A lucky imaging multiplicity study of exoplanet host stars II

10.4 MB·

by  C. Ginski

Mon.Not.R.Astron.Soc.000,1–19(2015) Printed8January2016 (MNLATEXstylefilev2.2)
A lucky imaging multiplicity study of exoplanet host stars II∗†
C. Ginski1,2‡, M. Mugrauer2, M. Seeliger2, S. Buder2,3, R. Errmann2,4, H. Avenhaus5,
D. Mouillet6,7, A.-L. Maire8, and S. Raetz9
1LeidenObservatory,LeidenUniversity,P.O.Box9513,2300RALeiden,TheNetherlands
6 2AstrophysikalischesInstitutundUniversitäts-SternwarteJena,Schillergässchen2,07745Jena,Germany
1 3Max-Planck-InstitutfürAstronomie,Königstuhl17,D-69117Heidelberg,Germany
0 4IsaacNewtonGroupofTelescopes,SantaCruzdeLaPalma,Spain
2 5DepartamentodeAstronomìa,UniversidaddeChile,Casilla36-D,Santiago,Chile
n 6UniversitéGrenobleAlpes,IPAG,F-38000Grenoble,France
a 7CNRS,IPAG,F-38000Grenoble,France
J 8INAF-OsservatorioAstronomicodiPadova,Vicolodell’Osservatorio5,35122Padova,Italy
7 9ScientificSupportOffice,DirectorateofScienceandRoboticExploration,EuropeanSpaceResearchandTechnologyCentre(ESA/ESTEC),
Keplerlaan1,2201AZNoordwijk,TheNetherlands

]
R
S Accepted2015XX.Received2015XX
.
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p
ABSTRACT
-
o The vast majority of extrasolar planets are detected by indirect detection methods such as
r transitmonitoringandradialvelocitymeasurements.Whilethesemethodsareverysuccess-
t
s fulindetectingshort-periodicplanets,theyaremostlyblindtowidesub-stellarorevenstellar
a companionsonlongorbits.Inourstudywepresenthighresolutionimagingobservationsof
[
63exoplanethostscarriedoutwiththeluckyimaginginstrumentAstraLuxattheCalarAlto

1 2.2m telescope as well as with the new SPHERE high resolution adaptive optics imager at
v theESO/VLTinthecaseofaknowncompanionofspecificinterest.Ourgoalistostudythe
4 influenceofstellarmultiplicityontheplanetformationprocess.
2 Wedetectedandconfirmed4previouslyunknownstellarcompanionstotheexoplanethosts
5
HD197037,HD217786,Kepler-21andKepler-68.Inaddition,wedetected11newlow-mass
1
stellar companion candidates which must still be confirmed as bound companions. We also
0
provide new astrometric and photometric data points for the recently discovered very close
.
1 binarysystemsWASP-76andHD2638.Furthermore,weshowforthefirsttimethatthepre-
0 viouslydetectedstellarcompaniontotheHD185269systemisaverylowmassbinary.Finally
6 weprovidepreciseconstraintsonadditionalcompanionsforallobservedstarsinoursample.
1
: Keywords: astrometry–planetsandsatellites:formation–(stars:)binaries:visual–tech-
v
niques:highangularresolution–stars:individual:HD185269–stars:individual:Kepler-21
i
X –stars:individual:Kepler-68–stars:individual:HD197037–stars:individual:HD217786.
r
a
1 INTRODUCTION (oftenintheorderofdaysorweeks),theyareblindtowide(sub-
) stellar companions at several tens or hundreds of au. However,
Weliveinagoldenageforextrasolarplanetdiscoveries.Inthepast
morethan50%ofallmainsequencestarsintheGalaxyandapprox-
decade several large radial velocity and transit surveys have dis-
imatelyhalfofallsolartypestarsareactuallymembersofstellar
coveredmorethan1200systemscontainingextrasolarplanets(ex-
multiplesystems(Mathieuetal.2000,Raghavanetal.2010).Itis
oplanet.eu,asofJuly2015).Whiletheseindirectdetectionmethods
thus of great interest to investigate the influence of stellar multi-
havebeenincrediblysuccessful,theyhaveafewinherentbiases.In
plicityonextrasolarplanetformationandorbitalevolution.
particular,whiletheyareverysensitivetoshort-periodcompanions
There have been a large number of theoretical and observational
studiesthatinvestigatedtheinfluenceofcloseandwidestellarcom-
panionsonthevariousstagesoftheplanetformationprocess.Itis,
∗ Based on observations collected at the German-Spanish Astronomical for instance, believed that close stellar companions will truncate
Centre,CalarAlto,Spain,operatedjointlybytheMax-Plank-Institutfür protoplanetary disks and shorten their dissipation timescale. This
Astronomie(MPIA),Heidelberg,andtheSpanishNationalCommissionfor hasbeenobservationallyconfirmede.g.byBouwmanetal.(2006)
Astronomy. whofoundasignificantlyreducednumberofdisksinbinarysys-
† BasedinpartonobservationsobtainedatParanalObservatoryinESO
temsintheirSpitzersurveyoftheyoungηChastarcluster.Other
program095.C-0273(A) studiessuchasKrausetal.(2012)findthatthiseffectisdependent
‡ E-mail:[email protected]
(cid:13)c 2015RAS
2 C.Ginskietal.
onthebinaryseparationwithsignificantdropsofdiskoccurrences CalarAltoof36ms).Wethenrecordedalargenumberofindividual
onlyobservedforsystemswithseparationssmallerthan∼40au. images(typically50000)ofwhichweonlyusedsubsetswiththe
Inadditiontotheinitialconditionsandtimescalesintheprotoplan- highestStrehlratio(Strehl1902)forfinalcombination.Thelucky
etarydisk,stellarcompanionsmightalsoinfluencetheaccretionof imagingtechniqueisdescribedindetailine.g.Lawetal.(2006).
planetesimalsbyexcitinghighereccentricitiesandvelocitieswhich AllluckyimagingobservationswereundertakenusingtheSDSSi
mightleadtomoredestructivecollisions(seee.g.Kley&Nelson filter.Theelectronmultiplyinggainoftheinstrumentwasadjusted
2007 or Paardekooper et al. 2008). However, recent studies find individuallyforeachtargettoenablehighsignal-to-noisewithout
thatthiseffectmightbemitigatedbythegravitationalforceofsuf- saturating the primary star. We also adjusted the focus of the in-
ficientlymassivedisks(Rafikov2013). strument several times during the night to ensure highest image
Finally,stellarcompanionsmighthaveamajorinfluenceontheob- quality.Inour2011,2013and2014observationsinvisitormode,
served semi-major axis, inclination and eccentricity distributions weusedthefullfieldofviewofthedetectorof24×24arcsecwith
ofextrasolarplanets.StudiesbyFabrycky&Tremaine(2007)and the shortest possible exposure time of 29.54ms in frame transfer
Petrovich (2015) suggest that Kozai-Lidov type interactions be- mode.Forthebrightesttargetsweusedshorterintegrationstimes
tween planets and stellar companions, in combination with tidal withoutframetransfermodeandlessoverallframesduetolarger
friction,mightexplainsomeoftheobservedextremeshortperiod overheads,i.e.significantlyincreasedreadouttime.Inthe2015ob-
orbits. Other studies (e.g. Naoz et al. 2011) suggest that such in- servationsinservice modetheinstrumentwasusedin windowed
teractions could explain very eccentric planet orbits or spin-orbit mode,readingonlyhalfofthefieldofview.Thisenabledshorter
misalignment. For a comprehensive overview of all these effects exposuretimesoftypically15.03ms.Detailsforeachsystemare
wesuggestthearticlebyThebault&Haghighipour(2014). giveninTab.1.
To study these effects, it is necessary to find the true fraction Datareductionoftheluckyimagingdataincludedflatfieldingwith
of multiple stellar systems amongst extrasolar planet host stars. skyflatstakenduringdawn,aswellasbiassubtraction.Biasframes
Diffraction- or seeing-limited imaging is a primary tool for this were taken before each science exposure with the same gain set-
purpose,inparticulartofindmultiplestellarsystemswithplanets tingsasthesciencetarget.Afterflatfieldingandbiassubtraction,
inS-typeoribts,i.e.theplanetsorbitoneofthestellarcomponents theStrehlratioineachimagewasmeasuredandthenonlytheim-
ofthesystem.Thisorbitconfigurationaccountsforthemajorityof ageswiththe10%,5%and1%bestStrehlratioswerealignedand
multiplestellarexoplanetsystems(seee.g.Roelletal.2012). combinedrespectively1.Forthefinaldatareductionweutilizedthe
Therehavebeenanumberofimagingstudiesinthepastsuchas nativeAstraLuxpipelineavailableatCalarAlto(describedindetail
Eggenbergeretal.(2007),Mugraueretal.(2007),Daemgenetal. byHormuthetal.2008),aswellasourownpipelineforthereduc-
(2009),Chauvinetal.(2011),Lillo-Boxetal.(2012),ormorere- tionofluckyimagingdata.Ourownpipelinewasusedinallthose
centlyDressingetal.(2014),Mugraueretal.(2014),Mugrauer& (few)caseswheretheCalarAltopipelineproducednooutputdue
Ginski(2015)andWöllertetal.(2015). tosoftwaremalfunction.Finalimageswithdetectedknowncom-
Inthisworkwepresenttheresultsofourongoingmultiplicitystudy panionsaswellasnewcompanioncandidatesareshowninFig.1
employingtheluckyimaginginstrumentAstraLux(Hormuthetal. and2.Weshowthe2013datawhenavailable,sinceitisingeneral
2008)attheCalarAlto2.2mtelescope.Inparticularwepresentre- ofslightlyhigherqualitythanthe2014dataduetobetterweather
sultsfor63systemsobtainedbetween2011and2015.Resultsprior conditions(highercoherencetime,noclouds).Toenhancethecon-
tothatcanbefoundinthefirstpublicationofoursurveyinGinski trastbetweenthebrightprimarystarsandthefaintcompanioncan-
etal.(2012).Ourtargetsarestarsaroundwhichanexoplanethas didates,wehaveemployedhighpassfilteringontheimages.
beendetectedbyradialvelocityortransitobservationsandwhich In addition, we did use SPHERE’s near infrared camera IRDIS
havenotyetbeenobservedwithhighresolutionimaging.Wefur- (Dohlenetal.2008)indualbandimagingmode(Viganetal.2010)
therlimitoursampletostarswithin∼200pc(withfewexceptions) toimagetheHD185269systeminY,JandH-bandwithbroadband
sothatweareabletoconfirmdetectedcompanioncandidatesvia filterson02-05-2015.Thespecificinterestinthissystemwastrig-
common proper motion analysis. In addition to our lucky imag- geredbyanobservedelongationofthecompanion’sPSFinourAs-
ingobservations,wecomplementourstudywithextremeadaptive traLuxobservations.Weusedtheminimalexposuretimeof0.84s
optics supported images from the new planet hunting instrument without coronagraph and with neutral density filter, which led to
SPHERE(Beuzitetal.2008)attheESO/VLT. onlyminorsaturationofthecoreoftheprimarystar’spointspread
Wederiveastrometricandphotometricdataofalldetectedcompan- function(PSF)inYandH-band,andnosaturationinJ-band.For
ioncandidatesandperformcommonpropermotionanalysisforall eachfiltersettingwetookatotalof20individualexposuresfora
systemswithmorethanoneobservationepoch.Finallyweprovide totalintegrationtimeof16.8s.Allindividualimagesineachband
detaileddetectionlimitsonallobservedsystems. were median combined and then flat fielded and dark subtracted.
Sincewedidnotapplyaditherpatterninthisveryshortobserva-
tionsequence,wethenusedabadpixelmask(createdfromflatand
darkframes)toeliminatebadpixels.Finallywecombinedbothim-
2 OBSERVATIONSANDDATAREDUCTION
agesofthedualimagingmodeineachband.Aresultingcombined
Theobservationspresentedinthisstudywereundertakenbetween colorimageisshowninFig.3.
July2011andMarch2015withtheluckyimaginginstrumentAs-
traLuxattheCalarAltoObservatory.Inaddition,wepresentdata
foronesystemwhichwastakenwiththenewSPHEREplanethunt-
ing instrument at the ESO VLT during guaranteed time observa-
tions(GTO)inMay2015.
Forourluckyimagingobservationsweusedshortexposurestimes
in the same order as the coherence time of the atmosphere (e.g. 1 Ifnototherwisestatedwegenerallyusedthebest10%imagesforsubse-
Hormuth et al. (2008) measure a speckle coherence times at the quentanalysis.
(cid:13)c 2015RAS,MNRAS000,1–19
Amultiplicitystudyofexoplanethoststars 3
Table1.ObservationsummaryofalltargetsobservedwithAstraLuxattheCalarAlto2.2mtelescope.Wegivethetotalintegrationtimeforeachtargetfora
frameselectionrateof10%.
Star R.A. DEC epoch #offrames exposuretime[ms] tot.integ.time[s] fieldofview[arcsec]
HD2638 002959.87274 -054550.4009 19-08-2014 50000 29.54 147.70 24×24
HD2952 003310.39467 +545341.9440 19-08-2014 50000 29.54 147.70 24×24
HD5608 005814.21893 +335703.1843 17-01-2013 50000 29.54 147.70 24×24
HD5891 010033.19204 +201732.9381 17-01-2013 50000 29.54 147.70 24×24
HD8574 012512.51565 +283400.1010 30-06-2013 50000 29.54 147.70 24×24
HD10697 014455.82484 +200459.3381 20-08-2014 50000 29.54 147.70 24×24
WASP-76 014631.8590 +024202.065 19-08-2014 50000 29.54 147.70 24×24
HAT-P-32 020410.278 +464116.21 19-08-2014 60000 29.54 177.24 24×24
HD12661 020434.28834 +252451.5031 20-08-2014 50000 29.54 147.70 24×24
HD13189 020940.17260 +321859.1649 20-08-2014 50000 29.54 147.70 24×24
HD13908 021814.56056 +653539.6988 19-08-2014 50000 29.54 147.70 24×24
HD15779 023209.42200 -010205.6236 17-01-2013 50000 29.54 147.70 24×24
HD285507 040701.22653 +152006.0989 20-08-2014 50000 29.54 147.70 24×24
HD290327 052321.56490 -021639.4302 10-03-2015 50000 15.03 75.15 12×12
HD40979 060429.94214 +441537.5940 10-03-2015 50000 29.54 147.70 12×12
HD43691 061934.67623 +410532.3113 10-03-2015 16383 15.01 24.59 12×12
HD45350 062845.71155 +385746.6670 10-03-2015 50000 15.01 75.05 12×12
OmiUma 083015.87064 +604305.4115 10-03-2015 20000 5.01 10.02 12×12
GJ328 085507.597 +013256.44 10-03-2015 50000 15.01 75.05 12×12
HD95089 105847.73629 +014345.1758 10-03-2015 32766 15.01 49.18 12×12
HD96063 110444.45463 -023047.5867 10-03-2015 50000 15.01 75.05 12×12
HD99706 112830.21370 +435759.6902 10-03-2015 50000 15.01 75.05 12×12
HD100655 113503.75349 +202629.5713 10-03-2015 50000 15.01 75.05 12×12
HIP57274 114440.96488 +305733.4552 10-03-2015 50000 15.01 75.05 12×12
HD102329 114646.64518 +032827.4563 10-03-2015 50000 15.01 75.05 12×12
HD106270 121337.28529 -093048.1691 10-03-2015 16383 15.01 24.59 12×12
HD113337 130146.92669 +633636.8092 10-03-2015 50000 15.01 75.05 12×12
HD116029 132039.54263 +243855.3080 30-06-2013 50000 29.54 147.70 24×24
132039.54263 +243855.3080 20-08-2014 60000 29.54 177.24 24×24
HD120084 134239.20186 +780351.9756 10-03-2015 50000 15.01 75.05 12×12
BetaUMi 145042.32580 +740919.8142 10-03-2015 20000 4 8.00 12×12
HD131496 145323.02871 +181407.4562 30-06-2013 50000 29.54 147.70 24×24
HD131496 145323.02871 +181407.4562 10-03-2015 50000 15.01 75.05 12×12
HD136726 151705.88899 +714926.0466 30-06-2013 50000 29.54 147.70 24×24
11UMi 151705.88899 +714926.0466 10-03-2015 50000 15.01 75.05 12×12
HD136512 152008.55879 +293658.3488 01-07-2013 50000 29.54 147.70 24×24
OmiCrB 152008.55879 +293658.3488 10-03-2015 50000 15.01 75.05 12×12
HD139357 153516.19886 +535519.7129 01-07-2013 50000 29.54 147.70 24×24
HD145457 161003.91431 +264433.8927 01-07-2013 50000 29.54 147.70 24×24
HD152581 165343.58257 +115825.4822 01-07-2013 50000 29.54 147.70 24×24
HAT-P-18 170523.151 +330044.97 30-06-2013 50000 29.54 147.70 24×24
170523.151 +330044.97 19-08-2014 65540 29.54 193.61 24×24
170523.151 +330044.97 20-08-2014 50000 29.54 147.70 24×24
HD156279 171223.20383 +632107.5391 01-07-2013 50000 29.54 147.70 24×24
HD163607 175340.49479 +562331.0417 30-06-2013 50000 29.54 147.70 24×24
HD163917 175901.59191 -094625.0798 30-06-2013 50000 29.54 147.70 24×24
HIP91258 183653.15422 +614209.0124 20-08-2014 50000 29.54 147.70 24×24
Kepler-37 185614.3063 +443105.356 19-08-2014 50000 29.54 147.70 24×24
Kepler-21 190926.83535 +384250.4593 01-07-2013 50000 29.54 147.70 24×24
190926.83535 +384250.4593 20-08-2014 50000 29.54 147.70 24×24
HD180314 191450.20890 +315137.2569 30-06-2013 50000 29.54 147.70 24×24
Kepler-63 191654.294 +493253.51 20-08-2014 50000 29.54 147.70 24×24
Kepler-68 192407.7644 +490224.957 01-07-2013 50000 29.54 147.70 24×24
192407.7644 +490224.957 19-08-2014 50000 29.54 147.70 24×24
Kepler-42 192852.556 +443709.62 30-06-2013 50000 29.54 147.70 24×24
HAT-P-7 192859.3616 +475810.264 19-08-2014 50000 29.54 147.70 24×24
HD185269 193711.74092 +282959.5055 30-06-2013 50000 29.54 147.70 24×24
193711.74092 +282959.5055 19-08-2014 50000 29.54 147.70 24×24
HD188015 195204.54338 +280601.3517 30-06-2013 50000 29.54 147.70 24×24
195204.54338 +280601.3517 20-08-2014 50000 29.54 147.70 24×24
HD190360 200337.40587 +295348.4944 01-07-2013 50000 29.54 147.70 24×24
(cid:13)c 2015RAS,MNRAS000,1–19
4 C.Ginskietal.
Table1.Continued
Star R.A. DEC epoch #offrames exposuretime[ms] tot.integ.time[s] fieldofview[arcsec]
HD197037 203932.96014 +421454.7845 01-07-2013 50000 29.54 147.70 24×24
203932.96014 +421454.7845 19-08-2014 50000 29.54 147.70 24×24
HD206610 214324.90004 -072429.7086 20-08-2014 50000 29.54 147.70 24×24
HD208527 215623.98467 +211423.4961 20-08-2014 50000 29.54 147.70 24×24
HD210277 220929.86552 -073255.1548 19-08-2014 50000 29.54 147.70 24×24
HD217786 230308.205 -002546.66 28-07-2011 50000 29.54 147.70 24×24
230308.205 -002546.66 30-06-2013 50000 29.54 147.70 24×24
230308.205 -002546.66 20-08-2014 50000 29.54 147.70 24×24
HD240210 231029.2303 +570146.035 01-07-2013 50000 29.54 147.70 24×24
HD219828 231846.73445 +183844.6021 30-06-2013 19214 29.54 56.76 24×24
HD220074 232014.37962 +615812.4578 19-08-2014 50000 29.54 147.70 24×24
HD222155 233800.30741 +485947.4907 01-07-2013 50000 29.54 147.70 24×24
(a)HD2638 (b)HAT-P-7 (c)HD185269
(d)WASP-76 (e)HAT-P-32
Figure1.Imagesofknownlow-massstellarcompanionstoexoplanethoststars,followedupinourmultiplicitystudy.Thehalosofthebrighthoststarswere
removedbyhigh-passfiltering.NorthisupandEastistotheleft.
3 ASTROMETRICCALIBRATIONAND ourastrometricmeasurementscanbecomparedbetweendifferent
MEASUREMENTS observation epochs as well as with measurements done with
different instrument, we took astrometric calibration images in
The most reliable method to determine if individual companion each observation epoch. In 2013 and 2014 we used the center of
candidatesareboundtothesystemsaroundwhichtheyarediscov- theglobularclusterM15forthispurpose.Inthe2015observation
ered is to ascertain if they exhibit the same proper motion as the epoch M15 was not visible and we imaged three wide binary
primarystarofthesystem.Forthispurposewearemeasuringthe systems instead (HIP72508, HIP80953 and HIP59585). To
separationandrelativepositionangle(PA)ofallnewlydiscovered calibrate the pixel scale as well as the orientation of the detector
companion candidates relative to the primary star. To ensure that
(cid:13)c 2015RAS,MNRAS000,1–19
Amultiplicitystudyofexoplanethoststars 5
(a)HD10697 (b)HD43691 (c)HD116029 (d)HAT-P-18
(e)Kepler-37 (f)Kepler-21 (g)Kepler-68 (h)Kepler-42
(i)HD188015 (j)HD197037 (k)HD217786
Figure2.ImagesofallnewlydetectedcompanioncandidatesduringthecourseofourmultiplicitystudywithAstraluxattheCalarAlto2.2mtelescope.
Spatialscalingofeachimageisindicated.Thecompanioncandidates(cc)aremarkedinallimages.Allimageswerehigh-passfilteredtoremovethebright
haloofthehoststar.NorthisalwaysupandEastistotheleft.
weusedasreferenceHSTobservationsofM15thatweretakenon each star relative to all other stars. This was done for 92 stars in
22-10-2011withtheWideFieldCamera3(WFC3,Kimbleetal. 2013 and 90 stars in 2014. We then used the known astrometric
2008). In the case of the binary stars, we used all measurements calibrationoftheHSTreferenceimagetocalculateanastrometric
of therespective systems inthe WashingtonDouble Star Catalog solutionforeachindividualmeasurement.Toexcludestarswitha
(Mason et al. 2001) as reference. We applied a linear fit to these strongpropermotionorpossiblymisidentifiedstarsweemployed
available measurements to correct for the slow orbital motion of sigma clipping. The final astrometric solution for the 2013 and
these wide binaries. For the calibration using cluster data, we 2014 observations is the median of all computed solutions. We
measuredindividualstarpositionsinourAstraLuximageandthe givetheresultsinTab.2.Thelisteduncertaintiesarethestandard
HST reference image with IDL2 starfinder (Diolaiti et al. 2000), deviationsofallastrometricsolutions.
which fits a reference PSF to each star position. The reference In the case of the binary stars, we only have two objects in the
PSFwascreatedfromthedataitself.Wethenusedourowncross field of view, thus we could not create a reference PSF from the
correlation routines to identify the same stars in both images. data.InsteadwearefittingatwodimensionalGaussiantothestar
Finally we calculated separations and relative orientations of positions. We checked that this approach is valid by comparing
similar measurements in the cluster images with the starfinder
results. The deviations between the two methods were typically
2 InteractiveDataLanguage much smaller than the measurement accuracy. The result of the
(cid:13)c 2015RAS,MNRAS000,1–19
6 C.Ginskietal.
Gaussian to both objects since there were no other objects in the
field of view to build a reference PSF. Also, it is problematic to
buildanaveragereferencePSFfromdifferentdatasets,sincethe
shapeofthePSFwillhighlydependontheatmosphericconditions
andtheheightofthetargetabovethehorizon.Toensurethatwe
obtained a stable fitting result, we repeated the fitting procedure
for each object at least 20 times with slightly different starting
positionsandfittingboxsizes.Forcompanioncandidatesthatwere
separated by less than 2arcsec from the bright primary stars, we
removedtheprimarystars’brighthalobyhighpassfilteringbefore
we measured the companion candidates position. All results are
listed in Tab. 3. The given uncertainties are the uncertainties of
theGaussianfittingaddedinquadraturetotheuncertaintiesofthe
astrometriccalibration.Multipleobservationepochswereavailable
forseveralsystems.Wediscussthesesystemsinthefollowingin
detailandtestifthecompanioncandidatesareco-movingwiththe
primarystars.
WASP-76
WASP-76 was observed by us only once in August of 2014.
Figure3.CompositecolorimageoftheexoplanethoststarHD185269and We detected a faint companion candidate ∼0.44arcsec to the
itscompaniontakenwithSPHERE/IRDISon02-05-2015.Red,greenand south-west of the star. Two months later in October of 2014, the
bluechannelsareH,JandY-banddatarespectively.IntheSPHERE/IRDIS target was observed also with AstraLux by Wöllert & Brandner
imagethelowmassstellarcompaniondiscoveredbyus(Ginskietal.2012) (2015),whoalsodetectedthiscompanioncandidateandclaimthat
isforthefirsttimeresolvedasalowmassstellarbinary.Northisupand it is likely a bound companion due to the decreasing likelihood
Eastistotheleft. of background objects with decreasing separation. We used their
discovery astrometric data point, along with our own astrometric
measurement,todetermineifitispossibletodrawconclusionson
binary calibration is also given in Tab. 2. We used the weighted the proper motion of the object relative to the primary star. The
averageofthethreesolutionscalculatedfromtheindividualbinary correspondingdiagramisshowninFig.4a.Inordertoachievean
systems. For the uncertainty, we conservatively assumed the accurate position measurement of this faint source, we employed
largest individual uncertainty that we measured. The uncertainty highpassfilteringontheimagestoremovethebrighthaloofthe
of the calibration includes the uncertainty of the linear orbital exoplanethost.
motionfitmentionedearlier.Wenotethatcalibrationsusingbinary Duetotheshorttimebaselineofonlytwomonths,andthelarge
stars are prone to systematic offsets due to unaccounted for (or uncertaintiesgivenbyWöllert&Brandner(2015)(presumablydue
underestimated) orbital motion of the systems. We thus caution toworseweatherconditionscomparedwithourowndetection),it
thattheresultofthe2015calibrationmightstillsufferfromsuch isnotpossibletodrawfirmconclusionsonthepropermotionofthe
anoffset. companioncandidate.However,wenotethatourownmeasurement
We have one companion candidate which was already observed isinprinciplemoreconsistentwiththeobjectbeinganon-moving
in July of 2011 for the first time. In this case we utilized the background source rather than a bound companion. Particularly
astrometriccalibrationderivedbyuswithclusterandbinarydata the1σdeviationofthetwoseparationmeasurementscouldbewell
inGinskietal.(2012). explainedbyparallacticdisplacementoftheprimarystarrelativeto
For the SPHERE/IRDIS data we used the astrometric solution apresumablydistantbackgroundsource.Anyfuturemeasurement
calculatedbytheSPHEREconsortiumfortheGTOruninwhich withasimilarprecisionasourownmeasurementofAugust2014
the data was taken. This astrometric calibration was derived willbeenoughtodeterminethestatusofthiscompanioncandidate.
from multiple observations of the globular clusters 47Tuc and
NGC6380, for which also precise HST reference observations HD185269
as well as proper motions for individual cluster members are
available. There is a small dependence of the pixel scale on the A low-mass companion to the HD185269 system was dis-
utilizedfilter;forourY-bandobservationsweused12.234±0.029 coveredbyuswithAstraLuxobservationsinGinskietal.(2012)
mas/pixand-1.78±0.13deg,whileweused12.214±0.029mas/pix withobservationsperformedbetween2008and2011.Wefollowed
for the J-band, and 12.210±0.029 mas/pix for the H-band (the uponthiscompanioninourcurrentstudywithobservationstaken
detector orientation is not influenced by the filter choice). In inJuly2013andAugust2014.Weshowtheimageobtainedinthe
addition,IRDISshowsasmallanamorphismbetweenthedetector 2013 observation epoch in Fig. 1. In this observation epoch we
xandydirection.Thiswasalsodeterminedfromobservationsof observedforthefirsttimethatthecompanionappearedextended
the globular cluster 47Tuc. To correct for this anamorphism we in north-east/south-west direction, while the PSF of the primary
multiplied the separation in y by a factor of 1.0062. A detailed starshowednosuchdistortion.Thispromptedustore-observethis
descriptionoftheIRDISastrometriccalibrationisgiveninMaire systemwithSPHERE/IRDIS.Themuchhigherresolutionextreme
etal.(2015). AOimagesofSPHEREshowforthefirsttimethatthecompanion
The measurements of the relative positions of companion candi- isactuallyaverylowmassbinarysystemitselfwithtwoapprox-
datestotheprimarystarswasalsodonebyfittinga2dimensional imately equally bright components (see Fig. 3). In addition to
(cid:13)c 2015RAS,MNRAS000,1–19
Amultiplicitystudyofexoplanethoststars 7
the (unresolved) follow-up astrometry performed with AstraLux, multiplesystemitself.
we measured the relative position of each binary component to
the primary star in all bands of the SPHERE/IRDIS observation. Kepler-37
We used again Gaussian fitting to determine the positions of all
objects. The primary star shows a very mild saturation of the Kepler-37 (KOI-245, KIC 8478994) was observed by us only
innermost 2-3 pixels in Y and H-band. We measured its position once in August of 2014. In this data set we discovered a wide
again multiple times to ensure that we reached a good fit (we fit (∼8.5arcsec) companion candidate south-south-west of the exo-
theflanksofthesaturatedPSFinthiscase).Finalresultsarelisted planethoststar.Kepler-37waspreviouslyobservedbyLillo-Box
inTab.4.Inaddition,weusedourmeasurementstocalculatethe etal.(2014),alsousingAstraLuxattheCalarAltoobservatory.In
weightedaverageofthepositionoftheBbcomponentwithrespect addition, it was targeted by Adams et al. (2012) using ARIES at
totheBacomponent.Wearriveataseparationof123.55±0.44mas theMMTobservatory.Bothstudiesdonotmentionthecompanion
(∼5au projected separation at a distance of 47.37±1.72pc, van candidate recovered in our own AstraLux image, since they are
Leeuwen2007)andapositionangleof214.87±0.21deg. focusingonclosecompanionswithin6arcsecoftheprimarystar.
SincetheSPHEREimageconfirmedthatHD185269Bisabinary, Sincetheobjectwaslocatedatsucharelativelylargeseparation,
were-examinedour2013AstraLuxobservationinordertoprovide wedecidedtocheckthe2MASS(Skrutskieetal.2006)surveyfor
an astrometric measurement of the relative binary position. This previousdetection.Whiletheobjectwasnotlistedinthe2MASS
is useful to determine the orbit of the binary and constrain its point source catalog, it was visible in the reduced J, H and K
massdynamicallyinlaterfollow-upstudiesofthesystem.Dueto images.Weextractedtheastrometricpositionfromtheindividual
the marginally resolved nature of the binary source in our 2013 2MASS images using Richardson-Lucy deconvolution and then
AstraLux data, Gaussian fitting proved to be difficult. Instead averaged the results over all bands. For details on the extraction
we used the primary star’s PSF as template and fitted it to the werefertoourrecentstudyMugrauer&Ginski(2015).Wefinda
two components of HD185269B using IDL starfinder. This fit separationof8.030±0.138arcsecandaPAof202.99±1.85degin
yielded a separation of 95.6±2.8mas and a PA of 221.1±1.3deg the 2MASS observation epoch of 1998.47. We used the 2MASS
of Bb relative to Ba, as well as separations of 4538±14mas and dataincombinationwithourmorepreciseAstraLuxmeasurement
4458±14masandPAsof8.39±0.17degand7.72±0.18degofBa totestifthediscoveredobjectisco-movingwiththeprimarystar.
and Bb relative to A. As expected for a system with such small ThecorrespondingdiagramisshowninFig.4b.Eventhoughthe
separation,weseestrongorbitalmotionbetweenthe2013andthe uncertainties of the 2MASS measurement are large compared
2015 observation epoch. Due to the non-optimal weather condi- to our AstraLux measurement, the position of the companion
tionsin2014,thecompanionisnotresolvedinour2014AstraLux candidate in the 2MASS epoch is consistent within 1σ with a
observation. At least one additional astrometric measurement is non-moving background object. By comparison, co-motion with
neededtoconstraintheorbitalelementsofthisbinarysystem. theprimarycanberejectedonthe4σlevel.Wethusconcludethat
the object is likely located in the distant background and is not
HD43691 physicallyassociatedwiththeKepler-37system.
HD43691 was imaged by us once in March of 2015. We de- Kepler-21
tected a companion candidate approximately 4.4arcsec to the
north-eastoftheexoplanethoststar.Sinceweonlyhaveoneepoch Kepler-21 (KOI-975, KIC 3632418) was observed in July of
it is not yet possible to determine if the object is indeed related 2013 and August 2014 with Astralux. A very close companion
to the HD43691 system. However, upon close inspection of the candidate at approximately 0.8arcsec was detected south-east of
companion candidate’s PSF we noticed that it appears extended theprimarystar.Inordertogetanaccuratepositionmeasurement
along an angle of roughly 135deg. A close-up of the companion ofthisfaintsource,weemployedhighpassfilteringontheimages
candidate’s PSF, as well as the primary stars’ PSF, is shown in to remove the bright halo of the exoplanet host. The resulting
Fig. 5. We actually see at least two distinct peaks in the PSF measurements were compared with the proper motion of the
(signal-to-noise ratio3 of 5.8 and 5.5, separation of ∼84mas, i.e. primarystar.ThecorrespondingdiagramisshowninFig.4c.Due
6.7auat80.4pc),whichwouldindicatethattheobjectitselfmay to the direction of motion of Kepler-21, no significant change
be a multiple system. We compared the companion candidate’s in separation would be expected for a co-moving object as well
PSFwiththePSFoftheprimarystartoexcludethatthisismerely as a non-moving background object. However, as can be seen
an effect caused by the observation conditions. However, the in the diagram, both types of objects diverge in expected PA.
primarystar’PSFappearscircularinthecenterwithahalothatis Our measurements of the PA of the companion candidate show
slightlyextendedinnorth-southdirection,i.e.weseenoindication no significant change in PA, consistent with common proper
for an intrinsic smearing of the PSF along the angle seen in the motion. We can reject the background hypothesis with 4.0σ.
companion candidate. We note that there appears to be a third We thus conclude that the object that we detected is most likely
peakdirectlynorthofthesouth-eastcomponentofthecompanion gravitationallyboundtoKepler-21Aandisthusa newlow-mass
candidate’sPSF.Thismightindeedbearesidualofanorth-south stellarcompanioninthissystem.
extended halo, as seen in the primarie’s PSF. The object might
hence be a binary or even trinary companion to HD43691A. Kepler-68
However, further observations are required to confirm that the
source is co-moving with the primary star and that it is indeed a Kepler-68 (KOI-246, KIC 11295426) was imaged by us also
inJulyof2013andAugustof2014.Awidecompanioncandidate
approximately 11arcsec to the south-east was detected in both
3 Thenoisewasdeterminedbycalculatingthestandarddeviationina5×5 observationepochs.UnfortunatelyKepler-68exhibitsonlyavery
pixelboxcenteredonthetwobrightestpeaksofthesource. small proper motion of -10.60±1.60 mas/yr in declination and
(cid:13)c 2015RAS,MNRAS000,1–19
8 C.Ginskietal.
-8.50±1.60 mas/yr in right ascension. Thus with only one year
Table2.Astrometriccalibrationofallobservationepochsasderivedfrom
of epoch difference it was not possible to assert whether the observationsofthecenteroftheglobularclusterM15.Duringour2015
companioncandidateisco-movingwiththeprimarystar.However, observationepochM15wasnotvisible;weinsteadusedbinarystars.We
sincethecompanioncandidateislocatedatawideseparation,we listthepixel-scale(PS)andthepositionangle(PA)ofthey-axisforall
checkedagainthe2MASScataloguetoseeifthesourcehadbeen observationepochs.
previously detected. We found that our companion candidate is
indeedcontainedinthe2MASSpointsourcecatalogueatarelative Epoch PS[mas/pix] PAofy-axis[◦]
position of 10.989±0.085arcsec and 145.45±0.44deg. Using this
30-06-2013 46.748±0.14 358.18±0.16
additionalobservationepochwetestedthecompanionforcommon
19-08-2014 46.864±0.10 358.15±0.12
proper motion with the primary star. The corresponding diagram 10-03-2015 46.834±0.13 357.66±0.15
is shown in Fig 4d. While the separation is inconclusive due to
the tangential direction of motion of the primary star, we would
have expected a significant change in PA of our measurements least one further observation epoch in good observing conditions
with respect to the 2MASS epoch if the companion candidate isrequired.
was a non-moving background object. Instead we find that all
measurementsareconsistentwithnochangeinPA.However,due HD197037
tothelargeuncertaintiesofthe2MASSepochwecanonlyreject
the background hypothesis with 2.1σ. We conclude that, given HD19037 was observed by us with AstraLux in two epochs
our data, it seems likely that the companion candidate is indeed inJuly2013andAugust2014.Wedetectedacompanioncandidate
boundtotheKepler-68system,butfurtherobservationsneedtobe approximately3.7arcsectothesouthoftheprimarystar.Usingthe
undertakentostrengthenthisconclusion. propermotionoftheprimarystar,wecalculatedtheexpectedpo-
sitionofanon-movingbackgroundobjectin2013giventhe2014
HD188015 measurement.ThecorrespondingdiagramisshowninFig.4j.The
astrometryinbothepochsisconsistentwithnosignificantchange
HD188015 was observed by us with AstraLux in July 2013 inrelativeposition.Wecanrejectthebackgroundhypothesiswith
andAugust2014.InFig.2iweshowour2013observationepoch. 4.8σinseparationand18.6σinPA.Weconcludethatthedetected
A total of 6 companion candidates are visible in the field of object is co-moving with HD19037A and is thus most likely a
view of AstraLux. The high density of objects in the field of newgravitationallyboundstellarcompaniontothesystem.
view compared to other systems is not entirely surprising since
HD188015islocatedinthedirectionoftheGalacticdisk(Galactic HD217786
latitude of +00.5428◦). We note that HD188015 has a known
low-mass stellar companion at ∼13arcsec and a PA of 85deg, HD217786 was observed by us on three different occasions
discoveredbyRaghavanetal.(2006)inSloanDigitalSkySurvey in July 2011 and 2013, as well as in August of 2014. In all
(SDSS, York et al. 2000) data. This companion is outside of the three observation epochs we detected a companion candidate
fieldofviewofourAstraLuxobservations. approximately 2.8arcsec to the south of the primary star. The
InAugust2014observationconditionswerenotasfavorableasin propermotionforthissystemiswelldeterminedtobe-88.78±0.84
2013 with shorter coherence times and thin cloud layers passing mas/yrindeclinationand-170.13±0.61mas/yrinrightascension
throughduringtheobservations.Thusonlythecandidatemarked (van Leeuwen 2007). We show the astrometric measurements as
as cc1 was re-detected with high signal-to-noise. Of the other well as the expected behavior of a background object in Fig. 4k.
five companions, four were detected marginally with cc5 being The PA of the companion candidate is not changing significantly
the exception. The marginal detections in 2014 did not allow for withtime.However,wedetectasmallincreaseinseparation.The
fitting of a Gaussian to the companion candidates. Instead we dashedlinesinthediagramshowtheexpectedchangeforacircular
have determined the center of light with a simple centroid for edge-on orbit. The data points are consistent with such a change
those four sources. This led to much larger uncertainties of the within 1σ. We note that even stronger changes in separation are
2014 astrometry. We nonetheless used the 2014 astrometry in possible for eccentric orbits. The change in separation is much
combination with the known proper motion of the primary star smaller than what would be expected from a background object
to determine if one or several of the companion candidates are andisalsoshowingthewrongdirection(forabackgroundobject
co-movingwiththeprimarystar.Thecorrespondingdiagramsfor theseparationshouldhavedecreasedfrom2011to2014).Infact
cc1 to cc6 (with the exception of cc5) are shown in Fig. 4e to wecanrejectthebackgroundhypothesiswith42.8σinseparation
Fig. 4i. For cc1 the available astrometry is more consistent with and 18.9σ in PA. We thus conclude that the discovered object is
a background object and we can reject common proper motion very likely bound to the system and emerges as new low-mass
withHD188015Aonthe3σlevel.Fortheremainingcompanion stellar companion. Due to the small change in separation, but no
candidateswecannotrejectcommonpropermotionorbackground changeinPA,weexpectthecompaniontobeinaclosetoedge-on
hypothesis with any significance. This is caused by the larger orbit configuration, but longer astrometric monitoring is required
uncertainty of the 2014 measurements in combination with the totestthishypothesis.
short time baseline of only one year. We note, however, that
the astrometry of cc2 and cc4 is more consistent with a distant
backgroundobject,whilethesameisnottrueforcc3andcc6.The
4 PHOTOMETRICMEASUREMENTSANDMASS
latter two remain completely inconclusive due to their opposite
DETERMINATION
behavior in separation and PA. We point out that this seemingly
mixed behavior could be caused by a non-zero proper motion of Todeterminethemassesoftheconfirmedcompanionsaswellas
these objects. To gain a better understanding of this system, at the possible companion candidates, we performed photometric
(cid:13)c 2015RAS,MNRAS000,1–19
Amultiplicitystudyofexoplanethoststars 9
222 206 132
positionangle[deg]222222211120114600882 Woellert & Brandner 2015 positionangle[deg]222222110000009940351289 A2MstAraSLSux positionangle[deg]111111112232222346089751
206 This work 197 123
8.6
0.445 0.792
separation[arcsec]0000000.......444444412341230000555 separation[arcsec]888888......403512 separation[arcsec]000000......777777758866466082
0.405 7.9 0.750
2014.0 2014.3 2014.6 2014.9 1998.0 2002.0 2006.0 2010.0 2014.0 2013.0 2013.5 2014.0 2014.5
time[year] time[year] time[year]
(a)WASP-76 (b)Kepler-37 (c)Kepler-21
146.8 298.0 306.2
positionangle[deg]111111144444445656656.......4466082 A2MstAraSLSux positionangle[deg]222222229999999967677677........44660882 positionangle[deg]333333000000555655......460082
145.2 296.2
304.8
separation[arcsec]111111111010110101.........909009090446608822 separation[arcsec]4444444.......112211205052720000555 separation[arcsec]1111111100001000........8798087946600822
1998.0 2002.0 2006.0 2010.0 2014.0 2013.0 2013.5 2014.0 2014.5 2013.0 2013.5 2014.0 2014.5
time[year] time[year] time[year]
(d)Kepler-68 (e)HD188015cc1 (f)HD188015cc2
268.75 114.8 111.00
positionangle[deg]222222266666667788778.......05052720000555 positionangle[deg]111111111111111124132423........44660082 positionangle[deg]111111110011001199009900........0505272700005555
10.04 4.20 7.28
separation[arcsec]1999990......898890460820 separation[arcsec]444434......010091460862 separation[arcsec]7777777.......02102014460082
9.76 3.92 6.96
2013.0 2013.5 2014.0 2014.5 2013.0 2013.5 2014.0 2014.5 2013.0 2013.5 2014.0 2014.5
time[year] time[year] time[year]
(g)HD188015cc3 (h)HD188015cc4 (i)HD188015cc6
Figure4.Propermotionanalysisforallcompanioncandidateswithtwoormoreobservationepochs.DatapointsareAstraLuxmeasurementsifnototherwise
marked.Thedashedlinesenclosetheareainwhichaco-movingcompanionwouldbeexpected.Thistakesintoaccountpossiblecircularorbitalmotionwith
thesemi-majoraxisgivenbytheprojectedseparationofthecompanion.Thegreyareaenclosedbythewobbledlinesistheareainwhichanon-moving
backgroundobjectwouldbeexpected,dependingonthepropermotionanddistanceoftheprimarystar.Thewobbleisintroducedbytheparallacticshiftin
theprimarypositionduetotheEarth’srevolutionaroundthesun.
(cid:13)c 2015RAS,MNRAS000,1–19
10 C.Ginskietal.
184.2 179
183.9 178
[deg]118833..63 [deg]117767
gle183.0 gle175
an182.7 an174
on182.4 on173
positi118812..81 positi117712
181.5 170
4.05 3.15
[arcsec]433...099005 [arcsec]333...010005
on3.85 on2.95
separati333...787005 separati222...898005
3.65 2.75
2013.0 2013.5 2014.0 2014.5 2011.0 2012.0 2013.0 2014.0
time[year] time[year]
(j)HD197037 (k)HD217786
Figure4.Continued.Propermotionanalysisforallcompanioncandidateswithtwoormoreobservationepochs.DatapointsareAstraLuxmeasurementsif
nototherwisemarked.Thedashedlinesenclosetheareainwhichaco-movingcompanionwouldbeexpected.Thistakesintoaccountpossiblecircularorbital
motionwiththesemi-majoraxisgivenbytheprojectedseparationofthecompanion.Thegreyareaenclosedbythewobbledlinesistheareainwhichanon-
movingbackgroundobjectwouldbeexpected,dependingonthepropermotionanddistanceoftheprimarystar.Thewobbleisintroducedbytheparallactic
shiftintheprimarypositionduetotheEarth’srevolutionaroundthesun.
measurements for all our observation epochs. Since the photom- model-gridweinterpolated(linearly)betweendifferentmodelages
etry depends on the gain settings of the detector as well as the andstarmagnitudes.Thefinalmassesforallconfirmedorpossible
observationconditionsandheightofthetarget,wedidnotrecord companioncandidatesarelistedalongwiththeirderivedabsolute
a photometric standard star and rather give relative photometric magnitude in Tab. 6. The listed uncertainties for the absolute
measurementsofthecompanions(andcandidates)totheirprimary magnitude include the uncertainty of the apparent magnitude of
stars. While the PSFs of all sources in one image are similar, thehoststar,aswellastheuncertaintyinthemeasureddifferential
they are changing with observing conditions and elevation of the magnitude and the uncertainty in the distance of the system. The
targetsaswell,thusitisnotpossibletobuildareferencePSFfor uncertainties listed for the masses of the objects also account for
photometric measurements from the data. We instead decided to the uncertainty of the system age. In the following we compare
performaperturephotometryonallsources.Weusedtheaperture our photometric measurements and mass determination for a few
photometrytool(APT,Laheretal.2012)forthesemeasurements. systemswithavailableliteraturevalues.
The aperture size was adjusted for each image individually to
encirclethemajorityofthefluxofthecompanioncandidates.The HD2638
sameaperturesizewasthenusedtogetthereferencemeasurement
from the primary star. In the cases where the faint sources were For the close stellar companion to HD2638 we find a differ-
locatedwithinthebrighthalooftheprimarystar,carewastakento ential magnitude of 3.11 ± 0.41mag in the SDSS i-band. Using
selectaskyapertureclosetothecompanionpositiontoaccurately this measurement along with the age, distance and apparent
subtractthecontributionoftheprimarytothefluxintheaperture. magnitude of the primary star, we find a mass of 0.425+0.067 M
−0.095 (cid:12)
In the case of the primary star, we used sky apertures with large for the companion. The companion was originally discovered by
separationsfromtheprimaryinordertonotoversubstractfluxdue Riddleetal.(2015)usingRobo-AOintheoptical.Theyhavetwo
tohalocontributions.AllresultsaregiveninTab.3. measurementsintheSDSSi-bandandfinddifferentialmagnitudes
Thepresenteduncertaintiestakeintoacco√untstatisticaluncertain- betweenprimaryandcompanionof3.39magand3.19mag(Riddle
ties, which were scaled with a factor of 2 to take into account etal.2015,Robertsetal.2015).Theydonotprovideuncertainties
the increased photometric uncertainty of electron multiplying for these measurements. However, given our own uncertainties,
CCDs. In addition, we consider uncertainties in the differential bothvaluesarewithin1σofourownmeasurement.Tocompare
magnitudes from changing aperture sizes, i.e. if we increase or our mass result with independent measurements, we used the K
s
decreasetheapertureradiusbyupto2pixels.Theseweretypically andJ-bandphotometricmeasurementsofthecompanion,provided
in the order of 0.04mag and were added in quadrature to the inthecharacterizationpaperoftheobjectbyRobertsetal.(2015).
statisticaluncertainties. To calculate a mass range we use again BT-SETTL models. We
To convert our photometric measurements to masses we used find an approximate mass range of 0.53M to 0.45M . While
(cid:12) (cid:12)
the BT-SETTL evolutionary models for low-mass stars, brown thisisslightlylargerthanourownSDSSi-bandresult,bothmea-
dwarfs and planets (Allard et al. 2011). These models take the surements are consistent within our 1σ uncertainties. The small
absolutemagnitudeandtheageofanobjectasinput.Tocompute discrepancy might be explained by a potential oversubtraction of
theabsolutemagnitudeofourconfirmed/possiblecompanionswe backgroundfluxinourSDSSi-bandimages.
usedtheapparentmagnitudeofthehoststarintheSDSSi-band,as
wellasthedistanceofthehoststar.Wethenassumethatthecom- HAT-P-7
panionsareofthesameageasthehoststar.Wesummarizethese
input values for all targets in our survey in Tab. 5. To get a finer We measure a differential SDSS i-band magnitude between
(cid:13)c 2015RAS,MNRAS000,1–19