Search for pulsating PMS stars in NGC 6383

A search for pulsating pre-main sequence (PMS) stars was performed in the young open cluster NGC 6383 using CCD time series photometry in Johnson B & V filters. With an age of only ~1.7 million years all cluster members later than spectral type A0 have not

2K.Zwintz

graphicstudyinacircularareaof～12arcminutesradiusandcouldcon rmEggen’sresults.Thecoreoftheclus-terwasinvestigatedbyFitzgeraldetal.(1978)usingpho-toelectricphotometrytoderiveanaveragecolourexcessE(B V)=0.33±0.02,aclusterdistanceof1.5±0.2kpc,anapparentdistancemodulusV MV=11.90±0.25cor-respondingto(m M)0=10.9(adoptedforouranalysis)andaclusterageof1.7±0.4Myr.Thespectralenergydis-tributionsofstarsinthecentralpartofNGC6383werestudiedbyTh´eetal.(1985)withphotoelectricphotome-tryintheWalravenWULBV,CousinsVRIandNear-IRJHKLMphotometricsystems.TheirderivedcolourexcessE(B V)=0.3±0.01anddistanceof1.4±0.15kpcagreewellwiththevaluesbyFitzgeraldetal.(1978).VandenAn-cker,Th´e&deWinter(2000)investigatedthecentralpartofthisclusterusinglow-resolutionCCDspectroscopyandcon- rmedthatallstarslaterthanA0lieabovetheZAMS.Theyalsoreportthatseveralclustermembersshowaninfraredex-cess,indicativeofthepresenceofcircumstellardust,heatedbythecentralstar.TheyfoundHαinemissiononlyfortheirstar#4andidenti editasanewHerbigAestar.

AllthesecharacteristicsmakeNGC6383anidealtargetforthesearchofPMSpulsatingstars.

3OBSERVATIONS&DATAREDUCTION

CCDphotometrictimeseriesinJohnsonB&V lterswereobtainedwiththe0.9mtelescopeattheCerroTololoInter-americanObservatory(CTIO),Chile,fromAug11toAug24,2001,usingthe2084x2046SITeCCDchip,whichpro-videsa eldofview(fov)of～13x13arcminutes.Intotal,53.25hoursphotometrycouldbeacquiredwithin8clearnights.

Thebasicreductions(biassubtraction, at- elding)wereperformedusingstandardIRAFroutines.TheMultiObjectMultiFrame(MOMF)softwaredevelopedbyKjeld-sen&Frandsen(1992)wasusedtoextractthephotometricsignal.MOMFisoptimizedtoanalyzephotometrictimese-ries(i.e.alargenumberofCCDframespernight)ofsemi-crowded eldsbycombiningpoint-spreadfunction ttingandaperturephotometry.MOMFdeterminesabsoluteandrelativemagnitudesofeachstaridenti edontheframesandtheircorrespondingstandarddeviations.Theabsoluteval-uesareraw,uncorrected,instrumentalmagnitudes,whereastherelativelightcurvesaredeterminedbysubtractingaweightedmeanofallstarsontheframe.Variableandnon-variable,extremelyredorbluestarsareusedtodeterminetheweightedmean,

requiringcolour-dependentextinctioncorrections(seeSect.3.1).

286starshavebeenidenti ed(seeFig.1)forwhichlightcurvesusingtheoptimumapertureproducingminimalpoint-to-pointscatterweregenerated.Nightlymeansweresubtractedtocorrectforzero-pointchangesandlong-termirregularlightvariationswhichprobablyareduetovariable～pxtheliteraturearegivenaccordingtothepublicationsbyFitzgeraldetal.(1978),e.g.NGC6383#F4,byTh´e(1965),e.g.NGC6383#T47,andEvans(1978),e.g.NGC6383#EV281.3.1

Colour-dependentextinction

Asystematice ectwasencounteredfordi erentiallightcurvesofsomestars.Towardstheendofthenightssomestarsbecamecontinuouslybrighter,butothersfainter.ThecorrespondingBouguerplots(i.e.magnitudevs.airmass)showedthatdi erentcoloursweretheexplanation.Hence,theextinctioncorrectionhadtoincludealsothecolour-dependentcoe cientk′(Sterken&Manfroid1992)in:m=m0 (k0+k′·CI)·X,

(1)

wherem0istheuncorrectedmagnitude,Xistheairmassandk0theprincipalextinctioncoe cient.Inourcase,thecolourindexCIwastakenas(B V).

The(B V)valuesavailableintheliteraturefor97starsshowaclearcorrelationwiththeslope,k,oftheBouguerplots(seeFig.2).Therelationbetweenthe97starswith(B V)fromtheliteratureandtheinstrumental(B V)valuesfromourobservationsismodelledbyaninversesecond-orderpolynomial(solidlineinFig.2).Thethreepolynomialcoe cientsevaluatetoa0= 0.712±0.052,a1=+0.692±0.084anda2=+0.177±0.029.(B V)instrvaluesforallstarscouldthenbetransformedtothestan-

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