A theoretical look at the direct detection of giant planets(4)

cos(α)=sin(θ+ω)sin(i)sin( ) cos( )cos(θ+ω).

Thisismerelyanapplicationofthelawofcosines.

Foracircularorbit,Risequaltothesemi-majoraxis(a).However,aplanetinaneccentricorbitcanexperiencesigni cantvariationinR,and,therefore,stellarinsolation(byafactorof(1+e(3)

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

theinterpretationofdirectdetectionsofEGPs.

AsSaturnitselfdemonstrates,dependinguponorbitalorientation,planetaryringscangreatlyaugmentre ectedlight[40,48].Theirpossiblepresenceisawildcardintheinterpre-tationofdirectEGPsignatures.Also,sinceΦ(α)iswavelength-dependent,thepotentiallylargevariationinre ectedoptical uxwithepochwillbecomplementedbyaninterest-ingvariationincolor.ThephasefunctionsΦ(α)arewavelength-dependent.Forexample,planetsshouldexecutetrajectoriesinthecolor-colorspaceV Rvs.B V,whereB,V,andRarethestandardblue,visible,andredbands.Thesetrajectorieswillbefunctionsofcloudparticlesize,amongotherthings,andwillbeusefulatmosphericdiagnostics.Similarbehaviorinthenear-andmid-infraredcolors,thoughmoremodest,maybeseen.

VI.GROUND-BASEDANDSPACE-BASEDTELESCOPESFORDIRECT

DETECTION

AsFig.2implies,thewiderangeofplanet/starcontrastratiosandspectraldiagnosticssuggestsdi erenttechnologicalsolutionstodirectdetection.Furthermore,therelativemer-itsofsearchingintheoptical,near-infrared,orthemid-infraredhaveyettobedetermined.Bothground-based(lessexpensive)andspace-based(morecapable)pathsarebeingpursuedandwhileadiscussionthatdoesjusticetothemanyinitiativeswhosegoalistheremotesensingofEGPsisfarbeyondthescopeofthisreview,wesummarizeafewrepresentativeapproaches.

EGPs,especiallyiftheyareyoung,massive,andclose,arebrightenoughthatcurrent8-to10-meterclassground-basedtelescopesornear-termspacetelescopes(suchastheJamesWebbSpaceTelescope(JWST)[49]witha6-meteraperture)mightbesensitiveenoughtopickuptheirlight.Thisisparticularlytrueinthenear-andmid-infrared(seeFigs.2and

3).However,undertheextremeglareofitsparentstarandatsmallangularseparationsoffrom～milliarcsecstoaroundanarcsecond(Table1),traditionaltelescopeopticsspillsfartoomuchlightinthevicinityoftheplanet.Themajorculpritsonthegroundarethe

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

turbulenceoftheatmosphere(“seeing”andscintillation),scatteringo dustandthespidermountofthesecondary,andimperfectionsinthemirror(s).Alsoatissueisthestabilityoftheopticalsystem.Evenforperfectoptics,thedi ractionpatternduetothe nitetelescopeapertureleavesacharacteristic“Airy”patternthatfora“Jupiter”at10parsecsaroundasolar-typestarwouldintheopticalbehundredsoftimesbrighter.

Hence,forlarge(8–10-meter)ground-basedtelescopes,(e.g.,thetwoKecks[50,51],thefourVLTs[52,53],thetwoGeminis[54],Subaru[55],thebinocularLBT[56,57])andmam-mothproposedtelescopes(e.g.,the100-meterOWL[58],the20-meterGMT[59],the30-meterGSMT[60]),speciale ortswillberequired.Theseincludeadaptiveoptics(AO)tocompensateforatmospheric uctuations(andmirrorimperfections)withmanyhundredsorthousandsoffast(millisecond)actuatorsandveryaccuratewavefrontsensing.Thelattercan,inprinciple,beachievedusingarti ciallaserguidestarsorstarsinthe eldofview.(WithAO,thereisusuallyabrightstarcloseenoughtoobviatetheneedforanarti cialbeacon.)InterferometrytonulloutthestellarlightisalsobeingpursuedbytheLBT,VLT,andKeck,andthedepthofthenulliscrucial,asistheangularregionoverwhichasu cientnullcanbeachieved.Finally,apodizingmasksand/orcoronagraphicspotstooccultthestarand,bydi ractiveinterference,redistributethestar’slightawayfromtheplanetarehighlydesirable(andmaybenecessary).NotethatsinceDomeCinAntarticahassomeofthebestseeingontheplanetandthequietestatmosphere,placingagiantnext-generationtelescopetheremayhaveitsadvantages[61].

AnEGPimagingsystemwillbejudgedbytheplanet/starcontrastratio,f,itcanachieveatagivenwavelengthandforagivenangularseparationfromthestar.Anglesof0.05to2.0arcsecsarecontemplated(Table1),withtherequisitecontrastsatsmalleranglesdeemedtoodi cultfor rst-generationimaging.Notethattheactualrequirementsareafunctionofdistancetothestar.AsFig.2indicatesfora5-Gyr/1-MJEGPat10parsecs,fsbetterthan10 4at10µmandbetterthan10 8intheopticalmightbenecessary.Atthe4-5µmbump,fsfrom10 5to10 8maybecalledfor.Fortunately,theseperformancegoalscanberelaxedformoremassiveandyoungerEGPsatangularseparationsgreaterthan

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

～0.1′′.Fortheroasters,almostindependentofmassandage,freaches10 3inthemid-

5infraredand<intheoptical,butatthecorrespondingmilliarcsecondseparationseven～10

thesecontrastratiosmaybetoochallengingforimaging.Figure5comparesthetheoreticallyrequiredcontrastratiosforthe ducial5-Gyr/1-MJEGPatvariouswavelengths(takenfromFig.2)asafunctionofangularseparationfromasolar-typestarat10parsecswiththeputativecapabilitiesofasampleofproposedimagingsystems,bothonthegroundandinspace.Contrastratiosfora0.5-Gyr/7-MJEGPintheHbandarealsoshown.Orbitandorientatione ectshavebeenignoredandlargeerrorbarsshouldbeassignedtoboththeoryandprojectedcapability.Inaddition,careshouldbetakentocomparetheoreticalnumberswithexperimentalhopesforthesamewavebands.Intheinterestsofbrevity,wehaveincludedmultiplewavebandsonFig.5.

Telescopesarestagesofcomponents(primarymirror,secondarymirror,lens,apertures,apodizingmasks,coronagraphs,etc.)thatinseriesactontheincident …… 此处隐藏：6277字，全部文档内容请下载后查看。喜欢就下载吧 ……