Science文献发现太阳系外双星系统中的行星

DirectimagingdiscoveryofaJovianexoplanetwithinatriple-starsystemKevinWagneretal.Science2016BackgroundDirectimagingallowsforthedetectionandcharacterizationofexoplanetsviatheirthermalemission.Mostdirectimagingsurveyshavetraditionallyexcludedvisualbinaryormultiplesystemswhoseseparationsarelessthanafewhundredastronomicalunits(au)duetotheassumptionthatsuchplanetarysystemswouldeitherbedisruptedorneverform,aswellastheincreasedtechnicalcomplexityofdetectingaplanetamongstthescatteredlightofmultiplestars.Multi-starsystemsareasnumerousassinglestars.Planetsonwideorbits(detectablebydirectimaging)mightarisemorefrequentlyinmulti-starsystemsduetoplanet-planetorplanet-starinteractions.BackgroundWidefieldX-rayimageoftheScorpius-CentaurusassociationconstructedfromthedataoftheROSATAllSkySurveyBackgroundmaps.TheyellowdotsmarkthepositionsofbrightX-raysourcesdetectedinthesurvey(onlyabout10%ofthebrightestX-raysourcesareshown).ThebluecirclesmarkthethreesubgroupsUpperScorpius,UpperCentaurus-Lupus,andLowerCentaurus-Crux(fromlefttoright).Toinvestigatethefrequencyoflong-periodgiantplanetsbotharoundsinglestarsandinmulti-starsystems,wearesamplingapopulationof~100youngsingleandmultipleA-typestarsinthenearbyUpperScorpius-Centaurus-Lupusassociation.TheScorpius-CentaurusAssociationiscenteredabout470light-yearsawayintheGouldBelt(thenearestOBassociationtotheSun)andcontainsseveralhundredstars,mostlyoftypeB.About12millionyearsagothemostmassivestarinUpperCentaurus-Lupuswentsupernova,creatingalargeshockwave.ThisshockwavepassedthroughtheUpperScorpiuscloudabout5millionyearsago,andtriggeredthestarformationprocessthere.Shortlyafter,thestrongwindsofthenumerousmassivestarsinUpperScorpiusstartedtodispersethemolecularcloudandhaltedthestarformationprocess.About1.5millionyearsagothemostmassivestarinUpperScorpiusexplodedasasupernova.DiscussionWeobservedHD131399onJune12,2015,obtainingawiderangeofnear-infraredspectralcoveragefromY–toK-band(0.95–2.25μm)anddiffraction-limitedimagingwithan8.2-mtelescopeapertureOurobservationsresultedinthediscoveryofHD131399Ab,apointsourcewithacontrasttoHD131399Aof10−5andprojectedseparationof0.84arcseconds,or82±6au.CharacterizationofHD131399AbFig.1.Near-infraredVLT/SPHEREimagesofHD131399AbandthehierarchicaltriplestarsystemHD131399ABC.Thecentralregionsthatareaffectedbythecoronagraphandresidualscatteredstarlightareblockedbyamask(dashed)in(A)to(D),withthelocationofstarAindicatedbythecrosshairs.(E)showsacompositeofthePSF-subtractedregion(dashed)superposedonthewidefieldK1imageshowingthestellarcomponentsofthesystem,whoseluminositiesareadjustedtotheleveloftheplanetforclarity.IneachimagetheluminosityofcomponentA(butnotBandC)hasbeensuppressedbytheuseofacoronagraph.Themass,effectivetemperature,andspectraltypeofthepreviouslyunresolvedBandC(exceptwherenoted)wereestimatedfromtheirK1luminosity.Theplanet’stemperatureandspectraltypeweredeterminedthroughspectralfitting.HD131399(HIP72940)isatriplesysteminthe16±1MyroldUpperCentaurus-Lupusassociation[UCL]atadistanceof98±7pcwhosebasicpropertiesaregiveninTable1.Isitabackgroundobject?WedetectHD131399AbwithasignaltonoiseratioinY(1.04μm),J(1.25μm),H(1.62μm),K1(2.11μm)andK2(2.25μm)of9.3,13.2,15.5,23.5and11.9,respectively.Followingastrometriccalibrations,wemeasureapositionaldisplacementtoHD131399AofΔα(rightascension)=12±8masandΔδ(declination)=6±8milliarseconds(mas)overtheeleven-monthbaseline.Thisallowsustorejectthehypothesisofabackgroundobject,whichwouldhavemovedrelativetoHD131399AbyΔα=27.3±0.6masandΔδ=28.8±0.6masduetotherelativelyhighpropermotionofthesystem.AssumingaKeplerianorbitfortheplanetwithasemi-majoraxisequivalenttoitsprojectedseparationof82auyieldsaperiodofroughly550years,whichforaface-oncircularorbitoverelevenmonthsisexpectedtoproduce~9masofrelativemotion,whichisconsistentwiththeobservations.TheboundplanethypothesisisalsosupportedbythelowprobabilityofdetectinganunboundobjectwithinUCLthathappenstoshareasimilarspectraltypetoHD131399Ab.“cold-start”or“hot-start”?Therearetwomainideasabouthowobjectsforminthemassrangeofgiantplanetsandbrowndwarfs.Cold-start:Solidcores,similarto,orsomewhatmoremassivethan,theterrestrialplanets,formfirst,andthenarunawayprocessofgasaccretionfollows.Inthispicture,onceasolidcoreof∼10Earthmasseshasdeveloped,therateatwhichitaccretes“atmosphere”increasesdramatically.Surroundinggasfromthediskflowsinwardandformsanaccretionshockneartheboundaryofthegrowingplanet.Thisshockcanbequiteluminous,andmightallowtheinfallinggastoradiateawaymuchofitsinitialheat.Hot-start:Theprotoplanetarydiskbecomesgravitationallyunstableandcollapsesintofragments,formingplanetsdirectly.CharacterizationofHD131399AbTheHversusH-Kcoloroftheplanetisinconsistentwiththecold-startscenario,inwhichtheplanethaslostsomefractionofitsinitialentropyduetoinefficientaccretionwhileconsistentwithhot-startmodelsincludingapartlycloudyatmosphereand/orsuper-solarmetallicity.Systematicinterpolationbetweenhot-startevolutionarytracksyieldsamassof4±1MJup,whichplacesHD131399Abfirmlyintheplanetarymassregime.Fig.2.Near-infraredspectrumofHD131399Ab.(A)HD131399Abspectrum(black)alongsidethebest-fitmodelatmosphereinred,withTeff=850Kandlog(g)=3.8cm/s2,showingwaterandmethaneabsorptionintheatmospherewiththeapproximateabsorptionregionsindicatedbythedashedlines.ThespectrumoftheT-typeexoplanet51Erib(16)isshowninblue,scaledby50%toroughlymatchtheluminosityofHD131399Ab.(B)Near-infraredspectrumofHD131399Abandspectraofstandardfieldbrowndwarfs,witheach1.4–2.4μmspectrumnormalizedindependentlyinλFλunits(equivalenttopowerperunitarea).ThetransitionbetweenLandTspectraltypes(Teff~2100–1300KandTeff~1300–600K,respectively)ismarkedbytheappearanceofH–andK-bandmethaneabsorptionintheatmospheresofthecoolerTdwarfs.InJversusJ–Hcolor-magnitudespace(Fig.3)thisappearsasbluercolor(morenegativeJ–Hcolor)comparedtothehotterLdwarfs.AtthethresholdoftheL/TtransitionthephotospherebecomesbrighterintheJ-band,assilicatecloudstransitionfromabovetobelowthephotosphere.HD131399AbandthetwootherdirectlyimagedT-typeexoplanetsfollowtheT-dwarfsequence,whichweinterpretasevidenceforasimilaritybetweenthemostlyorfullycloud-freeatmospheresoftheseexoplanetsandcoolfieldbrowndwarfs.HD131399AbistheclosestdirectlyimagedexoplanettotheL/TtransitionOrbitalcharacterizationofHD131399Weuseastrometricobservationsdatingbackto1897(tableS2)(26)tofittheorbittoagridofmodelsforabinarysystemusingthecenterofmassoftheBCsystem.OrbitalcharacterizationofHD131399Theorbitsforallfourcomponentsarestablefor100MyrandtheorbitofcomponentAbundergoesbothshortandlongtimescalevariationsduetoperturbationsfromBC.TheorbitalconfigurationofHD131399resultsinamoredynamicallyextremeconfigurationthanforanyknownexoplanetwithinabinaryormultiplesystem.Thecurrentastrometryalsopermitsunstableorbitsfortheplanet.Giventheyoungageofthesystem,theplanetmightbeonanunstableorbit,perhapsduetoplanet-planetorplanet-starscattering,andcouldyetbeejectedtobecomeafree-floatingplanetarymassobject.Thisisnotthemostlikelyscenario,asthetimescalefortheplanettosufferanejectionorcollisionisonlyafewMyr.FormationofHD131399Abandtheoriginofitslong-periodorbitWespeculatethattheplanetmayhavearrivedatitspresentorbitthroughoneofthreepossiblescenarios:ScenarioA)theplanetformedonashortorbitaroundstarA,andsubsequentlyunderwentaplanet-planetscatteringeventthatejectedittoitscurrentlong-periodorbit.ScenarioB)HD131399AbformedasacircumbinaryplanetaroundcomponentsBandCand