A newly identified Luminous Blue Variable in the galactic starburst cluster Westerlund 1

We present observations of the massive transitional star W243 in Westerlund 1. We find an apparent spectral type of early-mid A from our data, in contrast to an earlier classification of B2I, made from data obtained in 1981. The concurrent development of a

Astronomy&Astrophysicsmanuscriptno.

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Anewlyidenti edLuminousBlueVariableinthegalactic

starburstclusterWesterlund1

arXiv:astro-ph/0312058v1 2 Dec 2003J.S.Clark1andI.Negueruela212DepartmentofPhysicsandAstronomy,UniversityCollegeLondon,GowerStreet,London,WC1E6BT,England,UKDpto.deF´ sica,Ingenier´ adeSistemasyTeor´ adelaSe nal,UniversidaddeAlicante,Apdo.99,E03080Alicante,SpainReceived/AcceptedAbstract.WepresentobservationsofthemassivetransitionalstarW243inWesterlund1.We ndanapparentspectraltypeofearly-midAfromourdata,incontrasttoanearlierclassi cationofB2I,madefromdataobtainedin1981.Theconcurrentdevelopmentofarichemissionlinespectrumsuggestsaveryhighmasslossrate;thecontinuedpresenceofHeiemissionsuggestingthattheunderlyingstarremainssigni cantlyhotterthanimpliedbyitsapparentspectraltype.WesuggestthatW243isaLuminousBlueVariableundergoinganeruptivephase,leadingtoanextrememasslossrateandtheformationofapseudophotosphere.Keywords.stars:evolution-stars:variables:general-winds,out ows-supergiants1.IntroductionBeforebecominghydrogendepletedWolfRayets(WR),massivestarspassthroughaperiodofenhancedmassloss,whichremovesthebulkoftheH-richmantleovertheircores.However,passagethroughthe‘zoo’oftransitionalobjects-RedSupergiants(RSGs),YellowHypergiants(YHGs),LuminousBlueVariables(LBVs)andsgB[e]

stars-isatpresentpoorlyunderstood,largelyduetothe

rarityofsuchobjects.Nevertheless,allsuchobjectsap-

peartosharecertaincharacteristics,notablymassloss

ratesof≥10 5M⊙yr 1,signi cantlyslower,denserwinds

thaneithertheirOstarprogenitorsorWRprogenyand

highlyvariablesurfacetemperaturesandradii.Indeed,

signi cantvariabilityacrossallobservablephysicalparam-

etersappearstobethede ningcharacteristicofthisbrief

episodeofstellarevolution.Clearly,observationsofsuch

starsinclustersandtheirsubsequentplacementinHR

diagrams-fromwhichagesandprogenitormassescould

beinferred-wouldgreatlyenhanceourunderstanding.

Unfortunately,examplesofsuchclustersareunderstand-

ablyrareandpresentlyprovideratherpoorevolutionary

constraints.

Westerlund1(henceforthWd1;Westerlund1961)is

ahighlyreddenedclusterfoundatadistanceof～2.5kpc