Brown dwarfs and very low mass stars in the Hyades cluster : a dynamically evolved mass function

0.95 MB·English

by  J. Bouvier

Astronomy&Astrophysicsmanuscriptno.hyades˙rev1 (cid:13)c ESO2008
February2,2008
Brown dwarfs and very low mass stars in the Hyades cluster : a
dynamically evolved mass function.
J.Bouvier1,T.Kendall2,G.Meeus3,L.Testi4,5,E.Moraux1,J.R.Stauffer6,D.James7,J.-C.Cuillandre8,J.Irwin9,
M.J.McCaughrean10,I.Baraffe11,andE.Bertin12,⋆
1 Laboratoired’AstrophysiquedeGrenoble,ObservatoiredeGrenoble,BP53,F-38041Grenoble,France
8
2 CentreforAstrophysicsResearch,UniversityofHertfordshire,CollegeLane,HatfieldAL109AB,UK
0
3 AstrophysikalischesInstitutPotsdam,AnderSternwarte16,D-14482Potsdam,Germany
0
4 OsservatorioAstrofisicodiArcetri,INAF,LargoE.Fermi5,50125Firenze,Italy
2
5 EuropeanSouthernObservatory,KarlSchwarzschildStr.2,85748GarchingbeiMu¨nchen,Germany

n 6 SpitzerScienceCenter,CaliforniaInstituteofTechnology,1200EastCaliforniaBoulevard,Pasadena,CA91125,USA
a 7 DepartmentofPhysics&Astronomy,Box1807StationB,VanderbiltUniversity,Nashville,TN37235,USA
J 8 CFHTCorporation,65-1238MamalahoaHwy,Kamuela,Hawaii96743,USA
4 9 Harvard-SmithsonianCenterforAstrophysics,60GardenStreetMS-16,Cambridge,MA02138,USA
10 SchoolofPhysics,StockerRoad,Exeter,EX44QL,UK
] 11 CRAL,EcoleNormaleSupe´rieure,46alle´ed’Italie,69364LyonCedex07,France
h 12 Institutd’AstrophysiquedeParis,98bisbdArago,75014Paris,France
p
- Received;accepted
o
r ABSTRACT
t
s
a Aims.Weconductedasearchforbrowndwarfs(BDs)andverylowmass(VLM)starsinthe625Myr-oldHyadesclusterinorderto
[
derivethecluster’smassfunctionacrossthestellar-substellarboundary.

Methods.Weperformedadeep(I=23,z=22.5)photometricsurveyover16deg2 aroundtheclustercenter,followedupwithK-band
1
photometry tomeasuretheproper motion ofcandidate members, and optical andnear-IRspectroscopy of probable BDandVLM
v
members.
0
Results.Wereport thediscoveryofthefirst2browndwarfsintheHyadescluster.The2objectshaveaspectral typeearly-Tand
7
theiropticalandnear-IRphotometryaswellastheirproper motionareconsistentwiththembeingclustermembers.Accordingto
6
models,theirmassis50Jupitermassesatanageof625Myr.Wealsoreportthediscoveryof3newverylowmassstellarmembers
0
ofthecluster,andconfirmthemembershipof16others.Wecombinetheseresultswithalistofpreviouslyknownclustermembersto
.
1 buildthepresent-daymassfunction(PDMF)oftheHyadesclusterfrom50Jupitermassesto3M .WefindtheHyadesPDMFtobe
⊙
0 stronglydeficientinverylowmassobjectsandbrowndwarfscomparedtotheIMFofyoungeropenclusterssuchasthePleiades.We
8 interpretthisdeficiencyastheresultofdynamicalevolutionoverthepastfew100Myr,i.e.,thepreferentialevaporationoflowmass
0 clustermembersduetoweakgravitationalencounters.
: Conclusions.WethusestimatethattheHyadesclustercurrentlyhostsabout10-15browndwarfs,whileitsinitialsubstellarpopulation
v
mayhaveamountedupto150-200members.
i
X
Keywords.Stars:low-mass,browndwarfs–Stars:luminosityfunction,massfunction–(Galaxy:)openclustersandassociations:
r individual:Hyades(Melotte25)
a
1. Introduction rivedfromthemostmassivestarsdowntothesubstellardomain
inavarietyofenvironments:starformingregions(e.g.Luhman
The determination of the initial mass function (IMF), i.e.,
etal. 2003),youngopenclusters(e.g.Bouvieretal. 1998)and
the mass frequency distribution of objects formed in molecu-
in the field (e.g. Reid et al. 1999). As brown dwarfs continu-
lar clouds, yields strong constraints to star formation theories
ouslycooldownastheyevolve(Chabrieretal. 2000),theyare
(Padoan&Nordlund2002;Bate&Bonnell2005;Larson2005;
brighterwhenyounger.Thus,theIMFcouldbederiveddownto
Jappsenetal.2005).Inthelast10years,theIMFhasbeende-
a mass of a few Jupiter masses in star forming regions (Lucas
etal.2006;Caballeroetal.2007)anddowntoabout30Jupiter
Sendoffprintrequeststo:J.Bouvier masses in rich youngopen clusters (Morauxet al. 2003,2007;
⋆ PartlybasedonobservationsobtainedattheCanada-France-Hawaii Barradoetal.2004;deWitetal.2006).Whilealargenumberof
Telescope(CFHT)whichisoperatedbytheNationalResearchCouncil
fieldbrowndwarfshavealsobeendiscoveredwithinafewtens
ofCanada,theInstitutNationaldesSciencedel’UniversoftheCentre
of parsecs from the Sun, the derivation of the field IMF below
NationaldelaRechercheScientifiqueofFrance,andtheUniversityof
0.1 M still remainsuncertaindue to the lack of knowledgeof
Hawaii.PartlybasedonobservationsmadewiththeItalianTelescopio ⊙
Nazionale Galileo (TNG) operated on the island of La Palma by theagesofsubstellarobjects(Chabrier2002;Allenetal.2005;
the Fundacio´n Galileo Galilei of the INAF (Istituto Nazionale di Cruzetal.2007).
Astrofisica)attheSpanishObservatorio delRoquedelosMuchachos
oftheInstitutodeAstrofisicadeCanarias.Partlybasedonobservations Inapreviousseriesofpaper,wederivedthelowermassfunc-
madewithESOTelescopesattheLaSillaObservatories. tionofanumberofyoungopenclustersdowntoabout30M
Jup
2 J.Bouvieretal.:BrowndwarfsandverylowmassstarsintheHyadescluster:adynamicallyevolvedmassfunction.
(see Moraux, Bouvier & Clarke 2005 and Bouvier, Moraux &
Stauffer2005forshortreviews)anddiscussedthe implications
C4
ofanapparentlyuniversalclustermassfunctionforstarforma-
C3 C6
tionscenarios(Morauxetal.2007).Here,wereportanextended,
deepopticalsurveyoftheHyadesclusteraimedatdetectingsub- B2 C2 C5
stellar objects and deriving the cluster’s mass function across B1 C1 C10
B18
the stellar-substellar boundary.The motivations for this survey B3 C7 C9
are twofold. Firstly, all previoussearches for substellar objects B17 A7 A8 C8
in the Hyades have failed to report any positive brown dwarf B16 A6 A2 A3 B4
detections (Reid & Hawley 1999; Gizis, Reid & Monet 1999; A5 A1 A4 B5
B15
DinoclbubdieesetLaHl.02401082+).1T3,hewliothwaesstpmecatsrsalmtyempebeMrs8r.5epaonrdtedansoesftai-r C17 B14 A9 A10 BB67
matedmassof0.083M⊙(Reid&Hawley1999),the≃0.081M⊙ C16 B13 A13 A12 A11
companionsof a couple of 0.1 M⊙ Hyades probablemembers, C21 C15 B12 B11 B9 B8
LP 415-20 and LP 475-855(Siegler et al. 2003), and an unre- C24 C14 B10
solvedverylowmasscompanion(0.070-0.095M )inthespec- C20 C13
⊙
troscopicbinaryRHy403(Reid&Mahoney2000).Yet,thedis- C23 C19 C18
coveryof browndwarfsat an age of 625 Myr (Perrymanet al. C22
1998) would providea unique benchmarkto calibrate the sub-
stellarevolutionmodels.Secondly,theHyadesclusterisdynam-
ically evolved(Adamsetal.2002).A significantfractionofits
initiallowmasspopulationisthereforeexpectedtohavedrifted
away beyondthe cluster boundaries(e.g. Reid 1993).Deriving
the lower mass functionof such an evolvedcluster would pro-
videadirectmeasurementoftherateatwhichlowmasscluster Fig.1. The spatial distribution of CFHT12K Hyades fields is
membersevaporateandpopulatethefield.Inturn,thismeasure- shown by rectangles labelled with their names (cf. Table A.1).
mentwouldallow usto test the validity ofN-bodysimulations Dashed concentric circles are spaced by 1◦ around the cluster
ofthedynamicalevolutionofyoungclusters(e.g.Kroupa1995; center. Star symbols(red) are Hyadesprobablememberslisted
PortegiesZwartetal.2001). in Prosser& Stauffer’sdatabase,witha size relatedto theirlu-
The Hyades cluster (Melotte 25, α =04h26m54s, minosity.Large(blue)squaresindicatethearea previouslysur-
2000
δ =+15◦52′; l=180.05◦, b=-22.40◦) is the closest rich open veyedbyDobbieetal.(2002).
2000
cluster to the Sun. Perryman et al. (1998) derived its main
structural and kinematical propertiesbased on Hipparcosmea-
surements:adistanceof46.3±0.27pc,anageof625±50Myr,
also present follow up K-band imaging, as well as optical and
a metallicity [Fe/H] of 0.14±0.05, a present-day total mass
infraredspectroscopyobtainedforasubsetofoptically-selected
of about 400 M , a tidal radius of 10.3 pc, a core radius of
⊙
clustercandidatemembers.
2.5-3.0 pc and negligible extinction on the line of sight. The
large proper motion of the cluster (µ ≃100 mas yr−1) can be
easilymeasuredfromimagingsurveysoveratimeframeofonly
2.1.Theopticalsurvey
afewyears,whichhelpsinassessingcluster’smembership.
In Section 2, we describe the optical survey we performed Wide-field optical images were obtained in the I and z bands
over the central 16 deg2 of the Hyades cluster, as well as withtheCFHT12Kcamera,amosaicof12CCDarrayswitha
follow-up K-band photometry and both optical and infrared pixelsizeof0.21′′whichprovidesaFOVof42×28′(Cuillandre
spectroscopy.Thepropermotionofoptically-selectedcandidate et al. 2000). The survey was performed during several service
members is derived from optical and near-IR images obtained observingrunsbetweenSeptember2002andJanuary2003,and
2 to 3 years apart. In Section 3, we describe our selection of consists of 53mosaic fields coveringa totalof 17.3squarede-
candidatememberscombiningopticalandinrafredphotometry, grees over the cluster area. However, one CCD of the mosaic
as well as proper motion measurements and follow up spec- (CCD05)hadseriouscosmeticsproblemsduringtheserunsand
troscopy.We thus report21 probablemembers,of which 5 are yieldeduseless data, thusreducingthe effective surveyarea by
newand2arethefirstHyadesbrowndwarfs,withanestimated 1/12.Thedatesofobservationandthecoordinatesofthecenter
massof≃50M .InSection4,wediscussthespectralproper- ofthemosaicfieldsarelistedinTableA.1inAppendixA.The
Jup
tiesofthenewlyfoundlowmassHyadesmembers,andderivea areacoveredbythesurveyontheskyisillustratedinFigure1.
spectraltypeofT1andT2forthe2browndwarfcandidates.We The survey extends symmetrically around the cluster’s center,
proceedinderivingthepresent-daymassfunctionoftheHyades alonga4deg-widestripeofconstantgalacticlatitude,andupto
clusterfrom0.050to3.0M⊙,whichwefindtobestronglydefi- 3degreesawayfromtheclustercenteringalacticlongitude.
cientinverylowmassstarsandbrowndwarfscomparedtothe Short and long exposures (10s in I and z, 3×300s in I and
mass function of the younger Pleiades cluster. We discuss this 3×360sinz)wereobtainedforeachfield.Thisresultedinacon-
resultinthelightofN-bodysimulationsofthedynamicalevolu- tinuouslysampledrangeofImagnitudesfromI∼13,thesatura-
tionofyoungopenclusters. tion limit of short exposures, to a detection limit of I∼24. The
survey is at least 90% complete down to I∼23.0 and z∼22.5,
as computed from overlapping 12K fields (see Moraux et al.
2. Observationsanddatareduction
2003),alimitwhichvariesonlyslightlywithseeingconditions
We describe below the deep, wide field optical survey we per- (0.6-0.8′′ FWHM, see Table A.1). Photometric standard fields
formedontheHyadesclusteratCFHTintheIandzbands.We (Landolt1992)wereobservedseveraltimespernight,aspartof
J.Bouvieretal.:BrowndwarfsandverylowmassstarsintheHyadescluster:adynamicallyevolvedmassfunction. 3
thecontinuousmonitoringoftheCCDmosaicphotometriczero
pointsduringserviceobservingruns.
Individual images were processed through CFHT’s Elixir
reduction pipe-line (Magnier & Cuillandre 2004) including
bias subtraction, flat-fielding, fringe correction, scattered light
correction in z-band, and astrometric calibration (0.8′′ rms).
Multiple I- and z-band dithered long exposureswere then reg-
istered and co-added. PSF photometry was performed on both
shortandlongexposureswithamodifiedversionofSExtractor
(Bertin&Arnouts1996)fromaPSFmodelcomputedwiththe
PSFExsoftware(Bertinetal.,inprep.).Theresultingcatalogues
were photometricallycalibrated using the I- and z-band photo-
metriczeropointsofthecorrespondingnightsderivedfromthe
Landolt standard fields and extinction coefficients k and k of
I z
0.04and0.03mag/airmass,respectively.Inthez-band,thepho-
tometric zero points are computed from selected Landolt stan-
dardsofspectraltypeA0.Afewframesweretakenthroughthin
cirrusabsorption(afew0.01mag).Inthiscase,additionalIandz
exposureswereobtained(30sinI,36sinz)forthesamefieldson
subsequentphotometricnightsandusedtorecalibratetheorigi-
nalshortandlongexposureimages.
Note thateventhoughLandoltstandardshave beenused to
computethephotometriczero-points,theresultingI-bandmag-
nitudes are not in the Cousins system. This is due to the dif-
ferentpassbandsoftheCFHT12KandCousinsfilterswhichin-
ducescolorterms,especially forveryredobjects.These color-
termscanbeestimatedbyintegratingbothfilterpassbandsover
Baraffeetal.’s(1998)modelspectra.Thisyields: I = I for
12K c
(I − K) ≤ 2.0 and I = I − 0.034× (I − K) + 0.068 for Fig.2. (I, I-z) color-magnitude diagram of stellar like objects
c 12K c c
2.0 ≤ (I −K) ≤ 6.0.Wehavenotappliedthesecolortermson detected on the Hyades CFHT12K frames. 600 Myr NextGen
c
our photometry. Instead, 600 Myr theoretical isochrones from (upper)andDusty(lower)isochrones,shiftedtotheHyadesdis-
the Lyon models were specifically computed for the I- and z- tance,areshownassolidlinesandlabelledwithmass(M ).The
⊙
band CFHT filters in order to provide a consistent comparison horizontal dashed line illustrates the expected location of the
betweenmodelsandobservationsincolor-magnitudediagrams. stellar/substellar boundary along the cluster sequence. Hyades
We found slight systematic differences between the photo- candidate members (large red dots) were selected to the right
metric zero point of the various CCDs within the mosaic. The ofthedottedlinerunningclosetotheisochrones(seetext).For
CCD-to-CCDzeropointshiftswereempiricallyestimatedfrom clarity,only1 in10objectsof thebackgroundgalacticpopula-
the median (I-z) color of all stellar-like objects in each CCD, tionisshownleftwardsoftheselectionline.
withreferencetoCCD04.Thedistributionofcolorshifts,com-
puted over all images, has a median value of -0.005 mag and
a rms of 0.05 mag. The CCD-to-CCD zero point shifts were mics(smallerPSFFWHM).Eventually,theoutputphotometric
correctedfor by applyingthem to the z-bandmagnitudeof the catalogue contains over 930,000 non saturated stellar-like ob-
objects in the output photometric catalogues. The I-band mag- jectsdetectedinbothI-andz-bandsoverthe16squaredegrees
nitudewasleftunchanged.Smalloverlapsbetweenmostofthe we surveyed down to a detection limit of I∼z∼24. The corre-
observedfieldseventuallyallowedustocheckthequalityofthe sponding (I, I-z) color magnitude diagram (CMD) is shown in
photometriccalibration.Thiswasdonebycomparingthepairof Fig2.Atotalof125possibleHyadesmemberswereselectedin
magnitudemeasurementsavailable for everystar commonto 2 thisCMD fromtheirlocationrelativeto modelisochrones(see
overlappingfields. The rms accuracy of the I-band and z-band Section3.1).
photometryis foundto be of orderof 0.06mag for I=18-20in
longexposures,thenincreasesto0.08magintherangeI=20-22,
2.2.FollowupK-bandphotometry
and as the signal to noise ratio decreases further reaches 0.10
magatI∼22.5.Similarresults,scaledtothecorrespondingmag- Follow up K’-band imaging was obtained for 108 of the 125
nituderange,areobtainedforshortexposureimages. optically selected candidate members. About half of this sam-
The photometric cataloguesoriginally containednearly 2.3 ple was observedin Nov.04and the other half in Nov.05using
million non saturated objects detected on the short and long the 1k×1k CFHT IR camera (Starr et al. 2000). The exposure
exposures. Non stellar objects (galaxies, bad pixels, cosmics) timevariedfrom0.5to14minutesforcandidateswithImagni-
were rejected from the photometric catalogues on the basis of tudesrangingfrom13to23.Eachobjectwasditheredon7po-
the FHWM of their PSF. The seeing FWHM was measuredon sitionsonthedetector,sothatthemedianofthe7imageswould
each image from plots of the object’s magnitude against PSF provideanestimateoftheskybackgroundduringtheexposure.
FWHM.Stellar-likeobjectsliealongawelldefinedverticalline Individual images were dome flat fielded, sky subtracted, then
in theseplots, atthevalueofthe seeingFWHM (typically0.6- registered and added to yield the final K’-band image. K-band
0.8′′, i.e.,3-4pixelsFWHM).Conservativelimitstypicallybe- photometric standards from Hunt et al. (1998) were observed
tween 50 and 150% of the seeing FWHM were adopted to fil- everycoupleofhourseachnight.Aperturephotometrywasper-
ter out galaxies (larger PSF FWHM) and bad pixels and cos- formedoncandidatesandphotometricstandards.For2closevi-
4 J.Bouvieretal.:BrowndwarfsandverylowmassstarsintheHyadescluster:adynamicallyevolvedmassfunction.
NextGen NextGen
Dusty
Cond Cond
Dusty
Fig.3. Left:(I,I-z)CMDofopticallyselectedcandidates.Right:(I,I-K)CMDofopticallyselectedcandidatedfollowedupwith
CFHTIRintheK-band.Smalldots(left):17opticallyselectedcandidatewithoutfollowupIRphotometry.Largedots:optically
selectedcandidateswhosepropermotionisinconsistentwithHyadesmembership.Triangles:candidateswhosepropermotionis
consistent with Hyades membership (see text). The 2 most promising substellar cluster candidates are shown by large triangles.
NextGen(0.07-0.3M ),Dusty(0.04-0.07M )andCond(0.015-0.05M )isochronesareshownandlabelledwithmass.Inthe(I,
⊙ ⊙ ⊙
I-K)CMD,thedottedlineindicatesthelocusofM8-T5fielddwarfs(fromDahnetal.2002).Thermsphotometricerrorisshown
asbars.
sual binary systems (CFHT-Hy-15/17,CFHT-Hy-4/4c),a large andthederivedK-bandmagnitudeshaveanaccuracyoforderof
aperture was first used to estimate the system’s total flux, and 0.02mag.The(I,I-z)and(I,I-K)CMDsforthe108candidates
PSF photometry was performed to derive the flux ratio of the followedupintheK-bandareshowninFig3.
systemcomponents.
The derived K-band photometric zero point is 22.86 ±
0.03 mag, assuming an average extinction coefficient of 0.07 2.3.Astrometryandpropermotion
mag/airmass. Slight photometric variations occurred during
somenightsduetovaryingextinctionbythincirrus.Imagesob- ThepropermotionofopticallyselectedHyadescandidateswas
tained on these nights were recalibrated using 2MASS images computed from pairs of optical (I, z) and infrared (K) im-
of the same fields. In this case, the photometric accuracy is of ages obtained 2 or 3 years apart (Nov.04, Nov.05) under sim-
orderof0.05mag.Theothernightshadphotometricconditions ilar seeing conditions (0.6-0.8′′ FWHM). The optical images
J.Bouvieretal.:BrowndwarfsandverylowmassstarsintheHyadescluster:adynamicallyevolvedmassfunction. 5
multipliedbythepixelscale.Foreachcandidate,propermotion
was measured using different sets of reference stars within the
image,andfromboththeIandzimages.Thevariousmeasure-
mentswerealwaysfoundtobeconsistentwithinerrorsandwe
finally considered their weighted mean as the best estimate of
thecandidate’spropermotion.
The results are listed as µ and µ in Tables 1 and D.1.
αcosδ δ
Thermsaccuracyontheppmmeasurementisusuallyoforderof
10mas/yr.However,insomecaseswhenthedistributionofref-
erencestarsisasymetricrelativetothecandidate(i.e.whenthe
opticalcandidateliesclosetotheCCDedge),highersystematic
errorsmightbeexpected.Also,onecandidate(HA13c08-1866l)
was too faint in K (a probablenon member) to have its proper
motionmeasured.Thepropermotionvectordiagramof107op-
ticallyselectedHyadescandidatemembersisshowninFigure4.
2.4.OpticalSpectroscopy
Weobtainopticalspectroscopyfor3newHyadeslowmasscan-
didatemembersandfor2othercandidatesrecoveredinoursur-
vey but previously classified as non members (see Sect. 3.1).
Spectroscopic observations were performed from December 5
to 8, 2006 at ESO/NTT with EMMI spectrograph which pro-
vides echelle spectra covering the wavelength range from 580
Fig.4.Propermotionvectordiagramfor107opticallyselected
to 990 nm at a resolution of 9,800. Using NOAO/IRAF, the
candidates followed up in the K-band (see text). The expected
spectral orders were extracted, flat-field corrected, wavelength
proper motion for Hyades members is shown by the (red) box
calibrated using a Th-Ar lamp and corrected from the instru-
(Bryja et al. 2004). Within these boundaries, 23 optically se-
mental response computed from spectrophotometric standards
lected candidates(empty circles) are found to share the proper
observed during the same nights. Exposure times from 600 to
motionofthecluster,including2browndwarfs(largetriangles).
1200syieldedaS/Nratioofabout30perresolutionelementat
Typical rms errors on the ppm measurements are shown by a
850nm.
cross.
2.5.InfraredSpectroscopy
wereastrometricallycalibratedthroughCFHT’sElixirpipe-line
ThethreefaintestprobableHyadesmembers(Table1)wereob-
(Magnier & Cuillandre 2004) with an accuracy of 0.8′′ rms.
served with the NICS near infrared spectrograph at the 3.58m
The infrared images were astrometrically calibrated using the
TelescopioNazionaleGalileo(TNG).Weemployedthehighef-
WCStools package (Mink 2006) and 2MASS images as ref-
ficiency,verylow resolutionAmicispectroscopicmode,which
erences. We found the CFHT IR detector to be aligned along
yieldsa complete0.85-2.45µmspectrawith aresolvingpower
the North-South direction to within 1 deg and the pixel scale
of50 (Baffa etal. 2001;Oliva etal. 2003). Observationswere
to be 0.211′′, as mentionned in CFHT IR’s Reference Manual
carried out in service mode by the TNG staff on Oct. 14 and
(0.211±0.001′′). Some CFHT IR images had not enough com- 18, 2005. We used the 1′′ wide slit and total on-source inte-
mon objects with the shallower 2MASS images to allow a
gration times of 32, 56 and 40 minutes for CFHT-Hy-19, 20
straightastrometriccalibrationthroughthismethod.Inthiscase,
and 21 respectively. The data reduction procedure closely fol-
we adopted the average orientation and plate scale derived for
lowed the one described in Testi et al. (2001). Correction for
otherCFHTIRimagesandsimplycalibratedthelocationofthe
telluricabsorptionwasachievedusingobservationsoftheAS11
centerofthefield.
A0 star from the list of Hunt et al. (1998) obtained during the
Pairsofopticalandinfraredimageswherethenmatchedby
samenightsatasimilarairmassasourmaintargetsandwithan
celestial coordinatesto yield a list of common objects in the 2
identicalinstrumentalsetup.
images.The(X,Y)locationoftheobjectsonbothdetectorswas
measuredusingIRAF/DAOFINDpackage.Ageometricaltrans-
formationwasthencomputed(IRAF/GEOTRANS)whichmaps 3. Results
the(X,Y)coordinatesoftheopticaldetectorintothatoftheIR
detector.Thegeometricaltransformationwasderivedfromaset WeidentifiedHyadescandidatemembersfromtheirlocationin
of typically 10 to 20 reference stars evenly distributed around optical and IR color-magnitude diagrams (CMDs) and assess
theHyadescandidate.Thecandidateitselfisnotincludedinthe theirmembershipstatusfrompropermotionmeasurements.We
computationand an iterative procedureis applied to reject any describebelowthe selectioncriteria we usedand appliedto all
referencestarexhibitinghighresiduals.Theaccuracyofthege- stellarlikeobjectsdetectedontheopticalimages.
ometricaltransformationwastypically0.07pixelsrms.
The transformation was then applied to the Hyades candi-
3.1.ProbableHyadesmembers
date, mapping its optical (X,Y) coordinates into the IR plane.
The candidate proper motion was derived from the difference Possible Hyades members are selected from their location in
between the observed and mapped coordinates on the IR de- the (I, I-z) color magnitude diagram (CMD) relative to model
tector, divided by the time elapsed between the 2 images, and isochrones. The 600 Myr NextGen (Baraffe et al. 1998) and
6 J.Bouvieretal.:BrowndwarfsandverylowmassstarsintheHyadescluster:adynamicallyevolvedmassfunction.
Table1.ProbableHyadesmembersbasedonphotometryandpropermotion.
CFHT-Hy-# Internal RA(2000) Dec(2000) I I-z I-K µ µ Othernamea
αcosδ δ
name (mas.yr−1)
CFHT-Hy-1 HA10c00-33s 42312.5 154247 12.87 0.54 2.5 111 -9 VA262
CFHT-Hy-2 HC13c03-43s 42724 1476 12.91 0.46 2.41 123 -14 VA432
CFHT-Hy-3 HC06c00-511s 41833.8 182153 13.29 0.59 2.69 114 -65 LP414-158
CFHT-Hy-41 HC13c11-268s1 42822.4 134922 13.35 0.57 2.61 84 4 HAN430
CFHT-Hy-5 HA2Ic06-310s 42516.5 16187 13.4 0.55 2.45 87 -11 VA352
CFHT-Hy-6 HB10c09-178s 42431.5 135541 13.48 0.52 2.41 97 0 VA326
CFHT-Hy-7 HB8Ic01-117s 42056.1 145133 13.53 0.52 2.5 106 -17 VA203
CFHT-Hy-8 HA11c00-112s 4231.4 151341 13.59 0.52 2.43 103 -1 VA260
CFHT-Hy-9 HB12c04-432s 43042.5 143941 13.59 0.62 2.59 113 -7 HAN495
(—2 HB6Ic04-77s2 42232.8 155124 13.67 0.55 2.32 92 -22 VA241)
CFHT-Hy-103 HC04c10-516s3 42356.7 183819 14.17 0.6 2.68 102 -7 RHy191
CFHT-Hy-11 HC16c06-212s 43421 145116 14.39 0.57 2.65 82 0
CFHT-Hy-12 HB13c01-162s 43116.3 15012 14.44 0.64 2.7 94 -7
CFHT-Hy-13 HC01c00-279s 4195.7 173422 14.52 0.71 3.01 85 -1
CFHT-Hy-14 HA5Ic02-358s 4304.2 1646 14.71 0.69 2.76 99 -57 RHy281
CFHT-Hy-154 HA2Ic03-783s4 4276.4 162548 14.78 0.71 2.76 93 -16 RHy240A
CFHT-Hy-16 HC19c03-841s 4292.9 133759 14.81 0.82 3.19 104 -6 LH91,LHD0426+1331
CFHT-Hy-174 HA2Ic03-781s4 4276.6 162546 14.83 0.71 2.76 96 -15 RHy240B
CFHT-Hy-18 HC09c09-231s 41732.2 165659 15.2 0.75 3.17 123 -38 RHy138
CFHT-Hy-195 HC08c07-331s5 41724.8 163436 17.49 1.18 4.59 99 -28 2MASSWJ0417247+163436
CFHT-Hy-206 HC22c09-1735l6 43038.7 13957 21.58 1.79 5.5 135 -9
CFHT-Hy-217 HA9Ic04-1346l7 42922.7 153529 22.16 1.36 5.57 79 -18
aRef.:VA=vanAltena(1969);LP=Luytenetal.(1981);HAN=Hanson(1975);RHy=Reid(1992);LH=Leggett&Hawkins(1988);
LHD=Leggett,Harris&Dahn(1994);2MASS=Gizisetal.(1999)
1Afaintcompanionisseen3.2′′awayfromthisHyadesprobablememberbutitsI-Kcolorandpropermotionindicatesabackgroundstar.
2ClassifiedasanonmemberbyReid(1993).Spectroscopicallyconfirmedasanon-memberinthisstudy(seetext.)
3ClassifiedasanonmemberbyReid(1993).Spectroscopicallyconfirmedasamemberinthisstudy(seetext.)
4CFHT-Hy-15and17formavisualpair(RHy240AB).
5ClassifiedasanonmemberbyGizisetal.(1999),seetext.
6ThisprobableHyadesBDwasalsoobservedintheJandHbands,yielding(J-H,H-K)=(0.51,0.43).
7ThisprobableHyadesBDwasalsoobservedintheJandHbands,yielding(J-H,H-K)=(1.12,0.77).
Dusty (Chabrier et al. 2000) isochrones computed for CFHT bluewards.Chabrier et al. (2000)predictthat the transitionbe-
12KIandzfilterswereused.Inordertoaccountforphotomet- tween the Dusty and Cond models should occur at a tempera-
ricuncertaintiesandforthecluster’sdepth,theselectionbound- tureof1300-1400K,whichcorrespondstoamassof40Jupiter
arywastakentolie0.1magbluerthantheisochrones,withthe massesforHyadesmembers.
NextGentoDustytransitionoccurringatI≃18.5mag.The(I,I-
As an additional membership criterion, proper motion was
z) CMD is shownin Fig. 2togetherwith themodelisochrones
computedforallthecandidatesfollowedupintheK-band,ex-
labelledwithmassshiftedatadistancemodulusof3.33forthe
ceptone(HA13c08-1866l)whichwastoofaintinK andthere-
Hyadescluster(Perrymanetal.1998).Theselectionresultedin
foreaprobablenonmember.Thepropermotionvectordiagram
125possiblecandidatemembersfromI∼13to23,coveringthe
of 107 optically selected candidates is shown in Fig 4. While
mass range from 0.3 M to about 45 Jupiter masses. The stel-
⊙ most of the candidates scatter near the origin, with relatively
lar/substellarboundaryoccursatI∼17.8intheCMD.Ofthe125
smallpropermotionandthusmostlikelybackgroundfieldcon-
possiblecandidates,42arelowmassstars(0.1-0.3M ),4lieat
⊙ taminants, 23 candidates have a proper motion consistent with
thestellar/substellarboundary,and79lieinthesubstellarrange
Hyades membership. The ppm vector diagram also reveals 7
(M≤0.072M ).Thelistofopticallyselectedcandidatesisgiven
⊙ high proper motion objects (µ >100 mas/yr), most likely fore-
inTables1andD.1.
groundlate-typefielddwarfs.
Follow-upK-bandimagingof 108possible candidatespro- Inordertoanalysethedistributionofobjectsintheppmdi-
videdadditionalmembershipcriteria.The(I,I-K)CMDofthese agram,webuiltasyntheticppmvectordiagramfromsimulated
candidatesisshowninFig.3.Mostofthelowmassstellarcan- observationsspanning 1 sq.deg. in the direction of the Hyades
didatesqualifyaspossiblemembersinthisdiagramaswell,ex- cluster.Thesimulatedsamplewasobtainedfromthe Besancon
hibiting(I-K)colorsconsistentwithmodelpredictions.Incon- modelof galactic structure andkinematics(Robin etal. 2003).
trast, a large fraction of the optically selected substellar candi- We extractedfromthissamplealltheobjectslyinginthesame
datesappeartoobluecomparedtotheDustymodelisochrones, part of the (I, I-K) CMD as the optically selected Hyades can-
whichsuggeststheyareolderlatetypefielddwarfsunrelatedto didates.Theppmvectordiagramoftheresultingsyntheticsub-
thecluster.However,themodelisochronesbecomeincreasingly samplestronglyressemblestheobservedone,withmostobjects
uncertain at very low masses as dust develops and may con- scatteredaroundtheoriginwithaslightlyelongateddistribution
denseintheloweratmosphericlayers,whichresultsinasudden towards the south-east direction, and a few high velocity fore-
changeoftheI-Kindextowardsmuchbluercolors.Thelocation ground dwarfs. No object from the simulated sample is found
of low mass substellar cluster membersin the (I, I-K) diagram toliewithintheboundaryoftheHyadesclusterpropermotion.
maythusspanalargerangeofcolors,fromtheDustyisochrone Thisindicatesthatthecontaminationoftheobservedppmvector
J.Bouvieretal.:BrowndwarfsandverylowmassstarsintheHyadescluster:adynamicallyevolvedmassfunction. 7
CFHT-Hy-13
4
CFHT-Hy-13
CFHT-Hy-12
10 a
3
CFHT-Hy-12
CFHT-Hy-11
2
CFHT-Hy-11
CFHT-Hy-10 (RE 191)
5
CFHT-Hy-10 (RE 191)
1 VA 241
VA 241
0 0
6540 6550 6560 6570 6580 8150 8200
Fig.5.ThespatialdistributionofprobableHyadesmembersre- Fig.6. Optical spectra of 5 optically-selected Hyades very low
ported from our survey. Low-mass stellar members are shown massstellarcandidatemembers.TheH regionisshowninthe
α
aslargedots,andthe 2browndwarfsbytriangles.Thesepara- left panel, the NaI doublet in the right panel. The NaI doublet
tion of the 2 componentsof the visual binary CFHT-Hy-15/17 8195Åline of CFHT-Hy-12isaffected bya bad pixel,labelled
(RHy240AB)hasbeenexageratedforclarity.Thebackground with“a”.
shows the location of the 12K fields (the color code illustrates
0.5deg-wideringsaroundtheclustercenter.
ouslybeenreportedbyGizisetal.(1999)fromthe2MASSsur-
vey of the Hyades cluster. This candidate is discussed in more
diagramby foregroundfield dwarfs at the expectedlocation of details in the next section, as the evidence for membership is
Hyades members is small. We find only one optically selected somewhatinconclusive.Theremaining5probablememberswe
candidate(HA12c07-101l)withapropermotionconsistentwith reportare new. Theyinclude 3 low mass stars (∼0.14M ) and
⊙
Hyadesmembershipwhichistooblueinthe(I,I-K)CMDtobe 2 objects well within the substellar regime (∼0.050 M ). The
⊙
amember(cf.TableD.1).Weclassifieditasanonmember. spectra of the 3 new low mass stars exhibit H emission with
α
Eventually, 22 candidates consistently qualify as probable
equivalent widths between 4 and 6 Å, and deep NaI doublet,
Hyadesmembersonthebasisoftheiropticalphotometry,their
fullyconsistentwithmembership(cf.Fig.6).
(I-K) color and their proper motion. Their propertiesare listed
In addition, we found during this survey an extremely red
in Table 1 and their spatial distribution is shown in Fig. 5. Of
object, HB6Ic09 322l, with I=22.92 and I-K=7.55 ! The ob-
these, 14 were already listed as probable Hyades members in
ject appears point like in the IzK images but is not a Hyades
Prosser & Stauffer’s Open Cluster Database1 (P&S database)
member according to its proper motion (Table D.1). We note
basedonvariouscriteriaandareconfirmedhere.Theiralternate that this object lies only 3′ away from the Class 0 protostar
namesaregiveninTable1.Another3probablemembersinour
IRAM 04191+1522 (Andre´ et al. 1999) and from the Class
survey had been previously reported but classified as probable
I protostar IRAS 04191+1523 (Lee et al. 2002), both associ-
non members. One (CFHT-Hy-10 = RHy 191) is listed in the
ated to the molecular condensations MC 18a and 18b (Onishi
P&Sdatabaseandclassifiedasaprobablenonmemberbasedon
et al. 2002). This indicates that star formation is still active in
thelackofH emissioninalowresolutionspectrum(Stauffer’s
α this area located at the outskirt of the Taurus molecular cloud.
priv.comm.,quotedinReid1993).Thehigherresolutionspec-
HB6Ic09 322l, whose proper motion is consistent with Taurus
trumwe reportforthisobjectexhibitsweak H emission, with
α membership (Frink et al. 1997), could thus be a new, self-
EW(Hα)=3Å, and deep NaI 8183,8195Ålines, consistent with embedded Class I source. However, there are no 2MASS nor
membership (cf. Fig. 6). Another candidate (HB6Ic04 77s = IRAScounterpartsatthislocation,norisitdetectedbySpitzer
VA 241), also listed in the P&S database, was classified as a at24and70µm(Ph.Andre´,priv.comm.).
nonmemberbyReid(1993)basedon“(V-I)coloursinconsistent
withHyadesmembership”.Thehighresolutionspectrumweob-
tainforVA241showsH inabsorptionandshallowNaIdoublet 3.2.ComparisonwithpreviousHyadessurveys
α
lines,anonmemberindeed(Fig.6).Thelastone(CFHT-Hy-19)
Inordertochecktheconsistencyofourmembershipclassifica-
islocatedclosetothestellar/substellarboundaryandhasprevi-
tion of the 125 optically selected candidates, we searched the
literaturetocompareourresultswiththoseofpreviousdeepsur-
1 Open Cluster Database, as provided by C.F. Prosser
and J.R. Stauffer, and which currently may be accessed at veysoftheHyadescluster.
http://www.noao.edu/noao/staff/cprosser/, or by anonymous ftp to Dobbieet al. (2002)conducteda wide (10.5sq. deg.) opti-
140.252.1.11,cd/pub/prosser/clusters/. cal (I,z) survey of the Hyades cluster down to a completeness
8 J.Bouvieretal.:BrowndwarfsandverylowmassstarsintheHyadescluster:adynamicallyevolvedmassfunction.
limit of I∼20.3 mag. They report 20 possible candidate mem- 158,HAN 430,VA 352,VA 203,VA 326,VA 260,HAN 495,
bers, of which they confirm only one as being a (previously RHy 281, RHy 138, RHy 240A, RHy 240B, LH 91); 9 others
known)propermotionlowmassmember(h5078e=RHy297). aresaturatedonourshortexposureimages,withImagnitudesin
Ofthe20candidatestheyreport,8fallintheareawesurveyed. the rangefrom12.52to13.36andare thereforenotconsidered
Ofthese,4aretooblueinthe(I,I-z)CMDtobeselectedaspos- inourstudy(VA329,VA213,VA216,VA94,VA362,LP415-
siblemembers(h8644b,h1272b,h8711b,h5044b),and2aresat- 875, VA 763, VA 127, VA 368, all of which except VA 329
uratedon our images(h9448b,h4239b).The 2 remainingones have been spectroscopically confirmed as members, e.g., Reid
arerecoveredhereaspossiblemembersinthe(I,I-z)CMDbut &Mahoney2000);anotheroneisHAN150(VA95,RHy133,
are eventually rejected on the basis of their discrepant proper HC08c00 261s)whichwedorecoverinoursurveybuthastoo
motion(HB7Ic04 145s=h4046b;HB7Ic11 463s=h7334b). bluean(I-z)colortobeacandidatemember(I=15.72,I-z=0.4).
Reid & Hawley (1999, RH99) reported low resolution fol- It was already rejected by Reid et al. (1993) based on its too
lowupspectroscopyof12possiblelowmasscandidatemembers blue (V-I) color. The 6 remaining objects are the RH99 candi-
identified by Leggett & Hawkins (1989, LH) from a Schmidt dates discussed above, none of which qualify as a member in
surveyofthe centralregionsof thecluster. Based onradialve- our study. Hence, we recover all the candidate probable mem-
locitymeasurements,theyfindthatonlyoneofthe12starsisa berslistedintheP&Sdatabasewhichlieintheareacoveredby
probablelowmassclustermember(LH0418+13).Ofthe12LH oursurveyandprovideanewmembershipassessmentformost
objects,6fallintheclusterareawesurveyed.Ofthese,4aretoo of them. Another 6 objects listed as probable members in the
blueinour(I,I-z)CMDtobeselectedaspossibleclustermem- P&S database lie in the spatial limit of our survey but fall in
bers(LH0427+12,0418+15,0419+15,0420+15).Anotherone gapsbetweenourfields,asweavoidedtheimmediatevicinityof
(LH0422+17=HB1Ic01 386l)wasselectedasapossiblemem- brightstars.TheseareRHy159,RHy228,RHy230,RHy287
ber from its location in the (I, I-z) CMD but its proper motion (Reid1992)andLH0416+16,LH0429+15(Legget&Hawkins
proved inconsistent with membership. RH99 also measured a 1988). Two of these (RHy 228 and RHy 230) have been con-
discrepant radial velocity for this object, which appears to be firmed as members(Reid et al. 1995;Reid & Mahoney2000),
anunrelatedfielddwarf.Thelastobject(LH0424+15)fallson 2others(RHy159,RHy287)havenotbeenfollowedup(Reid
abadcolumnofaCCDpreventingusfromderivingitsphotom- 1992), and the 2 remaining ones have been shown to be non-
etry. members(Reid&Hawley1999).Conversely,asdiscussedabove
Gizis et al. (1999, GRM99) searched for very low mass (see Section 3.1),we recovered2 objects, previouslyclassified
stars and brown dwarfs in the central region of the Hyades asnon-membersintheP&Sdatabase,onewhichweconfirmas
cluster using the 2MASS survey. They selected 40 possible beinganon-member(VA241),theotherwhichweclassifyasa
candidates, including 29 new ones, from their infrared colors bonafideHyadesmember(CFHT-Hy-10=RHy191).
corresponding to spectral types M8-L4. From the follow up
low resolution spectroscopy they performed, none of these
candidates appear to be cluster members. Eleven GRM99 new 4. Discussion
candidatesfallintheareacoveredbyoursurvey.Ofthese11,10
4.1.LandTdwarfsintheHyadescluster
aretooblueinthe(I,I-z)CMDtobeselectedaspossiblecluster
members (2MASSW J0434462+144802, J0435489+153719, We report here the first 2 Hyades brown dwarf candidates
J0431500+152814, J0431322+152620, J0431420+162232, (CFHT-Hy-20, 21) as well as a previously detected very low
J0421175+153003, J0421521+151941, J0424045+143129, massobject(CFHT-Hy-19)closetothestellar-substellarbound-
J0430232+151436,J0423242+155954).Thelastone,however, ary.The3objectsarestrongcandidateclustermembersastheir
CFHT-Hy-19 = 2MASSW J0417247+163436, lies close to optical/infraredluminosityandcolorsareconsistentwithmem-
the stellar/substellar boundary in Fig. 2. We classify it as a bership,asistheirpropermotion.The2BDsarewellwithinthe
probableclustermemberbasedonitsopticalphotometry,(I-K) substellar domain with an estimated mass of about 50 Jupiter
colorandpropermotion,allbeingconsistentwithmembership. masses(seeTable2)whilethelowestmassstarhasanestimated
Gizisetal.(1999)dismissedthiscandidateasaclustermember mass around 0.08 M . A finding chart for the 2 substellar ob-
⊙
based on its low resolution spectrum (Sp.T. M8) exhibiting jects is provided in Fig. B.1. According to models, at an age
weak NaI 8200Å equivalent width, suggestive of low surface of 625Myr,these objectsshouldhavean effectivetemperature
gravity. They also fail to detect H in emission in their low around1600Kforthe2BDsandaround2400KfortheVLM
α
resolution spectrum. Comparing with theoretical models, they star,whichcorrespondstospectraltypeslate-L/early-Tandlate-
estimate a surface gravity of logg=4.4–4.7, somewhat lower Mrespectively.
thanexpectedforHyadesmembers(logg∼5.0).Theyconclude TheTNG/Amicilowresolutionspectraweobtainedforthese
itispresumablyayoung(10-30Myr),background(d∼150-200 3 objects, CFHT-Hy-19, 20 and 21 are shown in Figure 7. The
pc) substellar object (M≤0.04M ), possibly associated with portionsofthespectrathatareaffectedbythestrongesttelluric
⊙
the nearby Taurus star forming region. We find this possibility absorption(around1.4 and 1.9 µm) are not shown;the portion
unlikely however, as the proper motion we measure for this beyond 2.25µm and shortward of 1µm for CFHT-Hy-21 and
candidate ([99, -28] mas/yr) is much larger than the proper CFHT-Hy-20 are also not shown as the signal to noise ratio in
motion of Taurus SFR members (typically, [10,-20] mas/yr, the blue end of the spectra is too low and, with the integration
Frink et al. 1997). Furthermore, the candidate’s proper motion timechosenfortheseobjects,therededgeofthespectraisina
is fully consistent with Hyades membership. Hence, pending nonlinearregimeofthedetectorduetothehighskybackground.
additionalevidencefor (oragainst)membership,we retainthis The spectra of our targets have been compared with the
candidateasaprobablemember. Amici spectral library of field dwarfs compiled by Testi et
Finally, Prosser & Stauffer’s Open Cluster Database lists al.(2001)andTesti(2004).Whilethegravityoffielddwarfsis
147 probable members with V≥15.3 (I≥12.5).Of these, 30 lie expectedtobesomewhatdifferentfromthatofHyadesmembers,
in the area covered by our survey: 14 are recovered and con- attheAmiciresolution,thegeneralshapeofthespectrumshould
firmedhereasmembers(seeTable1,VA262,VA432,LP414- closelymatchforsimilarspectraltypes.Followingthisexpecta-
J.Bouvieretal.:BrowndwarfsandverylowmassstarsintheHyadescluster:adynamicallyevolvedmassfunction. 9
Table2.Massestimatesforthe12KHyadesmembers.
CFHT- I Mass(I) z Mass(z) K Mass(K)
Hy-# (M ) (M ) (M )
⊙ ⊙ ⊙
1 12.87 0.256 12.33 0.258 10.37 0.292
2 12.91 0.251 12.45 0.244 10.5 0.273
3 13.29 0.214 12.7 0.217 10.6 0.258
4 13.35 0.208 12.78 0.209 10.74 0.239
5 13.4 0.203 12.85 0.201 10.95 0.214
6 13.48 0.196 12.96 0.192 11.07 0.2
7 13.53 0.192 13.01 0.188 11.03 0.205
8 13.59 0.188 13.07 0.183 11.16 0.192
9 13.59 0.188 12.97 0.191 11 0.208
10 14.17 0.151 13.57 0.150 11.49 0.164
11 14.39 0.141 13.82 0.138 11.74 0.146
12 14.44 0.139 13.8 0.139 11.74 0.146
13 14.52 0.136 13.81 0.138 11.51 0.162
14 14.71 0.128 14.02 0.129 11.95 0.133
15 14.78 0.126 14.07 0.127 12.02 0.129
16 14.81 0.125 13.99 0.130 11.62 0.154
17 14.83 0.125 14.12 0.125 12.07 0.126
18 15.2 0.113 14.45 0.113 12.03 0.128
19 17.49 0.074 16.31 0.077 12.9 0.094
20 21.58 0.050 19.79 0.052 16.08 0.039
21 22.16 0.048 20.8 0.049 16.59 0.035
their volume to ours, we derive that about 0.06 early field T-
dwarfsshouldcontaminateoursurvey.Thisfurtherstrengthens
Fig.7. Near-infrared Amici low resolution spectra of CFHT- thelikelihoodthatthe2candidateswereporthereareindeedthe
Hy-19, 20 and 21 (solid lines from top to bottom). In each firstbrowndwarfsandthelowestmassmembersoftheHyades
panel we also show the closest matching field dwarf spectrum clusterknowntodate.
fromthelowresolutionAmicispectrallibrary(Testietal.2001;
Testi2004).
4.2.TheHyadesmassfunction
Our survey is complete in the mass range from less than 50
tion,wehavematchedourobservedspectrawiththemostsimi- Jupiter masses up to 0.20 M . We are thus able to derive
⊙
larfielddwarfspectraandderivedanapproximatespectraltype the cluster’s mass function across the stellar-substellar bound-
for our targets. In Figure 7 we show as dotted lines the spec- ary and well into the brown dwarf regime. The mass of the
traoftheclosestmatchingdwarfstogetherwithourtargetstars. probable members we report in Table 1 was derived using the
WefindanexcellentmatchwithdwarfswithspectraltypesM8, 600Myr NextGen(Baraffeet al. 1998)and Dusty (Chabrieret
T2andT1forCFHT-Hy-19,20and21respectively;neglecting al.2000)modelisochones,withthetransitionbetweenNextGen
the possible systematic offset introduced by comparing dwarfs andDusty takenat0.08M (T =2500K).These modelspro-
⊙ eff
of different gravity, the uncertainty of our procedure for spec- videMass-MagnituderelationshipsintheCFHTIandzsystem,
tral typing is within one subclass. It is worth pointing out that aswellasfortheK-band.Table2liststhemassofthecandidates
ournearinfraredclassificationofCFHT-Hy-19agreeswith the derivedfromtheirI,zandKmagnitudes(DM=3.33mag).Over
opticalM8classificationofGizisetal.(1999). themassrange0.05-0.25M ,massestimatesobtainedfromthe
⊙
As noted above, the 2 T-dwarfs we report here are strong I and z magnitudesagree to within 4%with no systematic off-
candidate Hyades members with their photometry and proper sets. In contrast,masses derivedfrom K magnitudeare usually
motion consistent with membership. Nevertheless, we proceed higher than those derived from the I magnitude by up to 20%
in estimating the probability that they could be unrelated field inthestellardomain,andarelowerbyuptothesameamountin
T dwarfsprojected onto the Hyades cluster. The 2 observedT- thesubstellardomain.Thediscrepancybetweenmassesobtained
dwarfs have a K-band magnitude of 16.08 and 16.59, respec- fromeither visualor near-IRmagnitudespresumablyresultsat
tively.T0-T2fielddwarfsatanageofafewGyrhaveanabso- leastinpartfrommodeluncertaintiesbutmightinsomecasesbe
luteK-bandmagnitudeof≃13.5(Chiuetal.2006).Thiswould relatedtothepresenceofunresolvedcompanions.Theseuncer-
thenlocate the contaminatingdwarfsata distancebetweenap- taintiesdonotsignificantlyaffecttheshapeofthemassfunction
proximately30 and 40 pc. Combinedwith the area coveredby we derivebelowandwe adoptthemasses obtainedfromthe I-
our survey, this corresponds to a volume of 65 pc3. From the bandmagnitudeinthefollowing.
combinationofthe2MASSandSDSSDR1surveys,Metchevet In order to build the complete present-day mass function
al.(2007)derivedanupperlimitof0.9×10−3pc−3 onthespace (PDMF)oftheHyadescluster,wecombinetheresultsofoursur-
densityofT0-T2.5dwarfsinthesolarneighborhood.Hence,we veyinthe0.05-0.20M massrangetotheP&Sdatabasewhich
⊙
wouldexpect≤0.06earlyfieldTdwarftocontaminateoursur- covers the mass range from about0.1 M to the most massive
⊙
vey.Anindependentempiricalestimateisprovidedbythedetec- Hyades members over the whole cluster area. Since we report
tionrateofearlyTdwarfsintheUKIDSSLASsurvey.Chiuet only3newmembers(overatotalof18,seeTable1and2)inthe
al.(2007)reportedthedetectionof3earlyT-dwarfsuptoadis- massrangefrom0.1to0.25M thatwerenotpreviouslylisted
⊙
tanceof 98pc over136deg2 in thesouthernsky.Scalingfrom intheP&Sdatabase,weassumethatthedatabaseisnearlycom-
10 J.Bouvieretal.:BrowndwarfsandverylowmassstarsintheHyadescluster:adynamicallyevolvedmassfunction.
Fig.8.ThemassdistributionofHyadesprobablemembers.Upperpanel:numberofsystemsper0.1logarithmicmassbin.Lower
panel:numberofsystemsperunitmassbin.Solidhistogram:massdistributionofHyadessystems.Themassdistributionderived
from our survey is shown as a red histogram in the 2 lowest mass bins (0.048-0.19 M ). It has been normalized to the mass
⊙
distributionoftheP&Sdatabaseoverthemassrange0.12-0.19M (seetext).Thedashedhistograminthelowestmassbin(0.048-
⊙
0.12M )assumesacoreradiusof7pcforbrowndwarfsintheHyadescluster(seetext).Dottedhistogram:massdistributionof
⊙
PleiadessystemsfromMorauxetal.(2003),normalizedtotheHyadesPDMFinthemassrange1-3M .
⊙
plete down to 0.1 M (Reid 1993). Conversely, among the 30 3 M , obtainedbycombiningthe resultsof our surveyand the
⊙ ⊙
lowmassprobablememberslistedintheP&Sdatabasethatlie P&Sdatabase.Notethatsincesub-arcsecondbinariesareunre-
in the area of our survey,8 are rejected as being non members solvedin seeing-limitedimagingsurveys(Patience et al. 1997;
(seeSection3.2).WethusestimatethecontaminationoftheP&S Reid & Gizis 1997), this mass distribution represents the sys-
databasebyfielddwarfstobeabout27%below0.3M .Weas- tems mass function. The mass function was built by counting
⊙
sumetheP&Sdatabasetobeuncontaminatedforbrighter,better thenumberofobjectsineachmassbinandstatisticallycorrect-
characterizedHyadesmembers.Themassoftheprobablemem- ingforthecontaminationoftheP&Sdatabasebylowmassfield
bers from the P&S database was estimated from their V-band dwarfs. In order to account for the different areas covered by
magnitudeasexplainedinAppendixC. the2datasets,thenumberofobjectsdetectedinoursurveywas
Figure 8 show the resulting mass distribution of Hyades normalizedtothenumberofobjectslistedintheP&Sdatabase
probable members over the whole mass range from 0.050 to overthecommonmassrange0.12-0.19M⊙wherethe2datasets