Multi-Color Simultaneous Photometry of the T-Tauri Star Having A Planetary Candidate CVSO 30

0.24 MB·

by  Masahiro Onitsuka

Publ.Astron.Soc.Japan(2014)00(0),1–6 1
doi:10.1093/pasj/xxx000
Multi-Color Simultaneous Photometry of the
T-Tauri Star Having A Planetary Candidate
CVSO 30
7
1 Masahiro ONITSUKA1,2, Akihiko FUKUI3, Norio NARITA,1,4,5, Teruyuki
0 HIRANO6, Nobuhiko KUSAKABE,1,5, Tsuguru RYU1,2 and Motohide
2
TAMURA,1,4,5
n
a 1NationalAstronomicalObservatoryofJapan,NINS,2-21-1Osawa,Mitaka,Tokyo181-8588,
J
Japan
6
2SOKENDAI(TheGraduateUniversityforAdvancedStudies),2-21-1Osawa,Mitaka,Tokyo

] 181-8588,Japan
P
3OkayamaAstrophysicalObservatory,NationalAstronomicalObservatoryofJapan,NINS,
E
Asakuchi,Okayama719-0232,Japan
.
h 4DepartmentofAstronomy,TheUniversityofTokyo,7-3-1Hongo,Bunkyo-ku,Tokyo,
p
113-0033,Japan
-
o 5AstrobiologyCenter,NINS,2-21-1Osawa,Mitaka,Tokyo,181-8588,Japan
r
t 6DepartmentofEarthandPlanetary Sciences,TokyoInstituteofTechnology,2-12-1
s
a Ookayama,Meguro-ku,Tokyo,152-8551,Japan
[
∗E-mail:[email protected]
1
v Received2016September7;Accepted2017January6
8
8
Abstract
5
1 We present three-band simultaneous observations of a weak-line T-Tauri star CVSO 30
0
(PTFO 8-8695), which is one of the youngest objects having a candidate transiting planet.
.
1 The data were obtained with the Multicolor Simultaneous Camera for studying Atmospheres
0
of Transiting exoplanets (MuSCAT) on the 188 cm telescope at Okayama Astrophysical
7
1 Observatory in Japan. We observed the fading event in the g2′-, r2′-, and zs,2-bands simul-
: taneously. As aresult,wefindasignificantwavelengthdependence offadingdepths ofabout
v
Xi 3.1%, 1.7%, 1.0% for the g2′-, r2′-, and zs,2-bands, respectively. A cloudless H/He dominant
atmosphereofahotJupitercannotexplainthislargewavelengthdependence. Additionally,we
r
a rule out a scenario by the occultation of the gravity-darkened host star. Thus our result is in
favorofthefadingoriginascircumstellardustclumporoccultationofanaccretionhotspot.
Keywords:planetarysystems—stars:individual(CVSO30,PTFO8-8695)—techniques:photometric
1 Introduction of 323+−29363 pc. CVSO 30has themass of0.44M⊙ (usingthe
Baraffe et al. 1998 model) or 0.34M⊙ (using the Siess et al.
CVSO30b(PTFO8-8695b)isacandidatetransitinghotJupiter
2000 model), the radius of 1.39R⊙, and the effective temper-
around a weak-line T Tauristar in the Orion OB1a/25-Ori re-
ature of 3470 K (Bricen˜o et al. 2005). The candidate planet
gion found by van Eyken et al. (2012). The age of CVSO 30
has the orbital period of 0.448413±0.000040 days which is
isestimatedas∼3Myr(Bricen˜oetal.2005) makingthisob-
comparabletotherotationalperiodofthehoststar,themassof
jecttheyoungestcandidateplanet. ThehoststarisanM3type
Mp ≤5.5±1.4MJup, theradius of1.91±0.21RJup,andthe
pre-main-sequencestarintheOrionOB1aregionatadistance
(cid:13)c 2014.AstronomicalSocietyofJapan.
2 PublicationsoftheAstronomicalSocietyofJapan,(2014),Vol.00,No.0
orbital semi-major axis of 0.00838±0.00072 AU(van Eyken fading shape is changing with time. We use the Multicolor
etal.2012). Simultaneous Cameraforstudying Atmospheres of Transiting
Incontrasttootherplanetarytransitingobjects,theobserved exoplanets (MuSCAT)(Naritaetal.2015)onthe188cmtele-
fadinglightcurvesofCVSO30changewiththeobservational scopeatOkayamaAstrophysicalObservatory(OAO)toinvesti-
epoch. Barnes et al. (2013) explained them in the combina- gatethewavelengthdependenceofafadingevent.Wedescribe
tion of the precession of planetary orbit’s ascending node and the observations and the analysis method in sections 2 and 3.
thestellargravitydarkeningeffectassumingsynchronizationof Section4showstheobservedlightcurves,theresultsoftransit
the stellar rotation and the planetary orbital motion. Kamiaka fittingandwavelengthdependence.InSection5,wediscussthe
etal.(2015)addedthephotometricdataandreanalyzedthelight originoftransit-likeevents. Finally,wesummarizeourresults
curves using the precession and the gravity-darkening model andfutureplansinsection6.
withouttheco-rotation.
On the other hand, Ciardiet al.(2015) observedaprimary
2 Observation
transitat4.5µmusingSpitzerIRACwithoutanysignofasec-
ondary eclipse. Inaddition, their radial velocity measurement We observed a fading event of CVSO 30 on 2016 Feb. 9
showednoevidenceoftheRossiter-McLaughlineffect.Yuetal. (UT).WeusedtheMuSCATonthe188-cmtelescopeatOAO.
(2015)observed13eventswithi′-bandandother13eventswith MuSCATcansimultaneouslytakethree-bandimagesusingtwo
I+z-bandoverthreeyears.Theyfoundthatthefadingperiodis dichroic mirrors and the SDSS g2′-, r2′-, and zs,2-band filters.
notconstant. Multiband simultaneous observations weredone MuSCATequipsthree1024×1024pixelsCCDswiththefield
for five fading events, of which r′- and I+z-bands observed ofviewof6′.1×6′.1.
simultaneously ontwoother nights, i′-andg′-bands observed Toachievethehighprecisionphotometry,wedidnotdither
on another two nights, and I+z- and H-bands observed on butfixedthetargetposition onthedetector,anddefocusedthe
onenight. Theirmultibandobservationsyieldedthatthedepth stellar PSF. The exposure times were 60 seconds for all the
of all but one fading events was deeper at short wavelengths; bands. We obtained 238, 236, and 236 frames for the g2′-,
oneofthefadingeventsinther′-andI+z-bandsshowedthe r2′-, and zs,2-bands, respectively, in 4.4 hours. Data reduction
same depth. Additionally, they tried to detect the secondary andaperturephotometry wereprocessedusingthecustomized
eclipseatinfraredandtheRossiter-McLaughlineffectbyhigh- pipelinebyFukuietal.(2011).
resolutionspectroscopy. However,theywereunabletofindthe
expected signature inboth observations but found strongvari-
3 Analysis
able Hα and Ca H & K lines. Hence Yu et al. (2015) argued
thatthetransit-likeeventswereunlikelytobecausedbyagiant Our analysis method is similar to that used by Fukui et al.
planet but rather caused by either starspots near the rotational (2016). First, we make light curves using different combina-
pole, circumstellar dust clump transit or occultation of an ac- tionsofcomparisonstarsandvariousapertureradii.Wechoose
cretionhotspot. Raetzetal.(2016) observed 33fadingevents thecomparisonstarswhicharenotsaturatednorvariablestars
and found that the period of the fading event was shortened. andtheapertureradiuswhichproducesalightcurvewithmini-
They also observed a fading event in the B- and the R-bands mumdispersionoutsidethefadingevent.Theselectedaperture
simultaneously,andfoundthatthelightcurveshowedthesame radiiare17,14, and12pixels fortheg2′-,r2′-,andzs,2-bands,
depth. Johns-Krulletal.(2016)observedexcessHαemission respectively, and the comparison stars are the same two stars
of CVSO 30. The excess Hα emission was not detected con- foreachband.Second,weconvertthetimesystemofModified
stantly. However, the observed velocity motion in transit was JulianDay(MJD)inUniversalTimeCoordinate(UTC)which
almost expected. They argued that this excess came from the is recorded in the FITS headers into Barycentric Julian Day
planetinthemiddleofmassloss. (BJD) in Barycentric Dynamical Time (TDB) using the algo-
Aspresentedabove,thetruenatureofCVSO30bisstillun- rithm of Eastman, Siverd, & Gaudi (2010). We also remove
clear, and both planet and non-planet scenarios have been ar- outliersthatseparatebeyond5σfrommeanrelativefluxofeach
gued. Thus further observational findings are still important band.Asaresult,weuse172,221,and220datafortheg2′-,r2′-
to reveal the identity of CVSO 30b. We focus on that in the , and zs,2-bands, respectively. The produced light curves are
previous studies both the presence and absence of the wave- showninthetoppanelsinFigure1.
length dependence inthedepth offadingevents havebeenre- Next,wefitthelightcurveswithatransitmodel,assuming
ported(Yu etal.2015; Raetz etal. 2016). Weherereportour that the fading event is caused by a transiting spherical body.
resultsofindependentthree-bandsimultaneoustransitphotom- Weusethe transit light curvemodel by Ohta, Taruya, & Suto
etry for a fading event of CVSO 30. The simultaneity is im- (2009)whichiscomparabletothequadraticlimb-darkeninglaw
portantbecauseCVSO30hasalargestellarvariabilityandits caseofMandel&Agol(2002). Thetransitmodelingparame-
PublicationsoftheAstronomicalSocietyofJapan,(2014),Vol.00,No.0 3
tersarethetimeofthetransitcenterTc,theratiooftheplanet Table1.Thebestfitparametersandthe
to thestellar radius Rp/Rs, the ratioof the semimajor axis to uncertainties
thestellarradius a/Rs, andtheimpactparameterb. Thevari- Parameter Value Uncertainty
ations seen outside the fading event seem to be caused by the Tc[BJDTDB] 2457428.0758 +−00..00001174
variabilityofCVSO30itselforcomparisonstars,withchang- a/Rs 1.730 ±0.061
b 1.054 +0.121
ingofairmass,shiftingofstellarimagesonthedetectorandso Rp/Rs(g2′) 0.344 −+−000...100187165
on.Wefitthetransitlightcurvesandbaselinemodelatthesame Rp/Rs(r2′) 0.271 +−00..100656
timeusingthecustomizedcodebyFukuietal.(2016)whichis Rp/Rs(zs,2) 0.206 +−00..100644
basedonNaritaetal.(2007)andNaritaetal.(2013). Atransit k0(g2′) 1.0103 +−00..000054534
andout-of-transit(OOT)modelisexpressedas k0(r2′) 1.0016 +−00..00001174
k0(zs,2) 0.99713 +−00..0000008704
F =(k0+XkiXi)×Ftr (1) kt(g2′) 0.010 +−00..005499
i=1 kt(r2′) 0.018 +−00..001197
whereF istherelativeflux,Ftristhetransitlightcurvemodel, kt(zs,2) −0.0489 +−00..00009837
{X} are time-dependent observed variables for example air-
mass,shiftsofstellarposition ontheCCDandsoon, {k}are Table2.Rp/Rsandtheuncertaintiesincase
coefficients(Fukuietal.2016). offixedvariousimpactparameterforeach
band
We fix the orbital eccentricity to zero and
Filter Impactparameterb Rp/Rs
the limb-darkening parameters to the values from
g′ 0.97 0.273+0.024
2 −0.026
Claret, Hauschildt, & Witte (2012), {u1, u2} = 1.05 0.342+0.025
−0.027
{0.8067, 0.0861}, {0.8423, −0.0160}, {0.2946, 0.3569} 1.18 0.451±0.028
fortheg2′-,r2′-,andzs,2-bands. Inaddition,weputaGaussian r2′ 0.97 0.205+−00..001112
prior for a/Rs = 1.685±0.064, based on the value of van 1.05 0.270±0.012
Eykenetal.(2012). 1.18 0.376+−00..001123
Forchoosing the most appropriate combination of freepa- zs,2 0.97 0.1427+−00..00008962
1.05 0.2057+0.0087
rameters,weoptimizetheparametersusingtheAMOEBAalgo- −0.0092
1.18 0.3099+0.0086
rithm(Pressetal.1992)andevaluatetheBayesianInformation −0.0090
Criteria (BIC: Schwarz 1978). The BIC value is defined by
BIC≡χ2+klnN, wherek isthenumberoffreeparameters, of theparameters between 15.87% and 84.13% of themerged
andNisthenumberofdatapoints.WecalculatetheBICvalues posteriordistributions.
foreachlight curveandparameter,thenwepickoutthecom-
binations tothe minimum BICvalue foreachband. For{X},
4 Results
wetestdifferentcombinations ofthetimet,thesquareoftime
t2,theairmassz,andtherelativestellarpositions ontheCCD Table 1 shows the best-fit transit parameters and uncertainties
∆x,∆y.BasedonminimumBIC,weadoptk0,ktforallbands. obtainedbytheMCMCanalysis.Notethatweobtainconsistent
Totakeintoaccountthetime-correlatednoise(so-calledred resultswhenwefitthelightcurvesthatareproducedbydiffer-
noise; e.g. Pont et al. 2006; Winn et al. 2008), we multiply entbaselinemodelswithcomparableBICvalues. Weshowthe
eachfluxuncertaintybythered-noisefactorβwhichistheratio transitlight curvesandthebest-fitmodelsofeachobservation
oftheobservedstandarddeviation ofthebinnedlightcurveto inFigure1.
theexpectedstandarddeviationoftheunbinnedandnon-time- van Eyken et al. (2012) and Yu et al. (2015) reported that
correlatedlightcurve. Thebinningsizesarebetween5and20 the fading depth of CVSO 30 changes with the observational
minutes. Weobtainthemedianofβ foreachbinning sizeand epoch. AccordingtoYuetal.(2015),thelossoflightwas20-
multiply the median β by the uncertainty of each data. The 30%largerinthebluerbandon2012Dec. 14atr′-andI+z-
typicalphotometricerrorsincludingβare2.1%,0.87%,0.54% band,on2014Jan. 9and2014Jan. 18ati′-andg′-band,and
fortheg2′-,r2′-,andzs,2-bands,respectively. on2014Jan. 19atI+z-andH-bandrespectively. However,
Afterthat,toestimatetheuncertaintyofthefreeparameters, the loss of light in r′ was essentially the same as in I+z on
weanalyzethechosenlightcurvebytheMarkovChainMonte 2012Dec.15.Wepresentthefadingdepthing2′-,r2′-,andzs,2-
Carlo(MCMC)methodfollowingNaritaetal.(2013)andFukui bandsobservedsimultaneously. Figure2givesthewavelength
etal.(2016).Wefirstanalyzeindependentlythelightcurvesfor dependenceofRp/Rs.Thetoppanelshowsthebest-fitparam-
eachband,andafterthatwejointlyanalyzeallthelightcurves. etersforRp/Rs andtheiruncertainties foreachbandinTable
We make the MCMC chain with 107 steps and first106 steps 1. Theuncertainties arestronglyinfluenced bytheimpactpa-
areexcludedasburn-in.Wedefine1σuncertaintiesastherange rameterbwhichisgrazing.WerecalculatetheRp/Rsandtheir
4 PublicationsoftheAstronomicalSocietyofJapan,(2014),Vol.00,No.0
1.04 g’2 r’2 zs2
1.02
Relative flux 0.9 18
0.96
0.94
1.04 g’2 r’2 zs2
1.02
Relative flux 0.9 18
0.96
0.94
0.04
Residuals- 00..00 022
-0.04
-0.05 0 0.05 0.1 -0.05 0 0.05 0.1 -0.05 0 0.05 0.1
BJD - 2457428 BJD - 2457428 BJD - 2457428
Fig.1.Toppanels:ThegraydotsaretherawlightcurvesofCVSO30observedbyg2′-,r2′-,andzs,2-bandfromtheleft.Theblackopencirclesare0.01days
binneddata.Thesolidlinesarethebest-fittransitmodels.MiddlePanels:Thelightcurvescorrectedbythebaselinecorrection.Bottompanels:Theresiduals
betweentheobserveddataandthebest-fitmodel.
uncertainties fixing the impact parameter at the median and 1
σupper/lowerlimitb={0.97,1.05,1.18} inordertohighlight 0.5
thewavelengthdependenceofRp/Rs(Figure2:bottompanel; 0.45
Table2).Astheresult,wefindthattheclearwavelengthdepen- 0.4
denceinRp/Rsisapparentregardlessofthevalueofb. 0.35
R/Rps 0.3
0.25
5 Discussion
0.2
EachofCVSO30’sfadinglightcurveshapesvariesintheob-
0.15
servational epoch, with someshowing largefading and others
0.1
showingsmallfading(vanEykenetal.2012).Therefore,multi- 400 500 600 700 800 900 1000
Wavelength [nm]
colorsimultaneousobservationsarenecessarytodiscusswave-
0.5
lengthdependenceoflightcurves.Thepreviousmulticolorob- b = 0.97
0.45 b = 1.05
servations are performed in two bands (Yu et al. 2015; Raetz b = 1.18
0.4
etal.2016). Inordertorevealprecisewavelengthdependence,
0.35
iwselaorbgseerrvaetisnhtohrrteeerbwaanvdeslesnimgtuhlst,an4e0o-u9s0l%y. Tlahregearppfaorreng2t′R−pz/sR,2s, R/Rps 0.3
and20-50%largerforr2′ −zs,2.Now,weconsiderwhetherthe 0.25
0.2
cloudlessatmospherecanexplainthiswavelengthdependence.
TheatmosphericscaleheightHiswrittenas 0.15
0.1
H= kBT (2) 400 500 600 Wavele 7n0g0th [nm] 800 900 1000
µg
wherekB istheBoltzmannconstant,T istheatmospherictem- rF2′ig-,.a2n.dTozps,2p-abnaenl:dTfrhoembTeasbtl-efit1p.aBroatmtoemtepraannedl:uTnhceerwtaaivnetylenogftRhpd/eRpesnidnegn2′c-e,
perature,µisthemeanmolecularweightwhichisµ≈2.3for ofRp/Rsattheimpactparameterfixedbyb={0.97,1.05,1.18}
H/He-dominatedatmosphere(e.g. deWit&Seager2013),and
PublicationsoftheAstronomicalSocietyofJapan,(2014),Vol.00,No.0 5
gisthelocalgravity.Weassumethattheplanetcandidateequi-
lfirbormiumuptpeemrpliemraittuorfeCofia1r8d0i0etKa,l.th(2e0p1l5an),etthareypmlaansestaorfy0r.a9dMiusJuopf 1 .10.0051 zgrs’’,222---bbbaaannnddd
x 1
iR2s.8⊙23R(0BJ0ruipckemfn˜rooomert0Ra.l0p.0/22R030s5Ro).fp.tThhiTsenhw,eortehrfkeorcaean,ldctuhsletaetlveladarrisarcataidolienuhsbeoeigftwh1te.3eHn9 Relative Flu 00 ..099.899559
ther2′-andthezs,2-bandcorrespondsto30H andthatbetween 0.98
theg2′-andthezs,2-bandis60H. Incomparison,thevariation 0.975-0.05 0 0.05 0.1 0.15 0.2
of HD 189733b from Sing et al. (2011, Fig. 14), the radius BJD-2457428
variation of HD 189733b between 630 nm and 880 nm corre-
Fig.3.Themodellightcurveincludedgravity-darkeningeffect.
sponding to the r2′- and the zs,2-band is 1.5 H, between 490
nm and 880 nm corresponding to the g2′- and the zs,2-band is Table3.Comparisonbetweentheobserveddepthandthe
2.5 H. In addition, we compare the wavelength dependence
gravity-darkenedtransitmodeldepth
withcloudless RayleighscatteringfollowingSouthworth etal. Filter observeddepth[%] gravity-darkenedmodeldepth[%]
(2015). Theslopeoftheplanetary radiusdepending onwave- g′ 3.0811+0.0151 1.98
2 −0.0035
lengthasafunctionisexpressedas r′ 1.7270+0.0108 1.89
2 −0.0025
zs,2 1.0191+−00..00008119 1.59
αH= dRp(λ) (3)
dlnλ
whereαisapower-lowcoefficient(α=−4forRayleighscat- ingdepthsistoolargeandcannotbeexplainedbythegravity-
tering), and λ is wavelength. The corresponding differential darkeningeffectalone.
radiusis63000kmforther2′-bandvsthezs,2-bandand130000 Thecandidateoriginsofthefadingeventsexceptagasgiant
km for theg2′-bandvs thezs,2-band. This result yields that α proposed by Yu et al. (2015) are starspots near the rotational
is -80 to -100. Therefore, the wavelength dependence of the pole,andorbitingoraccretingdust.AccordingtoGrankinetal.
CVSO30’sfadingeventistoolargetobeexplainedbytheat- (2008), weak-line T-Tauristarshave stable long-term periodic
mosphericRayleighscatteringofagasgiantplanet. variability for several years by starspots and stellar rotation.
We also discuss the influence of gravity darkening (von Suchlong-termstabilityis,however,theresultofstarspotmod-
Zeipel 1924). Barnes et al. (2013) and Howarth (2016) ex- ulation on so-called active longitudes despite shorter lifetime
plained the changes of the shape of transit light curve with (weeks)of eachstarspot. CVSO30hasbeenobserved todis-
gravity darkening. The intensity distribution on the gravity- playtransit-likefadingeventsoversixyearssincethefirstob-
darkenedstarvarieswithobservingwavelengths,andtherefore servationsbyvanEykenetal.(2012). Thestarspotsasacause
the transit light curves have wavelength dependence (Barnes offadingeventsareunlikelybecauseitisdifficultforthespots
2009). Now,wetestwhetherthewavelengthdependenceisex- toexistnearthepoleatalltimes.
plainableintermsofgravity-darkeningeffect.Wecalculatethe The remaining possibility for the wavelength dependence
g2′-, r2′-, and zs,2-band modellight curvewith gravity darken- wouldbeinfavorofatransitingdustclump. Asanexampleof
ing in the condition that thelargest wavelength dependence is dustwitharockyplanet,Rappaportetal.(2012)foundthatthe
expected.Thederivationoftheintensitydistributionisobtained shapeoftransitlightcurvesofKIC12557548changesintime.
bynumericalintegrationwiththeformulabyBarnes(2009).We In another example of a disintegrating planet, Sanchis-Ojeda
assume the strongest gravity darkening case, the stellar obliq- et al. (2015) signified the wavelength difference of the tran-
uity90◦ pole-onorbit, theimpactparameter0,Rp/Rs=0.11 sit depth by spectro-photometric observation in K2-22b. The
to approximate the observed fading depth in the r′-band. We exponent of the our extinction power law which is defined as
2
alsofixa/Rs withthebest-fitparameterinTable1,limbdark- −dlnA/dlnλ,whereAisfadingdepthobtainedby(Rp/Rs)2,
ening parameter with the same value in Section 3, the stellar is1.7±0.8. Thisvalueissimilartotheexponentoftheextinc-
rotationalvelocitywith120km/sobtainedbythestellarradius tion by interstellar medium 2.13±0.08 obtained by Damineli
andtherotationalperiodinvanEykenetal.(2012). etal. (2016). Thecandidate sources ofdust include adisinte-
Thederivedgravity-darkenedmodellightcurveisshownin gratingrockyplanet,acircumstellardisk,andanoccultationof
Figure 3. The transit depths with gravity darkening are2.0%, accretionhotspot.
1.9%,1.6%fortheg2′-,r2′-,andzs,2-bands,respectively(Table
3). Thewavelengthdependencecausedbygravitydarkeningis
6 Conclusion
weak and not able to reproduce the observed depths of 3.1%,
1.7%, 1.0% for the g2′-, r2′-, and zs,2-band, respectively. In Wehaveobservedthetransit-likefadingeventoftheweak-line
other words, the observed wavelength dependence of the fad- T-Tauri star CVSO 30 in the g2′-, r2′-, and zs,2-bands simulta-
6 PublicationsoftheAstronomicalSocietyofJapan,(2014),Vol.00,No.0
neously for the first time, using the MuSCAT instrument on Ohta,Y.,Taruya,A.,&Suto,Y.2009,ApJ,690,1
the 188 cm telescope at Okayama Astrophysical Observatory. Pont,F.,Zucker,S.,&Queloz,D.2006,MNRAS,373,231
Weperformthelightcurvesfittingusingtransitmodelwiththe Press,W.H.,Teukolsky,S.A.,Vetterling,W.T.,&Flannery,B.P.1992,
NumericalrecipesinFORTRAN.Theartofscientificcomputing
independent Rp/Rs, thesamea/Rs, andthesameimpactpa-
Raetz,S.etal.2016,MNRAS,460,2834
rameterforeachband.Wehavedetectedsignificantwavelength
Rappaport,S.etal.2012,ApJ,752,1
dependenceinCVSO30’sfadinglightcurve.Theresultoftran-
Sanchis-Ojeda,R.etal.2015,ApJ,812,112
sitlightcurvefittingshowsthelargewavelengthdependencein
Schwarz,G.1978,Ann.Statist.,6,2,461
transit depths of 3.1%, 1.8%, 1.1% for the g2′-, r2′-, and zs,2- Siess,L.,Dufour,E.,&Forestini,M.2000,A&A,358,593
bands, respectively. The wavelength dependence rules out a Sing,D.K.,etal.2011,MNRAS,416,1443
transiting gas giant scenario because of being too large to be Southworth,J.etal.2015,MNRAS,447,711
duetoahydrogen-dominatedatmosphereofahotJupiterorthe vanEyken,J.C.etal.2012,ApJ,755,42
vonZeipel,H.1924,MNRAS,84,665
gravity-darkeningeffect.Inaddition,thestarspotsasacauseof
Winn,J.N.etal.2008,ApJ,683,1076
fadingeventsareunlikelybecauseitisdifficultforthespotsto
Yu,L.etal.2015,ApJ,812,48
existseveralyearsnearthepoleatalltimes. Thusourresultis
infavoroftransitbycircumstellardustclumporoccultationof
anaccretionhotspotwhichwereintroducedbyYuetal.(2015).
Acknowledgments
TheauthorsthankYasunoriHoriforfruitfuldiscussions. A.F.andT.H.
is supported by the Astrobiology Center Project of National Institutes
of Natural Sciences(NINS) Grant Numbers AB281012 and JY280092.
N.N.acknowledgesasupportbyGrant-in-AidforScientificResearch(A)
(JSPSKAKENHIGrantNumber25247026). Thisworkwassupported
byJSPSKAKENHIGrantNumbers16K13791and16K17660. M.T.is
partlysupportedbyJSPSKAKENHIGrantNumber15H02063.
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