arXiv:astro-ph/9905030v1 5 May 1999ToappearinAstronomicalJournal
InfraredPhotometryofRedSupergiantsinYoungClustersintheMagellanic
Clouds
StefanC.Keller
ResearchSchoolofAstronomyandAstrophysics,TheAustralianNationalUniversity,Weston
CreekP.O.,ACT2611,Australia.
ABSTRACT
Wepresentbroad-bandinfraredphotometryfor52late-typesupergiantsinthe
youngMagellanicCloudsclustersNGC330,NGC1818,NGC2004andNGC2100.
Standardmodelsareseentodifferinthetemperaturetheypredictforthered
supergiantpopulationontheorderof300K.Itappearsthatthesedifferencesmost
probablyduetothecalibrationofthemixing-lengthparameter,αP,intheoutermost
layersofthestellarenvelope.DuetotheapparentmodeldependentnatureofαPwe
donotquantitativelycompareαPbetweenmodels.Qualitatively,wefindthatαPdecreaseswithincreasedstellarmasswithinstandardmodels.Wedonotfindevidence
forametallicitydependenceofαP.
Subjectheadings:galaxies:MagellanicClouds-stars:late-type-infrared:stars
1.Introduction
TheyoungclustersNGC330intheSMCandNGC1818,NGC2004andNGC2100have
beenthefocusofseveralpreviousstudiesaimedatunderstandingtheevolutionofintermediate
massstarsinalowmetallicityenvironment.Theseclustersarestandouttargetsforsuchstudies
giventheirrelativerichness.Theypresentasignificantsampleofstarsinevolvedblueandred
supergiantphases.
Athoroughexaminationoftheevolutionofintermediatemassstarsrequiresamatchingof
allpertinentfeaturesoftheobservedCMD.Thelocationoftheredsupergiantpopulationonthe
CMDisafundamentalinputintoanysuchstudy.WehaveobtainedHST/WFPC2photometryin
thevisibleandfar-UVforthesefourclusters(Kelleretal1999c).Withthisdatawewillclosely
examinethecompatibilitybetweenourobservationsandthepredictionsofstandardevolutionary
modelsinafuturepaper.Itisthegoalofthepresentworktoobtainaccuratetemperaturesand
luminositiesfortheredsupergiant(RSG)populationthroughanexaminationoftheirIRcolours.–2–
PreviousIRstudieswithintheseclustershavebeenlimited.AsampleoftheoutlyingRSGs
fromNGC330,2004and2100wereobservedbyMcGregor&Hyland(1984).AsimilarsamplewasalsotakenbyCarneyetal.(1985)inNGC330.Thesestudiesdrewuponthederivedtemperatures
andluminositiesoftheseRSGsasevidenceforalowmetalabundanceinNGC330.Subsequent
discussionhasreliedupontheseIRcoloursandtheB−VcoloursofRobertson(1974).
TheobservedpropertiesoftheRSGpopulationhasfeaturedprominentlyinmanyofthe
subsequentdiscussionsoftheevolutionaryimplicationsofthepopulationsoftheseclusters.Many
ofthesediscussionshavetosomeextentbeensupersededwiththeintroductionofnewopacities
andupdatedinputphysics.Thefirstdetailedapproachtoconstrainingthestellarevolutionary
modelsforintermediatemassstarswasmadebyStothersandChin(1992)intheirstudyofNGC
330.Theysetouttoconstrainthecriterionforconvectionandsemiconvectionandthedegreeof
convectivecoreovershoot.TheyfoundthatonlywiththeimplementationoftheLedouxcriterion
couldRSGsbeproducedatlowmetallicities.Theyusedtheluminositydifferencebetweenthe
blueandRSGstoruleoutsignificantamountsofconvectivecoreovershoot(d/Hp<0.2).Inthe
studyofNGC330byChiosietal(1995)itwasshownthatwithnewopacitiesandinputphysicsit
waspossibletoproducebluesupergiantswiththeSchwarzschildcriterionatlowmetallicity.Their
modelswereunabletomatchboththeMSturnoffandtheRSGluminositiessimultaneously;they
tookthistoindicatethattherewasasubstantialagespreadwithinthecluster.Inthestudyby
CaloiandCassatella(1995)ofNGC2004attentionwasdrawntothefactthattheRSGsappeared
underluminousrelativetotheupperMS.Thisresult,itwasacknowledged,wasnotdefinitiveand
requiredtheacquisitionofmoreaccuratetemperaturesfortheclusterRSGs.
TheRSGsofthepresentstudyprovideausefulcalibrationpointforthedegreeofconvection
instellarenvelopes.Thestandardtreatmentiswithinthemixing-lengththeory(seeforexample
Bohm-Vitense1958).Thiscontainstheaverageeddymixinglengthparameter,αP,whichis
variedtoachieveamatchbetweenobservedeffectivetemperaturesandthosepredictedbystellar
models.TheprecisevalueofαPisdifferentfrommodeltomodel;howeveritistypicallytreated
asaconstantoverawiderangeofmasses,luminositiesandmetallicities.Amongstlowmassstars
therearenosignsofadependenceofαPonstellarmass.ThestudyofStothersandChin(1995)
ofgalacticredgiantsandredsupergiantsinthemassrangeof5-13M⊙hasshownthatthere
isasignificantdecreaseinαPwithincreasedmass.Theysoughttoextendthisanalysistothe
lowermetallicityregimeavailablewithinNGC330toinvestigatethetrendofαPwithmetallicity
(StothersandChin1996).Thisstudywasinconclusiveinthisregardduetotheuncertainty
introducedbythedegreeofreddeningtotheclusterwhichhasbeenasourceofmuchdebate.We
claimthatcurrentdeterminationofthereddeningtoNGC330ismoreconvergentthandiscussed
inStothersandChin(1996).WequalitativelydiscusssomeoftheimplicationsfortrendsofαPwithmetallicityandstellarmass;however,aquantitativestudyofαPisbeyondthescopeofthe
currentworkandasweshallseemaynotbepossiblewithpresentstellarevolutionarymodels.
