19 95MNRAS.272. .630S Astronomy Group,DepartmentofPhysicsandAstronomy,UniversityLeicester,Road,LeicesterLEI7RH Accepted 1994September17.Received14;inoriginalformJuly8 I. A.SteeleandR.F.Jameson Mon. Not.R.Astron.Soc.272,630-646(1995) Optical spectroscopyoflow-massstarsandbrowndwarfsinthePleiades Jameson (1991,1993,hereafterHHJ91andHHJ93, by theproper-motionsurveyofHambly,Hawkins& The Pleiadesopenclusterpresentsauniqueopportunityto HHJ91, whoderiveluminosity andmassfunctionsforthe members ofthecluster.Thissituationhasnowbeenrectified lack ofmembershipinformationtoidentifythelowestmass In thepast,however,observationshavebeenhamperedby a chance ofdetectingsubstellar‘browndwarfs’inthecluster. (ra-M=5.65, Hambly1991)meanthatwehaveareal respectively). TheirtechniquewastousetheCOSMOS age (~70Myr,Harris1976;Patenaude1978)anddistance small scatterinageandmetallicity.Inaddition,thePleiades investigate anearbysampleoflow-massstarswhichhave with coordinates,findercharts andphotographicR/ (MacGillivray &Stobie1984)toscanfirst-andsecond- measuring machineoftheRoyalObservatory,Edinburgh magnitudes. cluster. HHJ93presentthe full proper-motioncatalogue, motions. Afulldescriptionofthetechniqueisgivenin (1986) wasthenusedtopairupimagesandsoderiveproper epoch Schmidtplatesofthecluster.ThesoftwareHawkins and identified 22 browndwarfcandidates. This identifica- presented infraredphotometry of68theHHJ91members 1 INTRODUCTION In Steele,Jameson&Hambly (1993, hereafterSJH93),we © Royal Astronomical Society • Provided by the NASA Astrophysics Data System ABSTRACT We presentlow-resolutionspectraintherange4600-9700Àofasamplelow- temperature, supportingtheiridentificationaspre-main-sequenceclustermembers. mass starsandbrowndwarfcandidatesinthePleiades.Weorderspectraintoa we discusshowtheseresultsaffectthestatusofourbrowndwarfcandidates,revising We investigatetheeffectofunresolvedbinarityontemperatureindicators,and temperature indicatorsforlow-massstarsisinvestigated.Wefindthatthemost temperature sequence,usingaleast-squaresminimizationprocedure,andassign the remainderbinary. the numberto11systemscontainingsuchcandidates,ofwhichthreearesingleand may havesurfacegravitiesslightlylowerthanmain-sequenceobjectsofthesame conclude that~46percentoflow-masssystemsinthePleiadesaremultiple.Finally, especially TiO,ratherthanatomictransitions.WealsofindthatthePleiadesobjects effective spectroscopictemperatureindicatorsarethosebasedonmolecularbands, spectral types.Theeffectivenessofvariousproposedspectroscopicandphotometric Key words:binaries:general-stars:low-mass,browndwarfsopenclustersand associations: individual:Pleiades. Bessell(1991). perature scaleandbolometriccorrectionspresentedby metry willbeonthenaturalUKIRTsystem.Anerrorwas band luminositywiththetheoreticalmodelsofStringfellow bluest objectsto~20percentforthereddest.Inaddition, magnitudes totheCousinssystem.Table1listscorrected made bySJH93inconvertingtheirCCDandphotographic I Cousins systemdefinedbyBessell(1986).AllJHKphoto- (1991), convertedtotheobservationalplaneusingtem- tion wasbasedoncomparisonoftheI—KcolourandI- magnitudes varieswithcolour,from~5percentforthe in SJH93.Wehavere-reducedourobservationsofthis the JHKmagnitudesofoneobject,HHJ36,arealsoincorrect /magnitudes fortheSJH93objects.Thedegreeoferrorin / object fromSJH93’s~3Myrtothemorereasonableageof object, andreviseitsphotometryto/=14.00±0.04, brown dwarfcandidates),Fig. 1plotsarevisedversionof all wavelengths).Thisincreasestheapparentageofthis magnitudes ontheprimaryconclusion ofSJH93(their22 of ~70Myr. object may,infact,beclassified asabinaryfortheclusterage //= 13.44±0.04,*:=13.17±0.04(i.e.0.68magfainterat SJH93’s fig.5, the/,I—Kcolour-magnitude diagram. ~ 40Myr,andinSection3.5 belowweshallseethatthis In thispaper,wewillquoteallRIphotometryonthe In ordertoinvestigatetheeffect oftheerrorinI 19 95MNRAS.272. .630S used onboth nights. Theabsolutefluxcalibration forthe resolution ofthespectrograph, aslitwidthof1arcsecwas Average seeingwas~1arcsec onthefirstnightand~2 graphic inorigin,andaccurateto~12percent. vations accurateto~1percent.Otherobservationsarephoto- using theFaintObjectSpectrograph(FOS)ofWilliam arcsec onthesecondnight.In order toobtainthefullspectral November 12-13.Theweather onbothnightswasclear. Allington-Smith etal.(1989). two orders.Thefirst(‘red’)orderextendsfrom4600to Herschel Telescope,LaPalma.Thisisahigh-efficiency In thispaper,wepresenttheresultsofobservationsobtained preparation) willpresenttheresultsofCGS41-2.5\im HHJ’s members.Acompanionpaper(Steeleetal.1994,in Table 1.CorrectedCousins/-bandphotometryfortheobjectsof spectrograph coveringaspectralrangeof3500-9700Àin therefore presentstheresultsofopticalspectroscopy31 Steele, Jameson&Hambly(1993).AsterisksindicateCCDobser- pixel. ThecharacteristicsoftheFOSarefullydescribedby disperser, covers3500-4900Àatadispersionof4.3per order, whichisseparatedfromthefirstusingacross- spectroscopy ofasmallersampletheseobjects. mass mainsequenceandthebrowndwarfcandidatesof 9700 Âatadispersionof8.7Àperpixel.Thesecond(‘blue’) 2 OBSERVATIONSANDDATAREDUCTION SJH93, spectroscopyisthenextlogicalstep.Thispaper in Section3.6below. paper onSJH93’sbrowndwarfcandidates,willbediscussed more recenttheoreticalmodelsandtheconclusionsofthis the ImagnitudesofSJH93,alongwithimplications brown dwarfcandidatesto13.Furthereffectsoftheerrorin greater thanthislimit,loweringthetotalnumberofapparent (i.e. M~0.08M)nowappeartohavemassesslightly masses. Theoveralleffectis,however,small.Severalofthe objects thatwerepreviouslyjustclassifiedasbrowndwarfs now appearslightlyolder,andthereforehavehigher Comparison withfig.5ofSJH93showsthatmostobjects o ID / HHJ236 14.79 HHJ291 14.45 HHJ2 17.30" HHJ5 17.04 HHJ413 13.38 HHJ320 14.22 HHJ153 15.20 HHJ390 13.73" HHJ127 15.47 HHJ81 15.75 HHJ139 15.34 HHJ391 13.52" HHJ321 14.21 HHJ89 15.74 HHJ319 14.22 HHJ297 14.37 HHJ27 16.44 HHJ25 16.48 HHJ204 14.96 HHJ410 13.43 HHJ425 13.17 The observationswereobtained onthenightsof1993 To investigatefurtherthepropertiesofPleiadeslow- © Royal Astronomical Society • Provided by the NASA Astrophysics Data System HHJ37 16.24 HHJ92 15.74 HHJ21 16.59 HHJ15 16.71 HHJ26 16.44 HHJ198 14.99 HHJ10 16.90 HHJ339 14.06 HHJ373 13.72 HHJ288 14.45 HHJ3 17.41" HHJ8 16.94 HHJ85 15.74 HHJ17 16.68 HHJ144 15.27 HHJ20 16.59 HHJ28 16.43 HHJ13 16.75 HHJ22 16.55 HHJ440 12.97 HHJ430 13.08 HHJ4 17.20" HHJ292 14.43 HHJ7 18.88" HHJ101 15.69 HHJ19 16.62 HHJ39 16.23 HHJ133 15.39 HHJ58 15.93 HHJ6 16.99 HHJ12 16.82 HHJ24 16.46 HHJ46 16.32" HHJ18 16.64 HHJ23 16.47 HHJ9 16.92 HHJ11 16.82 HHJ118 15.54 HHJ14 16.75 HHJ54 15.93 HHJ1 17.85* HHJ30 16.38 116 17.39* HHJ10 16.90 HHJ16 16.62* 18 17.51* 115 17.96 HHJ36 16.18* 125 17.75 14 17.85 HHJ79 15.75 13 17.32* 15 18.05 112 17.90 110 18.07 117 17.76 111 18.08 17 17.38* 114 18.05* 113 17.84 Low-mass starsandbrowndwarfsinthePleiades631 features at7186,8164,8227,8282,8952and8980À. from thespectra,ashumidityvariationsthrougheach We willthereforegenerallyignoretheregionabove9000 À These maybeintrinsictothespectraorterrestrialinorigin. Above 9000Ä,watervapourfeaturesdominatethespectra. night werelarge.Thespectrathereforeshowtheatmospheric tables ofKing(1985);however,noattemptwasmadeto pheric extinctioneffectswereremovedusingthe A’ and‘B’bandsat68677594Â,watervapour remove terrestrial0andwatervapourabsorptionfeatures observations ofstandardsfromOke&Gunn(1983).Atmos- the data,andinstrumentalresponsewascalibratedusing took place.Bothtungstenandskyflatswereusedtoflat-field Jameson (1995,inpreparation),(iii)alloftheobjectsfor in thediscussionthatfollows. which wehavealsoobtainedpartialorcomplete1-2.5pm will notdistinguishbetweenthesecategoriesinthe spectra, whichwasremovedbeforeanyfurtherreduction separating cross-disperserintroducesacurvatureintothe also UstedinTable2. order toprovidecomparisonspectrafortyping.Theseare (1991, hereafterK91)andLeggett(1992)wereobservedin spectra (Steeleetal.1995,inpreparation),and(iv)asample finder chartsandphotographicR/magnitudes.JHK were Faÿ,Stein&Warren(1974) and,forterrestrialfeatures, supported packagefígaro(Shortridge1991).Theorder- discussion thatfollows.Inaddition,asampleofMdwarf objects, ofcourse,fallintomorethanonecategory,andwe mine photometricallytobeaclusternon-member.Many osities, includingoneobject(HHJ144)whichSJH93deter- of objectsfromHHJ91withvariousR—Icoloursandlumin- for observation.Thesewere(i)asampleofthelowestmass spectral standardsfromKirkpatrick,Henry&McCarthy Pleiades membersidentifiedintheROSATPSPCfields photometry isgiveninSJH93fortheobjectspresent observed werealltakenfromHHJ93,whogivecoordinates, exposure timesandobservationdates.ThePleiadesobjects effects ofdifferentialatmosphericrefraction(Filippenko normalized spectraintheanalysisofSection3.All important, sincewewillingeneralbeconcernedonlywith emission, asignature ofyouth,further enhancing their objects identifiedbySJH93,(ii)asampleofX-ray-bright described previously.Severalsetsofobjectsweretargeted Section 1)fortheSJH93objectsisgiveninTable1,as objects observedwereatairmass<2.0,andinmostcases Stevenson (1994).Allofthe PleiadesspectrashowHa identification wasKirkpatrick etal.(1991);othersources 200068, 200556and200557byHodgkin,Steele& sample whichtheyobserved.Therevised/photometry(see the ‘parallacticangle’aspossibletocompensatefor second-night objectswillthereforebeunreliable.Thisisnot Fig. 2showsanexampleofone ofourspectrawithvarious spectral featuresidentified. The primarysourceforline 2 3.1 Lineidentiflcationsandspectralsequence 3 DISCUSSION 1982). C c < 1.3.Inallcases,carewastakentorotatetheslitasnear The datawerereducedusingroutinesfromtheStarlink- Table 2presentsalistoftheobjectsobserved,with 19 95MNRAS.272. .630S 632 LA.SteeleandR.F.Jameson further inthispaper, preferringtopostpone thistoHodgkin photometric non-memberof the cluster)doesnotshowHa motion membersareshownasverticalcrosses,non-members squares,andobjectswhosemembershipisuncertainasquestionmarks. applied bySJH93.Wewill not discusstheHaemission emission, validatingthephotometric membershiptests membership status.However, thespectrumofHHJ144(a Inclined crossesindicatelikely/-onlymembersofthecluster.Circles arehighermass,knownorsuspectedmembersfromStauffer(1982).The Figure 1.The/versusI-Kcolour-magnitudediagram.Thisis a correctedversionoffig.5Steele,Jameson&Hambly(1993).Proper- theoretical isochronesofStringfellow(1991)(solidanddashedlines) andBurrowsetal.(1993)(dottedline)areoverlayed. © Royal Astronomical Society • Provided by the NASA Astrophysics Data System al. (1995,inpreparation),where thisindicatorofchromo- although theoverall shapedeviatessignificantly froma observations ofthecoronae clustermembers. spheric activitywillbediscussed alongwithROSATX-ray As expected,thespectraaregenerally coolinappearance, 19 95MNRAS.272. .630S blackbody duemainlytomolecularabsorptionsofTiOand VO. Thesemolecularabsorptionsmakethedeterminationof blackbody fittingwillobviouslybeinappropriate.Thebest effective temperatures(T)forourobjectsdifficult.Simple this methodisthatitrequires acompleteinfrared1-2.5¡am opacity methodofJonesetal. (1994).Thedisadvantageof relevant opacitysources.ThiswasconsideredbyKirkpatrick approach istofitamodelatmosphereincludingallofthe spectrum. Although itispossibletoobtain such spectrafor method ofdetermining is theconstantwatervapour molecular opacities,notablywater vapour.Untilthemodel approach isrestrictedbyour limitedknowledgeofsome et al.(1993).Atpresent,however,itappearsthatsuchan atmosphere approachhasbeen properlyrefined,thebest en © Royal Astronomical Society • Provided by the NASA Astrophysics Data System (seconds)(1993Nov) HHJ3 180012 Object ExposureDate HHJ2 150013 HHJ9 150013 HHJ6 120013 with theFOS.HHJobjectsare Table 2.Listofobjectsobserved HHJ12 150013 HHJ7 150012 HHJ 14460013 HHJ36 150012 HHJ23 150013 HHJ18 150013 HHJ16 150012 HHJ161 60013 HHJ139 30013 HHJ101 80013 HHJ59 150012 HHJ58 80012 HHJ54 150012 HHJ48 150012 HHJ44 120013 HHJ37 100012 HHJ 19150013 HHJ298 60012 HHJ257 40013 HHJ250 60012 HHJ229 40013 HHJ111 100012 the text. spectral standardsasdescribedin (except HHJ144).GLobjectsare Pleiades proper-motionmembers GL806 2013 HHJ435 40013 HHJ429 30013 HHJ427 40013 HHJ390 40012 GL905 3012,13 GL875.1 1513 GL829 513 GL809 313 GL65AB 3013 HHJ436 40012 HHJ430 30013 HHJ391 40012 GL905.2 1512 Low-mass starsandbrowndwarfsinthePleiades633 faint Pleiadesobjects,itisaslowprocess,requiring~4hper considered inSteeleetal.(1995,preparation). follow theproceduredevisedbyK91.Thefollowingdescrip- methods ofdeterminingeffectivetemperature(suchas objects forwhichwehavecompleteinfraredspectrawillbe object. TheapplicationofthismethodtothetwoPleiades tion program,whichcompareseachnormalizedspectrum colours, linestrengthsetc.),itisnotnecessarytohavean flux differenceforeachspectralelementbetweenthetwo with alloftheothernormalizedspectrabycomputing tion ofthemethodisanabbreviatedversiononegiven until aminimizationofthesquareddifferencesbetweenit between 7490and7510À(aregionofnear-continuum). in theirpaper. absolute calibrationofL«.Itissufficientmerelytoorderthe between 0.75and1.25(typicallyintherange~0.95-1.05) perfect. Eachspectrumisthereforemultipliedbyanumber differences foreachpairofspectra.Becausenoiseinthe The spectraarethenrunthroughaleast-squaresminimiza- spectra fromhottesttocoolest.Toorderourspectra,we and thespectrumbeingmatchedisfound. spectra, thenormalizationat7500Ámaynothavebeen spectra. Itthencalculatesthesumofsquaresthese Section 2,whichcontained,inadditiontothePleiades tion oftwomethodswasused. neighbours intermsofL«.Toorderneighbours,acombina- program wereusedtomatchspectratheirnearest comparison wasdoneusingthe‘red’spectraoverrange sample, theselectionofMdwarfspectralstandards.The ‘ocular estimation’. process fortwosubregionsofthespectra,from5000to 5000-9000 À.Theleast-squaresvaluesoutputtedbythe from theabovemethod,andassignsspectraltypestoour neighbours. Thesecondmethod,whichwasfoundtobe described byK91andcanthereforebeusedtoordernearest comparison oftheoveraU‘redness’spectraas normalization constantsforthesetworegionsallowsa Pleiades objectsbaseduponthespectraltypesofM almost asreliable,wasasimpleoverlayingofthespectrafor value of1.0at7500Àand,forthepurposesthisfigure, dwarf standards.Thespectraareallnormalizedtoaflux(F ) 7000 Âandfromto9000Â.Comparisonofthe offset byeither1.0or1.5fromeachother.Thetemperature sequence isimmediatelyapparent. HHJ36 isincluded,seeSection1),couldnotbefittedinthe candidates inI-K(oncethecorrectedphotometryof here. Itistherefore unlikelythatHHJ18isvery coolandthat until 8000Äthespectraagree well.However,between8000 than thoseinHHJ12.Above 9000Â,HHJ18doesnot HHJ18. Theadditionalopacity isnotlikelytobedueTiO other objects.Fig.4showsthespectrumofHHJ18(solid spectral sequence,itsfluxdistributionbeingunlikeanyofthe appear particularlyred,being bluerthanHHJ2andHHJ6 or VO,sincebelow8000Âthese featuresseemnostronger and 9000Âsomeadditional opacitysourceappearsin line) andthatofHHJ12(dottedoverlaid.Notehow k However, inordertoinvestigatetheefficiencyofvarious First, wenormalizethespectratoameanFof1.0 The programwasrunonthefulldatasetdescribedin The firstmethodrepeatedtheleast-squaresminimization Fig. 3plotsallofour‘red’spectraintheorderresulting v One object,HHJ18,whichisthereddestofSJH93’s 19 95MNRAS.272. .630S 634 I.A.SteeleandR.F.Jameson that theregion8000-9000Àroughlycorrespondsto unresolved multiplicityintheobject).Wedo,however,note the additionalopacityisduetosomespeciesthatonlyforms of thisobject(inSection3.5belowweconsider,andruleout, propose noexplanationfortheanomalousfluxdistribution by SJH93. undoubtedly thecauseoflargeI-Kthisobjectnoted luminous atIcomparedtotheotherclustermembers.This is at verylowtemperatures.Atpresent,therefore,wecan Cousins Iband,andthereforeHHJ18willappearunder- We canusethespectralclassificationsderivedfrom based ontherelativestrengthsofvariousatomicand proposed temperatureindicatorsforlow-massstarsas method describedabovetotesttheeffectivenessofvarious 3.2 Spectroscopictemperatureindicators both temperatureandluminosity. Thefeaturesmeasuredby molecular featurestonearby ‘continuum’whichmeasure applied tothePleiades.K91proposefourratios(AD) to carryoutthemeasurements aregiveninTable3.K91 these ratiosandtheregionsover whichthefluxwassummed ing theratios.In Fig.5,weplotthemeasured ratiosagainst naturally convertedourspectra totheseunitsbeforemeasur- calibrate theirfluxesinwavelength units(F),andwehave x © Royal Astronomical Society • Provided by the NASA Astrophysics Data System us thateventhisfeaturewillnotbetotallyreliable. terrestrial 0Bband,andthelargespreadaroundM5shows dwarf samplebyK91.Inparticular,wenotethatallofthe trends intheseplotsaresimilartothatfoundforthefieldM discriminant ofFnappearstoberatioA,basedontheCaH {not giants),providingfurtherevidencethatourPleiades objects observedlieonthelocidefinedbyK91fordwarfs sample isnotcontaminatedbydistantgiants.Thebest spectral subclass.Inallcasesthegeneralappearanceand 6975-À feature.Thisfeatureis,however,contaminatedby a of Staufferetal.(1989).Theyalsodefinedsomeuseful small sampleofpossiblePleiadesmembersfromthesurvey tudes (Oke&Gunn1983).Theseare spectral indicesintermsofspectracalibrated'mABmagni- [TiOl^ABÍTlSOÍ-A^TSóO), [TiO] =A£(7590)-A£(8120), [TiO] =AB{1590)-A£(7560), 2 [TiO] =A£(8450)-AB{S120), [VO] =A£(7450)-AB(1560). All oftheseindicesmeasurethe depthofaparticularfeature relative topseudo-continuum pointsat either 7560or 2 3 4 Hamilton &Stauffer(1993,hereafterHS93)observeda 19 95MNRAS.272. .630S © Royal Astronomical Society • Provided by the NASA Astrophysics Data System 05 CD linsisiioQ +xrq^pazi^iujo^l Low-mass starsandbrowndwarfsinthePleiades635 CO C\2 in o o o O CO O O CD ST o o 05 o m o o o CO o o o o o o CD -c o ^ o o 05 o O O I—I £ tí 0) CD -t-J r—I cd > (U 0) Ö ÖjO

Figure 3. The spectral sequence. 19 95MNRAS.272. .630S 636 /.A.SteeleandR.F.Jameson © Royal Astronomical Society • Provided by the NASA Astrophysics Data System liraisuoQ +xnx^p9Zix^uuo>i CT> o o o O O O CO O S CO o o O lO o o o Q o 05 o o CO o o o o o CO o o o o LD o o ¿3 4-> r—I OjO tí Q) (D

Figure 3 - continued 19 95MNRAS.272. .630S not attributeanygreatimportance tothisdifference. spectral sequence, especiallyforourcoolest objects.Thisis inaccurate forcoolerstars(Jones etal.1994),andsowedo pseudo-continuum index, (1987). Thistemperaturescale hassincebeenshowntobe derived fromthetemperature scaleofBerriman&Reid (i.e. T).HS93foundthat[TiO] correlatedbestwithT [PC] =AB(1560)-,4£(8120) [TiO]! and[TiO]correlatebestwithourspectralsequence spectra. FromthisfigureitappearsthattheTiOindices 8120 Â.Inaddition,HS93usedthesetwopointstodefine a &n 3&n 4 The [VO]indexappearstocorrelate verywellwithour Fig. 6plotsthederivedvaluesoftheseindicesforour McCarthy (1991). Table 3.SpectralratiosfromKirkpatrick,Henry& Ä 7020-7050Á6960-6990À D 8567-8577Á8537-8547 C 8100-8130Â8174-8204 B 7375-7385Â7353-7363 Ratio NumeratorDenominator © Royal Astronomical Society • Provided by the NASA Astrophysics Data System Figure 4.ComparisonofthespectraHHJ12andHHJ18.Seetextfordetails. Til 7358 Call 8542 Nal 8183,8193Á CaH 6975Ä Feature Measured Wavelength (À) Low-mass starsandbrowndwarfsinthePleiades637 A possibleexplanationforthis willbediscussedinSection to havelowerintrinsicspread inthemthantheratiosofK91. the cooleststarsthan[PC] index. with itslongerwavelengthbase itseemsbetteratclassifying for thehotterM6HHJ3and7.Thisispresumablydueto 3.5 below. This indexisalsoplottedinFig. 6and,asmightbeexpected, by HS93.Fortheverycooleststars(M6.5),however, very strongTiOabsorptionfeaturesinthisregion.However, the spectra.ForHHJ6andHHJ2,whichwehaveclassified index appearstostartfallagain.Thiseffectcanbeseenin with ourspectralsequence.Thisindexwasalsofoundto as M6.5,thefluxin8100-8300Âregionislowerthan correlate wellwithT(againfromBerriman&Reid1987) between 7560and8120Â,alsoappearstocorrelatewell defined, basedonthefluxatpseudo-continuumpoint around 8900-9100ÀthefluxforM6.5objectsisgreater [PC] =A£(7560)-A£(9040). as expected,sinceforverycoolstarsitiswellknownthatthe 9040 À: than intheM6objects.AsecondPCindexwastherefore strength oftheVObandsincreasesrapidly(Bessell1991). cíí 2 We alsonoteherethatingeneral theindicesofHS93seem The [PC]index,measuringthenear-continuumslope 19 95MNRAS.272. .630S 638 /.A.SteeleandR.F.Jameson have derivedallowsustotestthatresult,andalsothe the regionofHandKbands.Thespectralsequencewe effectiveness ofothercoloursastemperatureindicators. Pleiades objectswhichhave//ÄphotometrybySJH93. and thefactthatpeakofa~2500-Kblackbodyliesin result, giventhelongerwavelengthbaseofI—Kcolour rehable measureofthanR—LThisisnotanunexpected diagram, SJH93claimedthattheI—Kcolourwasamore using thetransformsprovidedinappendix2Gofthatpaper. Filled squaresareoursampleoffielddwarfs,withphoto- /-// asafunctionofspectralsubclass.Opensquaressignify On thebasisofspreadinaplottedcolour-magnitude The relativelylargespreadattheselatespectraltypes(~0.4 metry fromLeggett(1992)convertedtotheUKIRTsystem 3.3 Photometrictemperatureindicators tor. Thiscolour, however,suffersfromphotometric errors mag) maybeduetounresolvedbinaritydistortingtheI— K less, althoughatthelatesttypesthisindexbeginstosaturate. able temperatureindicators.ThescatterinI-Kisslightly mag) ascomparedwithI-K(~ ±0.12mag),sinceboththe to thelargerphotometricerror oftheR-/colour(~±0.25 not appeartosaturate.Thislarge spreadmayinpartbedue colours ofsomeobjects.ThiswillbeconsideredinSection origin. 3.5 below.R—Ihasaslightlylargerintrinsicspread,butdoes R andImagnitudesofSJH93 weremainlyphotographicin < o Fig. 7thereforeplots(a)R-/,(b)I-K,{c)H-Kand(d) From theplots,bothR-IandI-Kappeartobereason- H-K alsoappearstobeareasonable temperatureindica- © Royal Astronomical Society • Provided by the NASA Astrophysics Data System Spectral Subclass Figure 5.RatiosAtoDversusspectralsubclassforthesequence. members, orsomeothereffectwhichwilldistorttheJ-H An obviousdifferencebetweenPleiadesandfieldstars is colours isoperating. metallicity variationsexistamongstlow-massPleiades would expectthemtohavelowersurfacegravitiesthanmain- estimated photometricerror(~±0.1mag).Thereforeeither Pleiades objectsalltohavethesamemetallicity;however, will haveasurfacegravityg=2.23g©(log4.79).Jones et models, wepredictthata0.1-MobjectatthePleiadesage reflecting theirmetallicitydifferences.Wewouldexpectthe measured (~0.3).J-Hisaknownmetallicityindicator that arelarge(~±0.1mag)comparedwiththevaluebeing sample arestillcontractingtowardsthemainsequence,we age. Sincetheyoung,lowermassPleiadesobjectsinour 3.4 Spectroscopicgravityindicators spread inthisplotisgreaterthancanbeexplainedbythe (Leggett 1992),thespreadinfieldstarsthisdiagram for agravitydifferenceoffactor 10(betweenlogg=5.75 width (EW)ofthedoubletincreasing withincreasinggravity. indicator ofsurfacegravityfor coolobjects,theequivalent sequence objectsofthesameT.UsingStringfellow’s(1991) Theoretical models fromAllard(privatecommunication), gravity oftheobjectsinoursample. Mould(1978)showsthat Using thisdoublet,wecan therefore estimatethesurface g=4.50 g(logg=5.09),i.e.,~2timesgreater. al. (1994)estimatethata0.1-Mmain-sequenceobjecthas and 4.75at3500K),theNai EW changesbyafactorof1.7. o cff 0 o The Naidoubletat8183and8195Âisawell-known 19 95MNRAS.272. .630S © Royal Astronomical Society • Provided by the NASA Astrophysics Data System Figure 6.TiO,VO andpseudo-continuumindices versusspectralsubclassforthe spectral sequence. (a) (c) (a) Low-mass starsandbrowndwarfsinthePleiades639 (d) 19 95MNRAS.272. .630S in Fig.7. Figure 8.Naiequivalentwidthversusspectralsubclass.Symbolsas 640 /.A.SteeleandR.F.Jameson which shehaskindlysupplied,showthatat2800Kfor a gravity changeoffactor5(logg=4.44to5.14)theNaiEW filled. Figure 7.(a)R-I,(b)I~K,(c)H-Kand(d)/-//versusspectralsubclass.Pleiadesobjectshaveunfilledsymbols,standard are would thereforepredictachange inNaiEWof~1.1-1.2. changes byafactorof1.2.For afactor2differenceingwe Note thatforthe lowermassobjectsofM5.5 the NaiEWfor squares, whereasthefieldobjects areshownasfilledsquares. subclass. Weestimateanerror intheEWdeterminationsof ~ 1Â.AgainthePleiades objects areshownasopen I In Fig.8,weplotNaiequivalent widthversusspectral .10 .14 .18 .22 .26 .30 .34 .38 .5 1.52.53.54.55.56.5 .5 1.52.53.54.55.56.5 © Royal Astronomical Society • Provided by the NASA Astrophysics Data System Spectral Subclass Spectral Subclass Spectral Subclass with thecoloursandluminositiesofourM3.5objectshave masses ~0.35M,andthatourM5.5objectshave (since Stringfellow’s1991modelsdonotextendabove have not.UsingthemodelsofD’Antona&Mazzitelli(1985) have reachedthemainsequence,whereasobjectsofM5.5 tion givenabove.Nodifferenceisapparentfortheobjects than forthePleiadesobjects,roughlyinlinewithpredic- the zero-agemainsequence.Henceourobservationsof 0.2 M),wefindthatforthePleiadesage(70Myr)objects earlier thanM4.ThisimpliesthatPleiadesobjectsofM3.5 between M3.5andM5.5areconsistentwithaPleiadesageof Pleiades objectsnearingthemainsequencesomewhere also notethatobjectsofabove0.3Mwilllieverycloseto the fielddwarfsissignificantlygreater(byafactorof~1.3) The discussionsofarhasassumedthatalloftheobjects observed aresingle.Thisisalmostcertainlynotthecase. 70 Myrandthemassesderivedfromtheoryfortheseobjects. Duquennoy &Mayor(1991)foundthat,formain-sequence naturally affectboththephotometry andthespectroscopyof Unresolved binarity(andhigher degreesofmultiplicity)will Richichi etal.(1994)find that, forlow-masspre-main- 3.5 Multiplicity sequence objects,~49per cent ofsystemsaremultiple. G dwarfs,~53percentofsystems weremultiple.Similarly, may bemodelled, andusethephotometricmodel tomakea our objects.Inthissection,we willdiscusshowtheseeffects 0 0 0 ~0.1 M.FromD’Antona&Mazzitelli(1985,fig.1)we 0 i .62 ffi .64 .56 .58 .60 .66 .68 .72 .74 .76 .50 .52 .54 .70 .5 1.52.53.54.55.56.5 .5 1.52.53.54.55.56.5 Spectral Subclass Spectral Subclass 19 95MNRAS.272. .630S temperature scaleandbolometric correctionofBessell (1991), tocreate artificialbinariesofallcombinations of converted totheobservational (I,I—K)planeusingthe Pleiades sample. quantitative estimateofthe multiplicity ofthelow-mass We haveusedthe70-Myrmodel ofStringfellow(1991), © Royal Astronomical Society • Provided by the NASA Astrophysics Data System Figure 9.TheI,I—Kdiagramshowingcombinationsofartificial binaries andthesingle-starsequence. in LO LO CO LO LO 2.6 . 2020 ' 2003 90 ■ . 2015 . 2009 . 2010 [ 2007 . 2006 . 2005 . 1515 ' 15 2.8 . 1509 . 1510 ■ 1503 . 1508 .1507 . 1506 . 1504 . 1505 , 1010 . 1009 . 09 . 0909 ■ 1008 Low-mass starsandbrowndwarfsinthePleiades641 I —K . 0908 .1007 . 08 . 0808/0907 .1006 objects from0.2to0.01M. These areplottedonFig.9as to giveasecond, ‘binary’sequence(BS)lying -0.75mag squares. Thelabelindicates the combinationused(e.g.,a apparent thattheeffectofbinaries ontheI,I—Kdiagramis 0.09/0.04-M binaryislabelled ‘0904’).AlsoplottedinFig. 9 isthe70Myrmodelforsingle objects.Itisimmediately 0 . 1003 o I 0906/0807 . 0903 ■ 1005 1004 - 07 ■ 0905/0806/0707 . 0904 . 0803 3.2 .. 0805/0706 . 0804 ■ 0606 . 0705 . 0703 3.4 , 0704 lO LO 'st- LO LO LO CO CO LO 19 95MNRAS.272. .630S mass ratios(e.g.,‘2003’)thattheartificialobjectsmoveaway Even ifthemassesofbinariesdiffersubstantially, 642 /.A.SteeleandR.F.Jameson keeping theobjectonBS.Itisonlyincaseofextreme lower totalluminosityiscompensatedforbyareddercolour, that theBSisnotformedonlybyobjectsofequalmasses. above the‘single-starsequence’(SSS).Itisimportanttonote Figure 10.TheI,I-Kdiagram again.Pleiadesobjects(crosses)arefromSteele,Jameson&Hambly (1993).Thesingle-starsequenceis shown asasolid line, thebinarysequenceasadashed line. © Royal Astronomical Society • Provided by the NASA Astrophysics Data System from theBStogapbetweenitandSSS.Notethat combinations involvingmassesof0.01or0.02Mlievery region oftheI,I-Kplanecoveredbythisdiagram(i.e. close totheSSSandarethereforenotplotted. 14.5 2.8,/>16.1).Therefore, usingStringfellow’s with I-K>3.0,/>16.4,and fortheBurrowsetal.model 0.0767 M,butforsimplicitywewillcontinuetousethe of systemswhichcontainatleast onebrowndwarf,aswell distort coloursonthisdiagram,creatingasecond‘binary all singlebrowndwarfsystems, lieinaregionredderand sequence’ (BS).FromFig.10,wenotethatallcombinations o 0 0 0 Burrows etal.(1993)havepublishedanewsetoflow- In Section3.5,weshowedthatunresolvedbinaritycan 2 2.22.42.62.833.23.4 © Royal Astronomical Society • Provided by the NASA Astrophysics Data System Low-mass starsandbrowndwarfsinthePleiades645 I —K predicting fewer browndwarfcandidatesthan theothers. by anyofthemodellinggroups, wecontinuetohavereason- fundamental misunderstanding whichisnotyetappreciated temperature scalesforcoolobjectsorthelithiumtestitself. indicator ofbrowndwarfstatusproposedbyReboloetal. not surprising,giventheirrelativelylowresolution.Higher mass objects(exceptHHJ18,seeSection3.1above).Thisis Stringfellow’s model, becauseitisapessimistic model, well (seeFig.1andSJH93).Therefore, unlessthereissome candidates HHJ3andHHJ14byMarcy,Basri&Graham (1992). Searchesforthisfinehavebeencarriedoutour resolution spectrawouldallowustosearchforthelithium radical differencesinappearancefromthehigher(stellar) able confidenceintheirresults. Wehavecontinuedtouse In thebrowndwarfregime modelstendtoagreefairly problem withthetheoreticalmodelsofbrowndwarfs, objects arethereforenotbrowndwarfs.Thisindicates a indicating thatsignificantlithiumdepletionhastakenplace, (1994). Inallcases,thelithiumlinehasnotbeendetected, (1994), andforHHJ10byMagazzu,Rebolo&Martin apparently duetonuclearhydrogen-burning;theseparticular may notalsocontainabrowndwarf. dwarf candidates,ofwhich13aresingleandbinary.We increases thistoatotalof26systemscontainingbrown also notethatobjectsabovethe0808pointinBSmayor None ofthespectraourbrowndwarfcandidatesshows We judgetheaccuracyof models byintercomparison. 19 95MNRAS.272. .630S forthcoming paper(Steeleetal.1995,inpreparation). Bessell (1991).Wewilldiscussthisproblemfurtherinour problem. Wehavediscussedthisabove,andalsoinSJH93, Disagreement betweentemperaturescalesisagreater and forthepurposesofthispaperwehavecontinuedtouse vary considerablyfromobjecttoobject,down~100m T Tauristar(UXTau,C,LiEW~600mA).Walteretal. point outthatingeneralthelithiumtestisbasedona for many‘naked’TTauristars,whichtheyregardasthetrue (1988) showthatLiequivalentwidths(EW)forTTauristars comparison ofthelithiumlineequivalentwidthwiththata Pleiades objectsobservedbyMagazzuetal.(1994)and precursors oflowermassobjects.TheLiupperlimitforthe present atitsinitialabundance,itstillmaynothavebeen Marcy etal.(1994)was~200mÂ.Therefore,evenifLi involving Li.Webelievethatfurtherworkisnecessaryin gravities. Thismayaffecttheabundanceofatomiclithium, our Pleiadesobjects,andsohaveconsiderablylowersurface observed. Inaddition,TTauristarsaremuchyoungerthan 646 I.A.SteeleandR.F.Jameson much furtherwork(boththeoreticalandobservational)in Li, anditmaybethatconvectiontakesLitothecorefaster modelling thischemistryinstellaratmospheresbeforethe We havenotattemptedtocalibratethesetemperatureindica- They alsoseemtoberelativelyresistanttheeffectsof the faintestandreddestknownproper-motionmembersof have identifiedcanbedismissed,andnotethattheyremain this fieldisnecessarybeforethebrowndwarfcandidateswe than existingmodelspredict.Wethereforeconcludethat of ourbrowndwarfcandidatesareintherange lithium testcanbeconsideredtrulyreliable.Further,all tors, sincethisrequiresdetailedmodelatmospheresorsome binarity. Inaddition,theI—Kcolourseemstoremainasa The primaryaimofthisworkhasbeentodeterminethe the Pleiades. 0.08 >M>0.06Mandshouldthereforeeventuallyburn since highergravitiesfavourtheformationofmolecules reasonable temperatureindicatorforknownsingleobjects, especially TiO,seemtobebest.Theseratiosapparently dates. Wefindthatratiosbasedonstrongmolecularbands, optical spectraforlow-massstarsandbrowndwarfcandi- ‘best’ indicatorsofeffectivetemperaturefromlow-resolution be exploredusingthemethodofJonesetal.(1994)for a other methodofdeterminingaccuratevaluesT.Thiswill and agoodmethodforinferringprobablebinarityinothers. suffer smallerscatterthanratiosbasedonindividuallines. 4 CONCLUSIONS membership testsofSJH93inselectinglow-massPleiades in preparation). small sampleoftheobjectsinthispaperSteeleetal.(1995, faintest andreddestobjects will remaingoodcandidates motion surveyofHHJ91combinedwiththephotometric which requirefurtherinvestigation atbothopticaland possible lowersurfacegravity comparedtomain-sequence members, fromtheevidenceofbothlargerHaemissionand infrared wavelengths. candidates presentedinSJH93, andconcludethattheir objects. Wehavealsodiscussed thestatusofbrowndwarf 0 cñ The finalunknownisthevalidityoflithiumtest.We In addition,weconfirmtheeffectivenessofproper- © Royal Astronomical Society • Provided by the NASA Astrophysics Data System use ofStarlinkcomputerfacilitiesattheUniversity ACKNOWLEDGMENTS Dhillon and‘Uli’fortheirassistanceatthetelescope.Wealso the RoyalObservatoriesonbehalfofPPARC.WethankVik Leicester. 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