Discovery of a ~23 Mjup Brown Dwarf Orbiting ~700 AU from the Massive Star HIP 78530 in Upper Scorpius

0.52 MB·English

by  David Lafrenière

DRAFTVERSIONJANUARY26,2011
PreprinttypesetusingLATEXstyleemulateapjv.11/10/09
DISCOVERYOFA∼23M BROWNDWARFORBITING∼700AUFROMTHEMASSIVESTARHIP78530IN
Jup
UPPERSCORPIUS
DAVIDLAFRENIÈRE1,2,RAYJAYAWARDHANA2,MARKUSJANSON2,CHRISTIANEHELLING3,SOERENWITTE4&PETERHAUSCHILDT4
DraftversionJanuary26,2011
ABSTRACT
1 We presentthe discoveryof a substellar companionon a wide orbit aroundthe ∼2.5M⊙ star HIP 78530,
1 whichis a memberof the 5 Myr-oldUpperScorpiusassociation. We haveobtainedfollow-upimagingover
0 two years and show that the companion and primary share common proper motion. We have also obtained
2 JHK spectroscopy of the companion and confirm its low surface gravity, in accordance with the young age
n of the system. A comparison with DRIFT-PHOENIX synthetic spectra indicates an effective temperature of
a 2800±200 K and a comparison with template spectra of young and old dwarfs indicates a spectral type of
J M8±1. Themassofthecompanionisestimatedtobe19-26MJup basedonitsbolometricluminosityandthe
predictionsofevolutionarymodels. Theangularseparationofthecompanionis4.5′′,whichatthedistanceof
4
2 theprimarystar,156.7pc,correspondstoaprojectedseparationof∼710AU.Thiscompanionfeaturesoneof
thelowestmassratios(∼0.009)ofanyknowncompanionatseparationsgreaterthan100AU.

] Subjectheadings:stars:pre–mainsequence—stars:low-mass,browndwarfs—stars:formation—planetary
R
systems
S
.
h
1. INTRODUCTION places it among the widest known substellar companions to
p
starsanditsextrememassratio–amongthelowestcurrently
- The recent direct imaging discoveries of very low mass
o known – is comparable to those of directly imaged planets,
substellarcompanionstostars–especiallythoseorbitingthe
r even though its mass is well above the deuterium-burning
t youngstars1RXSJ160929.1-210524(Lafrenièreetal.2008,
s 2010),HR8799(Maroisetal.2008),Fomalhaut(Kalasetal. threshold. Together with the companions mentioned above,
a this new substellar companion presents a good challenge to
2008),ABPic(Chauvinetal.2005),HNPeg(Luhmanetal.
[ allformationscenariosandcontributestoblurringthedistinc-
2007), and CT Cha (Schmidtetal. 2008) – have raised new
tionbetweengiantplanetsandbrowndwarfsevenfurther.
1 questions about the formation mechanisms of planets and
v brown dwarfs. These companions have separations of tens
6 toseveralhundredAUandmassratios.0.02relativetotheir 2. OBSERVATIONSANDDATAREDUCTION
6 primaries. Several theoretical ideas are under discussion to 2.1. Imaging
6 explain such properties, but they all run into some difficul-
4 Thediscoverypresentedinthispaperwasmadeaspartofa
ties. Core accretion models favor formation of giant plan-
. directimagingsearchfornewstellar andsubstellarcompan-
1 ets close to the ’snow line’, i.e., at <10 AU (Pollacketal.
ionsaroundabout90starsintheUpperScorpiusregion. The
0 1996). Gravitationalinstabilitycouldproducemassiveplan-
overall target sample was built by randomly selecting, from
1 etsatlargeradii,butwouldrequireunusuallylargeandmas-
thelistofUpperScorpiusstarsinCarpenteretal.(2006),an
1 sivecircumstellardisks(e.g.Vorobyov&Basu 2010). Frag-
equalnumberofstarsineachoffiveequallogarithmicmass
: mentation of pre-stellar cores during collapse does not lead
v bins over the range ∼0.15-5 M⊙; the spectral types range
tosuchextremebinarysystemseasily(e.g.Bateetal.2003).
i fromB0toM5. TheobservationsweremadeusingtheNIRI
X Thus, some theorists have recently proposed that dynamical
camera(Hodappetal.2003)andtheALTAIRadaptiveoptics
instabilities within planetary systems that originally formed
r system (Herriotetal. 2000) at the Gemini North Telescope.
a multiplegiantplanetscouldscattersomeofthemtolargesep-
Exceptforafewfainttargets,thetargetstarsthemselveswere
arations(Scharf&Menou2009;Verasetal.2009). However,
used for wave frontsensing. The field lens of ALTAIR was
the more massive planets are less likely to reach the widest
used to reduce the effect of anisoplanatism. The first epoch
orbits.
imagingofHIP78530wasdoneon2008May24inthenar-
Here we report on the direct imaging discovery, common
rowbandfilterK centeredat2.0975µm. Forskysub-
propermotionconfirmationandmulti-bandspectroscopyofa continuum
tractionwe used5 ditherpositionscorrespondingtothe cor-
∼23MJupcompanionseen∼700AUfromHIP78530,which ner and center of a square of side 10′′. At each position we
isa∼2.5M⊙ star(spectraltypeB9V)intheUpperScorpius obtainedone co-additionof twelve 0.5s integrationsin fast,
young association. The separation of this new companion
highread-noisemode,followedbyonesingle10sintegration
inslow,lowread-noisemode. Ateachpositionthisprovides
[email protected]
1Département de physique, Université de Montréal, C.P. 6128 Succ. an unsaturated image of the target star and a much deeper
Centre-Ville,Montréal,QC,H3C3J7,Canada imageofthefieldthatcanbereadilyspatiallyregisteredand
2Department ofAstronomyandAstrophysics, University ofToronto, scaledin flux. Thefull-width-at-half-maximumforthis data
50St.GeorgeStreet,Toronto,ON,M5S3H4,Canada
setis0.075′′andtheStrehlratiois0.34.
3SUPA,SchoolofPhysicsandAstronomy,UniversityofSt.Andrews,
NorthHaugh,St.AndrewsKY169SS,UK TheinitialimageofHIP78530revealedaninterestingfaint
4Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Ger- nearby source which, based on a comparison with observa-
many tions made by Kouwenhovenetal. (2005, 2007), was very
2 Lafrenièreetal.
likelytobeatrueco-movingcompanion.Toconfirmthecom-
monpropermotionofthiscandidateandgetadditionalpho- TABLE1
tometrymeasurements,follow-upimagingobservationsinJ, ASTROMETRICMEASUREMENTS
H and K′ were obtained on 2009 July 2 using the same in-
Epoch Band ρ(′′) P.A.(deg)
strument as before. To avoid saturation in these broad fil-
ters, we used a 512×512 subarray, and kept the strategy of 2008.3940 Kc2o.0n9t 4.529±0.006 140.32±0.10
ashort,unsaturatedimageimmediatelyfollowedbyalonger, 2009.4998 JHK′ 4.533±0.006 140.35±0.10
2010.6605 K2.09 4.536±0.006 140.27±0.10
saturatedimageateachof5ditherpositions. Foreachwave- cont
length, three sky frames were obtained at offsets of >15′′.
purpose.
Theshortexposuretimeswere0.055swith40co-additions,
ThereductionoftheNIFSdata,uptothereconstructionof
whilethe longexposureswere6 s, 5 s, and4 swith oneco-
thedatacube,wasmadeusingtheGeminiIRAFpipeline.The
addition in J, H and K′, respectively. To increase the sig-
stepscoveredinthispipelineareskybackgroundsubtraction,
nificanceofourcommonpropermotionconfirmationfurther,
flat-fieldandbadpixelcorrection,spatialandspectralcalibra-
anotherfollow-upobservationwasmadeon2010August30
tion, and data cube reconstruction. The spatial sampling of
usingtheKc2o.0n9t filter,fullframe,andoneco-additionoftwelve thereconstructeddatacubeis0.043′′pixel- 1. Theinstrumen-
0.6sintegrationsfollowedbyasingle10sintegrationateach
tal/telluric transmission correction and spectrum extraction,
of five dither positions. Other follow-updata were acquired
whichcouldbedoneusingtheGeminipipeline,wereinstead
in springand summer2010butowingto an overseendiffer-
doneusingcustomIDL.
ence in the instrumental setup, these data suffer from large
The center of each PSF was first registered to a common
systematicastrometricerrorsandarenotusedhere.
position in all spectral slices and all cubes of the sequence;
TheimagingdatawerereducedusingcustomIDLroutines.
thecenterpositionswerecalculatedbyfittinga2DGaussian
For the 2008 and 2010 imaging data, a sky frame was con-
function. Thespectrumofthesourcewasextractedbysum-
structedby taking the medianof the imagesat all ditherpo-
mingthefluxinacircularapertureofdiameter5pixels. The
sitions after masking out the regions dominated by the tar-
spectrumofthetelluricstandardwascorrectedforitsspectral
get’s signal, while for the 2009imaging data, the median of
slopeusinga9520Kblackbodycurve,andanyhydrogenline
the three sky frames was obtained. After subtraction of this
absorption was removed by dividing out a Voigt profile fit.
skyframe,theimagesweredividedbyanormalizedflat-field.
The companionspectrum was divided by the standard spec-
Then isolated bad pixels were replaced by the interpolated
trumtocorrectfortelluricandinstrumentaltransmission. Fi-
valueofathird-orderpolynomialsurfacefittothegoodpixels
nally,themedianofthe5spectrawasobtained.
in a7×7pixelsboxwhile clusteredbadpixelsweresimply
maskedout. Nexttheimagesweredistortioncorrectedusing 3. ANALYSISANDRESULTS
thedistortionsolutionprovidedbytheGeministaff.5 Finally,
A summaryofthepropertiesof HIP78530Aanditscom-
the long-exposure images were properly scaled in intensity
panion,takenfromtheliteratureorderivedinthissection,is
andmergedwiththeircorrespondingshort-exposureimages.
presentedinTable2.
ThecolorcompositeJHKimagefromthe2009follow-upob-
servationsisshowninfigure1.
3.1. Commonpropermotion
2.2. Spectroscopy The relative position of the companion and primary was
determined by fitting a 2D Gaussian function to each com-
The likely companion detected in the imaging data was
ponent. Thepixelpositionswereconvertedtoarcsecondsus-
very faint and, assuming it was a true companion, its mag-
ing the 21.4 mas pixel scale of NIRI with ALTAIR and the
nitude suggested a substellar mass. Thus, to verify its late-
field lens as indicated on the instrument web page6; the po-
type nature, we obtained spectroscopic observations of it in
sition angle of the image was taken from the FITS header.
H, K and J on 2009 July 2, 2009 July 3, and 2009 August
The astrometric errors, 6 mas in separation and 0.1◦ in po-
8, respectively, using the integral field spectrograph NIFS
sitionangle, were estimatedin Lafrenièreetal. (2010) using
(McGregoretal.2003)withtheALTAIRadaptiveopticssys-
thepositionofbackgroundstarsforasimilarsequenceofob-
tem at the Gemini North telescope. The J grating was used
servations; they are mostly dominated by residual distortion
withtheZJ blockingfilter,theH gratingwiththeJH block-
errors. We also note that the pixel scale of ALTAIR/NIRI
ingfilter andtheK gratingwiththe HK blockingfilter. The
with the field lens has not been extensively calibrated and
spectralresolvingpoweris∼6000inJ and∼5300inbothH
thatourvaluesmaybesystematicallydifferentfrommeasure-
andK. Giventhe largeangularseparationof thecompanion
(∼4.5′′), the primary star is not visible in the 3′′×3′′ NIFS ments made using other instruments or telescopes, but they
shouldbeinternallyconsistent.Theastrometrymeasurements
field-of-view. In each band, the companion was positioned
nearthecenterofthefield-of-viewandwasditheredby0.7′′ aregiveninTable1andshowninFig.2incomparisonwith
thechangesexpectedovertimeforadistantbackgroundstar,
along a line between each of five exposures to enable good
based on the propermotion and distance of the primarystar
sky subtraction. The individual exposure times were 480 s,
(takenfromvanLeeuwen2007). Ourmeasurementsarecon-
300sand240sinJ,H andK,respectively,inlowreadnoise
sistentwithcommonpropermotionbutinconsistent,atalevel
mode. AftertheJ andH sequences,theA0VstarHIP79229
of ∼6σ, with the changesexpectedfor a distant, motionless
wasobservedatasimilarairmassfortelluricandinstrumen-
backgroundstar. Thisclearlyindicatesthatthecompanionis
tal transmission correction, while the A0V star HIP 73820
co-movingwiththeprimary.
was observed just before the K band sequence for the same
Theaccuracyofourastrometrymeasurementscanbeveri-
5 Thedistortioncorrectionusediscentro-symmetricandisgivenbyr= fied usinga faintbackgroundstar detected bothin 2008and
r′+1.32×10- 5r′2,whererandr′arerespectively thedistortion-corrected
anddistortedradialpixeldistancesfromthearraycenter. 6http://www.gemini.edu/sciops/instruments/altair/field-lens-
Newsubstellarcompaniontoamassiveyoungstar 3
FIG.1.—ColorcompositeJHKimageofHIP78530anditssubstellarcompanion(bottomleft)obtainedwithNIRI/ALTAIRattheGeminiNorthtelescope.
Theintensityscalingofeachcolorisproportionaltotheincidentphotonfluxinequalfractionalbandwidths.
2010.Thissource,∼10.1magfainterthantheprimary,hada withtheprimarystar. Indeed,werethesourceabackground
separation of 9.451′′ and position angle of 43.49◦ at epoch star,itsseparationshouldhavechangedby- 0.1′′ anditspo-
2008.3940, and 9.493′′ and 43.48◦, respectively, at epoch sitionangleby- 3.3◦between2000and2010,whilethemea-
2010.6605. These values, showing a ∼7σ change in sepa- suredvaluesshowchangesof+0.01′′and+0.6◦,respectively.
ration and ∼1σ in position angle, are fully consistent with
expectationsforadistant,motionlessbackgroundstar. 3.2. Propertiesofthecompanion
The new companion we report in this paper was
Inthissection, in additiontocomparingourspectrumand
detected previously by Kouwenhovenetal. (2005) and
photometrymeasurementswiththoseofotherobjects,wealso
Kouwenhovenetal.(2007)indataobtainedin2000and2001
rely on comparisons with synthetic spectra to estimate the
withADONISattheESO3.6mtelescope.Theyreportasep-
physical properties of the companion. For the comparison
arationof4.54′′±0.01′′andpositionangleof139.7◦±0.3◦.
with models, we adopted the DRIFT-PHOENIX atmosphere
Theynotethattheywerenotabletoconcludewhetherthisob-
models (Witteetal. 2009; Hellingetal. 2008; Dehnetal.
jectwasaboundcompanionorabackgroundobjectbasedon
2007),whichcombineadetailedkineticmodelofdustcloud
HK photometry, and they have not followed up this source
formation with a radiative transfer code (Hauschildtetal.
further. Considering the uncertainties involved in compar-
1999;Baronetal. 2003). In contrasttoallotheratmosphere
ing our astrometry measurements with those made using
models which assume phase equilibrium between gas and
otherinstruments,asnotedabove,theastrometryreportedby
cloud particles, DRIFT-PHOENIX describes consistently the
Kouwenhovenetal. isinverygoodagreementwithoursand
formation of a stationary cloud by homogeneousnucleation
furthersupportsourconclusionthatthissourceisco-moving
andgraingrowth/evaporation,includinggravitationalsettling,
4 Lafrenièreetal.
broadH OabsorptionbandsandtheCOabsorptionbandhead
2
depthsare perfectlyreproducedbythe low gravityspectrum
atT =2600K.Thelowsurfacegravityisalsoapparentinthe
eff
depthoftheKIlinesintheJ band. Theeffectivetemperature
providingthebestfitdependsonthebandpassconsidered.As
mentioned previously, a temperature of 2600 K provides an
excellent fit in the K band. A temperature of 2600 K also
providesa slightly better fit in the J band, both for the con-
tinuum around1.2 µm and the depth of the KI lines. In the
H band,however,thebestfitisobtainedfortemperaturesof
∼2800-3000K.Takingtheseconsiderationsintoaccount,we
adoptaneffectivetemperatureof2800±200K.Wehavealso
donea chi-squareminimizationusingthe same set ofmodel
spectra, rather than a simple visual inspection, and reached
thesameconclusion. Inparticular,whentheminimizationis
done in all bands simultaneously, the best fit is found for a
FIG.2.—Measured(datapoints)separations(top)andpositionangles(bot-
tom)betweentheprimarystaranditscompanionasafunctionoftime. The temperatureof2800K.
solid lines show the expected position of a distant stationary background Figure4 showsthecompanionspectrum, ata higherreso-
sourceovertime,ascalculated fromthepropermotionanddistanceofthe lutionthistime,incomparisonwiththespectrumofaknown
primarystar.
M8 dwarf member of Upper Sco as well as with a model
spectrumwithoureffectivetemperatureestimate of2800K.
AmongtheUpperScobrowndwarfsidentifiedandspectrally
TABLE2
classifiedbyLodieuetal.(2008),thebestfittoourspectrum
PROPERTIESOFHIP78530AB
isobtainedforaspectraltypeofM8,whichcorrespondstoan
Value effective temperatureof ∼2700-2800K, consistent with our
Parameter Primary Companion previousestimate. Comparingourspectrumto thoseoffield
µαcosδ(masyr- 1)a - 11.38±0.59 ··· dwarfs from the IRTF spectral library (Rayneretal. 2009)
µδ(masyr- 1)a - 24.66±0.37 ··· alsoresultsinabestfitforspectraltypesofM7-M9,although
Distance(pc)a 156.7±13.0 ··· thereare differencesbetween the spectra owingto the lower
Angularseparation(′′) 4.533±0.006 gravity of the companionas noted earlier. The better agree-
Positionangle(deg) 140.3±0.1 ment of our spectrum with those of other objects in Upper
∆J(mag) 8.13±0.05
∆H(mag) 7.44±0.03 Sco,asopposedtofieldobjects,providesfurtherevidencethat
∆K′(mag) 7.27±0.03 thenewcompanionalsobelongstotheassociation.
∆K2.09(mag) 7.26±0.03 The companion spectrum is not only in good agreement
cont
J(mag)b 6.928±0.021 15.06±0.05 withthoseofmodelsandotherUpperScoobjectswithinthe
H(mag)b 6.946±0.029 14.39±0.04 individual bands, but it also shows a reasonable agreement
Ks(mag)b 6.903±0.020 14.17±0.04 in colors between the bands. Over a temperature range of
JHSp--eKcKtsrsa((mlmtayagpg)e) 00..0024B539±±V00..0034 00..82M928±±±001..0066 c2o6l0o0r–s3o0f0J0-KH,=th0e.5D0-R0I.F5T5-aPnHdOHEN-IXK=m0o.2d7e-l0s.3y8ie,ldtosbyentchoemtic-
Teff(K) ∼10500c 2800±200 pared with the companion colors of J- H=0.67±0.06 and
log(L/L⊙) ··· - 2.55±0.13 H- K=0.22±0.06. Assuming a small possible extinction
MPraosjesc(tMed⊙s)eparation(AU) ∼2.5d 710±600.022±0.004e of AV =0.5 mag toward this system (Carpenteretal. 2009),
the extinction-corrected colors would be J- H∼0.62 and
aFromvanLeeuwen(2007). H- K∼0.19.ThecolorsofM8dwarfsinUpperScofromthe
bFromthe2MASSPSCandourcontrastmeasurements.Differencesinfilter
bandpassesmayaddupto1-2%uncertainty. sample of Lodieuetal. (2008) are 0.58-0.65 and 0.38-0.57,
c From the spectral type based on the temperature scale of Sherryetal. respectively. Here, the agreement in J- H is good but our
(2004). companionis significantly bluer in H- K. We note that the
dFromthemodelsofD’Antona&Mazzitelli(1997).
differences between the NIRI (used for the companion con-
eFromthemodelsofBaraffeetal.(1998,2002)andBurrowsetal.(1997).
trast)and2MASS(usedfortheprimarystarmagnitude)filter
bandpassesaddanuncertaintyofatmost2-3%onthecolors
element depletion, and convective element replenishment
ofthecompanions,calculatedfromsyntheticspectra.
(Woitke&Helling2003,2004;Helling&Woitke2006). All
Agoodindicatorofsurfacegravityistheequivalentwidth
modelsusedassumesolarmetallicity,whichisreasonablefor
(EW) of the KI atomic line doublets at 1.168/1.177 µm
UpperSco(Mohantyetal.2004).
and 1.243/1.253 µm (e.g. McGovernetal. 2004). For
HIP 78530B, the EWs of these four lines are, respectively,
3.2.1. Temperature,surfacegravityandspectraltype
2.0±0.3 Å, 3.0±0.3 Å, 2.0±0.2 Å, and 2.1±0.2 Å.7 In
ThespectrumofHIP78530B,binneddownto aresolving theDRIFT-PHOENIXmodels,thereisaclearmonotonictrend
power of 400, is shown in figure 3 in comparison with syn- that the EW in these lines increases with increasing surface
thetic spectrafromthe DRIFT-PHOENIX atmospheremodels gravityforaconstanttemperature,seefigure5.Wehaveeval-
forvarioustemperaturesandsurfacegravities. Asvisibleon uatedtheEWformodelsof2800Kwithsurfacegravityrang-
thefigure,foralleffectivetemperaturesshown,thespectrum
of the companion is in better agreement with the lower sur- 7TocalculatetheEWs,weusedanintegrationintervalof10nmcentered
facegravityspectrum,asexpectedforayoungobjectinUp- oneach line and fitted the continuum as astraight line using 40nm-wide
perSco. ThisisparticularlystrikingintheK bandwherethe intervalsonbothsidesoftheline.
Newsubstellarcompaniontoamassiveyoungstar 5
FIG.3.—NIFSspectrumofHIP78530B(black)comparedwithsyntheticspectrafromtheDRIFT-PHOENIXatmospheremodels(redandblue)ofvarious
effectivetemperaturesandsurfacegravities.Fromthetoptothebottomrows,theeffectivetemperaturesare2600,2800and3000K,respectively.Theredcurves
areforlogg=4.0andthebluecurvesareforlogg=6.0. Thespectrawerebinnedtoaresolvingpowerof400andwerenormalizedseparatelyineachspectral
band.
ingfrom3.5to6.0,andinterpolatedthemeasuredEWofthe Thelocalgas-phaseandcloudchemistryisdeterminedby
companionfor each respectiveline. The mean and standard the local temperatureand pressure which we demonstratein
deviation of the four lines from this comparison indicate a Fig.6forallDRIFT-PHOENIXatmospheremodelsappliedin
surfacegravityoflogg=4.6±0.3,whichisrelativelylow,as Fig. 3. While the low gravity spectra in the 2600-3000 K
expectedand verifiedfor youngobjects(e.g.Brandekeretal. temperature range all provide a reasonable agreement with
2006). TheKIEWswemeasuredforthecompanionarealso the observed spectrum, figure 6 shows that the correspond-
comparabletothoseofotherM8-M9membersofUpperSco ingunderlyingatmospheric(T,p) profilesare verydifferent.
(usingspectrafromLodieuetal.2008)butonlyathirdtohalf This emphasises that medium differencesin the spectral en-
of the EWs of M7-M9 field dwarfs of comparable spectral ergydistributioncanhidelargevariationoftheatmosphere’s
types (using spectra from McLeanetal. 2003), clearly indi- structure.Themodelsat3000Kevendevelopasmalltemper-
catinglowsurfacegravityandmembershipinUpperScofor atureinversionhighupintheatmosphere,whereH Oisdis-
2
thenewcompanion(seealsofigure5). sociated,freeingupoxygenandlocallyenhancingtheopacity
comparedtothesurroundinglayers.8
3.2.2. Atmospherecompositionandstructure Of all the models shown on figure 3, only three do have
DRIFT-PHOENIX provideslocalgaspropertieslikethegas dust in their atmosphere: logg=4.5 with Teff =2600 K and
temperature T [K], the gas density ρ [g cm- 3], and the lo- logg = 6.0 with both Teff = 2600 K and 2800 K. Figure 7
shows the mean grain sizes for these models as function of
cal gas-phase composition, but also dust quantities such as
thenumberofdustparticlesn [cm- 3]ofmeangrainsizehai localtemperature;thelowesttemperaturesindicatetheupper
d
[cm] at each layer of the atmosphere. These quantities are
8 Suchatemperatureandpressureinversiondoesnotnecesserelyinvali-
needed to evaluate the radiation transfer through the atmo-
datetheassumptionofhydrostaticequilibriumwhichisappliedintheDRIFT-
spheretakingintoaccountconvectiveenergytransport,hence PHOENIXatmospheremodelsaswasshowninHellingetal.(2000)andin
tocalculatethesyntheticspectrumasshowninFig.3. Asplund(1998).
6 Lafrenièreetal.
FIG.4.—NIFSspectrumofHIP78530B(black)comparedwiththespectrumofUScoJ155419.99-213543.1(top),whichisanM8free-floatingbrowndwarf
memberofUpperSco(Lodieuetal.2008),andwithasyntheticspectrumfromtheDRIFT-PHOENIXatmospheremodelswithTeff=2800K,logg=4.5,and
solarmetallicity(bottom).Thespectrawerebinnedtoaresolvingpowerof1000andwerenormalizedseparatelyineachspectralband.
FIG.5.—SpectrumofHIP78530B(black)showingtheKIdoubletsat∼1.17µmand∼1.25µm.Inthetoppanels,thecompanionspectrumiscomparedwith
spectrafromtheDRIFT-PHOENIXmodelsforTeff=2800Kandlogg=3.5(red),4.5(blue),and5.5(green). Inthebottompanels,thespectrumiscompared
withthespectrumofUScoJ155419.99-213543.1(red,Lodieuetal.2008),ayoungM8browndwarfinUpperSco,andLP412-31(blue,McLeanetal.2003),a
fieldM8dwarf.TheshallowerKIlinesseeninthecompanionspectrumrelativetothefieldobjectindicateslowersurfacegravity. Thespectrawerebinnedtoa
resolvingpowerof∼3000.
Newsubstellarcompaniontoamassiveyoungstar 7
FIG.7.—Themeangrainsizesacrossthecloudsaltitudesinthethreedust-
formingmodelatmospheresplottedinFig.3.Thecloudgrainsremainrather
smallinallmodelsandresideintheopticalthinregionoftheatmosphere.
The age of the Upper Scorpius association is well con-
strained at 5 Myr and all stars appear to have formed in
a burst, over a period of at most 1-2 Myr (Preibischetal.
2002;Slesnicketal.2008). Foranageof5±1Myrandthe
mFoIsGp.h6er.e—mTohdeeltsemuspeedraitnurFe–igp.re3s.suTrheestTru-ctPurepsroffiolreasllvDaryRIcFoTn-PsiHdeOrEabNlIyXfaot-r abovecompanionbolometricluminosityestimate, theevolu-
thethreesetsofmodelsatdifferentTeff,althoughtheemergingspectrafrom tionmodelsofChabrieretal.(2000)indicateamassof∼21-
thethreelow-gravityatmosphereallprovideareasonableagreementwiththe 26M ,whilethoseofBurrowsetal.(1997)yieldamassof
Jup
observedcompanionspectrum. ∼19-25M ,seefigure8. Therangesofmassquotedreflect
Jup
onlytheuncertaintiesontheageandluminosityestimates,but
partoftheatmosphere. Thedustpersistsatahighertemper- ofcourse,largeruncertaintymaybepresentowingtothelack
ature in the high-gravitymodelsowingto an increased local ofabsolutecalibrationoftheevolutionmodels. Asvisibleon
density; however, these models are less relevantfor the new figure 8, the companionis currentlyin a phase where its lu-
companionfoundhere, which is youngand has low gravity. minosityismoreorlessconstantwithtimeduetodeuterium
Thusunlessthecompanionisonthelowendofourestimated burning;this phasewill last untilan age of ∼15Myr. Inter-
T range, it probably does not have dust in its atmosphere. estingly,thismeansthatthecompanionmass estimateis not
eff
Howeverifitisindeedcloserto2600Kandhasdustinitsat- strongly dependent on the age estimate, as is ordinarily the
mosphere,thegrainsizeswouldremainrathersmall,forming caseforsubstellarobjects.
a haze layer in the upper, optically thin layers of the atmo-
spheres(seeFig.7). Theonlyobjectswherehazelayershave 4. DISCUSSIONANDCONCLUDINGREMARKS
beeninferredfromtransitphotometry,however,areirradiated Due to the very large semi-major axis of the companion
giantgasplanets(seeSingetal.(2009)). Nodirectdetection orbit,itsexpectedorbitalmotionperyearisbelowtheastro-
ofhazeonbrowndwarfshasbeenobtainedtodate. metricprecisionof ourdata. To estimate the orbitalmotion,
wecanconsiderthehypotheticalcaseinwhichtheorbitiscir-
3.2.3. Luminosityandmass
cularandface-onasseenfromEarth,withasemi-majoraxis
We estimated the bolometric luminosity of the compan- of710AU.Giventhesystemmassofapproximately2.5M⊙,
ion by using its observed photometry in combination with this yields an orbital period of ∼12000years, which in turn
modelspectra.WefirstcomputedsyntheticfluxesintheJHK givesan angularmotion of 0.03◦ yr- 1 (or a linear motion of
bandsusingthe modelspectra, thenadjustedthosesynthetic ∼2.4 mas yr- 1). The angular motion over a decade under
fluxestothemeasuredvalues,andfinallyintegratedthescaled this assumption, 0.3◦, does come out to the same order of
model spectra over all wavelengths to obtain the total irra- magnitudeasthe2σdifferenceinpositionanglebetweenour
diance, which was then converted to bolometric luminosity imagesandthoseofKouwenhovenetal.(2005),0.6◦±0.3◦.
using the primary star distance. The synthetic average flux However, in reality, the orbital motion is likely to be much
densities in the JHK bands were computed using the rela- smaller, as the projected separation is typically smaller than
tive spectral response curves given on the 2MASS project therealsemi-majoraxis. Inaddition,theonlyway inwhich
webpage9. WerepeatedtheprocedureusingboththeDRIFT- theangularmotioncouldbelargerthan0.3◦ isiftheorbitis
PHOENIX andNextGen(Hauschildtetal.1999)modelspec- eccentricandthecompanionispresentlyclose toperiastron,
tra for rangesof Teff from 2600K to 3000K and logg from whichisaprioriunlikely. Aswenotein§3.1,ourastrometry
3.5to5.0.Weobtainedavalueoflog(L/L⊙)=- 2.55±0.13. is internally consistent for the NIRI data but not necessarily
The uncertainty quoted reflects the range of values obtained withrespecttootherinstruments,henceitisplausiblethatthe
for the different model parameters and accounts for the un- differencewithKouwenhovenetal. issimplyspurious.Thus,
certaintyonthedistanceoftheprimarystar. thereisnoconvincingevidenceoforbitalmotion,andnosuch
motionisexpectedovertherelevanttimescales.
9http://www.ipac.caltech.edu/2mass/releases/allsky/doc/sAecm6_a4sas.rhattimol–separationdiagramshowingHIP78530Band
8 Lafrenièreetal.
FIG.8.—Calculatedbolometricluminosityofthecompanion(blackpoint)
compared with evolutionary tracks from Baraffeetal. (1998, 2002) (red
curves)andBurrowsetal.(1997)(bluecurves). Thecurvesarelabeledwith FIG.9.—Massratioasafunctionofseparationforvarioussubstellarcom-
theirmassexpressedinunitsofsolarmass. panionstostars. TheredfilledcircleisHIP78530Bandtheredfilleddi-
amond is 1RXS J160929.1-210524b. The blue filled circles are all other
directlyimagedlow-masssubstellarcompanions(<25MJup)tostarsfrom
thecompilationgiveninLafrenièreetal.(2010),augmentedwiththenewly-
otherknownbrowndwarfandplanetarycompanionstostars
found companion Ross 458C (Goldmanetal. 2010; Scholz 2010). The
is shown in Figure 9. With a mass of ∼23 MJup and pro- largeblackfilledcircles aremoremassivedirectly imagedsubstellar com-
jected separation of ∼700 AU from its massive 2.5 M⊙ pri- panionsfromthecompilationofZuckerman&Song(2009). Thetriangles,
mary, HIP 78530B lies at the lower boundary of all known squareanddownarrowareHR8799bcde(Maroisetal.2008,2010),βPic
b(Lagrangeetal.2009,2010)andFomalhautb(Kalasetal.2008),respec-
companions with separations larger than 100 AU. It is un-
tively. Thesmall black circles indicate planets foundby the radial veloc-
clear whether these companions, whose mass ratios over- ityandmicrolensingtechniques fromtheExtrasolarPlanetsEncyclopaedia
lap with those of both more massive brown dwarf compan- (http://exoplanet.eu/).
ions and bona fide planets, formed in a planet-like or in a
stellar-like manner. Given the size of their orbits, it ap-
pears unlikely that they formed in situ in a planet-like man- quently in star-forming regions than around older stars. In
ner – either through the collapse of a gravitationally unsta- ouroverallsurveyofUpperScorpiusthatledtothediscovery
ble disk (e.g. Vorobyov&Basu 2010) or by the core accre- ofHIP78530Band1RXSJ1609-2105b,weobserved91stars
tion mechanism (e.g. Pollacketal. 1996). Indeed for both (masses0.15-5M⊙), so taken atface value, ourtwo discov-
scenarios the disks would need to be unusually large and eriesimplythatcompanionswithmassratiosbelow0.01and
massive, but even then, the formation timescale by core ac- separationsofhundredsofAUsexistin2.2-+15..95%(95%credi-
cretion would be prohibitively long while the efficiency of bility)ofstellarsystems. Consideringthatwedidnotachieve
disk instability to produce such companions is highly un- thesamesensitivityforalltargetsaswellasourincomplete-
certain (Vorobyov&Basu 2010; Kratteretal. 2010). Nev- ness to lower mass ratio companions, this number is only a
ertheless, it is possible that these companions did form in lowerlimit. Thestatisticsarenotyetsufficienttotellwhether
a planet-like manner much closer to the star, but were sub- there exists a difference for older objects. For example the
sequently kicked outward through gravitational interactions study of Lafrenièreetal. (2007), which targeted ∼200 Myr-
(e.g. Scharf&Menou 2009; Verasetal. 2009). In this sce- old GKM stars, enabled placing upper limits of ∼6% (95%
nario,accordingtothesimulationsofVerasetal.(2009),the credibility) for companions of ∼10 MJup (i.e. mass ratio
timescaleforinstabilitiestodevelopandsendplanetsonlarge ∼0.01); see also Chauvinetal. (2010) and Nielsen&Close
orbits may be quite short (0.01-1 Myr), with significant dy- (2010). Improving the statistics for both young and older
namical evolution occurringwithin the first few Myr. How- systems would thusbe a good way to investigatefurtherthe
ever,owingtofurtherevolution,planetsquicklyscatteredon importance of gravitational scattering in accounting for dis-
large orbits are likely to be ejected from the system after a tant companions. This approach is valid as long as the in-
few tensofMyr. Alternatively,these wideand low massra- ternal dynamics of the multiple planets dominates over in-
tio companions could form like stellar binaries, through the teractions with other stars or giant molecular clouds as the
fragmentationofapre-stellarcore(e.gBate2009;Bateetal. system travels through the galaxy, as these interactions can
2003). Numericalsimulationsindicatethatverylowmassra- alsostripoutwidecompanions.Thiscanberoughlychecked
tiocompanionscanindeedbeproducedbythisprocess,albeit using the results of Weinbergetal. (1987). Based on their
rarely,andthattheypreferentiallyhavelargeseparationsand figure2, with a/Mtot∼0.0014pc M⊙- 1, the disruptionlife-
aregenerallyfoundinhighordermultiplesystems.Futureob- timeofHIP78530ABduetoencounterswithstarsandgiant
servationsmay provideconstraintsto excludeor supportthe molecular clouds is larger than 10 Gyr; this is also the case
variousformationpossibilities of these wide low mass com- for1RXS J160929.1–210524Ab. In otherwords, stellar and
panions. For instance, if they formed in a planet-like man- molecularcloudsencountershavelittle impacton theevolu-
ner closer to the star and were subsequentlykicked outward tionofcompanionssuchastheoneswehavefound.
throughgravitationalinteractions, then it would be expected Thenewcompanionreportedinthispaperjoinsagrowing
that additional companions of similar mass or even heavier list of low-mass substellar companions (.25 M ) in wide
Jup
shouldbepresentinthesystems,atsmallerseparations. orbit (&100 AU) around stars, which now counts about ten
In addition, as mentioned above, distant companions pro- objects(seeLafrenièreetal.(2010)forarecentcompilation,
ducedthroughthisprocesswouldlikelybeeventuallyejected see also Fig. 9). These companions are found around pri-
from the system, such that they should be found more fre- mariescoveringawiderangeofmassesandevenaroundpri-
Newsubstellarcompaniontoamassiveyoungstar 9
mariesthatarethemselvesbinaries. Thus,whilethestatistics WethanktheGeministaffforhelpandsupportwiththeob-
on their frequencymay notbe veryaccurate, it seems likely servations. TheauthorsalsowishtothankMartenvanKerk-
thatwidelow-masssubstellarcompanionsarenotanunusual wijk,AlexisBrandeker,ChristianMaroisandÉtienneArtigau
outcomeofthestarformationprocess,yettheyremainhardto for useful discussion or help regarding some aspects of this
explainwithincurrenttheoreticalframeworks. Overthenext work. RJ acknowledges support from NSERC grants and a
fewyears, additionalsearchesfornew companionsand con- Royal Netherlands Academy of Arts and Sciences (KNAW)
tinuedeffortstocharacterizetheknownonesshouldallowus visiting professorship. Finally, we thank our referee, Dr.
tomakegoodprogresstowardexplainingtheirformation. KevinLuhman,forhelpfulsuggestionstoimprovethispaper.
REFERENCES
Asplund,M.1998,A&A,330,641 Lagrange,A.etal.2010,Science,329,57
Baraffe,I.,Chabrier,G.,Allard,F.,&Hauschildt,P.H.1998,A&A,337, Lagrange,A.etal.2009,A&A,493,L21
403 Lodieu,N.,Hambly,N.C.,Jameson,R.F.,&Hodgkin,S.T.2008,
—.2002,A&A,382,563 MNRAS,383,1385
Baron,E.,Hauschildt,P.H.,Allard,F.,Lentz,E.J.,Aufdenberg,J., Luhman,K.L.etal.2007,ApJ,654,570
Schweitzer,A.,&Barman,T.2003,inModellingofStellarAtmospheres, Marois,C.,Macintosh,B.,Barman,T.,Zuckerman,B.,Song,I.,Patience,J.,
IAUSymposium#210,ed.N.Piskunov,W.W.Weiss,&D.F.Gray,19 Lafrenière,D.,&Doyon,R.2008,Science,322,1348
Bate,M.R.2009,MNRAS,392,590 Marois,C.,Zuckerman,B.,Konopacky,Q.M.,Macintosh,B.,&Barman,T.
Bate,M.R.,Bonnell,I.A.,&Bromm,V.2003,MNRAS,339,577 2010,Nature,468,1080
Brandeker,A.,Jayawardhana,R.,Ivanov,V.D.,&Kurtev,R.2006,ApJ, McGovern,M.R.,Kirkpatrick,J.D.,McLean,I.S.,Burgasser,A.J.,Prato,
653,L61 L.,&Lowrance,P.J.2004,ApJ,600,1020
Burrows,A.etal.1997,ApJ,491,856 McGregor,P.J.etal.2003,inSPIEConferenceSeries,Vol.4841,ed.M.Iye
Carpenter,J.M.,Mamajek,E.E.,Hillenbrand,L.A.,&Meyer,M.R.2006, &A.F.M.Moorwood,1581–1591
ApJ,651,L49 McLean,I.S.,McGovern,M.R.,Burgasser,A.J.,Kirkpatrick,J.D.,Prato,
—.2009,ApJ,705,1646 L.,&Kim,S.S.2003,ApJ,596,561
Chabrier,G.,Baraffe,I.,Allard,F.,&Hauschildt,P.2000,ApJ,542,464 Mohanty,S.,Basri,G.,Jayawardhana,R.,Allard,F.,Hauschildt,P.,&
Chauvin,G.etal.2010,A&A,509,A52 Ardila,D.2004,ApJ,609,854
Chauvin,G.etal.2005,A&A,438,L29 Nielsen,E.L.,&Close,L.M.2010,ApJ,717,878
D’Antona,F.,&Mazzitelli,I.1997,MemoriedellaSocietaAstronomica Pollack,J.B.,Hubickyj,O.,Bodenheimer,P.,Lissauer,J.J.,Podolak,M.,&
Italiana,68,807 Greenzweig,Y.1996,Icarus,124,62
Dehn,M.,Helling,C.,Woitke,P.,&Hauschildt,P.2007,inConvectionin Preibisch,T.,Brown,A.G.A.,Bridges,T.,Guenther,E.,&Zinnecker,H.
Astrophysics,IAUSymposium#239,ed.T.Kuroda,H.Sugama, 2002,AJ,124,404
R.Kanno,&M.Okamoto,227–229 Rayner,J.T.,Cushing,M.C.,&Vacca,W.D.2009,ApJS,185,289
Goldman,B.,Marsat,S.,Henning,T.,Clemens,C.,&Greiner,J.2010, Scharf,C.,&Menou,K.2009,ApJ,693,L113
MNRAS,405,1140 Schmidt,T.O.B.,Neuhäuser,R.,Seifahrt,A.,Vogt,N.,Bedalov,A.,
Hauschildt,P.H.,Allard,F.,&Baron,E.1999,ApJ,512,377 Helling,C.,Witte,S.,&Hauschildt,P.H.2008,A&A,491,311
Helling,C.,Dehn,M.,Woitke,P.,&Hauschildt,P.H.2008,ApJ,675,L105 Scholz,R.2010,A&A,515,A92
Helling,C.,Winters,J.M.,&Sedlmayr,E.2000,A&A,358,651 Sherry,W.H.,Walter,F.M.,&Wolk,S.J.2004,AJ,128,2316
Helling,C.,&Woitke,P.2006,A&A,455,325 Sing,D.K.,Désert,J.,LecavelierDesEtangs,A.,Ballester,G.E.,
Herriot,G.etal.2000,inProc.SPIE,Vol.4007,AdaptiveOpticalSystems Vidal-Madjar,A.,Parmentier,V.,Hebrard,G.,&Henry,G.W.2009,
Technology,ed.P.L.Wizinowich,115–125 A&A,505,891
Hodapp,K.W.etal.2003,PASP,115,1388 Slesnick,C.L.,Hillenbrand,L.A.,&Carpenter,J.M.2008,ApJ,688,377
Kalas,P.etal.2008,Science,322,1345 vanLeeuwen,F.2007,A&A,474,653
Kouwenhoven,M.B.N.,Brown,A.G.A.,PortegiesZwart,S.F.,&Kaper, Veras,D.,Crepp,J.R.,&Ford,E.B.2009,ApJ,696,1600
L.2007,A&A,474,77 Vorobyov,E.I.,&Basu,S.2010,ApJ,714,L133
Kouwenhoven,M.B.N.,Brown,A.G.A.,Zinnecker,H.,Kaper,L.,& Weinberg,M.D.,Shapiro,S.L.,&Wasserman,I.1987,ApJ,312,367
PortegiesZwart,S.F.2005,A&A,430,137 Witte,S.,Helling,C.,&Hauschildt,P.H.2009,A&A,506,1367
Kratter,K.M.,Murray-Clay,R.A.,&Youdin,A.N.2010,ApJ,710,1375 Woitke,P.,&Helling,C.2003,A&A,399,297
Lafrenière,D.etal.2007,ApJ,670,1367 —.2004,A&A,414,335
Lafrenière,D.,Jayawardhana,R.,&vanKerkwijk,M.H.2008,ApJ,689, Zuckerman,B.,&Song,I.2009,A&A,493,1149
L153
—.2010,ApJ,719,497