Variable stars as tracers of stellar populations in Local Group galaxies Leo I and NGC 6822

Results are presented of a study of the variable star populations in the dwarf spheroidal galaxy Leo I and in the dwarf irregular galaxy NGC 6822, based on time series photometry obtained with the Wide Field Imager of the 2.2 m ESO/MPI telescope (Leo I) an

24L.Baldaccietal.:VariablestarsintheLocalGroup:LeoIandNGC6822

epochofstarformationwasincontrastwiththegeneraltrendoftheLocalGroupdwarfspheroidalgalaxies,allofwhichhavebeenfoundtocontainanoldstellarcompo-nent.Thepresenceofstarsasoldast>10GyrremainedunproveninLeoIuntilthestudybyHeldetal.(2000),whoreportedthe rstobservationalevidenceofawellex-tended(frombluetored)horizontalbranch(HB)intheexternalregionsofthegalaxy.Afurthercon rmationwasthe rstdetec-tionofaconspicuouspopulationsofRRLyraestarsinthegalaxy(Heldetal.2001,paperI).PaperIfocusedontheRRLyraestarsidenti edinonlytwochipsoftheWFImosaic.Herewepresentresultsonthevari-ablesdetectedinallthe8chipsofthemo-saic.

2.1.Observationsanddatareduction

Observations(40V,22Band5I15-minexposures)ofLeoIwereobtainedwiththeWideFieldImager(WFI)atthe2.2mESO/MPItelescope.The34×33ar-cmin eldofviewoftheWFIallowstofullycoverthegalaxyinjustoneexposure.Photometricreductionswereperformedus-ingDAOPHOTandALLFRAME(Stetson1994),andtheWFIreductionpipelinede-velopedbythePadovagroup(Rizzi&Held2003,inprep.).Candidatevariableswereidenti edusingISIS2.1(Alard2000),apackagebasedonanoptimalimagesub-tractionmethod.Inchip1,wherethesmallnumberofobjectsdidnotallowtorunISIS2.1,variableswereidenti edus-ingavariabilityindexrelatedtothelargescatteroftheirphotometricmeasurements.Becauseoftelescopepointingdriftsdur-ingtheobservationswehadtocutimagestoasamecommonareatoallowgeomet-ricalalignment,withalossofabout1/3intheskycoverage.Thetotalnumberofcandidatevariablesidenti edinLeoIis～1000.However,onlyabout300ofthemarepresentinboththeVandBmasterco-addedimagescreatedbyISIS.Thus,alargefractionoftheobjects aggedascandidatevariablescouldbespuriousidenti cations.

AllcandidatevariablesarebeinganalyzedwithGRATIS,acodedevelopedattheBolognaObservatory(seeClementinietal.2000)inordertochecktheiractuallightvariationandtodetermineperiods,ampli-tudes,meanmagnitudesandtypeclassi- cation,wheneverpossible.Thefullcata-logueofvariablestarsinLeoIwillbepre-sentedinaforthcomingpaper(Clementinietal.2003,inprep.).

2.2.ColorMagnitudeDiagrams

Figure1showsthecolormagnitudedia-gramsofthe8individualchipsofthemo-saic.Filledsquaresmarkthepositionofthecandidatevariables.Forapreliminaryqual-itativeanalysis,wewillconsiderhereonlychips6,7and8wheremostoftheLeoIstarsarelocated,withthegalaxycenterbeinginchip7.Thecolormagnitudedi-agramofchip7ismainlypopulatedbyanintermediateagepopulation.Theredgiantbranch(RGB)andtheredclumpstarsarethemostvisiblestructures.TheLeoIHBiscompletelyhiddenbythemainsequence,subgiantbranchandredclumpstarpopulations,andcanberecognizedthankstotheRRLyraestars(seeHeldetal.2001).SeveralothercandidatevariablesarepresentintheCMD.Themoststrik-ingfeatureisagroupofvariablesfoundinthestripabout1.4magbrighterthantheHB.TheirpositionisconsistentwiththelocationintheHRdiagramofthein-termediatemasshelim-burningpulsatingstarsthathavebeenfoundinseveralLocalGroupdwarfspheroidalgalaxies(seePritzletal.2002,andreferencestherein)andthataregenerallyreferredtoasAnomalousCepheids(Wallerstein&Cox,1984).Ontheotherhand,itseemsunlikelythatthesestarscouldbePopulationIICepheids,i.e.postHBstars(t>10Gyr,M≤1M⊙)com-ingfromthebluerpartoftheHBandcross-ingtheinstabilitystripintheirevolutiontotheAsymptoticGiantBranch.PopulationIICepheidsarecommonlyfoundinglob-ularclusterswithextendedbluetails,buthaverarelybeenfoundindwarfgalaxies.

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