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1991AJ 101. . 662B + 662 Astron.J.101 (2),February1991 0004-6256/91/020662-15$00.90 © 1991Am. Astron. Soc.662 we havemadespectroscopicand photometricobservations, tions H(=m+5log//) werederivedfortheLHS lated thesearchfornewredand whitedwarfs.Reducedmo- THE ASTRONOMICALJOURNAL decreasing Htoprovideanobserving list.Overthelast10yr ofcolorclassmor. These werethenorderedin log: Luyten1979;LHSAtlas: &Albers1979)stimu- faint redandbluehighproper-motion stars(theLHSCata- discoveries. late-M dwarfstodecidehowbestfollowupthesenew cant mass.Weneedtoreassessourknowledgeoftheknown possibility thatthefaintestMdwarfsmaycontributesignifi- found largernumbersofredderstars,whichraisesagainthe hypersensitized IVNandIIIaFplatestotheirlimits,have fainter Mstarscannotcontributeenoughmass.Hawkins function turnsoveratM4stars,andthatthecooler been madewhichconcludethattheobservedluminosity reside inthefaintredandbrowndwarfs.Severalstar-count analyses (e.g.,Luyten1968;Gilmore&Reid1983)have fact thatthe“missingmass”insolarneighborhoodcould dwarfs. can considerablyinfluencethesurfacelayersinlate-M lowest temperaturesbecausetheHOopacitywastoolarge. tions isinsecurebecauserealisticmodelatmospheresarenot spheres, butthesewillnecessarilynotincludetheeffectsof Allard (1990)andKuiarecomputingbetteratmo- atmospheres forMdwarfsbuttheseareinadequateatthe yet availableforMdwarfs.Mould(1976)computedmodel ences; however,comparisonbetweentheoryandobserva- ly, andwhetherevolutionarymodelingcanpredictdiffer- can distinguishbetweenthesegroupsofstarsobservational- the “browndwarfs.”Itisimportanttoestablishwhetherwe to theMainSequence,starsburninghydrogenon dwarfs willincludestarsundergoinggravitationalcollapse sons, bothobservationalandtheoretical.ThefaintestM “ spots”nornon-LTEphenomenasuchasflares,which Sequence, andstarswithtoolowamasstoburnhydrogen, R (1986) andHawkins&Bessell(1988),however,bypushing 2 The publicationofaccuratepositionsandfindingcharts A preeminentreasonforinterestintheMdwarfsis Low luminosityMdwarfsareofinterestformanyrea- © American Astronomical Society • Provided by the NASA Astrophysics Data System in goodagreementwiththeIGtheoreticalmainsequenceofStringfellow,andfainteststarscouldbe have HainemissionandappearvariablemagnitudeR—Icolor;oneisaflarestar.Theother M5. TwostarscoolerthanVB10,andsimilarintemperaturetoLHS2924havebeenidentified;both stars areofearlierspectraltypeandresembleW359VB8.TheobservedMj,I—Kmainsequenceis with spectraltype.FeHbandsnear9900Aareclearlyseeninthespectraofalldwarfstarslaterthan various colorsforthelate-Mdwarfsareinvestigated.OfallI—Kmostreliablycorrelates the basisoflargereducedredmagnitudesfromLHSCatalogue.Halfstarsinthreefaintest Mount StromloandSidingSpringObservatories,InstituteofAdvancedStudies,TheAustralianNationalUniversity,PrivateBag, Far-red spectraandVRIJHKphotometryhavebeenobtainedforasampleoflate-Mdwarfsselectedon —0.09 reddwarfsorlowermassbrowndwarfs. 1 magbinsarelate-Mstars,theotherredstarsmetallic-hydridesubdwarfs.Relationsbetween 1. INTRODUCTION Weston CreekPostOffice,ACT2611Canberra,Australia Received 26June1990;revised30October1990 THE LATE-MDWARFS VOLUME 101,NUMBER2 M. S.Bessell ABSTRACT by Bessell(1986);thelate-Mdwarfsaresystematically R/ systemsandtheKron-Cousinssystemarediscussed tems arediscussedinBessell(1983)and

5000. 6000. 7000. 8000. 9000. 10000 6000. 7000. 8000. 9000. 10000.

LAMBDA

Fig. 5. Spectra of some M giants to illustrate the Wing spectral types.

ent. The M7 spectrum is equivalent to the dwarf M8 type of some relative-absolute continuum flux measurements; a Boeshaar. estimate is often best found from some absorption There are underlying assumptions associated with assign- line profile or line ratios from different ionization states or ing and using spectral types, which unless agreed with, or the presence of some pressure sensitive molecular species; understood, cause unnecessary confusion. The fundamental and the composition is found from relative-absolute line or properties of most stars are their temperature, effective grav- band measurements, once the temperature and gravity are ity, and composition. The temperature is best obtained from known. It should not be expected that examination of the

© American Astronomical Society • Provided by the NASA Astrophysics Data System UD UD CM \—I PQ

FLAMBDA 667 M.S.BESSELL:THELATE-MDWARFS © American Astronomical Society • Provided by the NASA Astrophysics Data System The respectivespectraareofthe GL402 +GL406:M5;GL406 GL369 (LHS274):Ml;GL382 stars GL383(LHS2231):MO; to illustratetheTiOspectraltypes. FIG. 6.SpectraofsomeMdwarfs M7. (G53-29): M2;GL352 (LHS294, W358):M4; (LHS2151 ):M3;GL402 (LHS36, W359):M6;LHS2397a: 667 1991AJ 101. . 662B light. Itcanbearguedthattheseextendedatmosphereshave 668 M.S.BESSELL:THELATE-MDWARFS spheres ofthedwarfs;however,inarestrictedwavelength little incommonwiththeextremelythinandcompactatmo- complications. AllgiantswithspectralaterthanM6arepul- dwarfs. Inaddition,the“best”spectraltype,intermsof it isinthespiritoforiginalspectraltypingthatspectra provide definitivedetailsoftheeffectivetemperature,gravi- line andbandspectrumofastaratlowresolution,over what adwarfatmosphereofthesamelocaltemperature region atmostphases,theTiObandspectrumofLPVis represented bylargeamplitudevariablesnearminimum sationally unstable,andtheM8,M9,M10classesare gion nearthefluxmaximum.TheMregimehasadditional photospheric propertiesisobtainedfromthewavelengthre- deration, shouldhavethesamespectraltypeforgiantsand that lookbasicallyalikeinthespectralregionunderconsi- ty andcomposition.Butthespectral-luminositytypeshould restricted andnotnecessarilyoptimalwavelengthrange,can of dwarfs.Itshighpropermotionmustbeerroneous. clear thatthisstarcannotbeadwarfasitshowsnoneofthe probably lookslike. would looklike.Thereareobviousadvantageswhensearch- typical ofsomecooltemperature,andthereforeindicative type forgiantsanddwarfsrefertothesametemperature,but dance anomalies.Itisnotnecessarythatthesamespectral are anypeculiarities,suchasemissionlinesorspecificabun- in amonotonietemperaturesequence,andindicateifthere dwarfs oftheolddisk(near-solarcomposition),rankthem indicate whetherornotthestarresemblesgiantsand high gravityfeaturessuchasKi,NaI,FeH,characteristic sembles anlongperiodvariableofspectraltypeM10.Itis ing forthecoolestdwarfstoknowwhatanM9orM10dwarf others. Someofthesespectra,notablyLHS2607,showa band at8691A,althoughitisconfused withTiOabsorption. cussed thespectraofFeHinstars,andpublished features dominate. sions remainwherestrongTiObandsoccur,butthedwarf have dividedthespectrumofLHS2397aby A. ThesedetailsaremoreclearlyseeninFig.9,wherewe sure), namelyCaOHat5530-5570Âand6250Â,CaH tinuum; thereisalsoalmostcertainly evidenceofthe(1,0) er thaninthoseM5andM6stars, reaching50%ofthecon- LHS2397a, the(0,0)bandofFeHat9896Âismuchstrong- an M5andaM6dwarfbetween980010200Â.In BS2748 (L2Pup),ahaloLPVoftypeM7.Slightdepres- highlights obviousspectraldifferencesduetogravity(pres- sample ofstarsselectedfromahighpropermotioncatalog,it weak foritstemperature,andthestarismetaldeficient.Ina shown; Figure8showssomemorerecentCCDspectraof dwarfs, andwillbeveryusefulfor classification,orasalate- Examination ofalltheCCDspectra at9900Âindicatesthat 6383 Aand6700-7000Â,KIat76657699À,Na halo starsfoundsofarhaveearlierspectraltypesthanM5, is tobeexpectedthatmanywillhighspacemotionstars, prominent KifeaturewhichsuggeststhattheTiObandsare FeH isadominatingfeatureof thespectraoflatest-M 8194 Â,andinthelatestspectraltypes,FeHat9900-10100 discussed byBessell&Liebert(1991). and havethusbeenexcludedfromthispaper.Theywillbe or halostars,andthusquitemetalpoor;however,mostofthe The spectrumofLHS2067isalsoshowninFig.5;itre- Comparison ofthedwarfandgiantspectrainFig.5 In Fig.7theIDSspectraofsomelate-Mdwarfsare Nordh etal(1977)andWingal.havedis- © American Astronomical Society • Provided by the NASA Astrophysics Data System bands. InFig.10weshowtwoconsecutivespectrataken blueish continuumwhichfilledinthestrongyellow-redTiO between thetwoexposures;unfortunatelythiswasnotno- also shown.Clearlythestarunderwentaflare,duringor with theIDSin1982;differencebetweenthesespectrais cal, butLHS2924isslightlyredderoverall.Probst&Liebert bands. Thusflareactivitycanproducebrighterfluxesinthe emission, althoughnotasstronginLHS2397a. here thesamephenomenainLHS2065,whichhasHa phenomena inLHS2924,whichwasthenunique,butwesee the bandsinLHS2924andLHS2065arepracticallyidenti- less yellow-redfluxthanLHS2065butweakerVObands; bert etal.(1984).Thespectralongwardof7200Áareclear- ticed atthetime.TheHaemissionhadassociatedwithita LHS2065. LHS2397ahasstrongerTiOandFeHbands plotted divisionofLHS2397abyLHS2065andLHS2924 was anoverlyingbluecontinuum.Thisisemphasizedinthe found tobefainterthanVB10(Dahnetal1986).Thespec- M dwarfsignatureinintegratedspectra. The phenomenonofflaringinMstarshasgenerallybeen the verycoolsurfacewouldalsoresultinabrighteningof trum shownwastakenfromthepublishedspectrumofLie- LHS2065, andLHS2924(Probst&Liebert1983),thestar take placeinfullyconvectivestars,butLHS2397a(M7)is about M5,becauseoftheviewthatdynamoactioncouldnot considered toberestrictedspectraltypesearlierthan violet-blue-visual continuaandthefilling-inofdeepbands. yellow-red spectrum.Onecanalsoassumethathotspotson LHS2065 andLHS2924haveweakerbands,asthoughthere ly verysimilar,butshortwardofthislimitthespectraboth follow-up observationswouldbewellworthwhile. these starsatotherepochs,andradialvelocity from startostar,butitmayalsodiffertimetime,as difficult tobecertain.The“emission”differedinintensity of thestarslaterthanM6,butlowresolutionmakesit merous earlyMdwarfs. meant thattheywerelesslikelyinthepasttobemonitored comments thatthescarcityandfaintnessoflate-Mdwarfs shaar (1990)ontheotherhandreportsthateveryspectrum VB10 doesnotexhibitemissionlinesatminimumlight.Boe- although Herbigcommentsthatunlikethehotterflarestars ties thanpreviouslyconsidered(seee.g.,Rosneretal.1985), observations indicatethatflaringoccursatfainterluminosi- an opticalflareinthearchetypalM7dwarfVB10.These inVB8 (Tagliaferricia/.1990)andHerbig(1956)observed test flarestar.However,anx-raywasrecentlyobserved much coolerandfainterthanW359(M6)theformerfain- lished forMdwarfsoverthelast 20yr.InFigs.11(a)and seen inLHS2397a.Itisimportanttoobtainspectraofall for flareactivitycomparedtothebrighterandfarmorenu- of VB10thatshehasseendoesshowHainemission.Shealso for LHS2930andLHS2924which theydiscussinthetext. (1983) andLiebertetal.(1984)discussedtheweakband 11(b) areplottedthetemperatures derivedPettersen (1984) fortheGLstarsin appendix totheirpaper,and (1980) togetherwiththosederived byReid&Gilmore Other spectratakenofLHS2397ahavealsohadfilledin In Fig.9arecomparedthespectraofLHS2397a, There havebeenseveraltemperature calibrationspub- Ha emissionwasprobablypresentinmostofthespectra 3.3 TemperatureScaleandBolometricCorrections 668 UD UD CM \—I PQ 669 M.S.BESSELL:THELATE-MDWARFS

© American Astronomical Society • Provided by the NASA Astrophysics Data System FLAMBDA 6000. 7000.8000. 7000.8000. LAMBDA dwarfs. Fig. 7.IDSspectraoflate-M 669 Oï UD UD CM ï—i h) PQ 670 M.S.BESSELL:THELATE-MDWARFS

© American Astronomical Society • Provided by the NASA Astrophysics Data System FLAMBDA LAMBDA Fig. 7.(continued) 670 CMPQ UD 671 M. S. BESSELL: THE LATE-M DWARFS 671 O\—I

LAMBDA

Fig. 8. CCD spectra of M dwarfs.

These temperatures were derived by comparing broadband reff = 8510 — 4280*(/ — AT) fluxes with blackbody distributions. We have made the VRI 2 3 photometry for these stars except the latter two which are + 1090* (/-AT) -97* (/-/T) , from Probst & Liebert (1983). R—/is most sensitive to for 2.0sun. The program with my for 0.8<Æ — /<2.1, solar magnitudes of (BVRIJHKL *) = (5.49, 4.83, 4.45.

© American Astronomical Society • Provided by the NASA Astrophysics Data System UD UD CM \—I PQ 672 M.S.BESSELL:THELATE-MDWARFS

© American Astronomical Society • Provided by the NASA Astrophysics Data System FLAMBDA Fig. 9.CCDspectraofthecoolestknown late-Mdwarfs.Thespectraontherightareresultsofdivisionfluxes asindicated. LAMBDA 672 CMPQ UD 673 M. S. BESSELL: THE LATE-M DWARFS 673

\—I

LAMBDA

FIG. 10. IDS spectra of LHS2397a. The left and center spectra were taken 10 min apart. The right spectrum is the arithmetic difference between the two.

o OQ

R-i R-I

l-K l-K Fig. 11. Temperature calibrations for M dwarfs by Wing ( W ), Veeder ( V ), Reid, and Gilmore & Petterson. The points are some individual tempera- tures derived by Reid & Gilmore ( 1984) and Petterson ( 1980). FIG. 12. The bolometric correction to 7C vs /? — / and I — K diagram.

© American Astronomical Society • Provided by the NASA Astrophysics Data System 1991AJ 101. . 662B ing BC-colorrelationswereadopted: 4.11, 3.67,3.34,3.30,3.25)yieldedM-4.76.Thefollow- 674 M.S.BESSELL:THELATE-MDWARFS tude plane. above temperaturecalibrationsitispossibletoconvertthe These relationsareverysimilartothedataofReid&Gil- theoretical HRdiagramstotheobservationalcolor-magni- more (1984).Usingthesebolometriccorrectionsandthe brighter membercouldbeplacedontheuppermainse- by Dahnetal(1986)andWeis(1984),exceptthosefor quence. TheparallaxesarefromUSNOlistsassummarized and JHKphotometry,plussomebinarystarswherethe bol will notdiiferbyasmuchtheydofortheearlierdwarfs. addition, theR—/colorsoflate-typehaloandolddiskstars will befoundamongthefaintestlate-typedwarfs,andin Because oftheirrarity,itislesslikelythatextremehalostars known tobeatleast1magfainterthanthatfortheolddisk. and olddiskstarsinthesample;halomainsequenceis than fortheearlierdwarfs.Muchofscatter points aboutthelineismuchlessforlate-typeMdwarfs fainter M7starsat~Af=15.Theintrinsicscatterofthe quence forstarsbrighterthan=12wastakenfromWeis LHS1070 fromlanna&Bessell(1986).Themeanmainse- kindly providedbyConardDahn,andthoseforLHS288 LHS2924 andLHS2397a,whichwereprovisionalvalues dwarfs resultsfromthemixofmetal-deficient(halo)stars (1984); thiswasfittedbyapolynomialthroughtothe 7 3.4 TheObservedColor-MagnitudeandTheoreticalHR BC =0.96-0.126*(/-TO BC/ =0.70—0.21*(i£/) In Figs.13and14weplotdataforparallaxstarswithRI Models oflowmassstarspublishedbefore1985showed © American Astronomical Society • Provided by the NASA Astrophysics Data System 7 Fig. 13.TheMjR—Idiagramfor Mdwarfs. 23 2 for 2.0luminosity bins. stant, asstarsinequalmassbinsarespreadacrossmore in adramaticflatteningorturnovertheluminosityfunc- off thebottomofdiagram.Theeffectextension calibrations tolowertemperatures,themain-sequencemod- I —KdiagramascanbeseeninFig.14,wherethemain- the effectivetemperatureofmodelswerehotterthan ferent equationofstateandmolecularopacities.Formasses zitelli (1985)publishednewmodelscomputedusingadif- some ofthemcouldbebrowndwarfs,althoughVB8and tion eveniftheslopeofmassfunctionweretostaycon- the mainsequencetoextremelyfaintluminositieswillresult el for0.08^wouldlieatM=18.4and/—AT6.1,just observations. stars, aresultconfirmedbyStringfellow(1986);however, quence extendedtowellbelowtheluminosityofanyknown ted. Theageofthesemain-sequencemodelsrangesfrom1to city areinexcellentagreementwiththeobservedMjvs also containupdatedphysicsandconsidertheeffectsofvar- cantly fainterthantheearlieronesandinfactmainse- sequence modelsfor0.20,0.15,0.10,and0.09^areplot- ious grainopacities.ThemodelscomputedwiththeIGopa- 3.7 X10yr.Extrapolatingthetemperatureandbolometric between thecontractiontrack ofa0.08^#modelwhich low andd’Antona&Mazzitelli isthattheevolutionary 8 lar, therearenosignificantdifferences intheoverlapregion tended mainsequenceofhydrogen burningstars.Inparticu- tracks ofbrowndwarfsasthey contract willoverlaptheex- 7o o7 o o tion timesof10yrand4Xareindicated. tionary tracksfora0.08^dwarfand0.07^browndwarf;contrac- sequence ofStringfellow(1989)isshown(+),togetherwithevolu- Fig. 14.TheM,vs,I—KdiagramforMdwarfs.theoreticalmain o Recent theoreticalmodelsofmain-sequencedwarfshave The otherimportantprediction ofthemodelsStringfel- 3.5 TheFaintestStars-BrownDwarfs? 674 1991AJ 101. . 662B 9 7 9 8 9 8 in aHubbletime.Thetimescaletoreachgivenluminosity not stabilizebutcontinuetocool,reaching100timesfainter and thetrackofa0.07^browndwarfmodelwhichwill will eventuallystabilizeatMj=18.4afterabout2.10yr ing absolute/magnitudesof12.6and12.3,respectively,in is alsoverysimilar;theO.O8~#and0.07c^modelsreach- brown dwarfwhileithastheluminosityandtemperatureof parts ofthesetracksareplottedinFig.14. main sequenceandthechanceofdetectingaverylowmass likely tobefoundinthevicinityofhydrogenburning only theyoungestandmostmassiveofbrowndwarfsare of stateandotherphysicalparameters,butitisclearthat the browndwarfscontract(thecoolingtimescalevaries least 10timesyounger.Thelowerthemassmorequickly sequence starswithmassesbetween0.10and0.09^ dwarfs orreddwarfs.HBfoundthatthereddest(andfain- a normalMstar,isthereforelow. uncertain becauseofinadequateknowledgetheequation model afteronly2x10yr.Thesetimescalesaresomewhat cooling totheluminosityofO.O9^main-sequence roughly asthesquareofmass),a0.02~#browndwarf ages about2-4X10yr,orcontractinglowermassstarsat 675 M.S.BESSELL:THELATE-MDWARFS than theearlyMdwarfs.Fromlowervelocitydispersion this resultasindicatingthatthereddeststarswereyounger smaller motionsthanthebluerMdwarfs,andinterpreted test) starsintheirredstarsamplehad,ontheaverage, of thereddestdwarfs,HBestimateanageabout5X10yr compared to3X10yrforthebrighterMdwarfs.lanna& I —Kcolor;thelinesenclosingmajorityofstarshave H forthesampleofstarsinTable1areplottedagainsttheir fainter thanLHS2924,andverifyingtheextremenature survey givingitanabsoluteKmagnitudeof10.8,0.3mag Bessell (1991)measuredadistanceforthefainteststarin inferred fromthephotometryofHB. 10 yrand15.615.4,respectively,in4.10yr.Theinitial velocities thanthebluerstars.InFig.15reducedmotions vey ofLHSstarsarealsoconsistentwiththeirhavinglower o 0o (HB) inajR/colorsurveymayfactbecontractingbrown o 0 o r smaller motionsthantheothers. diagram forthestarsinTable1.The reddestdwarfsappeartohave Fig. 15.ReducedmotionsforLuyten redmagnitude{H)\sI—K R We seethereforethatstarslaterthanM6couldbemain- Some ofthestarsfoundbyHawkins&Bessell(1988) The propermotionsofthereddeststarsfoundinsur- © American Astronomical Society • Provided by the NASA Astrophysics Data System been drawnwiththesameslopeastheMvsl—Krelation. motions implyingthattheyhavesmallerpropermotions. These starsclearlydonotlieonanextensionoftheaverage relation definedbythebluerstars;theylieatsmallerreduced the M5andlaterclassessothatdwarfsgiantswith have beenobtained.Spectraltypesproposedfor been discoveredhavesmallpropermotionsandaretherefore VO bandsinthered.ThespectraofMdwarfsareclearly same MspectraltypewillhavesimilarappearanceTiOand tions andluminositiestoseewhetherthisobservationistrue is importanttofindmorefaintstarsandestablishtheirmo- lutionary contractionmodelsarecorrect,onewouldhave inferred tobeyoungcontractingstars.Ifthetheoreticalevo- which appearsatM5andstrengthensinlatertypes.Strong namely MgHinKstars,CaHearlyMdwarfsandFeH distinguishable fromspectraofgiantsbythehydridebands, to comparethesewiththetheoreticalisochrones. the lowermainsequenceinclusterswithdifferentagesand masses lowerthanO.1(L#.Itisalsoimportanttoobserve are old,thenonemustquestiontheinteriormodelsfor for alargersample.Iffewofthereddestandfaintestdwarfs expected theretobeamixtureofagesattheseluminosities.It because thetailonCCD/responseenablestobrighten R —Icolorappearstobebetterbecausephotographic that R—I(measuredphotoelectricallyatleast)mayreach old diskcompositionstarsfromM0toM6,butitappears FeH, andKI. ing ofTiOincomparisonwiththestrengthMgH,CaH, deficient Mdwarfsaredistinguishedbytherelativeweaken- lines ofKIandNaiarealsoveryevidentinMdwarfs.Metal a plateauforthelaterspectraltypes.(Thephotographic bluer. IRcolorssuchasJ—HandH—K&reusefultodis- late Mdwarfsmayalsobeaffectedbyspotsandplageactiv- more inthereddestdwarf.)TheR—Icolorsofsome so seriously.TheCCDR—Icoloralsocontinuestoredden does nothavethelongredtailofRthatoverlaps/band be inoperative. tinguish betweengiantsanddwarfsbutareaffectedbyme- ity whichweakenthebandsinVandRmakecolors R I —Kdiagram(forM4andlater)arewelldefinedindicat- tion surveyappeartohaveHaemission.Abrightflarewas types isI—K. tallicity andneedtobemeasuredwithprecisionforaccurate tions ofReid&Gilmore(1984), therecenttheoreticalHR metric parallaxes. ing thatthesecolorscanbeusedtomeasureaccuratephoto- fully convectivestarswheredynamoactionwasbelievedto ered. Thisisanotheroccurrenceofflareactivityinverycool, observed inLHS2397aoneofthefaintestMdwarfsdiscov- spectral typing.ThebestcolortouseforthelateM diagram ofStringfellow(1989) wasfoundtobeinexcellent agreement withtheobservedMjvsl —Kcolor-magnitude ilar magnitudesandcolorstothe reddestMdwarfsfoundin dwarfs withmassesbetweenO.O6^# andO.O8^hadsim- diagram. Stringfellow’sevolutionary tracksforredorbrown 0 c 0 The colorsandspectraofasamplethereddestdwarfs R —IandVaregoodindicatorsofspectraltypein It ispuzzlingthatalmostallthereddestdwarfshave Using thetemperatureandbolometric correctioncalibra- The MjvsR—Idiagram(forM0toM6)and Most ofthelate-Mdwarfsfoundinhighproper-mo- 4. CONCLUSIONS 675 1991AJ 101. . 662B 8 the LHScatalogsurveysandinHawkins&Bessell be found,moreparallaxesneedtomeasuredandbetter (1988) photometricsurvey,andwereconsistentwiththe spectra needtobeobtainedforradialvelocities,andlook dwarfs shouldbefollowedup.Moreredstarsneedto Hawkins &Bessell(1988)areyoungandmassivebrown ing boundaryconditionsprovidedbydetailedmodel-atmo- sphere analysesofthelate-typedwarfsdescribedhere. retical modelingofmain-sequencestarsisalsorequired,us- computed byAllard(1990)andKui(1990).Bettertheo- pected toresultfromnewmodelatmospheresthatarebeing More reliabletemperaturesforthelate-Mdwarfscanbeex- to checkthemass-radiusandmass-luminosityrelations. binary systems,isneededforlowmass(^<0.1^)stars dwarfs. Morebinarydata,frombotheclipsingandvisual more fundamentaldataonradiiandmassesoffaintM the faintestMdwarfs.Itisclearlyveryimportanttoobtain servation andtheoryliewiththetemperaturesmassesof estimated ageof~5X10yrfromtheirvelocitydispersions. Allard, F.1990,Ph.D.thesis,InstitütfürTheoretischeAstrophysik,Uni- Bessell, M.S.1986,PASP,86,1303 Bessell, M.S.1983,PASP,95,480 Bessell, M.S.1982,PASP,4,417 Berriman, G.,andReid,N.1987,MNRAS,227,315 676 M.S.BESSELL:THELATE-MDWARFS Johnson, H.R.1965,ApJ,141,170 Boeshaar, P.C.1990,privatecommunication Boeshaar, P.C.1976,Ph.D.thesis,TheOhioStateUniversity,Columbus, Bessell, M.S.,andWood,P.R.1984,PASP,96,247 Bessell, M.S.,andLiebert,J.1991,inpreparation Bessell, M.S.,andBrett,J.1987,PASP,100,1134 Bessell, M.S.1990,A&AS,83,357 lanna, P.A.,andBessell,M.S.1991,inpreparation Herbig, G.H.1956,PASP,68,531 Hawkins, M.R.S.1986,MNRAS,223,845 Eggen, O.J.1980,ApJS,43,457 D’Antona, F.,andMazzitelli,I.1985,ApJ,296,502 Cousins, A.W.J.1980,S.Af.Astron.Obs.Cire.,1,166 Boeshaar, P.C.1985,privatecommunication Bessell, M.S.,andWeis,E.W.1987,PASP,99,642 Luyten, W.J.1979,LHSCatalogue(University ofMinnesota,Minneapo- Luyten, W.J.1968,MNRAS,139,221 Liebert, J.,Boronson,T.A.,andGiampapa, M.S.1984,ApJ,282,758 Lacy, C.H.1977,ApJ,218,444 Kui, R.1990Ph.D.thesis,MSSSO,TheAustralianNationalUniversity Kron, G.E.1952,ApJ,115,301 lanna, P.A.,andBessell,M.S.1986,PASP,98,658 Hawkins, M.R.S.,andBessell,S.1988,MNRAS,234,177 Glass, I.S.1974,MNASSAf,33,53 Gilmore, G.,andReid,N.1983,MNRAS,202,1025 Eggen, O.J.1979,ApJS,39,89 Eggen, O.J.1976,ApJ,204,101 Eggen, O.J.1968,ApJS,16,49 Dahn, C.C.,Liebert,J.,andHarrington,R.S.1986,AJ,91,621 Mould, J.R.1976,A&A,48,443 Luyten, W.J.,andAlbers,H.1979,LHS Atlas(UniversityofMinnesota, 0 versität Heidelberg Ohio Minneapolis) lis) Finally, thepossibilitythatreddeststarsstudiedby The biggestuncertaintiesinthecomparisonbetweenob- © American Astronomical Society • Provided by the NASA Astrophysics Data System REFERENCES brown dwarfsuspectsandothercoolMdwarfs.Thephoto- for evidenceofanyspectroscopicdifferencesbetweenthe search forveryredobjects. 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