Table Of ContentAstronomy&Astrophysicsmanuscriptno.wasp11 (cid:13)c ESO2008
September26,2008
The sub-Jupiter mass transiting exoplanet WASP-11b
R.G.West1,A.CollierCameron2,L.Hebb2,Y.C.Joshi3,D.Pollacco3,E.Simpson3,I.Skillen4,H.C.Stempels2,P.J.
Wheatley5,D.Wilson6,D.Anderson6,S.Bentley6,F.Bouchy7,8,B.Enoch9,N.Gibson2,G.He´brard7,C.Hellier6,B.
Loeillet10,M.Mayor11,P.Maxted6,I.McDonald6,C.Moutou10,F.Pont11,D.Queloz11,A.M.S.Smith2,B.Smalley6,
R.A.Street12,andS.Udry11
8
0 1 DepartmentofPhysics&Astronomy,UniversityofLeicester,Leicester,LE17RH,UK
0 2 SchoolofPhysicsandAstronomy,UniversityofStAndrews,NorthHaugh,StAndrews,FifeKY169SS,UK
2 3 AstrophysicsResearchCentre,SchoolofMathematics&Physics,Queen’sUniversity,UniversityRoad,Belfast,BT71NN,UK
4 IsaacNewtonGroupofTelescopes,ApartadodeCorreos321,E-38700SantaCruzdelaPalma,Tenerife,Spain
p
5 DepartmentofPhysics,UniversityofWarwick,CoventryCV47AL,UK
e
6 AstrophysicsGroup,KeeleUniversity,Staffordshire,ST55BG
S
7 Institutd’AstrophysiquedeParis,CNRS(UMR7095)–Universite´Pierre&MarieCurie,98bisbvd.Arago,75014Paris,France
6 8 ObservatoiredeHaute-Provence,04870StMichell’Observatoire,France
2 9 DepartmentofPhysicsandAstronomy,TheOpenUniversity,MiltonKeynes,MK76AA,UK
10 Laboratoire d’Astrophysique de Marseille, OAMP, Universite´ Aix-Marseille & CNRS, 38 rue Fre´de´ric Joliot-Curie, 13388
] Marseillecedex13,France
h 11 ObservatoiredeGene`ve,Universite´deGene`ve,51Ch.desMaillettes,1290Sauverny,Switzerland
p 12 LasCumbresObservatory,6740CortonaDr.Suite102,SantaBarbara,CA93117,USA
-
o
r
t ABSTRACT
s
a
Wereportthediscoveryofasub-JupitermassexoplanettransitingamagnitudeV =11.7hoststar1SWASPJ030928.54+304024.7.
[
A simultaneous fit to the transit photometry and radial-velocity measurements yield a planet mass Mp = 0.53±0.07MJ, radius
1 Rp=0.91−+00..0036RJandanorbitalperiodof3.722465−+00..000000000086days.ThehoststarisofspectraltypeK3V,withaspectralanalysisyielding
v aneffectivetemperatureof4800±100Kandlogg=4.45±0.2.Itisamongstthesmallest,leastmassiveandlowestluminositystars
7 known to harbour a transiting exoplanet. WASP-11b is the third least strongly irradiated transiting exoplanet discovered to date,
9 experiencinganincidentfluxFp =1.9×108ergs−1cm−2andhavinganequilibriumtemperatureTeql=960±70K.
5
4
.
9
1. Introduction 2. Observations
0
8
2.1.WASPphotometry
0 Observations of planets that transit their host star represent
: the current best opportunity to test models of the internal
v The host star 1SWASP J030928.54+304024.7 (= USNO-
structure of exoplanets and of their formation and evolution.
i B1.0 1206-0003989 = 2MASS 03092855+3040249; hereafter
X Since the first detection of an exoplanetary transit signature
labelledWASP-11)wasobservedbySuperWASP-Nduringthe
r (Charbonneauetal. 2000; Henryetal. 2000) over fifty tran- 2004, 2006 and 2007 observing seasons, covering the inter-
a siting planetary systems have been identified. A number of
vals 2004 July 08 to 2004 September 29, 2006 September
wide-field surveys are in progress with the goal of detecting
09 to 2007 January 20 and 2007 September 04 to 2007
transitingexoplanets,forexampleOGLE (Udalskietal. 2002),
December12respectively.Thepipeline-processeddatawerede-
XO (McCulloughetal. 2005), HAT (Bakosetal. 2004), TrES
trended and searched for transits using the methods described
(O’Donovanetal.2006)andWASP(Pollaccoetal.2006).
in CollierCameronetal. (2006), yieldinga detectionofa peri-
TheWASPprojectoperatestwoidenticalinstruments,atLa odictransit-likesignaturewithaperiodof3.722days.Atotalof
Palma in the Northern hemisphere, and at Sutherland in South tentransitsareobservedindatafromallthreeobservingseasons
AfricaintheSouthernhemisphere.Eachtelescopehasafieldof (Table1;Figure1).
viewofjustunder500squaredegrees.TheWASPsurveyissen-
sitive to planetarytransitsignaturesin the light-curvesof hosts
in the magnitude range V∼9–13. A detailed description of the 2.2.Photometricfollow-up
telescopehardware,observingstrategyandpipelinedataanaly-
WASP-11wasfollowed-upwiththe2-mLiverpooltelescopeon
sisisgiveninPollaccoetal.(2006).
La Palma as part of the Canarian Observatories’ International
In this paper we report the discovery of WASP-11b, a Time Programme for 2007-08.We used the 2048×2048 pixel
sub-Jupiter mass gas giant planet in orbit about the host star EEVCCD42-40imagingcameragivingascale of0.27arcsec-
1SWASPJ030928.54+304024.7.WepresenttheWASPdiscov- onds/pixelin2×2binmodeandatotalfieldofviewof∼4.6×4.6
ery photometryplus higher precision optical follow-upand ra- arcminutes2.Observationsweretakenduringthetransitof2008
dial velocity measurements which taken together confirm the January 14, and consist of 656 images of 10 seconds exposure
planetarynatureofWASP-11b. in the Sloan z′ band. The night was non-photometricand with
2 R.G.Westetal.:Thesub-JupitermasstransitingexoplanetWASP-11b
Table1.WASP-NsurveycoverageofWASP-11 Table2.RadialvelocitymeasurementsofWASP-11
Season Camera N N T P BJD RV σ v Inst
pts tr 0 RV span
BJD-2400000.0 (days) (UT) (kms−1) (kms−1)
2004 103 1756 4 53240.921696 3.7220 2454462.395 4.8689 0.0185 NOT
2006 144 2679 3 54056.140758 3.7223 2454463.456 4.8725 0.0203 NOT
2007 146 2750 2 54346.4883 3.7226 2454465.404 4.9208 0.0258 NOT
2007 147 729 1 - - 2454466.440 4.8262 0.0244 NOT
2454466.443 4.9486 0.0246 NOT
2454491.424 4.9339 0.0220 NOT
2454508.3700 4.8910 0.0103 0.011 SOPHIE
2454509.3534 5.0104 0.0084 0.000 SOPHIE
2454510.3813 4.8989 0.0120 0.025 SOPHIE
2454511.3092 4.8515 0.0076 -0.002 SOPHIE
2454511.3800 4.8330 0.0106 -0.002 SOPHIE
2454511.4206 4.8235 0.0143 -0.035 SOPHIE
2454512.3848 4.9482 0.0096 0.022 SOPHIE
2.3.Radialvelocityfollow-up
InitialspectroscopicobservationswereobtainedusingtheFIbre-
fedEchelleSpectrograph(FIES)mountedonthe2.5-mNordic
Optical Telescope. A total of five radial velocity points were
Fig.1.SuperWASP-N photometryofWASP-11 fromthe 2004, obtained during 2007 December 27–31 and 2008 January 25.
2006 and 2007 seasons. The data have been de-trended using WASP-11wasobservedwithanexposuretimeof1800sgivinga
the sysrem scheme described in (CollierCameronetal. 2006) signal-to-noiseratioofaround70–80at5500Å.FIESwasused
andareplottedherephase-foldedonthebest-fitperiodfromthe in medium resolution mode with R=46000 with simultaneous
MCMCanalysis(section3). ThAr calibration.We used the bespoke data reductionpackage
FIEStool to extract the spectra and a specially developed IDL
line-fitting code to obtain radial velocities with a precision of
20–25ms−1.
seeingvaryingfrom0.9to2.2arcsecduringthefourhourlong RadialvelocitymeasurementsofWASP-11werealsomade
observingrun. withtheObservatoiredeHaute-Provence’s1.93mtelescopeand
Theimageswerebiassubtractedandflat-fieldcorrectedwith the SOPHIE spectrograph (Bouchy&TheSophieTeam 2006),
a stackedtwilightflat-fieldimage.All thescience imageswere over the 8 nights 2008 February 11 – 15; a total of 7 usable
also corrected for the fringing effect. The autoguider did not spectrawereacquired.SOPHIEisanenvironmentallystabilized
work during our observations and a maximum positional shift spectrographdesignedtogivelong-termstabilityatthelevelof
of 17.5 arcsec of the stars within the frame was noticed. After a few ms−1. We used the instrument in its medium resolution
aligningthe imageswith respectto the first targetimage,aper- mode,acquiringsimultaneousstarandskyspectrathroughsep-
turephotometrywereperformedaroundthetargetandcompari- arate fibreswith a resolutionofR=48000.Thorium-Argoncal-
sonstarsusinganapertureof20pixels(5′.′4)radius.Threebright ibration images were taken at the start and end of each night,
non-variablecomparisonstars were available in the targetfield andat2-to3-hourlyintervalsthroughoutthenight.Theradial-
withwhichtoperformdifferentialphotometry. velocitydriftneverexceeded2–3ms−1,evenonanight-to-night
Further observations of WASP-11 were made with the basis.
KeeleUniversityObservatory60cmThorntonReflectoron2008 Conditions during the SOPHIE observing run were photo-
February09and13.Thistelescopeisequippedwitha765×510 metric throughout, though all nights were affected by strong
pixelSantaBarbaraInstrumentGroup(SBIG)ST7CCD atthe moonlight. Integrations of 1080s yielded a peak signal-to-
f/4.5Newtonianfocus,givinga0.68arcsecond/pixelresolution noise per resolution element of around ∼30–40. The spectra
anda8.63×5.75arcminutefieldofview.Conditionswerepho- were cross-correlated against a K5V template provided by the
tometricthroughoutbothnights,althoughthetransitofFebruary SOPHIEcontrolandreductionsoftware.
9endedatanairmassof4andcryogenicsproblemsonthenight InallSOPHIEspectrathecross-correlationfunctions(CCF)
ofFebruary13mayhaveledtosomefrostingontheCCDdewar werecontaminatedbythestrongmoonlight.Wecorrectedthem
windowduringthefirstfewexposures.Trackingerrorsandspu- byusingtheCCFfromthebackgroundlight’sspectrum(mostly
rious electronic noise mean that systematic noise is introduced theMoon)intheskyfibre.WethenscaledbothCCFsusingthe
intothesystematanestimatedlevelof4millimagwithperiod- differenceofefficiencybetweenthetwofibres.Finallywesub-
icitiesof2(wormerror)and20minutes(presentlyofunknown tractedthecorrespondingCCFofthebackgroundlightfromthe
origin).Nocorrectionshavebeenappliedfortheseeffects. starfibre,andfittedtheresultingfunctionbyaGaussian.Thepa-
Altogether (237 + 276)× 30s observations in the R band rametersobtainedallowustocomputethe photon-noiseuncer-
were obtained. After applying corrections for bias, dark cur- taintyofthecorrectedradialvelocitymeasurement(σ ),using
RV
rent and flat fielding in the usual way, aperture photometry on therelation
WASP-11 and the comparison star USNO-B1.0 1207-0040657
were performed using the commercial software AIP4Win σ =3.4p(FWHM)/(S/N×Contrast)
RV
(Berry&Burnell 2005). The resulting lightcurves from both
LiverpoolTelescopeandKeele60cmobservations(Figure2top OverallourSOPHIERVmeasurementshaveanaveragephoton-
panel)confirmthepresenceofatransit. noiseuncertaintyof10.3ms−1.Themeasuredbarycentricradial
R.G.Westetal.:Thesub-JupitermasstransitingexoplanetWASP-11b 3
Fig.3.Thelinebi-sectoragainstvelocityforWASP-11,showing
noevidenceofcorrelation.
Table3.SystemparametersofWASP-11derivedfromasimul-
taneousMCMCanalysisoftheavailablephotometricandradial-
velocitymeasurements.Quoteduncertaintiesdefinethe1σcon-
fidenceintervals.
Transitepoch(HJD),T 2454473.05586±0.0002
0
Orbitalperiod,P 3.722465+0.000006 days
−0.000008
(R /R )2 0.0162+0.0003
p ⋆ −0.0002
Transitduration 2.556+0.029 hours
−0.007
Impactparameter,b 0.054+0.168 R
−0.050 ∗
Reflexvelocity,K 0.0821±0.0074 kms−1
1
Centre-of-massvelocity,γ 4.9077±0.0015 kms−1
Orbitaleccentricity,e ≡0.0
Orbitalinclination,i 89.8+0.2 deg
−0.8
Orbitalseparation,a 0.043±0.002 AU
Stellarmass,M 0.77+0.10 M
⋆ −0.08 ⊙
Stellarradius,R 0.74+0.04 R
⋆ −0.03 ⊙
Planetradius,R 0.91+0.06 R
p −0.03 J
Planetmass,M 0.53±0.07 M
p J
logg (cgs) 3.16+0.04
p −0.05
Planetdensity,ρ 0.69+0.07 ρ
p −0.11 J
PlanetT (A=0;f=1) 960±70 K
eql
3. Systemparameters
3.1.Stellarparameters
Inordertoperformadetailedspectroscopicanalysisofthestel-
lar atmospheric properties of WASP-11, we merged the avail-
able FIES spectra into one high-qualityspectrum, carefullyre-
moving any radial velocity signature during the process. This
merged spectrum was then continuum-normalized with a very
Fig.2. The best-fit model from the simultaneous MCMC fit to loworderpolynomialtoretaintheshapeofthebroadestspectral
the available photometry (top panel) and radial velocity data features.Thetotalsignal-to-noiseofthecombinedspectrumwas
(lowerpanel).Thefittedzero-pointoffsetbetweentheNOTand around200perresolutionelement.Wewerenotabletoinclude
SOPHIE radial-velocity measurements (5.4 ± 0.4ms−1) is re- theSOPHIEspectrainthisanalysis,becausethesespectrawere
movedinthisplot. obtainedwiththeHE(high-efficiency)modewhichisknownto
sufferfromproblemswithremovaloftheblazefunction.
For our analysis we followed the same procedure as for
the spectroscopic characterization of WASP-1 (Stempelsetal.
2007) and WASP-3 (Pollaccoetal. 2008). We used the pack-
ageSpectroscopyMadeEasy(SME,Valenti&Piskunov1996),
which combines spectral synthesis with multidimensional χ2
velocity(Table2,Figure2lowerpanel)showasinusoidalvaria- minimization to determine which atmospheric parameters best
tionofhalf-amplitude∼ 90ms−1 aboutacentre-of-massRVof reproduce the observed spectrum of WASP-11 (effective tem-
∼4.9kms−1,consistentwiththepresenceacompanionofplan- peratureT ,surfacegravitylogg,metallicity[M/H],projected
eff
etary mass. The period and ephemeris of the RV variation are radial velocityvsini, systemic radial velocity v , microturbu-
rad
consistentwiththoseoffoundbythetransitsearch. lence v and the macroturbulence v ). For a more detailed
mic mac
An analysis of the line-bisector spans shows no significant descriptionofthespectralsynthesisandourassumptionswere-
correlationwithradialvelocity(Figure3),aswouldbeexpected fertoStempelsetal.(2007).
if the observed radial velocity variations were due to a diluted The four spectral regions we used in our analysis are (1)
eclipsingbinaryorchromosphericactivity(Quelozetal.2001). 5160–5190Å, covering the gravity-sensitive Mg b triplet (2)
4 R.G.Westetal.:Thesub-JupitermasstransitingexoplanetWASP-11b
Fig.4.A comparisonbetweentheobservedFIESspectrumofWASP-11andthe calculatedspectrumobtainedfromspectralsyn-
thesiswithSME.Thewhiteregionsareexcludedfromthespectralanalysis,mainlybecauseofthepresenceoftelluricabsorption.
Lightshadedregionswereusedtodeterminethecontinuumlevel,andtheremainingdarkshadedregionstodeterminethestellar
atmosphericparameters.
5850–5950Å, with the temperature and gravity-sensitive Na the best-fitting models. The best-fit parameters (Table 3) show
i D doublet (3) 6000-6210Å, containing a wealth of differ- WASP-11btohaveamass M = 0.53±0.07MJ andaradiusof
ent metal lines, providing leverage on the metallicity, and (4) R=0.91+−00..0063RJ.
6520–6600Å, covering the strongly temperature-sensitive H-
alphaline.A comparisonbetweentheobservedFIESspectrum
4. Discussion
and the synthetic spectrum is shown in Figure 4. The spectral
analysis yields an effective temperature Teff = 4800± 100K, The system parameters derived here place WASP-11b towards
logg = 4.45±0.2,[M/H] = 0.0±0.2andvsini < 6.0kms−1. the lower end of the mass range of known transiting planets,
These parameters correspond to spectral type of K3V. A close falling approximately mid-way between the masses of Jupiter
examination of the region around the Li i 6708 shows no evi- and Saturn. The host star WASP-11 is also amongst the small-
denceofsuchafeature,suggestingthatthelithiumabundanceis est and lowest luminosity stars known to host a transiting
verylow. planet, however it is relatively nearby and thus quite bright
(V = 11.7). WASP-11b is irradiated by a stellar flux F =
p
1.9 × 108ergcm−2s−1 at the sub-stellar point making it the
3.2.Planetparameters
third least heavilyirradiatedtransiting planetafter GJ436b and
To determinethe planetaryandorbitalparametersthe SOPHIE HD17156b.WecomputeanequilibriumtemperatureforWASP-
and NOT FIES radial velocity measurements were combined 11bofT (A = 0; f = 1) = 960±70K, which makesit more
eql
with the photometry from WASP and the Liverpool Telescope typicalofthebulkofknownexoplanetsthanofthe“hotJupiter”
in a simultaneous fit using the Markov Chain Monte Carlo classmostcommonlyfoundbythetransitmethod.
(MCMC) technique. The details of this process are described Theoreticalmodelsoftheatmospheresofhotgiantexoplan-
inPollaccoetal.(2008).Aninitialfitshowedthattheorbitalec- ets (Fortneyetal. 2006; Burrowsetal. 2007) have shown that
centricity(e = 0.086+0.070)waspoorlyconstrainedbytheavail- heavyirradiationcan lead to the developmentof a temperature
−0.062
abledataandnearlyconsistentwithzero.Wethereforefixedthe inversion and a hot stratosphere. This is due to the absorption
eccentricity parameter at zero in a further fits. Figure 2 shows of stellar flux by an atmospheric absorber, possibly TiO and
R.G.Westetal.:Thesub-JupitermasstransitingexoplanetWASP-11b 5
Fig.6.Planetarymass-radiusrelationsasafunctionofcoremass
and system age, interpolated from the models of Fortneyetal.
(2007).
Fig.5. The position of WASP-11 in the R/M1/3 − T plane.
eff
EvolutionarytracksforasolarmetallicitystarfromBaraffeetal.
(1998)(upperpanel)andGirardietal.(2000)(lowerpanel)are ationaroundtheorbitwillbecorrespondinglylowerinWASP-
plotted along with isochrones for ages 10Myr (solid), 1Gyr 11b, removinga potentially complicatingfactor when compar-
(dashed), 5Gyr (dot-dashed), 10Gyr (dotted). Evolutionary ing follow-up observations with predictions from atmospheric
masstracksareshownfor0.7,0.8,0.9and1.0M⊙. modelsdevelopedassumingsteady-stateirradiation.
To estimate the age of the WASP-11 we compared the ob-
served stellar density and temperature against the evolution-
arymodelsoflow-andintermediate-massstarsofGirardietal.
VO. In both sets of models the magnitude of the incident stel- (2000)andBaraffeetal.(1998).InFigure5weplottheposition
lar flux is the key controlling variable determining whether a ofWASP-11intheR/M1/3 versusT planeatopisochronesof
eff
given extra-solar giant planet (EGP) will possess a hot strato- different ages from the two models. For such a cool star, the
sphere. Recent observations by Machaleketal. (2008) of sec- isochronesarecloselyspacedinthisparameterplaneduetothe
ondary transits of XO-1b using the Spitzer Space Telescope slowpost-main-sequenceevolutionoflate-typestars.Thesetsof
suggest the presence of a temperature inversion in the atmo- isochronesfromthetwomodelsoverlapinthisregime,andboth
sphere of that exoplanet. On the other hand analogous obser- modelssuggestthesamemassandageforthehoststar.WASP-
vationsofHD189733b(Charbonneauetal. 2008) shownoevi- 11fallsabovethe10Gyrisochroneforbothmodels,thoughitis
dence for an inversion, despite the irradiating fluxes of XO-1b consistentwiththisagewithintheerrors.Theverylowlithium
and HD189733b being almost identical (Fp = 0.49×109 and abundance also points toward WASP-11 being >∼ 1–2Gyr old
F = 0.47×109ergcm−2s−1 respectively). This strongly sug- (Sestito&Randich 2005). We investigated using gyrochronol-
p
geststhattheincidentstellarfluxisnotthesolecontrollingpa- ogy to age the host star, following Barnes (2007), however we
rameter determiningthe presence of the inversion,a likelihood wereunabletomeasureadefiniterotationalperiod.Norotation
which the authors of the atmosphere models readily point out modulationwasdetectedinthelightcurvetoanamplitudelimit
themselves. Further observations of planets particularly in the ofa fewmilli-magnitudes.Thespectralanalysisfurnishesonly
low-irradiationregimearerequiredtohelpparameterisethether- an upper-limit to vsini, so no rotational period can be deter-
mal inversion.WASP-11b is amongst the nearest and brightest mined in that way. Taken together these factors are all consis-
low-irradiationEGPsmakingitagoodcandidateforsuchstud- tent with WASP-11 being an old star, older than maybe 1Gyr,
ies.MoreoverwenotethattheorbitaleccentricityofWASP-11b howeveritisnotpossibletobemoredefinitethanthatwiththe
is muchlowerthanthe othertwo brightlow-irradiationtransit- availabledata.
ingexoplanets,GJ436bandHD17156b(e = 0.15ande = 0.67 Fortneyetal.(2007)presentmodelsoftheevolutionofplan-
respectively). As a consequence the secular variation in irradi- etary radius over a range of planetary masses and orbital dis-
6 R.G.Westetal.:Thesub-JupitermasstransitingexoplanetWASP-11b
tances,andundertheassumptionofthepresenceofadensecore
of various masses up to 100M . To compare our results with
⊕
the Fortney et al. models we plotted the modelled mass-radius
relation as a function of core mass in Figure 6. To account for
the lower-than-Solar luminosity of the host star WASP-11 we
calculatedtheorbitaldistancea = a(M /M )−3.5/2 atwhicha
⊙ ⋆ ⊙
planet in orbit about the Sun would receive the same incident
stellar flux as WASP-11b does from its host. We then interpo-
latedthemodelsofFortneyetal.tothiseffectiveorbitaldistance
(a = 0.068 for WASP-11b). As the age of the WASP-11 sys-
⊙
temispoorlyconstrainedwecompareourresultswiththemod-
elledmass-radiusrelationat300Myr,1Gyrand4.5Gyr.Wefind
thattheradiusofWASP-11bisconsistentwiththepresenceofa
densecorewithamassintherangeM ∼42–77M forasys-
core ⊕
temageof300Myr,M ∼33–67M at1Gyr,andM ∼22–
core ⊕ core
56M at4.5Gyr.
⊕
Acknowledgements. TheWASPConsortiumconsistsofastronomersprimarily
fromtheQueen’sUniversityBelfast,Keele,Leicester,TheOpenUniversity,and
StAndrews, theIsaac Newton Group(LaPalma),the Instituto deAstrof´ısica
de Canarias (Tenerife) and the South African Astronomical Observatory. The
SuperWASP-N and WASP-S Cameras were constructed and operated with
funds made available from Consortium Universities and the UK’s Science
and Technology Facilities Council. SOPHIE observations have been funded
by the Optical Infrared Coordination network (OPTICON), a major interna-
tionalcollaborationsupportedbytheResearchInfrastructuresProgrammeofthe
EuropeanCommission’sSixthFrameworkProgramme.FIESobservationswere
madewith theNordic Optical Telescope, operated onthe island ofLaPalma
jointly by Denmark, Finland, Iceland, Norway, and Sweden, in the Spanish
Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de
Canarias. Weextendourthanks totheDirectorandstaffoftheIsaacNewton
Group of Telescopes for their support of SuperWASP-N operations, and the
Director and staff ofthe Observatoire de Haute-Provence for their support of
theSOPHIEspectrograph.TheLiverpoolTelescopeisoperatedontheislandof
LaPalmabyLiverpoolJohnMooresUniversityintheSpanishObservatoriodel
RoquedelosMuchachosoftheInstitutodeAstrofisicadeCanariaswithfinan-
cialsupportfromtheUKScienceandTechnologyFacilitiesCouncil.
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WASP-16b: A new Jupiter-like planet transiting a southern solar
analog
T. A. Lister1, D. R. Anderson2, M. Gillon3,4, L. Hebb5, B. S. Smalley2,
9
0 A. H. M. J. Triaud3, A. Collier Cameron5, D. M. Wilson2,6, R. G. West7, S. J. Bentley2,
0
D. J. Christian8,9, R. Enoch10,5, C. A. Haswell10, C. Hellier2, K. Horne5, J. Irwin11,
2
Y. C. Joshi8, S. R. Kane12, M. Mayor3, P. F. L. Maxted2, A. J. Norton10, N. Parley10,5,
g
u F. Pepe3, D. Pollacco8, D. Queloz3, R. Ryans8, D. Segransan3, I. Skillen13, R. A. Street1,
A I. Todd8 S. Udry3, P. J. Wheatley14
3 [email protected]
]
P
E
. ABSTRACT
h
p
We report the discovery from WASP-South of a new Jupiter-like extrasolar planet, WASP-
-
o 16b,whichtransits its solaranaloghoststarevery3.12days. Analysisofthe transitphotometry
r andradialvelocity spectroscopic data leads to a planet with R =1.008±0.071R andM =
t p Jup p
s 0.855±0.059MJup,orbitingahoststarwithR∗ =0.946±0.054R⊙ andM∗ =1.022±0.101M⊙.
a
Comparison of the high resolution stellar spectrum with synthetic spectra and stellar evolution
[
models indicates the host star is a near-solar metallicity ([Fe/H]= 0.01 ± 0.10) solar analog
1 (T =5700±150 K, logg=4.5±0.2) of intermediate age (τ =2.3+5.8Gyr).
v eff −2.2
7 Subjectheadings: planetarysystems: individual: WASP-16b—stars: individual()—stars: abundances
9
2
1. Introduction
0 1Las Cumbres Observatory, 6740 Cortona Drive Suite
.
8 102,Goleta,CA93117,USA There are currently over 300 known exoplan-
0 2Astrophysics Group, Keele University, Staffordshire, ets15withthemajorityofthemdiscoveredthrough
9 ST55BG,UK
the radial velocity technique. A growing number
0 3Observatoirede Gen`eve, Universit’deGen`eve, 51Ch.
ofexoplanetsinrecentyearshavebeendiscovered
: desMaillettes,1290Sauverny,Switzerland
v 4Institutd’AstrophysiqueetdeG´eophysique,Universit´e through the transit method. Transiting exoplan-
i
X deLi`ege,All´eedu6Aouˆt,17,Bat. B5C,Li`ege1,Belgium ets are particularly valuable as they allow param-
r 5SUPA,SchoolofPhysicsandAstronomy,Universityof eters such as the mass, radius and density to be
a St Andrews, North Haugh, St Andrews, Fife KY16 9SS, accuratelydetermined andfurther studies suchas
UK
transmissionspectroscopy,secondaryeclipse mea-
6CentreforAstrophysics&PlanetaryScience,Schoolof
surementsandtransittiming variationsto be car-
Physical Sciences, University of Kent, Canterbury, Kent,
ried out.
CT27NH,UK
7Department of Physics and Astronomy, University of There are severalwide angle surveys that have
Leicester,Leicester,LE17RH,UK been successful in finding transiting exoplanets
8Astrophysics Research Centre, School of Mathematics
& Physics, Queen’s University, University Road, Belfast,
22,770SouthWilsonAvenue, Pasadena, CA91125,USA
BT71NN,UK
9CaliforniaStateUniversityNorthridge18111Nordhoff 13Isaac Newton Group of Telescopes, Apartado de
Correos 321, E-38700 Santa Cruz de la Palma, Tenerife,
Street,Northridge,CA91330-8268, USA
Spain
10DepartmentofPhysicsandAstronomy,TheOpenUni-
14DepartmentofPhysics,UniversityofWarwick,Coven-
versity,MiltonKeynes,MK76AA,UK
tryCV47AL,UK
11Harvard-SmithsonianCenterforAstrophysics,60Gar-
15http://exoplanet.eu
denStreet, Cambridge,MA,02138USA
12NASA Exoplanet Science Institute, Caltech, MS 100-
1
around bright stars, namely HAT (Bakos et al. sit fit, is shown in Figure 1. In order to better
2002),TrES(Alonso et al.2004),XO(McCullough et al. constrain the transit parameters, follow-up high
2005) and WASP (Pollacco et al. 2006). The precisionphotometricobservationswiththe Swiss
WASP Consortium conducts the only exoplanet 1.2m+EULERCAM on La Silla, were obtained in
search currently operating in both hemispheres the I band on the night of 2008 May 04 and are
c
although HATnet is planning a southern exten- shown in Figure 2.
sion and several groups are planning searches
from Antarctica (e.g. Strassmeier et al. 2007; 2.2. Spectroscopic observations
Crouzet et al. 2009).
In order to confirm the planetary nature of the
We reportthe discoveryfromthe WASP-South
transitsignal,weobtainedfollow-upspectroscopic
observatoryofa ∼0.86M mass companionor-
Jup observationwiththeSwiss1.2m+CORALIEspec-
biting a V ∼ 11.3 close solar analog WASP-16
trograph. The data were processed through the
(=TYC 6147-229-1,USNO-B1.0 0697-0298329).
standard CORALIE reduction pipeline as de-
scribed by Baranne et al. (1996) with an addi-
2. Observations
tional correction for the blaze function. Fourteen
radial velocity measurements were made between
2.1. Photometric observations
2008 March 10 and 2008 August 04 and an ad-
WASP-South, located at SAAO, South Africa, ditional sixteen between 2009 Feb 19 and 2009
is one of two SuperWASP instruments and com- June 03 (see Table 1) by cross-correlating with
prises eight cameras on a robotic mount. Each a G2 template mask. The resulting radial veloc-
camera consists of a Canon 200mm f/1.8 lens ity (RV) curve is shown in Figure 3. The low
with an Andor 2048×2048 e2v CCD camera giv- amplitude RV variation clearly supports the exis-
ing a field of view of 7.8◦ × 7.8◦ and a pixel tence of a planetary mass companion. In order to
scale of 13.7′′. Exposure times were 30s and the rule out a non-planetary explanation for the ra-
same field is returned to and reimaged every 8– dial velocity variation such as a blended eclipsing
10 minutes. Further details of the instrument, binaryorstarspots,weexaminedthe line-bisector
survey and data reduction pipelines are given in spans. Contamination from an unresolved eclips-
Pollacco et al. (2006) and the candidate selection ing binary will cause asymmetries in the spec-
procedure is described in Collier Cameron et al. tral line profiles and line bisector span variations
(2007) and Pollacco et al. (2008) and references (Queloz et al. 2001; Torres et al. 2005). As can
therein. be seen from the lower panel of Figure 3, there
WASP-16 was observed for a partial season in is no sign of variation with phase of the bisector
2006, a full season in 2007 and a further par- spans and their amplitude is much smaller than
tial season in 2008 with the distribution of data the radial velocity variation. This supports the
pointsas3324points(2006),6013(2007)and4084 conclusion that the radial velocity variations are
(2008). The 2007 light curve revealed the pres- due to a planet orbiting the star and not some
enceofa∼1.3%dipwithaperiodof∼3.11days. other cause.
The transit coveragein the other two seasonswas
verysparse,particularlyin2006,andthereisonly 3. WASP-16 System Parameters
evidence for 2 partial transits in the 2008 data.
3.1. Stellar Parameters
WASP-16wasafairlystrongcandidateforfollow-
up despite the small number of transits, passing The individual CORALIE spectra are of rela-
the filtering tests of Collier Cameron et al.(2006) tively low signal-to-noise,but when co-addedinto
with a signal to red noise ratio, Sred =9.38 (with 0.01˚Asteps they give a S/N of around70:1 which
Sred > 5 required for selection), ‘transit to anti- is suitable for a photospheric analysis of the host
transitratio’∆χ2/∆χ2− =2.5(∆χ2/∆χ2− ≥1.5 star. In addition, a single HARPS spectrum was
required for selection) and no measurable ellip- used to complement the CORALIE analysis, but
soidal variation. thisspectrumhadrelativelymodestS/Nofaround
The SuperWASP light curve showing a zoom 50:1. The standard CORALIE/HARPS pipeline
of the transit region, along with the model tran- reduction products were used in the analysis.
2
Table 1
CORALIE radial velocities for WASP-16.
Time of obs. Rad. Vel. σ Bisector span
RV
(BJD-2450000) (kms−1) (kms−1) (kms−1)
4535.864842 -1.99772 0.01591 0.00306
4537.849158 -1.96688 0.00853 -0.04553
4538.858364 -2.00734 0.00899 -0.03129
4558.780835 -1.83336 0.00723 -0.02779
4560.709473 -2.00513 0.00725 -0.02403
4561.688137 -1.82730 0.00785 -0.03998
4589.705102 -1.84255 0.00875 -0.04520
4591.706755 -2.03571 0.00892 -0.03221
4652.495906 -1.82493 0.00808 -0.03209
4656.551645 -2.02421 0.00787 -0.02555
4657.577293 -1.96640 0.00957 -0.01827
4663.539741 -2.02961 0.00969 -0.02661
4664.616769 -1.78590 0.01108 -0.04350
4682.521501 -1.98118 0.00754 -0.02123
4881.869213 -2.02245 0.00813 -0.02760
4882.801025 -1.83289 0.00823 -0.03739
4884.737094 -2.04565 0.00778 -0.01672
4891.805707 -1.90043 0.00798 -0.01009
4892.723980 -1.83413 0.00891 -0.02116
4941.728231 -1.88737 0.00748 -0.04134
4943.730102 -2.04677 0.00753 -0.01825
4944.739293 -1.91359 0.00860 -0.02245
4945.799895 -1.85815 0.00807 0.01502
4947.745317 -1.93960 0.00741 -0.03134
4948.673112 -1.82992 0.00743 -0.06231
4972.707323 -1.93123 0.00854 -0.03631
4975.733486 -1.93144 0.01100 -0.01416
4982.647535 -1.83433 0.01036 -0.02677
4984.642389 -2.04210 0.00892 -0.04270
4985.694776 -1.81561 0.00802 -0.02406
3
