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AstronomischeNachrichten,4February2013 A multiwavelength view of star-disk interaction in NGC 2264 ⋆ A.M. Cody1,⋆⋆,J.R.Stauffer1,G.Micela2,A.Baglin3,andtheCSI2264Team◦ 1 SpitzerScienceCenter,CaliforniaInstituteofTechnology,1200ECaliforniaBlvd.,Pasadena,CA91125,USA 2 INAF-OsservatorioAstronomicodiPalermo,PiazzadelParlamento1,90134Palermo,Italy 3 LESIA,UniversitPierreetMarieCurie,UniversitDenisDiderot,ObservatoiredeParis,92195MeudonCedex,France 3 ReceivedXXXX,acceptedXXXX 1 PublishedonlineXXXX 0 2 Keywords openclustersandassociations:individual(NGC2264),stars:variables:TTauristars,infrared:stars,stars: n a pre-mainsequence,accretion J Variabilityisasignaturepropertyofcoolyoungstars,particularlyforthosesurroundedbydisks.Traditionalsingle-band 1 timeseriesdisplaycomplexfeaturesassociatedwithaccretion,diskstructure,andaccompanyingstellaractivity,butthese 3 processesarechallengingtomodel.Tomakeprogressinconnectingobservedtimedomainpropertieswiththeunderlying physicsofyoungstarsandtheirdisks,wehaveembarkedonanunprecedentedmultiwavelengthmonitoringcampaign:the ] CoordinatedSynopticInvestigationofNGC2264(”CSI2264”).BeginninginDecember2011,CSI2264hasacquired30 R continuousdaysofmid-infraredtimeseriesfromSpitzer,simultaneousopticalmonitoringfromCoRoTandMOST,X-ray S observationswithChandra,aswellascomplementarydatafromanumberofground-basedtelescopes.Theextraordinary . photometricprecision,cadence,andtimebaselineoftheseobservationsenabledetailedcorrelationofvariabilityproperties h atdifferentwavelengths,correspondingtolocationsfrom thestellarsurfacetotheinnerAUofthedisk.Wepresentthe p earlyresultsoftheprogram,anddiscusstheneedforfurthermodelingeffortsintoyoungstarsandtheirdisks. - o r Copyrightlinewillbeprovidedbythepublisher t s a [ 1 The promiseofmultiwavelengthtime known for decades (e.g., Joy 1945) that young stellar ob- series monitoring ofyoung stars jects (YSOs) display prominentoptical brightness fluctua- 1 v tionsontimescalesfromdaystoyears.Lightcurvescontain 2 The canonical picture of a young accreting star (see Hart- not only regularsinusoidalpatterns, but abruptand unpre- 2 mann 1998) involves emission at a wide range of wave- dictable changesas well (e.g.,Cody& Hillenbrand2010). 0 lengths,characterizingvariousregionsfromthecentralob- Morerecently,ithasbecomeevidentthatmanyoftheseob- 0 . ject to the outerreachesof its disk. Stellarflux comespri- jectsalsoexhibitsignificantvariationsinthenear-andmid- 2 marily in the optical, with some contributions from mag- infrared,suggestive of changesin emission from the inner 0 neticspotsandflares.Wherethemagnetosphereanchorsto diskitself(Morales-Calderońetal.2011;Rebull2011). 3 1 the surface, accretion material from the disk is thought to A key question is how to connect variability with the : funnel along columns before colliding with a shocked re- physicalconfigurationandprocessesrelevanttoYSOs.Ini- v gion where ultraviolet radiation is produced. Further out, tialattemptstocorrelatetheopticalandnear-infraredtime- i X emissionlinessuchasHαarisefromionizedgasintheac- domainpropertiesofyoungstarswithmodels(e.g.,Herbst r cretion flow. The disk is heated by the centralstar, and its etal.1994;Carpenteretal.2001)haverevealedphotomet- a innermost portions (d < 1 AU) reradiate at near-infrared ric behavior that is consistent with variable accretion, hot wavelengths.Aninnerwallmaycastashadowontheouter andcoolphotosphericspots,orvariableobscurationbycir- parts, which emit in the mid to far-infrared according to cumstellar material. Yet with limited wavelength coverage thelowerdusttemperaturesfoundthere.Themeasuredflux ortemporallysparsedata,thesescenarioscouldnotbedis- at all of these wavelengthsis furtherdependenton the ob- tinguishedunambiguously.FurtherworkonclassIIsources server’s aspect angle to the star/disk system as well as on by Eiroa et al. (2002), Bary et al. (2009), and Espaillat et therotationperiodsoftheemittingregions. al. (2011)uncoverednear-IRandmid-IR fluxchangesim- While this model involves a relatively static geometry, plicatingdiskthermalandstructuralchangesontimescales youngstarsandtheirdisksconstituteanincrediblydynamic fromdaystoyears.Ithasbeenproposedthatthemorerapid environment,asborneoutbyvariabilitystudies.Ithasbeen variationsreflectchangesintheheightoftheinnerdiskwall (Hirose&Turner2011;Keetal.2012).Additionalmodel- ⋆ Based on data from the Spitzer and CoRoT missions. The CoRoT ing efforts such as those by Dullemond et al. (2003), Fla- spacemissionwasdevelopedandisoperatedbytheFrenchspaceagency herty&Muzerolle(2010),andRomanovaetal.(2011)have CNES,withparticipationofESA’sRSSDandScienceProgrammes,Aus- beguntoofferdescriptionsofinnerdiskdynamicsandstar- tria,Belgium,Brazil,Germany,andSpain. ⋆⋆ Correspondingauthor:e-mail:[email protected] disk interaction but require more extensive input from ob- ◦ http://csi2264.ipac.caltech.edu servationsonvariedtimescalesandwavelengths. Copyrightlinewillbeprovidedbythepublisher 2 A.M. Codyetal.:Amultiwavelengthviewofstar-diskinteractioninNGC2264 Table1 Opticalandinfraredspace-basedobservations insightsgleanedfromthe combinationof opticalandmid- infraredtimeseries. Telescope #oftargets Precision(mag) Cadence(min) Spitzer/map 1000 0.01–0.03 100 Spitzer/stare 540 0.001-0.01 0.1or1 3 Initialresults from CSI 2264 CoRoT 500 0.0005–0.01 8.5 MOST 67 0.001-0.01 0.4or0.85 3.1 Periodicandsemi-periodicvariability Periodic variability in YSOs is usually attributed to rota- 2 The Coordinated Synoptic Investigationof tional modulation of the light curve by magnetic spots on NGC 2264 the stellar surface. Many of our light curves display sinu- soidalvariationsinbothbands,andthisisparticularlychar- Wehaveembarkedonanunprecedentedexplorationofyoung acteristicofthetypeIII(i.e.,noinfraredexcess)objectsthat starvariabilityviahigh-precision,simultaneousopticaland arepresumedtolackdisks.Typically,theIRACamplitudes infraredtimeseriesmonitoringofYSOsinNGC2264.This are lower than those in the optical CoRoT light curve, but few-Myr-oldcluster containssome 2000knownmembers, the variationsbecomemorecomparabletowardlaterspec- many of which have disks (Rebull et al. 2002; Dahm & tral type. A handfulof objects have more than one signif- Simon 2005). It was previously monitored in the optical icant period in their light curve, suggesting that spots are (Lamm et al. 2004; Cieza & Baliber 2007) as well as a presentatmultiplelatitudes,orthatweareobservingabi- “shortrun”(23days)withtheCoRoTsatellite(Favataetal. nary system involving two active stars with different rota- 2010).Resultsfromthelatterprogramhavecontributedvi- tionperiods.A numberofeclipsingbinariesalsolie in the tallytoourunderstandingofthecomplexitiesofYSOvari- NGC 2264 field, and follow-up monitoring is expected to abilityatontimescalesfromminutestoweeks(e.g.,Alencar provide precise stellar parameters for a number of them, etal.2010,Zwintzetal.2011). along with new benchmarks for comparison with models Ourcampaign–theCoordinatedSynopticInvestigation at youngages. At the brightendof the sample, asteroseis- of NGC 2264 (“CSI 2264”)– combines the power of pre- mology of cluster δ Scuti stars will continue to unveil the cision space-based photometry with the benefits of multi- propertiesoftheirinteriors(e.g.,Zwintzetal.2011). wavelength monitoring.The programcommencedin early A subsetof objectsdisplaysemi-periodicvariabilityin December2011, with roughly30 continuousdays of mid- both bands that is not necessarily consistent with the spot infraredphotometryfrom Spitzer/IRACand40continuous modulationscenario.Muchofthisislargeamplitude(>0.1 daysofopticalmonitoringwithCoRoT,targetingthecentral magnitudes) and involves a combination of repeating fea- degreeofthecluster.Four∼1-dayblockswerededicatedto turesaswellassmalleramplitudedeviationswhichappear monitoringoftwo 5.2′ regionsneartheclustercenterwith anddisappearon∼1daytimescales.Wesuspectthatsome Spitzer’s high precision (<1%) staring mode, whereas the of this behaviormay be explainedby the periodicpassage remainingtargetswerevisitedinmappingmode(1–3%pre- of obscuring disk material by the face of the central star. cision).Complementingtheseobservationswere40daysof This was the idea put forth to explain the “AA Tau” phe- high-cadence,high-precisiontimeseriesof67ofthebright- nomenon,whichwashighlightedinapreviousCoRoTshort est cluster members with the Microvariability and Oscil- rundatasetonNGC2264byAlencaretal.(2010).Apromi- lations of STars telescope (MOST; Walker et al. 2003), as nentexampleofAATautypebehavioroccursinNGC2264 well as 350 ks (∼4 days) of Chandra/ACIS X-ray moni- memberV354Mon,whichdisplays∼0.25magnitudefluc- toring simultaneous with the Spitzer staring observations. tuationsintheopticaland∼0.06magnitudefluctuationsin Inaddition,synopticground-basedopticalandnear-infrared the infrared. The variations in the two bands mirror each dataintheU throughKbandswasacquiredsimultaneously otherquitewellwhenthosefromthe4.5µmbandarescaled withanumberofinstruments,includingR∼17,000spec- up bya factorof4.0,as seen in Fig. 1. Supposingthatthe trafromtheVLT/Flamesmulti-objectspectrographaswell brightnessvariationsareexplainedsolelybychangingdust asopticalphotometryfromtheUSNO40-inchtelescope(I extinctioninthelineofsighttothestar,wearriveatared- band),andtheCanada-France-HawaiiTelescopeMegaCam deninglaw,A4.5/AV,thatisapproximatelyfivetimeslarger (U,Rband).Thebulkofthisauxiliarymonitoringcontin- than the standard interstellar predictions(e.g., Indebetouw uedthroughFebruary2012. et al. 2005).Taking into accountthe 4.5 µm flux from the Thespace-borneinstrumentsinvolvedinCSI2264have disk itself would only raise this value further. Thereforeif ahistoryofprovidingexquisiteprecisionphotometrictime these semi-periodicchangesare indeeddue to obscuration series at minute cadences (see Table 1). Since NGC 2264 by disk material, then we can infer that its dust properties is the only young open cluster available for simultaneous aresignificantlydifferentfromthoseofISMgrains. monitoringbySpitzerandCoRoT,weexpectthecombined In contrast, other cases of semi-periodic variability in- datasettoprovideinsightsintothedynamicenvironmentof volverelativelycolorlessbrightnessfluctuations.Theopac- young stars and their disks for years to come. The project ityofobscuringmaterialmayvaryfromstartostarandplay is still in its early stages, and we present here some initial aroleindeterminingthecolortrendsintheirlightcurves. Copyrightlinewillbeprovidedbythepublisher ANheaderwillbeprovidedbythepublisher 3 10.2 5] 4. g [ 10.3 a M 10.4 CoRoT optical, scaled by 0.25 Spitzer 4.5 microns 10.5 0 10 20 30 40 HMJD-55896.5 Fig.1 TheopticallightcurveofNGC2264memberV354MonfromCoRoT(blackcurve),withfluxscaledbyafactor of0.25andshiftedsoastohavethesamemedianoftheSpitzer/IRAC4.5µmdata.Thetwolightcurvesdisplayvariability thatiswellbutnotperfectlycorrelatedontimescalesof ∼5days.Thescalefactorissignificantlydifferentfromtheratio ofA4.5/AV expectedfromaninterstellarextinctionlaw(∼0.05). 10.4 10.6 5] 4. g [ 10.8 a M 11.0 CoRoT optical Spitzer 4.5 microns 0 10 20 30 40 HMJD-55896.5 Fig.2 Anexampleofuncorrelatedvariabilityinopticalandmid-infrared.TheCoRoTdataismedianshiftedtomatch theSpitzerzeropoint.ThecontrastbetweenthisbehaviorandthehighdegreeofcorrelationinFig.1mightbeaccounted forbyadifferentaspectangletothediskand/oralargerfractionofthe4.5µmluminositycomingfromtheinnerdisk,as opposedtothecentralstar. 3.2 Aperiodicvariability thisclassvariesfromlarge-amplitudeexcursionson3–5day timescalestomoresmoothlychangingbrightnessonlonger Themajorityofdisk-bearingstarsinNGC2264displaysig- timescales. In cases for which fluctuations take place over nificant variability at both optical and mid-infrared wave- severaldays,variabilitybehaviorevolvesfasterthanthedy- lengths thatcontainsaperiodic features. In most cases, the namicaltimescaleofthe innerdisk.Thismid-infraredflux behaviorinthetwobandsappearsatleastsomewhatcorre- variation may involvea combinationof structural, dynam- lated. This is particularly evident for YSOs whose optical ical, and thermal variations in the disk but currently lacks lightcurvesdisplaydeep(>0.1mag),semi-periodicfading moredetailedexplanation. eventsconsistentwiththe AATau phenomenon.However, wealsoidentifymoreextremecasesofvariability,inwhich Many of the cluster members monitored are accreting, thebrightnessinboththeopticalandmid-infraredvariesby basedonstrongHαemissionandU-bandexcesses.Theac- morethan20%,butfluctuationsappearcompletelyuncorre- cretionprocesslikelyproceedsinbursts,andchangesinac- latedatthesetwodifferentbands.Wepresentanexamplein cretionluminositymaybeanothersourceofstochasticvari- Fig.2.Thelargestamplitudeinfraredvariabilityintheseob- ability inYSOs. We tentativelyidentifya numberofcases jectsoccursonlongertimescales(5daysormore)thanthat for which the optical brightness undergoes abrupt bright- seen in the semi-periodicobjects.Mostof theopticallight ening events consistent with accretion changes. Flux out- curves, on the otherhand, contain high amplitude dip-like burststypicallylastseveraldays,withlowerlevelstructure orundulatingfeatureson1–5daytimescales. onshortertimescales.Nearlyalloftheseobjectsdisplaysig- Afurthersubclassofvariablesexhibitslarge-amplitude nificantUVexcesses,asinferredfromourCFHT/Megacam variation (>0.2 magnitudes) in the IRAC bands but rela- U-band dataset. Figure 4 illustrates an example of behav- tivelylittlevariationasseenbyCoRoT.Anexampleisshown iorthatweattributetoaccretion.WhereIRACphotometry inFig.3.Theappearanceofthemid-infraredlightcurvesin isavailableandunsaturated,themid-infraredfluxtypically Copyrightlinewillbeprovidedbythepublisher 4 A.M. Codyetal.:Amultiwavelengthviewofstar-diskinteractioninNGC2264 10.6 5] 10.7 4. g [ Ma 10.8 CoRoT optical 10.9 Spitzer 4.5 microns 0 10 20 30 40 HMJD-55896.5 Fig.3 Anexampleinwhichmid-infraredvariabilityappearswithlargeamplitude,whereasopticalbrightnessfluctuations occuratamuchlowerlevel. 9.0 9.2 5] 4. g [ 9.4 a M 9.6 CoRoT optical Spitzer 4.5 microns 9.8 0 10 20 30 40 HMJD-55896.5 Fig.4 NGC2264memberforwhichabruptincreasesinopticalbrightnessaresuggestiveofaccretionbursts. displayssimilarabruptincreases,probablyreflectiveofdisk Bary,J.S.,Leisenring,J.M.,Skrutskie,M.F.:2009,ApJ706,168 heatingfollowinganincreaseinaccretionluminosity. Carpenter, J. M., Hillenbrand, L. A., Skrutskie, M. F.: 2001, AJ 121,3160 Cieza,L.,Baliber,N.:2007,ApJ671,605 4 Summary Cody,A.,HillenbrandL.:2010,ApJS191,389 Dahm,S.,Simon,T.:2005,AJ129,829 The CSI 2264 project offers great potential to unlock the Dullemond, C.P.,vandenAncker,M.E.,Acke, B.,vanBoekel, multiwavelengthtime-domainpropertiesofYSOsacrossa R.:2003,ApJ594,47 Eiroa,C.,etal.:2002,A&A384,1038 range of stellar and disk properties. Further investigation Espaillat,C.,etal.:2011,ApJ,728,49 of this high-precision, high-cadencedataset is expected to Favata,F.,Micela,G.,Alencar,S.,Aigrain,S.,Zwintz,K.:2010, shedlightonthephysicalmechanismsofvariabilityandpo- HighlightsinAstronomy15,752 tentially revealthe propertiesof otherwise inaccessible in- Flaherty,K.,Muzerolle,J.:2010,ApJ719,1733 nerdiskregions.Theeventualinclusionofhigh-resolution Hartmann,L.:1998,Accretionprocessesinstarformation,Cam- spectroscopicdataforasubsetoftargets,aswellasphotom- bridgeUniversityPress,CambridgeUK etry at other bands from the X-ray through near-infrared, Herbst,W.,Herbst,D.K.,Grossman, E.J.,Weinstein,D.:1994, will enhance these efforts. Ultimately, reduced data from AJ108,1906 Hirose,S.,Turner,N.J.:2011,ApJ732,30 the campaignwill be madepubliclyavailable froma web- Indebetouw,R.,etal.:2005,ApJ619,931 site hosted at the NASA/Caltech Infrared Processing and Joy,A.H.:1945,ApJ102,168 AnalysisCenter. Ke,T.T.,Huang,H.,Lin,D.N.C.:2012,ApJ745,60 Lamm, M. H., Bailer-Jones, C. A. L., Mundt, R., Herbst, W., Acknowledgements. Thisworkisbasedonobservationsmadewith Scholz,A.:2004,A&A215,125 the CoRoT satellite and the Spitzer Space Telescope. Spitzer is Morales-Calderoń,M.,etal.:2011,ApJ733,50 operatedbytheJetPropulsionLaboratory,CaliforniaInstituteof Rebull,L.etal.:2002,ApJ123,1528 Technology under a contract with NASA. Support for this work Rebull,L.2011:PASP448,5 wasprovidedbyNASAthroughanawardissuedbyJPL/Caltech. Romanova,M.M.,Ustyugova,G.V.,Koldoba,A.V.,Lovelace,R. V.E.:2011MNRAS416,416 References Walker,G.,etal.:2003,PASP115,1023 Zwintz,K.,etal.:2011,ApJ729,20 Alencar,S.,etal.:2010,A&A,519,88 Copyrightlinewillbeprovidedbythepublisher