Hardy MR Schreiber SG Parsons C Caceres G Retamales Z Wahhaj D Mawet H Canovas L Cieza TR Marsh MCP Bours VS Dhillon A Bayo Received accepted To appear in ApJ Letters Based on observations collected at the European Southern Observatory Ch ID: 78799
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{2{ABSTRACTVariationsofeclipsearrivaltimeshaverecentlybeendetectedinseveralpostcommonenvelopebinariesconsistingofawhitedwarfandamainsequencecom-panionstar.Thegenerallyfavouredexplanationforthesetimingvariationsisthegravitationalpullofoneormorecircumbinarysubstellarobjectsperiodicallymovingthecentreofmassofthehostbinary.Usingthenewextreme-AOinstru-mentSPHERE,weimagetheprototypeeclipsingpost-commonenvelopebinaryV471Tauinsearchofthebrowndwarfthatisbelievedtoberesponsibleforvariationsinitseclipsearrivaltimes.WereportthatanunprecedentedcontrastofmH=12:1ataseparationof260maswasachieved,butresultedinanon-detection.Thisimpliesthatthereisnobrowndwarfpresentinthesystemunlessitisthreemagnitudesfainterthanpredictedbyevolutionarytrackmodels,andprovidesdamagingevidenceagainstthecircumbinaryinterpretationofeclipsetimingvariations.InthecaseofV471Tau,amoreconsistentexplanationisof-feredwiththeApplegatemechanism,inwhichthesevariationsareprescribedtochangesinthequadrupolemomentwithinthemain-sequencestar.Subjectheadings:stars:individual(V471Tau)|planet-starinteractions|binaries:eclipsing|browndwarfs|whitedwarfs|binaries:close {4{modelemployedtoexplaintheeclipsetimingvariations(ETVs)seeninthePCEBNNSer(Beuermannetal.2010)successfullypredictedneweclipsearrivaltimes(Beuermannetal.2013;Marshetal.2014),providingsupporttothecircumbinaryinterpretationbutraisingquestionsregardingtheformationofthesethirdobjects.Zorotovic&Schreiber(2013)favourascenarioinwhichthecircumbinaryobjectsformasaconsequenceofthecommonenvelopeevolution,inaso-calledsecondgenerationscenario.ThisisbasedonthendingthatnearlyallPCEBswithlongenoughcoverageshowETVs,yetonlyasmallfractionofmainsequencebinariesseemtohostcircumbinarysubstellarobjects.Indeed,Schleicher&Dreizler(2014)wereabletodevelopamodelinwhichasecondgenerationprotoplanetarydiskformsduringcommonenvelopeevolutionandproducesgiantplanetsthroughthediskinstabilitymodel.Incontrast,Bear&Soker(2014)prefertherstgenerationscenario,inwhichtheobjectsformatasimilartimetotheirmain-sequencehosts,andsurvivethecommon-envelopephase.Theyclaimthatifasecondgenerationscenarioweretrue,toolargeafractionofthecommonenvelopemasswouldhavetoformintosubstellarcompanions.However,beforeinvestigatingpossibleformationscenariosfurther,wemustexercisecautionwiththethirdbodyhypothesis.AlthoughthecircumbinaryobjectmodelhasprovedsuccessfulinthecaseofNNSer,thisisanexception.IngeneralthepredictionsfromproposedplanetarysystemsaroundPCEBsdisagreewithmorerecenteclipsetimingmeasurements(Parsonsetal.2010b;Boursetal.2014),andsomeoftheproposedplanetarysystemsaredynamicallyunstableonveryshorttimescales(Wittenmyeretal.2013;Horneretal.2012,2013).ThefailureofallcircumbinaryobjectmodelsexcepttheoneforNNSerimplieseitherthatourtimingcoverageisinsucient,orthattheremustbeanalternativemechanismdrivingETVs.Toprogresswiththissituation,ithasbecomevitalthatthecircumbinarycompanion {6{thetelescopeandusedexposurestimesof5s.TherewerenosuitablybrightnearbystarsinULTRACAM'seldofviewtouseforcomparison.Wethereforeusedthei0bandobservations,wheretheeclipseisextremelyshallow,asacomparisonsourcefortheu0banddata.Thisresultsinalargecolourtermintheresultinglightcurve,butdoesremovevariationsintheconditionsanddoesnota ecttheeclipsetimings.Insomeobservationstheconditionsweregoodenoughthatnocomparisonsourcewasrequired.ThesedatawerereducedusingtheULTRACAMpipelinesoftware(Dhillonetal.2007)andtheresultanteclipselightcurveswerettedwithadedicatedcodedesignedtotbinariescontainingwhitedwarfs(Copperwheatetal.2010).ThemeasuredeclipsetimeswerethenheliocentricallycorrectedandarelistedinTable.1.2.2.SPHEREobservationsTheimagingdatatestingtheexistenceofthebrowndwarfwereacquiredusingtheextremeadaptiveopticsinstrument,SPHEREattheUT3NasmythfocusoftheVLT,on2014December11.Anearliersetofobservationswasperformedon2014December6,butisnotusedherebecauseofpoorerdataquality.SPHEREisabletoprovideHandK-bandimageswithStrehlratios�90%.Theintegraleldspectrograph(IFS)andinfrareddual-bandimagerandspectrograph(IRDIS)wereusedsimultaneouslyintheIRDIFSTable1.ULTRACAMeclipsetimesforV471Tau CycleHMJD(TDB)Uncertainty(seconds) 28485512.28405841.7628865532.08898851.5929115545.11859421.6229155547.20336082.37 {8{(e.g.Irwin1959)wherecisthespeedoflightanda12isthesemi-majoraxisofthebinarystar'sorbitaroundthecommoncenterofmassofthetriplesystem.Theotherparametersdenetheorbitofthethirdbody,i.e.itsinclinationi3,theorbitaleccentricitye3,argumentofperiastron!,andtrueanomaly3.AsshownbyMarshetal.(2014),strongcorrelationscanexistbetweenorbitalparametersandtheproblemishighlydegenerateunlessalargenumberofhighprecisioneclipsetimingmeasurementsareavailable.OnlyourrecentULTRACAMmeasurementsprovidepreciseeclipsetimings,withuncertaintiesof1.8s,whereasthetimingsintheliteraturehavebeenassignedlargeerrorestimatesof15sforthesakeofcaution.ToproperlyidentifynotonlythebesttparametersbutalsotheiruncertaintiesweperformedaMarkovChainMonte-Carlo(MCMC)simulationtotheeclipsetimes.Thepredictionofthebest-tmodelcanbeseeninFig.1,(topleftpanel)alongsideallarchivalobserved-minus-calculatedeclipsetimes(Kundra&Hric2011)andthenewtimesreportedinthispaper.Thisbest-tmodelcorrespondstoabrowndwarfofmass0.0440.001Mandsemi-majoraxis12.80.16AU.Whiletheparametersofthebrowndwarfintheonebodytarewellconstrainedbytheeclipsetimes(Fig.2),theresidualsarefarfromrandomandsuggestanothermechanismmayalsobeatwork.Totestthispossibility,weperformedanotherMCMCwith2companionstoaccountforthesedeviations.Thisfurtherallowedustotestwhetherthebrowndwarfcausingthemainvariationcouldbeatasmallerseparationorbelessmassive,whichwouldmakeithardertodetect.TheresultingbesttisshowninthebottomleftpanelofFig.1.ThederivedorbitalparametersforbothcaseswerethenprojectedontotheskyusingadistancetoV471Tauof50pc,asmeasuredbyHipparcos(vanLeeuwen2007),toobtainapredictedseparationforthebrowndwarfcompanioninDecember2014.Thesimulation