Table Of Content(cid:13)
January25,2009
LettertotheEditor
Ground-based K-band detection of thermal emission from the
exoplanet TrES-3b
E.J.W.deMooij1 andI.A.G.Snellen1
LeidenUniversity,Postbus9513,2300RA,Leiden,TheNetherlands;e-mail:[email protected]
9 ABSTRACT
0
0 Context.SecondaryeclipsemeasurementsoftransitingextrasolarplanetswiththeSpitzerSpaceTelescopehaveyieldedseveraldirect
2 detectionsofthermalexoplanetlight.SinceSpitzeroperatesatwavelengthslongwardof3.6µm,arguablyoneofthemostinteresting
partsoftheplanetspectrum(from1to3µm)isinaccessiblewiththissatellite.Thisregionisatthepeakoftheplanet’sspectralenergy
n distributionandisalsotheregimewheremolecularabsorptionbandscansignificantlyinfluencethemeasuredemission.
a Aims.So far, 2.2µm K-band secondary eclipse measurements, which are possible from the ground, have not yet lead to secure
J
detections.TheaimofthispaperistomeasurethesecondaryeclipseoftheveryhotJupiterTrES-3binK-band,andinadditionto
3 observeitstransit,toobtainanaccurateplanetradiusinthenearinfrared.
1 Methods.WehaveusedtheWilliamHerschellTelescope(WHT)toobservethesecondaryeclipse,andtheUnitedKingdomInfrared
Telescope(UKIRT)toobservethetransitofTrES-3b.Bothobservationsinvolvedsignificantdefocusingofthetelescope,aimedto
] producehigh-cadencetimeseriesofseveralthousandframesathighefficiency,withthestarlightspreadoutovermanypixels.
P
Results.WedetectthesecondaryeclipseofTrES-3bwithadepthof 0.241 0.043%( 6σ).Thiscorrespondstoaday-sidebrightness
E temperature of T (2.2µm)=2040 185K, which is consistent with−current±models of∼the physical properties of this planet’s upper
. atmosphere. TheBcentre of the ec±lipse seems slightly offset from phase φ=0.5 by ∆φ= 0.0042 0.0027, which could indicate that
h − ±
theorbitofTrES-3bisnon-circular.AnalysisofthetransitdatashowsthatTrES-3bhasanear-infraredradiusof1.338 0.016R ,
p ± Jup
showingnosignificantdeviationfromopticalmeasurements.
-
o
Keywords.techniques:photometric–stars:individual:TrES-3–planetarysystems
r
t
s
a
[ 1. Introduction and Snellen&Covino (2007). Two partial eclipses of
HD209458b were observed in K-band by Snellen (2005),
1
During the secondary eclipse of an extrasolar planet (the while the telescope was significantly defocused to avoid
v
8 moment the planet moves behind its host star), the contri- saturation of the array by the bright star. Although milli-
7 bution from the direct planet light can be measured. Until magnitude precisions were reached, nothing was detected,
8 now such measurements have been the domain of the Spitzer probably because of the lack of baseline on both sides of
1 Space Telescope, which have led to the first detections of the eclipse (Snellen 2005). Subsequently, OGLE-TR-113b
. thermal emission from extrasolar planets (Charbonneauetal. was observed (Snellen&Covino 2007), which has the great
1
2005; Demingetal. 2005). Until now a handful of planets advantage that it has several other stars in its field. It was
0
9 has been observed at several wavelengths between 3.6µm and shown that randomly offsetting the telescope randomises the
0 24µm, with one of the highlights so far being the measure- photometric errors down to 0.1-0.2% per hour. It resulted in a
: mentoftheday/night-sidetemperaturecontrastoftheexoplanet tentativedetectionofitssecondaryeclipseof0.17 0.05%.
v HD189733b(Knutsonetal.2007). In this letter we present our results for K-b±and photom-
i
X Unfortunately, a very interesting part of the broad spectral etry of the transit and secondary eclipse of the planet TrES-
r energydistributionofhotJupitersbetween1and3µmisshort- 3b (O’Donovanetal. 2007). This planet is significantly more
a ward of the observingwindow of Spitzer. This region contains suitableforsecondaryeclipsephotometrythanprevioustargets,
theoverallpeakoftheplanet’semissionspectrum,andcanalso since it is in a very close orbit (P 1.31 days) and its radius is
∼
be strongly influenced by molecular bands (e.g. from water, significantlyinflated(R 1.3R ).Furthermorethereisanearby
Jup
∼
methane and CO) in the planets’ upper atmosphere. However, referencestar,whichcanbeusedfordifferentialphotometry.In
ground-basedobservations,whichcouldaccessthisspectralre- section2wepresentourobservationsanddatareduction.Insec-
gion, have thus far proven very challenging. Several methods tion3wepresentanddiscussourresults.
havebeentried.Richardsonetal.(2003)usedadifferentialspec-
troscopicmethod,searchingforthewavelengthdependentshape
oftheplanetspectrum,butnoeclipsewasdetected.Inaddition, 2. Observations,datareductionandanalysis
observationsofthesecondaryeclipseofTrES-1binL-bandwith
2.1.ThetransitofTrES-3b
theNIRIspectrographonGeminiNorthbyKnutsonetal.(2007)
onlyresultedinanupperlimit. We have observed the transit of the exoplanet TrES-3b with
Photometric attempts to detect the secondary eclipse of theUnitedKingdomInfraredTelescope(UKIRT)usingitsFast
extrasolar planets have been presented by Snellen (2005) TrackImager(UFTI;Rocheetal.2003)onJune20,2008.The
Fig.1.K-bandlightcurvesfromtheUKIRTtransitobservations. Fig.2.K-bandlightcurvesfromtheWHTsecondaryeclipseob-
The top panel shows the lightcurve of the reference star and servations.Thetoppanelshowstherawlightcurvefortherefer-
TrES-3bbeforecorrectingforthe positionaldependenceof the encestarandTrES-3b.Themiddlepanelshowsthelightcurveof
flux.Themiddlepanelshowsthe fluxesofbothstars corrected TrES-3bdividedbythatofthereferencestar.Thebottompanel
forthiseffect.Thedashedlineisapolynomialfittothefluxof showsthefinal,fullycorrected,lightcurve.
the reference star. The bottom panel shows the final corrected
unbinnedlightcurveofTrES-3b.
Thedata wasflatfielded usinga skyflat constructedfroma
seriesofframestakenjustbeforethestartoftheobservationsof
observations, carried out in queue scheduling, lasted for al- TrES-3.Nodarksubtractionwasperformed,becauseanydark-
most 4 hours, starting 0.6 hour before ingress and ending current was seen as a contribution to the sky background and
∼
1.7 hours after egress. The field of view of UFTI is 93 93 removedtogetherwiththesky.Thepositionsofthehotandcold
∼ ×
arcsec, with a pixel scale of 0.091 arcsec/pixel. Since the dis- pixelsweredeterminedfromboththeflatfieldandseparatedark
tancebetweenTrES-3(K=10.61)andthenearbyreferencestar frames. These pixels were replaced by the values of 3rd order
(2MASSJ175225.15+373422.1,K 9.77)is 4arcminutes,we polynomial surfaces fitted to the 7 7 pixels surrounding these
were forced to alternate observati∼ons betwe∼en the target and pixels. ×
the reference, in order to correct for time dependent atmo- To determine the sky background,first all stars in the field
sphericand instrumentaleffects. To reduceoverheadsfromde- were masked using circular masks of 30 pixels. A median sky
tectorread-out,thedetectorwaswindowedtoasinglequadrant valueforthemaskedimagewascalculatedandsubtractedinor-
(512 512 pixels, 46.6 46.6 arcsec) which increased the cycle dertoremovetemporalfluctuationsinthebackground.Thefinal
× ×
speed. During each sequence we repeated a nine point dither sky level was determined by averaging the 27 masked frames
pattern three times before switching to the other star. The ex- fromonecycle,andremovedfromtheimages.
posuretimewas5secondsfortheTrES-3,and2secondsforthe Subsequently,aperturephotometrywasperformedusingthe
referencestar. With a typicaloverheadof5 secondsper frame, APERprocedurefromtheIDLAstronomyUser’sLibrary1with
thetimerequiredtocompleteanentiretarget-referencesequence anapertureradiusof42pixels.Anypossibleresidualskyfluctu-
was 8minutes. ationswerecorrectedforbymeasuringthebackgroundvaluein
∼
The telescope was significantly defocused to 1) reduce in- a42to70pixelannulusaroundtheobject.
trapixelvariations,2)minimisetheinfluenceofflatfieldinaccu- Theresultsfromtheaperturephotometrycanbefoundinthe
racies, and 3) prevent the count levels from reaching the non- toppanelofFigure1.Boththefluxfromthetargetandtheref-
linear range of the detector. Due to the fact that the telescope erencestarwasfoundtobeafunctionofditherposition,there-
optics had not yet been realignedafter the recentswitch to the forethedatawerenormalisedforeachditherpositionseparately.
Cassegrain focus, the defocused telescope produced a signifi- These corrected lightcurves are shown in the middle panel of
cantly asymmetric PSF, which could be covered by a circular
apertureof 40pixels( 4”)inradius. 1 http://idlastro.gsfc.nasa.gov
∼ ∼
Figure1.Toremovetimedependenteffects,wefittedahighor-
der polynomial to the binned lightcurve of the reference star.
This fit was used to correctthe lightcurveof TrES-3. The final
unbinnedlightcurveforTrES-3isshowninthebottompanelof
Figure1.
2.2.ThesecondaryeclipseofTrES-3b
On July 3-4, 2008 the secondary eclipse of TrES-3 was ob-
served with the Long-slit Intermediate Resolution Infrared
Spectrograph(LIRIS;Acosta-Pulidoetal.2002)attheWilliam
Herschell Telescope (WHT) on La Palma, with the observa-
tionslastingfortheentirenight,startingabout 3hoursbefore,
∼
andending 4hoursaftertheexpectedcentreofthesecondary
∼
eclipse (at 1:31UT on July 4th). We used the full 1024 1024
×
pixel array for these observations, which combined with the
pixel size of LIRIS of 0.25”/pixel, yields a field of view of
256 256 arcseconds. This larger field of view enabled us to
×
placebothTrES-3andthereferencestar simultaneouslyonthe
detector, making these observations 80% more efficient rela-
tivetothoseobtainedwithUKIRT.W∼euseda9pointditherpat- Fig.3. The 9-point binned lightcurve of the transit of
tern,includingsmallrandomoffsetsfromthenominalpositions TrES-3b. The solid line shows the best fitting model with
ofeachditherpoint.We usedan exposuretime of7.5seconds, RP=1.338 0.016RJup.Thebottompanelshowstheresidualsaf-
±
withtypicaloverheadsof7secondsperframe. tersubtractingthismodelfit.
AsfortheUKIRTobservations,wedefocusedthetelescope,
whichresultedinaringshapedPSFwitharadiusofabout4pix-
els.
Since the LIRIS detector is non linear at high countlevels,
we applied the following empirical non-linearity correction to
thedatabeforeflatfielding:
1
F = F (1)
true meas· 1 c F6
meas
− ·
with F thefluxaftercorrection,F themeasuredfluxand
true meas
c=6 10 29aconstantdescribingthestrengthofthecorrection
−
·
term.Thevalueofcwasdeterminedbyminimisingthenoiseof
thereferencestarovertheentirenight,andcomparingthisvalue
tonon-linearitymeasurementstakenbytheLIRISteam2.
All frames were corrected for crosstalk between the quad-
rants, which was found to be present in a co-add of all the re-
duced science frames at a level of 10 5 of the total flux along
−
arowofaquadrant.Thiscorrectionwasdoneonarowbyrow
basis. A flat field was created from a series of domeflatstaken
withthedomelightbothonandoff.Theseweresubtractedfrom
eachother(toeliminatestructurefromtheemissionbythedome
andtelescope)andaveraged. Fig.4.Final,9-pointbinnedlightcurveofthesecondaryeclipse.
Thesmallnumberofhotandcoldpixelsinthe imagewere Thesolidlineisthebestfittingmodelwith∆F= 0.241 0.043%
− ±
replacedbya medianoftheneighbouringpixelsin a 3 3grid. andanoffsetfromphase0.5of∆φ= 0.0041 0.0018.Thebot-
Thesky wasdeterminedfromthe combinationof the9×images tom panel shows the residuals after−subtracti±ng the best fitting
ineachdithersequence,forwhichallthestarsweremaskedout. model.
We used a circularaperturewith a radiusof 13 pixelsto deter-
minethestellarfluxandanannulusof30to90pixelstodeter-
minethe levelofpossible residualflux in the backgroundafter In a similar way a small dependenceon airmass was removed.
sky-subtraction. The corrected unbinnedlightcurveof TrES-3b can be foundin
TheindividualrawlightcurvesofTrES-3bandthereference thelowerpanelofFigure2.
starareplottedinthetoppanelofFigure2.Beforefurtheranal-
ysiswedividedthelightcurveofTrES-3bythatofthereference
star,showninthemiddlepanelofFigure2.Therelativefluxesof
3. Resultsanddiscussion
thetwostarsarecorrelatedwiththeirpositionsonthedetector.
Theseeffectsarecorrectedforbyfittinglinearfunctionsbetween 3.1.ThetransitofTrES-3b
thex,y-positionsandtherelativefluxfordataoutsideoftheex-
The final transit lightcurve binned per 9-point dither cycle is
pectedeclipse.Thesefitsarethenappliedtothewholedataset.
showninFigure3.TheRMS-noiseleveloutoftransitis0.0060,
2 http://www.ing.iac.es/Astronomy/instruments/liris/liris detector.html whilethetheoreticalnoiselevelfortheseobservationsis0.0029,
which is actually dominated by the detector’s readnoise of subsequently becomes 0.190+0.034, with an offset in phase of
− 0.037
26e−/pixel,duetothelargeapertureused. ∆φ = 0.0036+0.0022 (χ2/ν=1.3−9). Although the significance of
− 0.0020
Since a dependence of the transit depth on wavelength can the fitis reduc−ed,we still detectthe secondaryeclipse ata 6σ-
revealcharacteristicsoftheplanetatmosphere,wefittedthera- level.
dius of TrES-3b, keeping the parameters of the host star and Because it is clear that red noise dominates the uncertain-
the planet’s orbit (mean stellar density, orbital inclination and ties in our measurements, we tried to characterise the uncer-
ephemeris) fixed with respect to the values of Sozzettietal. taintyusingthe”residualpermutation”bootstrapmethodasused
(2008).ForthelimbdarkeningweusedparametersfromClaret by Winnetal. (2008). The best fitting model was subtracted
(2000),assumingatemperatureofT =5750Kforthehoststarof from the data, after which the residuals were shifted between
TrES-3b,ametalicityoflog[Fe/H]= 0.2andasurfacegravity 1 and 1800 points and added to the model light curve. These
−
of log(g)= 4.5. The transit was modelled using the IDL pro- light curves were refitted for each individual shift. The result-
cedures from Mandel&Agol (2002), resulting in a radius of ing distributions of parameter values representbetter estimates
1.338 0.013R atχ2/ν=1.58.Thereducedχ2 issignificantly oftherealuncertaintiesinthedata.Withthisanalysiswefindan
Jup
±
largerthanunity.Sincewefittedtothebinnedlightcurve,thisin- uncertaintyof4.310 4intheeclipsedepthand2.710 3forthe
− −
· ·
dicatesthatthenoisedoesnotscalewith √N,meaningthatwe offsetinphase,whichweuseintheremainderofthepaper.
haveanadditional,unknown,sourceofcorrelatednoise.Toob- Assumingablackbodymodelforthethermalemissionspec-
tainabetterestimateoftheuncertaintyintheplanetradius,we trumof TrES-3b,we can estimate the day-sidebrightnesstem-
forcedχ2/ν=1byscalingupourerrorsinthebinnedlightcurve perature in the K-band. For the stellar temperature, we use
buannifdorrmadlyiu.sInshthoiwsswnaoyswigenoifibtcaainntRdPev=i1a.t3io3n8±fr0o.m01t6hReJoupp.tTichaelrKa-- Tdeeffp=th5u6s5i0n±g7th5eKfu(Sllolzigzhetttciuertvael.w2e0o0b8)ta.iFnroTmB=th2e04fi0tto1t8h5eKec(l9ip0s%e
dius of 1.336+0.031R , as measured by Sozzettietal. (2008). confidenceinterval). ±
0.037 Jup
Theuncertain−tyinourK-bandradiusappearssmaller,butthisis Due to the high incident stellar flux, Fortneyetal. (2008)
solely due to the fact that we keep the parameters for the host classifythisplanetasbelongingtothepMclassofplanets.This
starandtheplanet’sorbitfixed. class is expected to have an inversion layer in their upper at-
We show that we can obtain the K-band radius of an exo- mospheres and to show a large day/night contrast. Comparing
planetataprecisionof 1%.Thiscouldbefurtherimprovedby thedepthofthesecondaryeclipsewiththeirmodel(theirFigure
observingthereference∼starandtargetatthesametimebothbe- 14),weseethatourresultsareindeedconsistentwiththismodel.
causeoftheincreaseinthenumberofframesduringtransitand However, since we have measured the brightness temperature
becauserapidatmosphericfluctuationscanbecorrectedfor. ofTrES-3batonlysinglewavelength,Spitzermeasurementsat
longerwavelengthsareneededtoconfirmthepresenceofanin-
versionlayerandthusclassifyTrES-3basapMplanet.
3.2.ThesecondaryeclipseofTrES-3b
Thefully correctedlightcurvebinnedbyone 9pointdithercy- 4. Conclusions
cle is shown in Figure 4. Before binning, and after subtracting
the bestfitting model(see below), we clippedthe lightcurveat We havedetectedthesecondaryeclipse oftheexoplanetTrES-
0.01,removing43ofthe1800points(2.3%),ofwhich3points 3binK-bandatalevelof 0.241 0.043%,indicatingabright-
±(0.9%)duringtheeclipse.Thebinnedlightcurvewasfittedusing ness temperature of TB(2.2−µm)=2±040 185K.The eclipse tim-
±
the Mandel&Agol (2002) procedures, with all the parameters ingshowsasmalloffsetfromφ=0.5,whichwouldimplyanon-
fixedtothetransitmodel,exceptthecentreandthedepthofthe circularorbit,butthisneedstobeconfirmedbySpitzerobserva-
eclipse(ofcoursewithoutlimbdarkening). tions.AlsotheradiusofTrES-3bisdeterminedinK-bandtobe
1.338 0.016R ,showingnodeviationsfromopticalmeasure-
The bestfitted lightcurveis over-plottedinthe toppanelof Jup
±
ments.
Figure4, withthebottompanelshowingthe residuals.Theout
ofeclipseRMSnoiseis0.0038,whiletheexpectationfromthe-
Acknowledgements. Weare grateful to the UKIRT observers and the staff of
oreticalnoisestatisticsis0.0015.Thedominantsourceofnoise
boththeUKIRTandWHTtelescopes.TheUnitedKingdomInfraredTelescope
inthesenoisecalculationsistheskycontribution.Thetotalflux is operated by the Joint Astronomy Centre on behalf of the Science and
fromthe sky within the apertureis, evenfor the brighterrefer- Technology Facilities CounciloftheU.K.TheWilliam HerschelTelescopeis
encestar, 50%higherthanthestellarflux. operatedontheislandofLaPalmabytheIsaacNewtonGroupintheSpanish
∼ Observatorio del Roque de los Muchachos of the Instituto de Astrof´ısica de
The fitted depth of the secondary eclipse is
Canarias.
∆F = 0.241 0.021%, with a reduced chi-squared of
χ2/ν=1.7−2. As±for the transit data, the reduced χ2 for the
fit to the binnedlightcurveis greaterthan unity,indicatingthat References
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