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AampA 568 A127 (2014)DOI 1010510004-6361201424106ccopy ESO 2014

Astronomyamp

Astrophysics

Physical properties of the WASP-67 planetary systemfrom multi-colour photometry

L Mancini1 J Southworth2 S Ciceri1 S Calchi Novati34 M Dominik5 Th Henning1 U G Joslashrgensen67H Korhonen867 N Nikolov9 K A Alsubai10 V Bozza411 D M Bramich12 G DrsquoAgo411

R Figuera Jaimes513 P Galianni5 S-H Gu1415 K Harpsoslashe67 T C Hinse16 M Hundertmark5D Juncher67 N Kains17 A Popovas67 M Rabus181 S Rahvar19 J Skottfelt67 C Snodgrass20

R Street21 J Surdej22 Y Tsapras2123 C Vilela2 X-B Wang1415 and O Wertz22

1 Max-Planck Institute for Astronomy Koumlnigstuhl 17 69117 Heidelberg Germanye-mail mancinimpiade

2 Astrophysics Group Keele University Staffordshire ST5 5BG UK3 International Institute for Advanced Scientific Studies (IIASS) 84019 Vietri Sul Mare (SA) Italy4 Department of Physics University of Salerno Via Giovanni Paolo II 84084 Fisciano Italy5 SUPA University of St Andrews School of Physics amp Astronomy North Haugh St Andrews KY16 9SS UK6 Niels Bohr Institute University of Copenhagen Juliane Maries vej 30 2100 Copenhagen Oslash Denmark7 Centre for Star and Planet Formation Geological Museum Oslashster Voldgade 5-7 1350 Copenhagen Denmark8 Finnish Centre for Astronomy with ESO (FINCA) University of Turku Vaumlisaumllaumlntie 20 21500 Piikkiouml Finland9 Astrophysics Group University of Exeter Stocker Road EX4 4QL Exeter UK

10 Qatar Foundation PO Box 5825 Doha Qatar11 Istituto Nazionale di Fisica Nucleare (INFN) Sezione di Napoli 80126 Napoli Italy12 Qatar Environment and Energy Research Institute Qatar Foundation Tornado Tower Floor 19 PO Box 5825 Doha Qatar13 European Southern Observatory Karl-Schwarzschild-Straszlige 2 85748 Garching bei Muumlnchen Germany14 Yunnan Observatories Chinese Academy of Sciences 650011 Kunming PR China15 Key Laboratory for the Structure and Evolution of Celestial Objects Chinese Academy of Sciences 650011 Kunming PR China16 Korea Astronomy and Space Science Institute Daejeon 305-348 Republic of Korea17 Space Telescope Science Institute 3700 San Martin Drive Baltimore MD 21218 USA18 Instituto de Astrofiacutesica Facultad de Fiacutesica Pontificia Universidad Catoacutelica de Chile Av Vicuntildea Mackenna 4860 7820436 Macul

Santiago Chile19 Department of Physics Sharif University of Technology PO Box 11155-9161 Tehran Iran20 Max Planck Institute for Solar System Research Justus-von-Liebig-Weg 3 37077 Goumlttingen Germany21 Las Cumbres Observatory Global Telescope Network 6740B Cortona Drive Goleta CA 93117 USA22 Institut drsquoAstrophysique et de Geacuteophysique Universiteacute de Liegravege 4000 Liegravege Belgium23 School of Physics and Astronomy Queen Mary University of London Mile End Road London E1 4NS UK

Received 30 April 2014 Accepted 28 June 2014

ABSTRACT

Context The extrasolar planet WASP-67 b is the first hot Jupiter definitively known to undergo only partial eclipses The lack of thesecond and third contact points in this planetary system makes it difficult to obtain accurate measurements of its physical parametersAims By using new high-precision photometric data we confirm that WASP-67 b shows grazing eclipses and compute accurate esti-mates of the physical properties of the planet and its parent starMethods We present high-quality multi-colour broad-band photometric observations comprising five light curves covering two tran-sit events obtained using two medium-class telescopes and the telescope-defocusing technique One transit was observed through aBessel-R filter and the other simultaneously through filters similar to Sloan gprimerprimeiprimezprime We modelled these data using jktebop The phys-ical parameters of the system were obtained from the analysis of these light curves and from published spectroscopic measurementsResults All five of our light curves satisfy the criterion for being grazing eclipses We revise the physical parameters of the wholeWASP-67 system and in particular significantly improve the measurements of the planetrsquos radius (Rb = 1091 plusmn 0046 RJup) anddensity (ρb = 0292 plusmn 0036 ρJup) as compared to the values in the discovery paper (Rb = 14+03

minus02 RJup and ρb = 016 plusmn 008 ρJup) Thetransit ephemeris was also substantially refined We investigated the variation of the planetrsquos radius as a function of the wavelengthusing the simultaneous multi-band data finding that our measurements are consistent with a flat spectrum to within the experimentaluncertainties

Key words planetary systems ndash stars fundamental parameters ndash techniques photometric

Based on data collected with GROND at the MPG 22 m telescopeand DFOSC at the Danish 154 m telescope Full Table 2 is only available at the CDS via anonymous ftp tocdsarcu-strasbgfr (130791285) or viahttpcdsarcu-strasbgfrviz-binqcatJA+A568A127

1 Introduction

WASP-67 b (Hellier et al 2012) is a transiting extrasolar planet(TEP) discovered by the SuperWASP group (Pollacco et al2006) orbiting a K0 V star (V = 125 mag) every 461 d It is

Article published by EDP Sciences A127 page 1 of 9

AampA 568 A127 (2014)

Table 1 Details of the transit observations presented in this work

Instrument Date of Start time End time Nobs Texp Tobs Filter Airmass Moon Aperture Scatterfirst obs (UT) (UT) (s) (s) illum radii (px) (mmag)

GROND 2012 06 04 0300 1050 162 7090 110120 Sloan gprime 214rarr 101rarr 122 98 34 50 80 108GROND 2012 06 04 0300 1050 162 7090 110120 Sloan rprime 214rarr 101rarr 122 98 38 60 85 056GROND 2012 06 04 0300 1050 162 7090 110120 Sloan iprime 214rarr 101rarr 122 98 40 60 85 072GROND 2012 06 04 0300 1050 162 7090 110120 Sloan zprime 214rarr 101rarr 122 98 40 60 85 064DFOSC 2013 06 22 0430 0833 136 100 110 Bessel R 112rarr 101rarr 117 97 20 35 55 048

Notes Nobs is the number of observations Texp is the exposure time Tobs is the observational cadence and ldquoMoon illumrdquo is the fractionalillumination of the Moon at the midpoint of the transit The aperture sizes are the radii of the software apertures for the star inner sky and outersky respectively Scatter is the rms scatter of the data versus a fitted model

an inflated (ρb ρJup) hot Jupiter (a sim 005 AU) on a graz-ing orbit (impact parameter b gt 09) causing the transit lightcurve to have an atypical V shape Hellier et al (2012) found thatWASP-67 b satisfies the grazing criterion (X = b + RbR gt 1)by 3σ which makes it the first TEP definitively known to havea grazing eclipse1 In this particular configuration the secondand third contact points (eg Winn 2010) are missing and thelight curve solution becomes degenerate This hampers accu-rate measurements of the photometric parameters of the systemConsequently Hellier et al (2012) measured the radius of theplanet with a large uncertainty of sim20 In such cases high-quality light curves are mandatory to reduce the error bars tolevels similar to those of other known TEPs

Here we present the first photometric follow-up study ofWASP-67 since its discovery paper The main aim of this studyis to refine the physical parameters of the system and ephemerissetting the stage for a more detailed study in the near futureWASP-67 is located in field 7 of the K2 phase of the NASArsquosKepler mission2 and will be observed continuously for approxi-mately 80 d in late 2015

2 Observations and data reduction

A complete transit of WASP-67 b was observed on 2012 June 4(Table 1) using the Gamma Ray burst Optical and Near-infraredDetector (GROND) instrument mounted on the MPG3 22 mtelescope which is located at the ESO observatory in La Silla(Chile) GROND is an imaging system capable of simultane-ous photometric observations in four optical (similar to Sloangprime rprime iprime zprime) and three NIR (J H K) passbands (Greiner et al2008) Each of the four optical channels is equipped with aback-illuminated 2048 times 2048 E2V CCD with a field of viewof 54prime times 54prime at 0158primeprime pixelminus1 The three NIR channels use1024 times 1024 Rockwell HAWAII-1 arrays with a field of viewof 10prime times 10prime at 06primeprime pixelminus1 The telescope was autoguided dur-ing the observations which were performed with the defocusingtechnique (Southworth et al 2009)

Another complete transit of WASP-67 b was observed on2013 June 22 by using the DFOSC imager mounted on the154 m Danish Telescope which is also at the ESO obser-vatory in La Silla during the 2013 observing campaign bythe MiNDSTEp consortium (Dominik et al 2010) The instru-ment has a E2V44-82 CCD camera with a field of view of137prime times 137prime and a plate scale of 039primeprime pixelminus1 The observations

1 Other TEPs which might undergo grazing eclipses are WASP-34(Smalley et al 2011) and HAT-P-27WASP-40 (Beacuteky et al 2011Anderson et al 2011)2 httpkeplersciencearcnasagovK23 Max Planck Gesellschaft

Table 2 Excerpts of the light curves of WASP-67

Telescope Filter BJD (TDB) Diff mag Uncertainty

ESO 22 m gprime 2 456 082655745 000061 000043ESO 22 m gprime 2 456 082657102 000142 000043ESO 22 m rprime 2 456 082655745 000083 000038ESO 22 m rprime 2 456 082657102 000101 000033ESO 22 m iprime 2 456 082655745 000069 000041ESO 22 m iprime 2 456 082657102 000117 000043ESO 22 m zprime 2 456 082653032 minus000041 000048ESO 22 m zprime 2 456 082654390 minus000117 000048DK 154 m R 2 456 465694278 000066 000141DK 154 m R 2 456 465695528 000033 000141

Notes This table is available at the CDS A portion is shown here forguidance regarding its form and content

were performed through a Bessel R filter the telescope was de-focused and autoguided and the CCD was windowed to reducethe readout time With the applied defocus the diameter of thePSF of the target and reference stars was sim12primeprime which is similarto that for the GROND images

The optical data collected from both telescopes were re-duced using defot an idl4 pipeline for time-series photometry(Southworth et al 2009) The images were debiased and flat-fielded using standard methods and then subjected to aperturephotometry using the aper5 task and an optimal ensemble ofcomparison stars Pointing variations were followed by cross-correlating each image against a reference image The shape ofthe light curve is very insensitive to the aperture sizes so wechose those that yielded the lowest scatter The relative weightsof the comparison stars were optimised simultaneously with ade-trending of the light curve to remove slow instrumental andastrophysical trends This was achieved by fitting a straight lineto the out-of-transit data for the DFOSC data and with a fourth-order polynomial for the GROND data (to compensate for thelack of reference stars caused by the smaller field of view)

The final differential-flux light curves are plotted in Fig 1and tabulated in Table 2 In particular the GROND light curvesin the top panel of Fig 1 are reported superimposed to high-light the differences of the light-curve shape and the transitdepth along the four passbands Contrary to what is expected forhigher-inclination systems (eg Knutson et al 2007) the transitdepth gradually increases moving from blue to red bands Thisphenomenon happens because the planet only covers the limb of

4 idl is a trademark of the ITT Visual Information SolutionshttpwwwittviscomProductServicesIDLaspx5 aper is part of the astrolib subroutine library distributed by NASAon httpidlastrogsfcnasagov

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L Mancini et al Physical properties of WASP-67 b

002 001 000 001 002 0030980

0985

0990

0995

1000

1005N

orm

alis

edFl

ux

Sloan g

Sloan r

Sloan i

Sloan z

002 001 000 001 002 0030980

0985

0990

0995

1000

1005

Orbital phase

Nor

mal

ised

Flux

Sloan r

Bessel R

Gunn r

Fig 1 Light curves of WASP-67 b eclipses Top panel light curves obtained with GROND in gprimerprimeiprimezprime showing how the transit light curve shapechanges with wavelength The transit in the gprime band is shallower than the other bands as expected for a grazing eclipse as limb darkening isstronger at bluer wavelengths Bottom panel light curves obtained with DFOSC in the R-band (June 2013 brown open circles) with GROND inthe rprime-band (June 2012 yellow points) and with the Euler 12 m telescope in the r-band (July 2011 green open squares Hellier et al 2012) Thelight curves are superimposed to highlight variations in transit shape between the three measurements

the star (as this is a grazing eclipse) which is fainter in the bluepart of the optical spectrum than the red one due to the strongerlimb darkening Thus we expect to see shallower eclipses in thebluest bands for this system

The DFOSC Bessel-R light curve is shown in the bottompanel of Fig 1 and is superimposed with the GROND Sloan-rprimelight curve and that from Hellier et al (2012) which was ob-tained with the Euler 12 m telescope through a Gunn-r filterThis panel highlights the slight variation of the transit depth be-tween the DFOSC and GROND light curves the Euler data aremore scattered and agree with both Slight differences can becaused by the different filters used or by unocculted starspotsThe latter hypothesis suggests a variation of the starspot activityof the WASP-67 A during a period of two years which is rea-sonable for a 5200 K star

Similar to some previous cases (Nikolov et al 2012 Manciniet al 2013b 2014) the quality of the GROND NIR data were notgood enough to extract usable photometry We were only able toobtain a noisy light curve in the J band that which if we considerthe particular transit geometry of the WASP-67 system returnedvery inaccurate estimates of the photometric parameters in the

light-curve fitting process (see next section) in comparison withthe optical ones

3 Light-curve analysis

Our light curves were modelled using the jktebop6 code (seeSouthworth 2012 and references therein) which represents thestar and planet as biaxial spheroids for the calculation of the re-flection and ellipsoidal effects and as spheres for calculation ofthe eclipse shapes The main parameters fitted by jktebop arethe orbital inclination i the transit midpoint T0 and the sumand ratio of the fractional radii of the star and planet rA + rb andk = rbrA The fractional radii are defined as rA = RAa andrb = Rba where a is the orbital semimajor axis and RA and Rbare the absolute radii of the star and the planet respectively

Each light curve was analysed separately using a quadraticlaw to model the limb darkening (LD) effect Due to the difficultyof measuring accurate LD coefficients in TEP systems with im-pact parameters b ge 08 (Muumlller et al 2013) the WASP-67 A LD

6 The source code of jktebop is available at httpwwwastrokeeleacukjktcodesjktebophtml

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0 20 40 60 80 100 120 140

100

50

0

50

100

Cycle number

Res

idua

lss

ec

Fig 2 Residuals for the timings of WASP-67 b at mid-transit versus a linear ephemeris The two timings based on the observations reported byHellier et al (2012) are plotted using open circles while the other timings (this work) are plotted with filled circles

Table 3 Times of WASP-67 b at mid-transit and their residuals versusa linear orbital ephemeris

Time of minimum Cycle Residual ReferenceBJD(TDB)minus2 400 000 No (d)

55 83360357 plusmn 000032 2 0000510 155 83360237 plusmn 000033 2 minus0000690 256 08278067 plusmn 000034 56 minus0000578 356 08278126 plusmn 000016 56 0000012 456 08278135 plusmn 000019 56 0000102 556 08278145 plusmn 000019 56 0000202 656 46577729 plusmn 000016 139 minus0000064 7

Notes References (1) Euler 12 m telescope (Hellier et al 2012) (2)Trappist 06-m telescope (Hellier et al 2012) (3) GROND gprime-band(this work) (4) GROND rprime-band (this work) (5) GROND iprime-band (thiswork) (6) GROND zprime-band (this work) (7) Danish 152-m telescope(this work)

coefficients were fixed to their theoretical values (Claret 2004b)We also assumed that the planetary orbit is circular (Hellieret al 2012) We included the coefficients of a linear (DFOSC)or fourth (GROND) polynomial versus time in the fits to fullyaccount for the uncertainty in the de-trending of the light curves

We also considered the two light curves obtained with theEuler 12 m and Trappist 06 m telescopes which were reportedin Hellier et al (2012) To present a homogeneous analysis were-fitted these two light curves using jktebop in the same man-ner as for our own data

As in previous works (Mancini et al 2013abc 2014) weenlarged the error bars of the light curve points generated byour reduction pipeline Such a process is necessary because theaper algorithm which is used to perform aperture photometrytends to underestimate the true uncertainties in the relative mag-nitude measurements This is a typical situation in time-seriesphotometry where additional noise sources such as red noiseare not accounted for by standard error-estimation algorithms(eg Carter amp Winn 2009) We therefore rescaled the error barsfor each eclipse to give a reduced χ2 of χ2

ν = 1 and then again byusing the β approach (eg Gillon et al 2006 Winn et al 2008Gibson et al 2008)

31 Orbital period determination

We used our photometric data and those coming from the dis-covery paper (Hellier et al 2012) to refine the orbital period ofWASP-67 b The transit time for each of the datasets was ob-tained by fitting with jktebop and uncertainties were estimatedusing Monte Carlo simulations All timings were placed on theBJD(TDB) time system and are summarised in Table 3 The plot

of the residuals is shown in Fig 2 The resulting measurementsof transit midpoints were fitted with a straight line to obtain afinal orbital ephemeris

T0 = BJD(TDB)2 455 82437424(22)+ 46144109(27) E

where E is the number of orbital cycles after the reference epochwhich we take to be that estimated by Hellier et al (2012) andquantities in brackets denote the uncertainty in the final digit ofthe preceding number The quality of fit χ2

ν = 190 indicatesthat a linear ephemeris is not a perfect match to the observa-tions However considering that our timings cover only threeepochs it is difficult to claim systematic deviations from the pre-dicted transit times Future Kepler data will enlarge the numberof observed transit events of WASP-67 b and may rule in or outpossible transit timing variations

32 Photometric parameters

The GROND light curves and the jktebop best-fitting modelsare shown in Fig 3 A similar plot is reported in Fig 4 for thelight curves from the Danish Telescope and Hellier et al (2012)The parameters of the fits are given in Table 4 Uncertainties inthe fitted parameters from each solution were calculated from5500 Monte Carlo simulations and by a residual-permutation al-gorithm (Southworth 2008) The larger of the two possible errorbars was adopted for each case The error bars for the fits to in-dividual light curves are often strongly asymmetric due to themorphology of the light curve The final photometric parameterswere therefore calculated by multiplying the probability densityfunctions of the different values This procedure yielded errorbars which are close to symmetric for all photometric param-eters and are given in Table 4 The values obtained by Hellieret al (2012) are also reported for comparison Due to their lowerquality we did not use any of the GROND-NIR light curves toestimate the final photometric parameters of WASP-67

4 Physical properties

Similarly to the Homogeneous Studies approach (Southworth2012 and references therein) we used the photometric param-eters estimated in the previous section and the spectroscopicproperties of the parent star (velocity amplitude KA = 0056 plusmn0004 km sminus1 effective temperature Teff = 5200 plusmn 100 K andmetallicity

[FeH

]= minus007 plusmn 009 Hellier et al 2012) to revise

the physical properties of the WASP-67 system using the abs-dim code

We iteratively determined the velocity amplitude of theplanet (Kb) which yielded the best agreement between the mea-sured rA and Teff and the values of RAa and Teff predicted by a

A127 page 4 of 9

L Mancini et al Physical properties of WASP-67 b

002 000 002 004092

094

096

098

100

Orbital phase

Nor

mal

ised

Flux

g 047 Μm

r 062 Μm

i 077 Μm

z 093 Μm

002 000 002 004092

094

096

098

100

Orbital phase

g 047 Μm

r 062 Μm

i 077 Μm

z 093 Μm

Fig 3 Left-hand panel simultaneous optical light curves of the WASP-67 eclipse observed with GROND The jktebop best fits are shown assolid lines for each optical data set The passbands are labelled on the left of the figure and their central wavelengths are given on the rightRight-hand panel residuals of each fit

002 001 000 001 002

094

095

096

097

098

099

100

Orbital phase

Nor

mal

ised

Flux

r Euler

R Danish

Iz Trappist

002 001 000 001 002

094

095

096

097

098

099

100

Orbital phase

r Euler

R Danish

Iz Trappist

Fig 4 Left-hand panel light curves of the WASP-67 eclipses observed in Gunn-r with the Euler telescope (Hellier et al 2012) in Bessell-R withthe Danish telescope (this work) and with an I + z filter with the TRAPPIST telescope (Hellier et al 2012) The filters and the name of eachtelescope are labelled on the figure The jktebop best fits are shown as solid lines for each optical dataset Right-hand panel residuals of each fit

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Table 4 Parameters of the jktebop fits to the light curves of WASP-67

Telescope Filter rA + rb k i rA rb

MPG 22 m Sloan gprime 00831+00061minus00036 01323+00192

minus00058 8630+020minus039 00734+00040

minus00029 000972+000200minus000074

MPG 22 m Sloan rprime 00827+00023minus00019 01345+00061

minus00035 8631+011minus014 00729+00016

minus00015 000980+000065minus000043

MPG 22 m Sloan iprime 00823+00027minus00020 01337+00061

minus00034 8534+012minus017 00726+00020

minus00016 000970+000069minus000044

MPG 22 m Sloan zprime 00865+00040minus00027 01424+00139

minus00065 8609+017minus025 00757+00025

minus00019 001078+000143minus000075

Danish 154 m Bessel R 00868+00038minus00026 01445+00151

minus00070 8608+016minus025 00758+00023

minus00018 001095+000148minus000077

Euler 12 m Gunn r 0102+0013minus0013 0229+0150

minus0080 8509+086minus087 00828+00021

minus00047 00189+00127minus00074

Trappist 06 m I + z filter 00854+00054minus00035 01310+00149

minus00047 8616+020minus035 00755+00038

minus00028 000989+000164minus000065

Final results 00846 plusmn 00012 01379 plusmn 00030 8620 plusmn 007 007455 plusmn 000083 001023 plusmn 000034

Hellier et al (2012) 01345+00048minus00019 858+03

minus04

Notes The final parameters given in bold are the weighted means of the results for the datasets Results from the discovery paper are included atthe base of the table for comparison The Euler and TRAPPIST data sets are from Hellier et al (2012) while the others are from this work

Table 5 Final physical properties of the WASP-67 planetary system compared with results from Hellier et al (2012)

This work (final) Hellier et al (2012)

Stellar mass MA (M) 0829plusmn 0050 plusmn 0037 087 plusmn 004Stellar radius RA (R) 0817plusmn 0019 plusmn 0012 087 plusmn 004Stellar surface gravity log gA (cgs) 4533plusmn 0014 plusmn 0007 450 plusmn 003Stellar density ρA (ρ) 1522 plusmn 0049 132 plusmn 015Planetary mass Mb (MJup) 0406plusmn 0033 plusmn 0012 042 plusmn 004Planetary radius Rb (RJup) 1091plusmn 0043 plusmn 0016 14+03

minus02

Planetary surface gravity gb (m sminus2) 845 plusmn 083 50+12minus23

Planetary density ρb (ρJup) 0292plusmn 0036 plusmn 0004 016 plusmn 008Planetary equilibrium temperature T primeeq (K) 1003 plusmn 20 1040 plusmn 30Safronov number Θ 00457plusmn 00037plusmn 00007Orbital semimajor axis a (au) 00510plusmn 00010plusmn 00008 00517 plusmn 00008Age Gyr 87 +127

minus73+55minus86 20+16

minus10

Notes Two sets of errorbars are given for the results from the current work the former being statistical and the latter systematic

set of theoretical stellar models for the calculated stellar massand[

FeH

] Statistical errors were propagated by a perturbation

analysis and the overall best fit was found by evaluating resultsfor a grid of ages We assessed the contribution of systematic er-rors from theoretical stellar models by running solutions for fivedifferent grids of models (Claret 2004a Demarque et al 2004Pietrinferni et al 2004 VandenBerg et al 2006 Dotter et al2008) The final set of physical properties was calculated by tak-ing the unweighted mean of the five sets of values found fromthe different stellar models and the systematic errors were takento be the maximum deviation of a single value from the meanThe physical parameters of the WASP-67 planetary system aregiven in Table 5

Table 5 also shows the values obtained by Hellier et al(2012) for comparison We find a smaller radius for the starwhich is attributable to the better constraint on the stellar densityfrom our high-precision light curves We also obtain a signifi-cantly smaller planetary radius and hence a larger surface grav-ity and density This is due partly to the smaller stellar radiuscombined with a comparable measurement of k (Table 4) andpartly to an inconsistency among the RA Rb and k values foundby Hellier et al (2012) The latter issue arises because Hellieret al (2012) quote the median value of each fitted parameterfrom Markov Chain Monte Carlo simulations rather than givingthe set of parameters corresponding to the single best-fitting linkin the Markov chain (D R Anderson priv comm)

5 Variation of the planetary radiuswith wavelength

If it were not for the difficulty of measuring its radius WASP-67 b would be a good target for studies of the planetary atmo-sphere due to its low surface gravity However its moderateequilibrium temperature (T primeeq = 1003 plusmn 20 K) indicates thatthe planet should belong to the pL class (Fortney et al 2008)implying that we do not expect to measure large variations ofthe planet radius with wavelength As the GROND instrumentis able to cover different optical passbands we used our data toprobe the terminator region of the planetary atmosphere

Following our method in previous works (Southworth et al2012 Mancini et al 2013b) we re-fitted the GROND lightcurves with all parameters except k fixed to the final valuesgiven in Table 4 This approach maximizes the precision ofestimations of the planetstar radius ratio by removing com-mon sources of uncertainty We find the following values k =01369 plusmn 00063 for gprime k = 01384 plusmn 00026 for rprime k =01381 plusmn 00024 for iprime and k = 01387 plusmn 00023 for zprime Theseresults are shown in Fig 5 where the vertical errorbars repre-sent the relative errors in the measurements and the horizon-tal errorbars show the FWHM transmission of the passbandsused For illustration we also show the predictions from a modelatmosphere calculated by Fortney et al (2010) for a Jupiter-mass planet with a surface gravity of gb = 10 m sminus2 a base ra-dius of 125 RJup at 10 bar and T primeeq = 1000 K The opacity of

A127 page 6 of 9

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0130

0135

0140

0145

Rb

RA

400 500 600 700 800 900 1000

02

04

06

08

10

wavelength nm

Effi

cien

cy

Fig 5 Variation of the planetary radius interms of planetstar radius ratio with wave-length The black diamonds are from the tran-sit observations performed with GROND Thevertical bars represent the errors in the mea-surements and the horizontal bars show theFWHM transmission of the passbands usedThe observational points are compared with asynthetic spectrum (see text for details) Totalefficiencies of the GROND filters are shown inthe bottom panel The blue boxes indicate thepredicted values for the model integrated overthe passbands of the observations

strong-absorber molecules such as gaseous titanium oxide (TiO)and vanadium oxide (VO) was removed from the model Our ex-perimental points agree with the prominent absorption featuresof the model (sodium at sim590 nm and potassium at sim770 nm)and being compatible with a flat transmission spectrum do notindicate any large variation of the WASP-67 brsquos radius

6 High-resolution image

Eclipsing binary star systems are a common source of false pos-itives for transiting planets detected by wide-field photometryThe host star can have a gravitationally bound companion orits light can be contaminated by a background eclipsing binarywhich is coincidentally at the same sky position Both cases canmimic a planetary-transit signal Faint close stars may also con-taminate the PSF of the target star thus slightly lowering thedepth of the transit and causing us to underestimate the radius ofboth the TEP and its host star Finally these faint nearby starscould also affect the radial velocity measurements of the starand thus the measured mass of the planet (eg Buchhave et al2011)

To check if WASP-67 A is contaminated by any faint com-panion or background stars we observed it on 20140421with the Andor Technology iXon+ model 897 EMCCD LuckyCamera mounted at the Danish 154 m telescope The imagingarea of this camera is 512 times 512 pixels and each 16 μm pixelprojects to 0primeprime09 on the sky which gives a 45 times 45 arcsec2 fieldof view The camera has a special long-pass filter with a cut-onwavelength of 650 nm which corresponds roughly to a combi-nation of the SDSS iprime + zprime filters (Skottfelt et al 2013)

Figure 6 shows the resulting image WASP-67 A is the brightstar in the centre of the image Figure 7 shows the central re-gion of the image and it can be seen that two stars (A and B)occur approximately 45primeprime and 60primeprime northeast of WASP-67 AThe plate scales and inner apertures of DFOSC and GROND(Table 1) are such that both stars are inside the defocused PSFsof WASP-67 However they are much fainter than WASP-67 Awith Δ(iprime + zprime) = 76 mag and 79 mag respectively They there-fore contribute only 01 and 007 of the total flux in eachimage so have a negligible effect on our results

Fig 6 Lucky Camera image of WASP-67 The image size is 45 times45 arcsec2 and is shown in a logarithmic flux scale with north upand east to the left The FWHM of the image is 0primeprime54 The triangu-lar PSF comes from the telescope in very good seeing The extra fluxnorth-west of WASP-67 A is not a real contaminating flux source butan optical ghost from the star caused by internal reflections within thebeamsplitters

In the eventuality that the two faint nearby stars are intrinsi-cally very blue objects they could have affected our gprime-band ob-servations by more than the amount given above Measurementof a colour index from multiple high-resolution images would al-low this possibility to be investigated As a worst-case scenarioif both contaminants have Teff = 30 000 K and are located at suchas distance as to contribute 01 of the flux in the Lucky Camera

A127 page 7 of 9

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A

B

25e+0613e+0664e+0532e+0516e+0581e+0441e+0421e+0421e+0411e+0411e+04

Fig 7 Central part of the Lucky Camera image in Fig 6 The imageis shown in with a logarithmic flux scale with north up and east to theleft Two faint stars A at sim44primeprime and B at sim60primeprime north-east of WASP-67 A are evident Values in the colour bar refer to the number of countsin ADU

passband the contamination in the gprime-band would be 11 Thisfigure remains too small to be important to the current analysis

7 Kepler-K2 observations

A more extensive study of the WASP-67 planetary system is an-ticipated as this object will be observed by the Kepler satelliteduring its K2 phase To explore the impact of these forthcomingobservations we have generated a synthetic light curve match-ing the K2 data characteristics and subjected it to the same mod-elling process as for the real data presented in the current work

We calculated a model light curve for the best-fitting pho-tometric parameters (Table 4) using jktebop and for quadraticLD coefficients appropriate for the Kp passband (Claret 2004b)This was extended over the full duration of the observations forfield 2 (as the schedule for field 7 is not yet set) and numer-ically integrated to the duration of the short-cadence (588 s)and long-cadence (294 min) data types obtained by KeplerGaussian random noise was added to each data point equiva-lent to a scatter of 100 parts per million per six-hour time in-terval (Howell et al 2014 their Fig 10) Data points outsideorbital phases minus002 to 002 were discarded for computationalconvenience

The synthetic light curves were fitted with jktebop usingthe same treatment as our real data sets for WASP-67 with theexception that we numerically integrated the model for the long-cadence simulated data to match its sampling rate (Southworth2011) We find that the uncertainties in the resulting photometricparameters are quite similar between the two cadences whichis due to the relatively smooth brightness variation through thepartial eclipse of WASP-67 They are also similar to those of ourfinal parameters in Table 4 suggesting that the Kepler data willnot allow a substantial improvement in the measured physicalproperties of WASP-67 This result was unexpected but can beexplained by the larger scatter of the Kepler data (083 mmag forshort-cadence) versus our best light curves (see Table 1)

One possibility which is much better suited to K2 obser-vations is the detection of the rotational period of WASP-67 Adue to spot-induced brightness modulations WASP-67 A is acool star (5200 K) but no spot modulation was detected in theSuperWASP light curve to a level of roughly 1 mmag The data

acquired by K2 may allow the rotational period to be estimatedwhich is useful for dynamical and tidal studies

8 Summary and conclusions

We have presented the first follow-up study of the planetary sys-tem WASP-67 which is based on the analysis of five new lightcurves of two transit events of WASP-67 b The first transit wasobserved simultaneously with GROND through Sloan gprime rprime iprimezprime filters the second was observed in Bessell-R with DFOSCThe transits were monitored roughly one and two years respec-tively after the reference epoch used by Hellier et al (2012)Both transit events were observed in telescope-defocusing moderesulting in a photometric precision of 048minus108 mmag per ob-servation We modelled our new and two published datasets us-ing the jktebop code By estimating the impact parameter b andthe ratio of the planetstar radii we found that the criterion fora grazing eclipse b + k gt 1 is satisfied for all the light curvesconfirming that the eclipse is grazing

We used the results of the light-curve analysis to substan-tially improve the measurements of the physical properties ofthe planet and its host star (Table 5) Compared to the discoverypaper (Hellier et al 2012) we find a significantly smaller radiusand a greater density for WASP-67 b We obtain Rb = 1091 plusmn0046 RJup versus 14+03

minus02 RJup and ρb = 0292plusmn 0036 ρJup versus016 plusmn 008 ρJup Our revised physical properties move WASP-67 b into a quite different region of parameter space Figure 8shows the change in position in the planet mass-radius plot (toppanel) and in the planet mass-density plot (bottom panel) Therevised positions are marked with a green circle while the redcircle indicates the old values from Hellier et al (2012) The val-ues of the other TEPs were taken from the TEPCat catalogue7For illustration the bottom panel of Fig 8 also shows 10 Gyrisochrones of exoplanets at 0045 AU orbital separation froma solar analogue (Fortney et al 2007) The plot suggests thatWASP-67 b should have a more massive core than previouslythought

As an additional possibility offered by the GROND data wemade an attempt to investigate possible variations of the radiusof WASP-67 b in different optical passbands Our experimentalpoints are compatible with a flat transmission spectrum and donot indicate any large variation of the planetrsquos radius The grad-ual increase of the transit depth moving from the GROND gprimeto zprime band which is opposite to the case for higher-inclinationsystems is explicable in that WASP-67 b only produces grazingeclipses Due to stronger limb darkening these are shallower inthe blue bands than in the red ones

Acknowledgements This paper is based on observations collected with theMPG 22 m and the Danish 154 m telescopes both located at ESO Observatoryin La Silla Chile Operation of the Danish telescope is based on a grant to UGJby the Danish Natural Science Research Council (FNU) GROND was builtby the high-energy group of MPE in collaboration with the LSW Tautenburgand ESO and is operated as a PI-instrument at the MPG 22 m telescopeWe thank David Anderson and Coel Hellier for useful discussions and thereferee for a helpful report JS (Keele) acknowledges financial support fromSTFC in the form of an Advanced Fellowship CS received funding from theEuropean Union Seventh Framework Programme (FP72007-2013) under grantagreement No 268421 MR acknowledges support from FONDECYT post-doctoral fellowship N3120097 TCH would like to acknowledge KASI grant2014-1-400-06 HK acknowledges support by the European Commissionunder the Marie Curie Intra-European Fellowship Programme in FP7 S-HGand X-BW would like to thank the financial support from National NaturalScience Foundation of China (No 10873031) and Chinese Academy of Sciences(project KJCX2-YW-T24) OW thanks the Belgian National Fund for Scientific

7 httpwwwastrokeeleacukjkttepcat

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L Mancini et al Physical properties of WASP-67 b

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Fig 8 Top panel masses and radii of the known TEPs The grey pointsdenote values taken from TEPCat Their error bars have been sup-pressed for clarity WASP-67 b is shown with red (Hellier et al 2012)and green (this work) points with error bars Dotted lines show wheredensity is 25 10 05 025 and 01 ρJup Bottom panel the mass-density diagram of the currently known transiting exoplanets (takenfrom TEPCat) Four planetary models with various core masses andanother without a core (Fortney et al 2007) are plotted for comparison

Research (FNRS) JS and OW acknowledge support from the Communauteacutefranccedilaise de Belgique minus Actions de recherche concerteacutees ndash Acadeacutemie universi-taire Wallonie-Europe KA MD and MH acknowledge grant NPRP-09-476-1-78 from the Qatar National Research Fund (a member of Qatar Foundation)This publication was aided by NPRP grant X-019-1-006 from the QatarNational Research Fund (a member of Qatar Foundation) The reduced lightcurves presented in this work will be made available at the CDS (httpcdswebu-strasbgfr) The following internet-based resources were used inresearch for this paper the ESO Digitized Sky Survey the NASA AstrophysicsData System the SIMBAD data base operated at CDS Strasbourg France andthe arXiv scientific paper preprint service operated by Cornell University

References

Anderson D R Barros S C C Boisse I et al 2011 PASP 123 555Beacuteky B Bakos G Aacute Hartman J et al 2011 ApJ 734 109Buchhave L A Latham D W Carter J A et al 2011 ApJS 197 3Carter J A amp Winn J N 2009 ApJ 704 51Claret A 2004a AampA 424 919Claret A 2004b AampA 428 1001Demarque P Woo J-H Kim Y-C Yi S K 2004 ApJS 155 667Dotter A Chaboyer B Jevremovic D et al 2008 ApJS 178 89Dominik M Joslashrgensen U G Rattenbury N J et al 2010 Astron Nachr

331 671Fortney J J Marley M S amp Barnes J W 2007 ApJ 659 1661Fortney J J Lodders K Marley M S amp Freedman R S 2008 ApJ 678

1419Fortney J J Shabram M Showman A P et al 2010 ApJ 709 1396Gillon M Pont F Moutou C et al 2006 AampA 459 249Gibson N P Pollacco D Simpson E K et al 2008 AampA 492 603Greiner J Bornemann W Clemens C et al 2008 PASP 120 405Hellier C Anderson D R Collier Cameron A et al 2012 MNRAS 426

739Howell S B Sobeck C Haas M et al 2014 PASP 126 398Knutson H A Charbonneau D amp Noyes R W 2007 ApJ 655 564Mancini L Southworth J Ciceri S et al 2013a AampA 551 A11Mancini L Nikolov N Southworth J et al 2013b MNRAS 430 2932Mancini L Ciceri S Chen G et al 2013c MNRAS 436 2Mancini L Southworth J Ciceri S et al 2014 AampA 562 A126Nikolov N Henning Th Koppenhoefer J et al 2012 AampA 539 159Muumlller H M Huber K F Czesla S et al 2013 AampA 560 A112Pietrinferni A Cassisi S Salaris M amp Castelli F 2004 ApJ 612 168Pollacco D L Skillen I Collier Cameron A et al 2006 PASP 118 1407Skottfelt J Bramich D M Figuera Jaimes R et al 2013 AampA 553 A111Smalley B Anderson D R Collier Cameron A et al 2011 AampA 526

A130Southworth J 2008 MNRAS 386 1644Southworth J 2011 MNRAS 417 2166Southworth J 2012 MNRAS 426 1291Southworth J Hinse T C Joslashrgensen U G et al 2009 MNRAS 396 1023Southworth J Mancini L Maxted P F L et al 2012 MNRAS 422 3099VandenBerg D A Bergbusch P A amp Dowler P D 2006 ApJS 162 375Winn J N 2010 in Exoplanet ed S Seager (The University of Arizona Press)

56Winn J N Holman M J Torres G et al 2008 ApJ 683 1076

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