The GROUSE project III: Ks-band observations of the thermal emission from WASP-33b

4.8 MB·English

by  E. J. W. de Mooij

Astronomy&Astrophysicsmanuscriptno.demooij˙wasp33 (cid:13)c ESO2013
January16,2013
The GROUSE project III: Ks-band observations of the thermal
⋆
emission from WASP-33b
E.J.W.deMooij1,2,M.Brogi1,R.J.deKok3,I.A.G.Snellen1,M.A.Kenworthy1,andR.Karjalainen4
1 LeidenObservatory,LeidenUniversity,Postbus9513,2300RA,Leiden,TheNetherlands;
2 DepartmentofAstronomyandAstrophysics,UniversityofToronto,50St.GeorgeStreet,Toronto,ONM5S3H4,Canada;e-mail:
[email protected]
3 SRONNetherlandsInstituteforSpaceResearch,Sorbonnelaan2,3584CAUtrecht,TheNetherlands;
4 IsaacNewtonGroupofTelescopes,ApartadodeCorreos321,E-38700SantaCruzdelaPalma,CanaryIslands,Spain
3
1 Preprintonlineversion:January16,2013
0
2 ABSTRACT

n Context.Inrecentyears,day-sideemissionfromaboutadozenhotJupitershasbeendetectedthroughground-basedsecondaryeclipse
a observationsinthenear-infrared.Thesenear-infraredobservationsarevitalfordeterminingtheenergybudgetsofhotJupiters,since
J
theyprobetheplanet’sspectralenergydistributionnearitspeak.

5 Aims.TheaimofthisworkistomeasuretheK -bandsecondaryeclipsedepthofWASP-33b,thefirstplanetdiscoveredtotransitan
s
1 A-typestar.Thisplanetreceivesthehighestlevelofirradiationofalltransitingplanetsdiscoveredtodate.Furthermore,itshost-star
showspulsationsandisclassifiedasalow-amplitudeδScuti.

] Methods.As part of our GROUnd-based Secondary Eclipse (GROUSE) project we have obtained observations of two separate
P
secondaryeclipsesofWASP-33bintheK -bandusingtheLIRISinstrumentontheWilliamHerschelTelescope(WHT).Thetelescope
s
E wassignificantlydefocusedtoavoidsaturationofthedetectorforthisbrightstar(K∼7.5).Toincreasethestabilityandthecadence
. oftheobservations,theywereperformedinstaringmode.Wecollectedatotalof5100and6900framesforthefirstandthesecond
h
nightrespectively,bothwithanaveragecadenceof3.3seconds.
p
Results.Onthesecondnighttheeclipseisdetectedatthe12-σlevel,withameasuredeclipsedepthof0.244+0.027 %.Thiseclipse
- −0.020
o depthcorrespondstoabrightnesstemperatureof3270+115 K.Themeasuredbrightnesstemperatureonthesecondnightisconsistent
−160
r withtheexpectedequilibriumtemperatureforaplanetwithaverylowalbedoandarapidre-radiationoftheabsorbedstellarlight.For
st theothernighttheshortout-of-eclipsebaselinepreventsgoodcorrectionsforthestellarpulsationsandsystematiceffects,whichmakes
a thisdatasetunreliableforeclipsedepthmeasurements.Thisdemonstratestheneedofgettingasufficientout-of-eclipsebaseline.
[
Keywords.techniques:photometric–stars:individual:WASP-33–planetsandsatellites:atmospheres

1
v
0
1. Introduction trast is most-favourable for observations with ground-based
8
telescopesthroughtheavailableatmosphericwindows.
3
Inrecentyears,therehavebeenmanymeasurementsofthermal
3
emissionfromtheatmospheresofhotJupiters,especiallyinthe From the combination of the measurements at multiple
.
1 mid-infraredusingthe SpitzerSpaceTelescope(e.g.thereview wavelengths, a picture is emerging that there are (at least) two
0 byDeming2009).TheseSpitzerobservationsprobethethermal types of hot Jupiter atmospheres, those which show a thermal
3
emission of hot Jupiters redward of the peak of their spectral inversion,andthosewhichdonot.Ithasbeenproposedthatthe
1
energydistribution(SED),andthusmeasuretheplanet’slightin presence of the inversion layer is set by the stellar irradiation,
:
v theRayleigh-Jeanstailoftheiremissionspectrum. where at high levels of irradiation the planet’s stratosphere is
i hot enough to keep a strongly absorbing compound in the gas
X Observations in the near-infrared, on the other hand,
phase,whileatlowerirradiationlevelsthecompoundcondenses
typically probe the planet’s emission spectrum around or
r outanddisappearsfromthegas-phase(e.g.Burrowsetal.2007;
a even blue-ward of its peak, and therefore provide essential
Fortneyetal.2008).
informationontheplanet’stotalenergybudget.Duringthepast
three years several measurements of planetary emission short-
Knutsonetal. (2010) proposed an alternative scenario for
ward of 2.5µm have been obtained (deMooij&Snellen
thepresenceorabsenceofastrongabsorberinthehighestlayers
2009; Sing&Lo´pez-Morales 2009; Gillonetal. 2009;
of the planetaryatmosphere. In their scenario the absorber can
Rogersetal. 2009; Andersonetal. 2010; Alonsoetal. 2010;
bedestroyedbystrongUVemissionfromtheplanet’shost-star,
Gibsonetal. 2010; Crolletal. 2010a; Lo´pez-Moralesetal.
duetostellaractivity.Forhigherlevelsofstellaractivity,which
2010;Crolletal.2010b,2011;deMooijetal.2011;Smithetal.
resultinahigherUVflux,theabsorbingcompoundispossibly
2011; Ca´ceresetal. 2011; Demingetal. 2012), most of these
removed,resulting in a non-invertedatmosphere.Note that the
are in the K-band (λ=2.15µm) where the planet-to-star con-
inferenceofaninversionlayerhasrecentlybeenquestionedby
Madhusudhan&Seager (2010), who point out that for several
⋆ Photometrictimeseriesareonlyavailableinelectronicformatthe planets there is a degeneracybetween the atmospheric temper-
CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via aturestructureandthechemicalcompositionoftheplanet’sat-
http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/ mosphere.
1
DeMooijetal.:TheGROUSEprojectIII:ThesecondaryeclipseofWASP-33b
In this paper we present observations of two secondary
eclipses of the very hot Jupiter WASP-33b in K -band. These
s
are part of the GROUnd-based Secondary Eclipse project
(GROUSE), which aims to use ground-based telescopes for
exoplanet secondary eclipse observations in the optical and
near-infrared. As part of this project we have already pub-
lishedK -banddetectionsofthesecondaryeclipsesofTrES-3b
s
(deMooij&Snellen2009)andHAT-P-1b(deMooijetal.2011)
WASP-33b (CollierCameronetal. 2010) is currently the
only known planet to transit an A-type star (T =7430±100K)
eff
whichit orbitsin ∼1.22days. ThismakesWASP-33bthe most
irradiated planet known to date, with an irradiation of 1.2·1010
erg/sec/cm2. This high level irradiation results in an expected
day-side equilibrium temperature of 3250K. Recent observa-
tions by Smithetal. (2011) indeed show a very high bright-
ness temperature at 0.9µm of 3466±140K. Additionally, since
thehost-starisrelativelyhot,theexpectedUVfluxitreceivesis
also high, makingit an ideal candidateto investigatethe influ-
enceofahighUVfluxonthetemperaturestructureofaplanet’s
atmosphere.
In addition to being the first transiting planet discoveredto
orbitanA-typestar,WASP-33bisalsothefirstplanettotransit
a pulsating star. In the discovery paper, CollierCameronet al.
(2010) find evidence for non-radial pulsations in their spectral
time-series,andtentativelyclassified WASP-33asa γ Doradus
pulsator, which is a class of non-radial pulsators with periods
of ∼0.3 days or longer (see e.g. Handler&Shobbrook 2002).
Recently, Herreroetal. (2011) analysed photometric time se-
ries for WASP-33 and found a pulsation period of 68.6 min- Fig.1.RawlighcurvesforWASP-33andthereferencestar(mul-
utes, which, when converted to the pulsation parameter Q, the tipliedby2.08forplottingpurposes)forthenightofAugust18,
productofthepulsationperiodandthesquare-rootofthemean 2010(toppanel)andforthe nightofSeptember20,2010(bot-
stellar density (e.g. Breger 1990; Handler&Shobbrook2002), tom panel).The verticaldashed lines indicate the expected be-
iscomparabletothatofδScutistars,andwelloutsidetherange ginning and end of the targeted eclipse. The solid, grey, lines
ofγDoradusstars.Theobservedstellarpulsationshaveamea- show the airmass during the nights, scaled to match the stellar
surableimpactonthetransitandeclipsemeasurementsforthis fluxinthefirsthouroftheobservations.
planet(e.g.Herreroetal.2011;Smithetal.2011;Demingetal.
2012).
InSect.2wepresentourobservationsanddatareduction.In
Sect.3stellarpulsationsandthelightcurvefittingarepresented.
Subsequently we discuss the results in Sect. 4, and finally we
tions were performed in staring mode. Since this method does
willgiveourconclusionsinSect.5.
notallow forbackgroundsubtractionusing the science frames,
asetofskyframeswereobtainedaftertheobservationsonboth
nightsforsky-subtractionpurposes.
2. Observationsanddatareduction
OnAugust18(NightI)theobservationsstartedat00:45UT
2.1.Observations and lasted for ∼4.5 hours. The weather conditions during the
nightwerephotometric,ascanbeseenfromtherawlightcurves
The secondary eclipse of WASP-33b was observed on two
showninthetoppanelofFig.1.Atotalof5100scienceframes
nights, on August 18, 2010 and September 20, 2010, in the
wereobtainedwithanaveragecadenceof3.3seconds.Thefirst
K -band with the Long-slit Intermediate Resolution Infrared
s threeframesofasequenceofframes1 areknowntosufferfrom
Spectrograph(LIRIS; Acosta-Pulidoetal. 2002) instrumenton
theresetanomaly,whichisseenasananomalousstructureinthe
theWilliamHerschelTelescope(WHT)onLaPalma.
background.These frames are therefore excluded from further
The pixel scale of LIRIS is 0.25 arcsec per pixel, yielding
analysis,whichresultsinatotalof4994frames.
a field-of-view of 4.2 by 4.2 arcminutes, large enough to ob-
serve both WASP-33 and a reference star of similar brightness TheobservationsonSeptember20(hereafternightII)were
simultaneously.Since WASP-33 is very bright, exposuretimes taken between 22:35 UT and 05:00UT.During the first part of
of1.5secondswereusedinordertoavoidsaturationofthede- theobservationstheconditionswerephotometric,howeverdur-
tector. As an additionalmeasureto preventsaturation,the tele- ingthelastfewhoursoccasionalcloudsmovedacrosstheimage,
scope was strongly defocused. This is a well proven strategy absorbingupto65%ofthelight(seeFig.1).Atotalof6693sci-
alsousedforotherGROUSEobservations(deMooij&Snellen enceframeswereobtainedwithanaveragecadenceof3.3sec-
2009; deMooijetal. 2011), which should also reduce the im- ondsperframe,excluding207framesduetothereset-anomaly.
pact of flat-field inaccuraciesby spreading the light overmany
pixels, thereby minimizing the impact from uncorrected pixel-
to-pixelsensitivityvariations.Tokeeptheobservationsasstable 1 The observations consisted of multiple sequences of 100-200
as possible, and in orderto reducethe cycle time, the observa- frames
2
DeMooijetal.:TheGROUSEprojectIII:ThesecondaryeclipseofWASP-33b
2.2.Datareduction
The data-reduction for both nights was performed in the same
way. All frameswere correctedforcrosstalk along rows of the
detector,whichispresentatalevelof10−5ofthetotalfluxalong
the rows of all four quadrants. Subsequently we performed a
non-linearity correction on all the frames using our own non-
linearitymeasurementswhichwerecreatedfromasetofdome-
flatsataconstantlevelofilluminationbutwithvaryingexposure
times.Afterthesecorrectionstheimageswereflat-fieldedusing
aflat-fieldcreatedfrombrightanddarktwilightflats.
Abackgroundmapwasconstructedfromthesetofdithered
images obtained after the eclipse observations. These images
were reduced in the same way as the science images, and, af-
ter filtering out the discrepant pixels in time to remove (faint)
stars, were subsequently combined. The resultant background
mapwasthenscaledandsubtractedfromtheindividualscience
images.
Afterbackgroundsubtraction,aperturephotometrywasper-
formedonbothWASP-33andthereferencestarusinganaper-
tureof18and26pixelsfornightIandnightIIrespectively.Any
residual sky backgroundwas determined in annuli between 30
and50pixelsfornightIandbetween40and60pixelsfornight
II.Thefluxintheannuliwasclippedat5σtoavoidoutliersinthe
background(suchashotpixels)fromaffectingthedata.Finally,
thelightcurveofWASP-33wasnormalisedwiththatoftheref-
erencestar,andtheresultantlightcurvesforthetwonightsare
showninthetoppanelsofFigs.3and4.
Fig.2.Normalisedperiodogramsofthelightcurvesforthetwo
3. Correctionforsystematiceffectsandstellar separatenights.Thetoppanelshowstheperiodogramfornight
pulsations IandthebottompanelshowstheperiodogramfornightII.The
dashed lines in both panels indicate the periodsused in the fit-
3.1.Stellarpulsations
ting.
CollierCameronetal. (2010) noted that the host-star of
WASP-33b is a pulsator. Observationsby Herreroetal. (2011)
indicatedadominantperiodof68.5minutes.Smithetal.(2011)
is found at 52.1 minutes, while three weaker peaks are found
observedasecondaryeclipseofWASP-33binanarrowbandfil-
at 43.3, 65.3 and 83.9 minutes. The signals at all the periods
ter at 0.91µm, and found three pulsation periods in their data,
in both datasets havea false-alarm probability,estimated using
at 53.62, 76.52and 41.85minutes, all with amplitude between
Monte-Carlosimulations,ofbelow0.1%,indicatingthatthepe-
0.4mmagand0.9mmag.ObservationsbyDemingetal.(2012)
riodicities are very likely real, although they might not be as-
showedvariouspulsationperiods,68minutesforobservationsof
trophysicalin origin.In bothdatasets we find a periodicsignal
two separate events in the mid-infraredfrom the Spitzer Space
at ∼65 minutes, which differs from the period of 68.5 minutes
Telescope,71minutesforoneground-basedeventintheK-band,
found by Herreroetal. (2011) and the 68 minute period from
146minutesforobservationsin the J-band and 54 & 126 min-
thetwoSpitzerdatasetsfromDemingetal.(2012).However,the
utesforasecondsetofK-bandobservations.Foralltheirperiods
shorttimespancoveredduringeachnightisnotsufficienttoget
theamplitudeswerelargerthan1mmag,withthetwonightsof
a very tight constraint on the period, and therefore the periods
K-banddatashowingamplitudesinexcessof2mmag.
couldbeconsistentwith 68minutes.The∼43minuteperiodis
Forourdata, the stellar pulsationsare clearly visible in the
seen in the measurements from both our nights, as well as in
light curve for night I, while for night 2 the variability is less
the data from Smithetal. (2011). The periodaround ∼52 min-
apparent.Inordertodeterminetheperiod(s)ofthestellarpulsa-
utes is foundin bothoursecondnightof data as well as in the
tions, a periodogramof the light curves was created. Since the
datafromSmithetal.(2011)andDemingetal.(2012),although
planetaryeclipsesignalandpossiblesystematiceffectscaninflu-
thereitisnotthedominantfrequency.Wecaution,however,that
encethe periodsfoundin the data,botha scaled eclipse model
the periodogramsused for the frequencyanalysis were created
aswellasamodelforthesystematicsbasedoninstrumentalef-
withdatathatwasonlypartiallycorrectedforsystematiceffects,
fects(seesection3.2)werefittedtothedatausingasimplelinear
andthereforecanstillbeinfluencedbyresidual(quasi)periodic
regressionalgorithmandsubsequentlydividedoutbeforedeter-
systematiceffects.
miningtheperiodogram.ThegeneralisedLomb-Scargleformal-
ism from Zechmeister&Ku¨rster (2009) was used to construct
the periodogram, and the results for both nights are shown in
3.2.Lightcurvefitting
Fig.2.
FornightIthestrongestpeakintheperiodogramisfoundat Since the lightcurveis the result of a combinationof three ef-
aperiodof64.5minutes,andthereisaweakerpeakat43.2min- fects,thestellarvariability,systematiceffectsrelatedtoboththe
utes.FornightII,therearefourpeaksvisible,thestrongestpeak instrumentandtheEarth’satmosphereandthesecondaryeclipse
3
DeMooijetal.:TheGROUSEprojectIII:ThesecondaryeclipseofWASP-33b
Table1.FittedparametersanduncertaintiesfromtheMCMCanalysisofthelightcurvesofWASP-33b.
Parameter Night1 Night2
unit Instrumentalparameters Polynomial Instrumentalparameters Polynomial
F /F (%) 0.140±0.007 0.092±0.017 0.245±0.009 0.246±0.018
p ∗
x 0.06±0.01 — -0.12±0.02 —
airmass -0.76±0.03 — 0.34±0.03 —
sky 0.27±0.03 — -0.53±0.03 —
c — 0.42±0.01 — 0.10±0.01
1
c — -0.09±0.02 — -0.01±0.02
2
c — 0.10±0.01 — -0.13±0.01
3
P (minutes) 63.95±0.39 62.00±0.51 52.65±0.39 52.28±0.51
1
P (minutes) 43.22(fixed) 43.34(fixed)
2
P (minutes) — 65.29(fixed)
3
P (minutes) — 83.90(fixed)
4
A (%) 0.095±0.004 0.082±0.005 0.056±0.006 0.058±0.006
1
A (%) 0.041±0.004 0.042±0.004 0.017±0.005 0.014±0.005
2
A (%) — 0.011±0.006 0.011±0.006
3
A (%) — 0.031±0.006 0.013±0.006
4
Notes.Baselinecoefficentsfortheinstrumentalmodelarethex-position,airmassandskylevelandforthepolynomialbaselinethecoefficients
arec toc .P toP aretheperiodsusedforthestellarpulsations,andA toA arethecorrespondingamplitudes.
1 3 1 4 1 4
ofWASP-33b,afitforallthreeeffectsisperformedsimultane- Beforefitting,outlierswereremovedbyexcludingallpoints
ously. that were morethan 0.9%away froma mediansmoothedlight
curvewithaboxsizeof51points,aswellasallpointsforwhich
For the stellar pulsations the period of the dominant mode
is left as a free parameter, although with a penalty for the χ2 the flux of the individual stars, corrected for airmass, dropped
oftheform(P-P )2/σ2,withP theperioddeterminedfromthe below90%.Inthiswayatotal21and414pointswereexcluded
0 p 0 duringthefirstandsecondnightrespectively.Inaddition,there
periodogram,and σ set to 1 minute. The periods of the other
p is a feature present in both light curves at the same time after
modeswaskeptfixedtoperiodsfoundintheanalysisofthepe-
the start of the observations(after 0.1795±0.0025days) that is
riodogramasdescribedintheprevioussection.Forallthemodes
not at an identical point during the planet’s orbit and therefore
theoffsetin phaseandtheamplitudeofthe pulsationswere al-
most likely due to an, as yet, unidentified instrumental effect.
lowedtovaryfreely.
Excluding all the points that were obtained during this feature
For the fitting of systematic effects two different methods removesanadditional128frames.
were used. Forthe first method,the systematic effects are con-
Thelightcurveswerefittedwith9freeparameters(1forthe
sidered to be due to the change of position on the detector,
eclipse, 3 for the systematic effects and 5 for the stellar pulsa-
the airmass and the difference in sky background between the
tions) fornightI, and 13 free parametersfornight II (due to 2
twoquadrants.Thisissimilartowhatwasusedintheprevious
additional periods found in the data). The two nights were fit-
papers from the GROUSE project (deMooij&Snellen 2009;
tedseparatelyusinga Markov-ChainMonteCarlo method.Per
deMooijetal. 2011). In the second method,the systematic ef-
night,5 sequencesof2millionstepsweregenerated,trimming
fectsaremodelledusingloworderpolynomials,asalsoused in
thefirst200,000pointstoavoidanycontaminationfromtheini-
deMooijetal. (2012) for the near-infraredtransit observations
tial conditions. The chains were combined after checking that
ofGJ1214b.
theywerewellmixed(Gelman&Rubin1992).
The secondary eclipse was modelled using the
Mandel&Agol (2002) formalism. We used the parameters
from CollierCameronetal. (2010) for the impact parameter, 4. Resultsanddiscussion
semi-major axis, orbital period and planet-to-star size ratio,
while the orbit of the planet is assumed to be circular. This The best fit values of the eclipse depth and their fornal uncer-
assumptionisreasonablesincetheplanetorbitsextremelyclose taintiesfornightIare0.140±0.007%and0.092±0.017%forthe
to its host-star, which should result into a rapid damping of fit with instrumental parameters and polynomials respectively,
eccentricity.In addition,on the secondnightan eclipse-shaped while for nightII the best-fit eclipse depths are 0.245±0.009%
dipin the lightcurvecenteredon φ ∼0.5isreadilyvisible (see and0.245±0.018%forthetworespectiveanlyses.Theseresults
therightpanelofFig.3). aregiveninTable1.
4
DeMooijetal.:TheGROUSEprojectIII:ThesecondaryeclipseofWASP-33b
Fig.3.LightcurvesforthesecondaryeclipseofWASP-33bforthenightI(leftpanels)andnightII(rightpanels).Toppanels:the
lightcurvesof WASP-33 normalisedwith those of the referencestar, overplottedis the best fitting ’full’ modelwith a low order
polynomialbaseline correction,stellar pulsationsand the eclipse. Middle panels: The lightcurvescorrected forthe trends in the
baseline and stellarpulsations, clearly showingthe transit. Bottom panels:The residualsaftersubtractingthe best-fitmodel.The
thickpointswitherrorbarsinthesefiguresshowthedatabinnedby50points.Theverticaldashedlinesshowtheexpectedtimesfor
firsttofourthcontact.
Thedifferencesfoundbetweenmethodsinthefirstnightare of0.255±0.028%and0.242±0.035%forthefitwithinstrumen-
significantlylargerthantheuncertaintiesintheeclipsedepthas tal parameters and polynomials respectively. To assess the im-
estimated from the MCMC analysis. First of all the first night pact of (uncorrected)red noise on the measured eclipse depths
suffers from a strong peak, possibly due to stellar pulsations, inanotherway,theresidualpermutationmethodwasused(e.g.
right in the middle of the eclipse. We attribute this to a prob- Gillonetal.2007).Thebestfitmodelissubtractedfromthelight
lemwiththeobservationsonnightI.Therelativelyshortout-of- curve, and these residuals are then shifted by n points, wrap-
eclipsebaselinesavailableforthefirsteclipseobservationham- ping the light curve around, so that the points that are shifted
perstheremovalofthesystematiceffectsfromthestellarpulsa- beyond the end of the lightcurve are inserted at the beginning.
tions,aswellasfromtheinstrumentalandatmosphericeffects. Thebestfit modelis thenaddedback tothe data, andthisnew
This is clearly illustrated when looking at the correlations be- light curve is fitted again. The interval between 16% and 84%
tween the parameters used for the removal of the systematics of the distribution of the best-fitting eclipse depths is used for
and the eclipse depth, as shown in Figs. A.1 to A.4. We there- the1-σuncertaintiesontheeclipsedepth.Tospeeduptheresid-
foreconcludethatthefirstnightofdataisnousableforareliable ualpermutationanalysis,insteadofaddingbackthefullmodel,
eclipsemeasurement. which includes the stellar pulsation, trends in the baseline and
eclipsedepth,weonlyusedthetrendsinthebaselineandeclipse
To assess the impactofcorrelatednoise,we redidtheanal- depth,sincethecorrelationbetweentheparametersforthe stel-
ysis for night II after binning the data by 50 points (∼3 min- lar pulsations and the eclipse depth is weak. From the residual
utes). Although overall the parameters are the same, we find permutation analysis we also find larger uncertainties for both
largeruncertaintiesintheeclipsedepthswiththebest-fitvalues
5
DeMooijetal.:TheGROUSEprojectIII:ThesecondaryeclipseofWASP-33b
Fig.4.SameasFig.3butthesystematiceffectsarenowmodelledbylinearrelationswithinstrumentalparameters.
decorrelation methods, with eclipse depths of 0.244+0.027% for band and in the IRAC 3.6µm and 4.5µm bands, obtaining
−0.020
abaselinefittedwithinstrumentalparametersand0.249+0.033% eclipse depths of 0.27±0.04%, 0.26±0.05% and 0.41±0.02%
−0.052
for a polynomial baseline fit. In all cases the uncertainties are respecively. These brightness measurements correspond to
higher than for the MCMC analysis of the unbinned data but a brightness temperature of 3490±140 K, 3415±130 K,
comparabletotheMCMCanalysisofthebinneddata,whichis 2740±225 K, 3290±100 K for the SII, Ks, IRAC 3.6µm and
expectedinthepresenceofrednoise. 4.5µmrespectively.Althougha planet’sbrightnesstemperature
Since there is a strong correlation between the coefficients can be a strong function of wavelenght, most of the measure-
for the polynomial baseline fit and the eclipse depth (see mentsareconsistentatthe1σlevel.
Fig. A.2), we use the fit of the baseline with instrumental pa-
AscanbeseeninthemiddlerightpanelofFig.4,theeclipse
rameters for the remainder of the paper, since the correlation
appearsto endearlierthan expectedfromthe model.Although
betweendifferentparametersis muchweaker.We note thatthe
systematic effects are the most likely cause of this, it is worth
polynomial baseline correction for this night gives the same
noting that a narrowerwidth of the eclipse would indicate that
eclipsedepth,howeverwithalargeruncertainty.
theorbitofWASP-33biseccentric.Ifthisisthecase, bycom-
The measured eclipse depth of 0.244+0.027 % corresponds
−0.020 bining the ratio between the transit and secondary eclipse du-
to a brightness temperature in the Ks-band of 3270+−111650 K. rations with the time of mid-eclipse, a direct measurement of
This brightness temperature was calculated using the solar- both the eccentricity and the argument of periastron is possi-
metallicity NextGen models (Hauschildtetal. 1999) interpo- ble(e.g.Charbonneauetal.2005).Althoughafullfitisbeyond
lated to the stellar parameters of WASP-33 determined by the scope of this work,we can estimate the changein duration
CollierCameronetal.(2010)(Teff=7430K,log(g)=4.294). from the light curve. The duration of the secondary eclipse is
Currently there are several other measurments of the sec- shorterthan the transit durationby 0.01in phase, such that the
ondary eclipse of WASP-33b, Smithetal. (2011) obstained eclipsedurationcorrespondsto ∼90%oftransitduration.From
data in a narrowband filter at ∼9100Å, showing a depth of this we estimate esin(ω)∼0.05. Since the ingress appears to be
0.09±0.016%. Demingetal. (2012) observed both in the Ks- at the expected time, the time of mid-eclipse is in this case
6
DeMooijetal.:TheGROUSEprojectIII:ThesecondaryeclipseofWASP-33b
Fig.6. Equilibrium temperature of WASP-33b for different
albedo (A) and re-radiation factors (f). Solid contours show
lines of constant temperatures at 150 K intervals. Overplotted
are lines of constant temperature for the measured brightness
temperatures (dashed lines), labeled with the bandpass they
were observed (D12 indicates the K -band measurement from
Fig.5. Spectral energy distribution of WASP-33b. Top panel: s
Demingetal.(2012).Thelinelabeled“Avg.”indicatestheline
EclipsedepthsinK -band(thiswork)andinthe SII -filter
s 0.91µm forconstanttemperatureoftheeffectivetemperaturedetermined
fromSmithetal.(2011).Bottompanel:brightnesstemperatures
from the literature measurements. Vertical (dashed) lines indi-
in the two bands. Overplotted in both panels are the expected
catethere-radiationfactorsforahomogeneousday-sidetemper-
eclipsedepths/brightnesstemperaturesforazero-albedohomo-
ature(f=1/2),andforaninstantlyre-radiatingday-side(f=2/3).
geneousday-side (solid line), for an instantly re-radiatingday-
side(dashedline)andforthebest-fiteffectivetemperature(dot-
tedline). Lo´pez-Morales&Seager(2007):
R 1/2
T =T ∗ [f(1−A )]1/4 (1)
p ∗(cid:18) a (cid:19) B
also slightly earlier than expected. From the shift we estimate
With R the stellar radius, a the semi-major axis, A the bond
ecos(ω)∼0.008.Combining these two estimates we find an ec- ∗ B
albedoand1/4<f<2/3,wheref=1/4isforahomogenoustem-
centricityof∼0.05.Weagaincautionthatsystematicscaneasily
peraturedistriubutionacross the planetand f=2/3is forinstant
giverisetoanapparentnon-zerodeterminationoftheeccentric-
re-radiation.
ity,whichisforinstanceseenforthesecondaryeclipseofTrES-
InFig.6weshowasimplemodeloftheequilibriumtemper-
3b (deMooij&Snellen 2009; Fressinetal. 2010; Crolletal.
atureasafunctionofalbedoandre-radiationfactor.Inaddition
2010b). We therefore do not advocate this non-zero eccentric-
weshowlinesofconstanteffectivedaysidetemperaturewiththe
ityscenariobasedonthesedata.
observedbrightnesstemperaturesandderivedeffectivetempera-
tureoverplotted.Ascanbeseenthemeasurementsrequireavery
lowalbedoandaveryshortre-radiationtimescale,suchthatall
4.1.Alowalbedoandrapidre-radiationofincidentlight
thestellarfluxisabsorbedandrapidlyre-radiatedwithout hav-
With the exception of the Spitzer 3.6µm measurement from ingtimetoadvecttothenight-sideoftheplanet.Thisisconsis-
Demingetal.(2012),allofthecurrentlyavailableeclipsemea- tent with the findings of Cowan&Agol (2011), who study the
surementsforWASP-33bpointtowardsaveryhotday-sidetem- albedo and redistribution efficiencies for a large sample of hot
perature. If we assume that the measured brightness tempera- Jupiters,andfind thatthe hottestplanets(intheirsample) have
turesarerepresentativeofWASP-33b’sequilibriumtemperature, a low albedoand a low efficiencyof the advectionof absorbed
andarenotgenerateddeepinsidetheplanetsatmosphere,where stellarfluxtotheplanet’snightside.
the temperaturesare evenhigher,we can constrainthe planet’s The low redistribution efficiency suggests that the re-
equilibrium temperature to T =3298+66K (see also Fig. 5). radiationtimescalesareshort,andthattheplanetprobablyhasan
eff,p −67
Note that the basic assumption that the brightness temperature inversion layer (Fortneyetal. 2008). Knutsonetal. (2010) hy-
equals the effective temperature does not necessarily has to be pothesisethatanincreaseintheUV-fluxfromanactivestarcan
the case, but detailed modellingof the available measurements cause a shift in the photochemistry such that the efficient ab-
isbeyondthescopeofthispaper.Fromthisequilibriumtemper- sorberisremovedfromthegas-phase.ThehighincidentUV-flux
aturewecanfurtherconstrainthere-radiationfactorandalbedo. onWASP-33bwouldargueagainstthis.Itshouldbenotedthat
Forthere-radiationfactorweusedthefdescription,asusedby for active stars most of the UV flux is emitted in the Lyman α
7
DeMooijetal.:TheGROUSEprojectIII:ThesecondaryeclipseofWASP-33b
line,whileforWASP-33itislikelythattheUVcontinuumemis- Herrero,E.,Morales,J.C.,Ribas,I.,&Naves,R.2011,A&A,526,L10+
siondominates.ToinvestigatetheinfluenceoftheUV-radiation, Knutson,H.A.,Howard,A.W.,&Isaacson,H.2010,ApJ,720,1569
photochemical modelling will be necessary (e.g. Zahnleetal. Lo´pez-Morales,M.,Coughlin,J.L.,Sing,D.K.,etal.2010,ApJ,716,L36
Lo´pez-Morales,M.&Seager,S.2007,ApJ,667,L191
2009).
Madhusudhan,N.&Seager,S.2010,ApJ,725,261
Mandel,K.&Agol,E.2002,ApJ,580,L171
Rogers,J.C.,Apai,D.,Lo´pez-Morales,M.,Sing,D.K.,&Burrows,A.2009,
5. Conclusion ApJ,707,1707
Sing,D.K.&Lo´pez-Morales,M.2009,A&A,493,L31
We have presented our results of Ks-band observations of the Smith,A.M.S.,Anderson,D.R.,Skillen,I.,CollierCameron,A.,&Smalley,
secondary eclipse of WASP-33b, the most irradiated planet B.2011,MNRAS,416,2096
Zahnle,K.,Marley,M.S.,Freedman,R.S.,Lodders,K.,&Fortney,J.J.2009,
knowntodate.Themeasuredeclipsedepthforthesecondnight
ApJ,701,L20
is 0.244+0.027 %, which results in a brightness temperature of
−0.020 Zechmeister,M.&Ku¨rster,M.2009,A&A,496,577
3270+115 K. Thishigh brightnesstemperature,if representative
−160
for the planet’s equilibrium temperature, requires a very low
albedoandahigh(f&0.5)re-radiationfactor.
AppendixA: CorrelationplotsfortheMCMC
CombiningourK -bandmeasurementwiththemeasurement
s
of Smithetal. (2011), we can fit a simple blackbody function analysis
to the spectral energy distribution, and determine an effective
temperatureofT =3290+66K.
eff,p −67
We also find that stellar pulsations of the δ Scuti host-star,
WASP-33, appears to have switched modes between the two
nights, which are located a month apart, and also differ from
the measurements by Herreroetal. (2011). We caution, how-
ever, that this could be due to systematic effects which could
alsohavestrongperiodicities.
Themeasurementsonthefirstnightsufferfromstrongresid-
ual systematics and stellar pulsations that cannot be fully cor-
rectedduetotheshortout-of-eclipsebaseline,anddemonstrates
theneedforobservingatargetforaslongaspossible.
Acknowledgements. WearegratefultothestaffoftheWHTtelescopefortheir
assistance with these observations. The William Herschel Telescope is oper-
ated on the island of La Palma by the Isaac Newton Group in the Spanish
Observatorio del Roque de los Muchachos of the Instituto de Astrof´ısica de
Canarias.
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8
DeMooijetal.:TheGROUSEprojectIII:ThesecondaryeclipseofWASP-33b
Fig.A.1.CorrelationsbetweendifferentparametersfromtheMCMCanalysisfornightI.Thebaselineismodelledusingapolyno-
mialwithcoefficientsc toc .Thecorrelationbetweentheparametersandtheamplitudes ofthestellarpulsationatlowersignifi-
1 3
cancearenotshown,sincetheirimpactonanyofthemeasuredparameterswillbeminimal.Theamplitudeforstellarpulsationis
forapulsationperiodof62.00minutes.
9
DeMooijetal.:TheGROUSEprojectIII:ThesecondaryeclipseofWASP-33b
Fig.A.2.sameasFig.A.1butnowfornightII.Theamplitudeforstellarpulsationisforapulsationperiodof52.28minutes.
10