A search for hot massive extrasolar planets around

arXiv:astro-ph/0502376v118Feb2005AsearchforhotmassiveextrasolarplanetsaroundnearbyyoungstarswiththeadaptiveopticssystemNACO1E.Masciadri,R.Mundt,Th.Henning,C.AlvarezMax-PlanckInstitutf¨urAstronomie,K¨onigstuhl17,D-69117Heidelberg,Germanye-mailcontact:[e-mail]´esLaboratoriodeAstrof´ısicaEspacialyF´ısicaFundamental,INTA,Apdo.Postal50727,E-28080Madrid,SpainABSTRACTWereportonasurveydevotedforthesearchofexo-planetsaroundyoungandnearbystarscarriedoutwithNACOattheVLT.Thedetectionlimitfor28amongthebestavailabletargetsvs.theangularseparationfromthestarispresented.Thenon-detectionofanyplanetarymasscompanioninoursurveyisusedtoderive,forthefirsttime,thefrequencyoftheupperlimitoftheprojectedseparationplanet-stars.Inparticular,wefindthatin50%ofcases,no5MJplanet(ormoremassive)hasbeendetectedatprojectedseparationslargerthan14AUandno10MJplanet(ormoremassive)hasbeendetectedatprojectedseparationslargerthan8.5AU.In100%ofcases,thesevaluesincreaseto36AUand65AUrespectively.Theexcellentsensitivityreachedbyourstudyleadstoamuchlowerupperlimitoftheprojectedplanet-starseparationcomparedwithpreviousstudies.Forexample,fortheβPictorisgroup,(∼12Myr),wedidnotdetectany10MJplanetatdistanceslargerthan15AU.Apreviousstudycarriedoutwith4mclasstelescopesputanupperlimitfor10MJplanetsat∼60AU.Forourclosesttarget(V2306Oph-d=4.3pc)itisshownthatitwouldbepossibletodetecta10MJplanetataminimumprojectedseparationfromthestarof1AUanda5MJplanetataminimumprojectedseparationof3.7AU.Ourresultsarediscussedwithrespecttomechanismsexplainingplanetformationandmigrationandtoforthcomingobservationalstrategiesandfutureplanetfinderobservationsfromtheground.Subjectheadings:binaries:close—planetarysystem—stars:low-mass,browndwarfs1.IntroductionAtthepresenttimemorethan100exo-planetsareknownwhichhavebeenmostlydiscoveredbytheindirectmethodofthevariationoftheradialvelocityinducedbyplanetsorbitingaroundthecentralstar2.Itcanbeshownthatyoungplanets1BasedonobservationscollectedattheEuropeanSouthernObservatory,Chile.Program70.C−0777D,70.C−0777Eand71.C−0029A2GenevaChronologyCatalog2004-(AO)imag-ingtechniquesat8-10metertelescopesiftheyaresufficientlydistantfromtheirparentstar.Sinceitisreasonabletoassumeacoevalitybetweentheparentstarandtheplanet,wecanretrievefromat-mosphericalmodels(Burrowsetal.1997,Baraffeetal.2003)thatplanetshavingamassintherange3-10MJandorbitingaroundyounglate-typestars(10-200Myr)haveatypicalbrightnesscontrastwithrespecttotheparentstaroftheor-derof102-106,correspondingtoamagnitudedif-ferenceofΔMof5-15mag.Thismeansthata1planetcanbedetectedatafewtensofAUsfromparentstarshavingdistances≤50pc.Firstre-sultsonthesearchforyoungandmassiveplanetsarereportedbyNeuh¨auseretal.(1997)using4mclasstelescopesandMacintoshetal.(2001)andKeisleretal.(2003)using10mclasstelescopes.Recently(Chauvinetal.2004),acompanionof∼5±2MJwasdetectedataround50AUfromthecentralstar2M1207,averylatetypestar(M8)inTWAHydrae(∼10Myr).Thisobjectis,atthepresenttime,themostinterestinggiantplanetcandidateeverdetectedandfurtherconfirmationoftheorbitalmotionisneeded.Fromthepointofviewofthescientificmoti-vationsrelatedtothiskindofsearchitiscer-tainlyanimportantchallengetoimagedirectlyaplanetaroundanotherstar,sinceitcanpro-videadirectevidencefortheexistenceofsuchaplanet.Atthepresenttimeonlyafewexo-planetshavebeendetectedwiththephotometrictechniquecalled’transit’(HD209458b-Charbon-neauetal.2000,OGLE-TR-113(Konackietal.,2004,Bouchyetal.2004),OGLE-TR-56(Torresetal.,2004),OGLE-TR-132(Bouchyetal.2004),OGLE-TR-111(Pontetal.,2004)andTrRes-1(Alonsoetal.,2004)).Moreover,directimagingiscomplementarytootherdetectionmethodsliketheradialvelocityvariationtechnique.Thelatterissensitivetoobjectsorbitingatclosedistancesfromtheparentstar(theradialvelocityinducedbythepresenceofacompanionisindeedpropor-tionaltotheinverseofthesquarerootoftheor-bitaldistance)whiletheAOimagingpresentedherewouldbesensitivetoplanetsorbitingatmuchlargerdistances.Accordingtostatisticalresultsretrievedbyra-dialvelocitystudies(Marcyetal.2003),mostoftheknownexo-planetsorbitatdistancessmallerthan1AUandthesemi-majoraxesofexo-planetsfoundsofararenotlargerthan∼6AU.Sincetheradialvelocitytechniqueisheavilybiasedtowardsshortseparations,thederivedmajor-axisdistribu-tioniscertainlynotrepresentative.Notmuchisknownatthepresentaboutpotentialplanetsthatcouldexistatdistanceslargerthanabout6AU.Wenotethattheformationofexo-planetsisquiteunlikelyatdistancesofafewtensofAU.Attheselargedistancesthedensityoftheplanetesimalsislowandthegravityfromthecentralstarisweak.Thismakestheprocessofplanetformationineffi-cientandslow(Thommesetal.,1999,2002).How-ever,differentmodelspredictthetransfer(jump)ormigrationofplanetstodistanceslargerthan5-10AUfromthecentralstars.Amongthesemod-els,werecallthegravitationalscatteringmodelwhichclaimsthatinmulti-planetarysystems,duetogravitationalinteractionsbetweentheplanets,itcanhappenthatthelowestmassobjectsareejectedonhyperbolictrajectoriesandsomeplan-etscanmoveonstableorbitsof