A theoretical look at the direct detection of giant planets

Astronomy is at times a science of unexpected discovery. When it is, and if we are lucky, new intellectual territories emerge to challenge our views of the cosmos. The recent indirect detections using high-precision Doppler spectroscopy of now more than on

Atheoreticallookatthedirectdetectionofgiantplanetsoutside

theSolarSystem

AdamBurrows

arXiv:astro-ph/0501484v1 22 Jan 2005DepartmentofAstronomy,TheUniversityofArizona,Tucson,AZ85721e-mail:aburrows@as.arizona.eduI.HEADINGAstronomyisattimesascienceofunexpecteddiscovery.Whenitis,andifwearelucky,newintellectualterritoriesemergetochallengeourviewsofthecos-mos.Therecentindirectdetectionsusinghigh-precisionDopplerspectroscopyofnowmorethanonehundredgiantplanetsorbitingmorethanonehundrednearbystarsisanexampleofsuchrareserendipity.Whathasbeenlearnedhasshakenourpreconceptions,fornoneoftheplanetarysystemsdiscoveredtodateislikeourown.However,thekeytounlockingaplanet’schemical,structural,andevolutionarysecretsisthedirectdetectionoftheplanet’slight.Ireviewtheembryonictheoryofthespectra,atmospheres,andlightcurvesofirradi-

atedgiantplanetsandputthistheoryintothecontextofthemanyproposedastronomicalcampaignstoimagethem.

II.INTRODUCTION:THENEWLY-DISCOVEREDWORLDS

Directdetectionofanextrasolarplanetrequiresthatitsdimlightbeseparatedfromundertheglareofitsbrightparentstar.However,suchhigh-contrastimaging(e.g.,apartin107 10inthevisible)hasnottodatebeenachieved.Instead,thevastmajorityofknownextrasolargiantplanets(EGPs)havebeendiscoveredfromthegroundusingtheindirecttechniqueofhigh-precisionstellarspectroscopy[1–3].Duetogravitationalattraction,an

Astronomy is at times a science of unexpected discovery. When it is, and if we are lucky, new intellectual territories emerge to challenge our views of the cosmos. The recent indirect detections using high-precision Doppler spectroscopy of now more than on

orbitingplanetinducesaDopplerwobbleinitsparentstar.Iftheplanetismassiveandcloseenough,theperiodicvariationinthestellarspectrallinescanbemeasured.Theplanet’speriod(P),eccentricity(e),orbitalsemi-majoraxis(a),andprojectedmass(Mpsin(i)),whereiistheinclinationoftheorbit,cantherebybedetermined.ThelargerMpsin(i),thelargerthesignal.Thisisthereasonthe rstplanetsdetectedweretheEGPs.Terrestrialplanets,suchasEarthandVenus,are～300timeslighterthanJupiter,whileicegiants,suchasUranusandNeptune,are～20timeslighter.

BeforeIdelveintothephysicaltheoryofEGPsandtheirdirectdetection,IsummarizethebasicfactsoftheknownmembersoftheEGPfamily.The rstextrasolargiantplanetculledwas51Pegb[1]anditisinatight4.2-dayorbit,onehundredtimesclosertoitsprimarythanisJupitertotheSun.Todate,morethan140EGPs/planetshavebeendiscovered,morethan25ofwhichareinmorethan10multiplesystems.55Cancrihousesaquadruplet[4],oneofwhichhasamassnearthatofNeptune(～17Earthmasses),υAndhouseatriplet,andGJ876housesadoubletinatwo-to-oneorbitalresonance.(Wefollowtheconventionbywhichtheplanet’snameisgivenbythestar’sname,withanappendedlower-caseletter,eitherb,c,ord,indiscoveryorder.)

TheprojectedmassesoftheknownDopplerplanetsvaryfrom～0.06(!)MJtoabove10MJ,whereMJisaJupitermass,whichis318Earthmassesorroughly10 3solarmasses.Themoremassiveobjectsmaybebrowndwarfswithadi erentprovenance(seeBox).Radial-velocity(Doppler)techniquescannotdistinguishEGPsandNeptune-massplanetsfrombrowndwarfs.TheorbitalperiodsoftheknownEGPsspanavastrangefrom～1.2daysto～12years,theirsemi-majoraxesextendfrom～0.022AUto～6.0AU,whereanAUisanAstronomicalUnit,thedistancebetweentheEarthandtheSun,andtheirorbitaleccentricitiesvaryfrom0.0toabove0.9.Forcomparison,Jupiterresides5.2AUfromourSun,hasanorbitalperiodof～12years,andhasanorbitaleccentricityof～0.05.Table1providesthesebasicdataforarepresentativesubsetofthecurrentEGPbestiary.Theextremelyclose-inEGPs,suchas51Pegb,τBoob,HD209458b,andOGLE-TR56b

[5–7],wereasurprise,butnolesssothanwastheheterogeneityofthemassesandorbital

Astronomy is at times a science of unexpected discovery. When it is, and if we are lucky, new intellectual territories emerge to challenge our views of the cosmos. The recent indirect detections using high-precision Doppler spectroscopy of now more than on

propertiesoftheemergingEGPfamily.Tobesure,theDopplertechniqueselectsforthecloserrepresentatives,buttheymustexisttobedetected.Aswouldbeexpectedduetotidaldissipation,theclose-inEGPswithorbitaldistancessmallerthan～0.06AUallhavenearlycircularorbits.

Thereseemstobeacorrelationbetweentheprobabilityof ndinganEGPandthemetallicityofitsparentstar.The“metallicity”ofastaristhemassfractionofelements,suchascarbon,oxygen,nitrogen,neon,magnesium,siliconandiron,thatareheavierthanhelium.Hydrogenandheliumpredominateinstarsandgiantplanets,comprising～98%bymassoftheSun.Themoresuper-solartheheavy-elementcompositionofthepotentialparent,themorelikelyweareto ndanEGPinorbit.Thismaybeahintconcerningtheprocessesofgiantplanetformation,andisinkeepingwiththe3-5×solarexcessesmeasuredinJupiterandSaturn.Thecurrentcensusrevealsthatthereisa～5%apriorichanceof ndingagiantplanetbytheDopplertechniquearoundanearby(<～50parsecs≡160light-years)star,buta～20%chanceof ndingonearoundastarwithatleasttwicetheSun’smetallicity(J.Valenti&D.A.Fischer,inpreparation).

Presumably,theinclinationsofEGPorbitsaredistributedrandomlyonthesky.Hence,theprobabilitythattheorbitisedge-on(i=90 )isapproximatelyR /(2a),whereR isthestellarradius.Giventhis,theclose-inEGPshavethelargestchanceoftransitingthestellardisk,duringwhichtimethestarwilldimbyafraction(Rp/R )2,whereRpistheplanet’sradius.SinceRJ(theradiusofJupiter,～7.14×104kilometers)isroughly10%oftheradiusoftheSun,thisratioisexpectedtoberoughly1%.A1%dimmingiseasilydetectablefromtheground.Ata=0.045AUandadistance(d)of47parsecs,theplanetaroundtheF8V/G0VstarHD209458wasthe rstofonlyahandfulofEGPsthatarenowknowntotransittheirprimariesandaperiodicdimmingatthe～1.6%levelwasmeasured

[8–10].ThetransitofHD209458lasts …… 此处隐藏：5979字，全部文档内容请下载后查看。喜欢就下载吧 ……