg1985)to~530pcat20 scale heightincreaseswithgalactocentricradiusowingtothe of thesupershellsendswhenshellradiusbecomescompa- shells inspiralgalaxiestendtoincreasewithgalactocentric homogeneous ambientISM,inorderthattheyremaincoher- seems torequirethatthesupershellsaredevelopinginafairly supershell isthepressure-drivenphase[eq.(3)],itskinetic the theoreticalmodelofBruhweileretal(1980)inwhich produced bymultiplesupernovaebecausetheenergies(>10 shell isH. obvious shellssurroundingthemcouldbeexplainedifthe Ç andn.Inspiralgalaxiesboth(Pageletal1979) n Milky Wayasthelimb-brightenedrimsofsupershellsthat 194 McCRAYANDKAFATOSVol.317 irregular galaxyhaslargescaleheightandlowdensity as a decrease withincreasinggalactocentricradius. metallicity thanthegiantspirals. Thus,forexample,the (N* £i/200)'fortheradius atwhichradiativelossesbecome (Hindman 1967),equation (12)givesR~2.5kpc neighboring galaxies.Theirregulargalaxiestendtohavelower the gaslayerhasbeendisturbedbytidalinteractions with result ofthelowmassgalaxy,andpossiblyalsobecause supershells inirregulargalaxies.Theinterstellargas an important. Webelievethatthese factorsmayexplainwhythe c 0 s 7 LMC, with£~0.3(Dufour 1984)andn~035cm 2 0 2 0 0 Magellanic Cloudscontainso manyspectacularsupershells. 5 c 0 Another effectthatfavorsthedevelopmentoflargershellsin Bruhweiler etal(1980)explainedwhytheradiiofsuper- Heiles (1984)hasassertedthatthesupershellscannotbe All oftheseeffectsconspiretofavorthedevelopment of © American Astronomical Society • Provided by the NASA Astrophysics Data System -3 might encounterrelatively high-density(n~1cm), by photoevaporationofclouds(Elmergreen1976;McKee, Van “ homogenization”oftheISMaroundayoungOBassociation giant shellcanforminathree-phaseISMasresult of which comeinclusters,thenmostofthecoronalgasdisk model (Liszt1983;Lockman,Hobbs,andShull1986;Kulk- primarily inlarge-scalesheetlikeratherthencloudlikestruc- waves cannoteasilycircumventthewarmH1ifitisdistributed model iscertainlynotrealisticiftheISMhasstructure ambient interstellargaswithin lessthan~50pc.Allsub- associations, onlythefirst supernova fromtheassociation Buren, andLazareff1984). mated (cf.Shull1987). factor ofholesfromsupershellsshouldbemorethan~90%in might besubstantiallylessthanthatestimatedbyMcKeeand rate inthegalacticdiskisdominatedbyTypeIIsupernovae, possibilities, notnecessarilyexclusive.First,ifthesupernova the modelrequiressomequalitativerevision.Thereareseveral arni andHeiles1987;CowieShull1987),suggestingthat pervasive andsmooththanpredictedbytheMcKee-Ostriker tures. supershells wouldbehardtoproduce.Ontheotherhand,blast right outofthediskbeforeitbecomesradiative,andcoherent blast wavefromasinglesupernovaexplosioncouldpropagate and entrainingcloudsasitdoes.Insuchamedium,eventhe propagate veryrapidlythroughthecoronalgas,overtaking low-density coronalgas.Inthatcase,thesupershellwould clouds withwarmH1mantlesareembeddedinasubstrateof envisioned byMcKeeandOstriker(1977),inwhichcool inputs arestillveryuncertainandmayhavebeenoveresti- novae mayactuallybe“Typelb”supernovaefromPopulation little impactonthediskgas.Furthermore,manyTypeIsuper- energy directlyintothegalacticcoronaandhaverelatively galactic H1disk,sothatmanyofthemwilldischargetheir probably haveasubstantiallylargerscaleheightthanthe Heiles 1987).TheTypeIsupernovaefromPopulationIIstars novae ontheISMisknottyandstillunresolved,however(cf. tion oftherelativeimpactTypeIandIIsuper- randomly distributed,willmakeanadditionalcontributionto Type IsupernovaefromPopulationIIstars,whichshouldbe vations. Furthermore,notallsupernovaeareclustered.The the solarneighborhood,muchgreaterthanindicatedbyobser- galactic corona,Heiles(1987)hasestimatedthatthecovering any case.Evenallowingfortheventingofsupershellsinto randomly distributedinspaceandtime. vent mostoftheirenergyintothegalacticcorona(cf.Cowie through thethinH1diskfairlyearlyintheirevolutionand should befoundintheinteriorsofsupershells.Moreover, supershells developinanISMoffairlyuniformdensity.Our I stars(Branch1986).Finally,supernovaratesandenergy the porosityofISMwhichmaybesubstantial.Theques- Ostriker (1977)ontheassumptionthatsupernovaeare supershells intheinnerpartsofspiralgalaxiesshouldburst 1987). Thusthevolumefractionofcoronalgasindisk 0 If, asweassume,mostsupernovae inthediskcomefrom In themodelpresentedabove,wehaveassumedthat Cowie andJeffrey(1983)havepointedoutthatacoherent However, thelowapparentporosityofISMisapuzzlein Recent observationsindicatethatthewarmH1ismore b) StructureoftheInterstellarMedium c) MissingSupernovaRemnants 1987ApJ. . .317. .190M -3 -3 -17 -3 have well-formedspiralarms, thespiraldensitywavemecha- Hunter 1984)oftenrotatenearly asrigidbodiesanddonot galaxies. SincetheMagellanic irregulars(Gallagherand that wehavedescribedin§IV islikelytodominateinirregular main triggerofstarformationinspiralgalaxies(Linand Shu tion mayoccurasasecondary mechanisminsomeinstances. (n ~1cm)ISM.Itisclearthatthedensitywavesare the necessary totriggergravitationalinstabilityinahomogeneous other kindofshock,suchasaspiral-armdensitywave)may be neighborhood. Therefore,weseethatsupershells(orsome mation. Indeed,rotationandshearinadiskgalaxy will can compressthegasbyamodestfactorwilltriggerstar for- suppress gravitationalinstabilityoflow-density(n100pc)scales. into anexistingmolecularcloudcomplex.Elmegreen(1982, 0cr seems clearthatOBstarscandriveawaveofstarformation scales hasbeendiscussedbyBlauw(1964),Elmegreenand CT Lada (1977),Lada,Blitz,andElmegreen(1979),others:It pulsars have“missing”supernovaremnants. pulsars intheMilkyWayseemstoexceedformationrateof (Kafatos etal.1980;Tomisaka,Habe,andIkeuchi1981).This 5i0s supernova remnantscontainingpulsars(HelfandandBecker phenomenon mayhelptoexplainwhytheformationrateof 1 brightness andescapedetectionbyopticalorradiosurveys 1985a, b;1986)hasdiscussedavarietyofevidenceforpropa- supernovae withinsupershellsmayhaveverylowsurface radius lessthan50pc.TheblastwavescreatedbyTypeII II supernovaetocreatewell-formedsupernovashellswith typically, say,N*~20,onewouldexpectonly~5%ofType Thus, ifallTypeIIsupernovaeareformedinassociationswith large cavityoflow-densitycoronalgas,asdiscussedin§III. has verymassivestarswithstrongstellarwinds)willoccurina No. 1,1987SUPERSHELLSANDSTARFORMATION195 sequent supernovae(andeventhefirstone,ifassociation 1984). PerhapsmostoftheTypeIIsupernovaethatproduce 7-1 Clearly, theISMinspiralgalaxiesmusthavereachedastate In contrast,themechanismfor propagatingstarformation Following MuellerandArnett(1976),GerolaSeiden Evidence forpropagatingstarformationonlocal(<50pc) ~ 10yr,implyingapropagationvelocity~20kms.We © American Astronomical Society • Provided by the NASA Astrophysics Data System d) PropagatingStarFormation - 1 7 6 -3 supershell-induced starformation willbecomecompelling. We believethatwhensuchobservations aremade,thecasefor much moredetailedandsystematic observationsofthespatial structure andrelationshipof starsandgasinnearbygalaxies. neous (“cloudy”)ISM.Perhaps moreimportant,weneed consider theoreticalmodels for supershellsinaninhomoge- of supershellsandtheirroleinstarformation,wemust also and irregulargalaxies.Inordertoassesstheactualimportance approximation totheactualevolutionofsupershellsinspiral supersonic (^15-50kms^velocitiesintheHnregions. surprisingly, theburstsofstarformationareassociated with nism isatwork(cf.Gerola,Seiden,andSchulman1980). Not statistical excursionsandsuggestthatsomeinfectiousmecha- systems seemmuchgreaterthanonemightexpectfromnormal galaxies, e.g.,IZw18andII40(SargentSearle1970; values. Thisinferenceisalsotrueforsomecompactbluedwarf Searle andSargent1972).Thestar-formationratesinthese otherwise themetallicityofgaswouldexceedobserved substantially (factor>10)greaterthanthelong-termaverage; local (~1kpc)regionsofthesegalaxies.Furthermore,insome evidence thatlargeburstsofstarformationhaveoccurredin formation historyinmostMagellanicirregularsmaybefairly Magellanic irregularsthecurrentstar-formationratemustbe steady whenaveragedoverlongtimes.However,thereisclear pc) ofOBstarsinNGC4449pointedoutbyBothun(1986). in Magellanicirregularsisthespectacularring(diameter~500 about thesametimeifa«0.4kms“. that suchashellwouldfirstbecomegravitationallyunstableat created inatime£~2x10yr,andfromequation(17)wesee equation (3),weseethatasupershellwithR~750pccouldbe culprit; withage~3x10yr,itistooyoung.)Then,from and thatn=0.1cm.(ConstellationIIIitselfcannotbethe the supershellwascreatedbyanassociationwithN*~300, IV istheresidue.Toillustratethatthispossible,assume result ofgravitationalinstabilityasupershell,whichloop N* ~300.Webelievethattheseassociationscouldbethe an IMFwithß=1.6.Thuseachassociationtypicallyhas and Feitzinger1983;Isserstedt1984),implyingN*~6000for et al1984;Dopita,Mathewson,andFord1985),hasaradius which surroundsconstellationIIIandalargeHihole(Rohlfs great arcofbrightbluestarsstretchingsome600pc. s these isconstellationIII(WesterlundandMathewson1966),a (Shapley’s “constellations”)thataresuggestiveofpropagating s star formation(BraunsfurthandFeitzinger1983;Isserstedt 0 remnants abound.Manyofthestellarassociationsin ipheries, whereOBassociations,giantshells,andsupernova dence forsecond-generationstarformationaroundtheirper- Magellanic Cloudsareorganizedintolarge-scalesystems ment ofverylargesupershells,owingtotheirlowmetallicities and extendedgasdistributions. galaxies provideparticularlyfavorablesitesforthedevelop- ~20 young(t<1.3)associations(Lucke1974;Braunsfurth 1984; Dopita1986;Feitzinger1986).Themostspectacularof ~750 pc.Itcontains~700bright(M<>—4)OBstarsin We havealreadypointedout(§Ya)thatMagellanicirregular ble, andinsomecasesfargreater,thanthattheMilkyWay. galaxies thestar-formationrateperunitgasmassiscompara- nism isnotavailabletotriggerstarformation.Yetinthese 7 v Clearly, theidealizedtheoryoutlinedhereisatbestacrude According toGallagherandHunter(1984),thestar- Another goodexampleofsupershell-inducedstarformation For example,thelargestsupershellinLMC,loopIV, Indeed, thesupershellsinMagellanicCloudsshowevi- 1987ApJ. . .317. .190M galaxy formation. protogalaxies. 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