19 94MNRAS.210. .4 99B A llr23 415 6 5 3 2 6 1 A ROSATsurveyofhotDAwhitedwarfsinnon-interactingbinary Max--InstitutfürExtraterrestrichePhysik,Giessenbachstrasse,D-8046Garching,Germany M. A.Barstow,f^:J.B.Holberg,*T.Fleming,C.Marsh,*tD.Koester Mullard SpaceScienceLaboratory,DepartmentofandClimatePhysics,UniversityCollegeLondon,HolmburyStMary,Dorking,Surrey Accepted 1994April25.Received20;inoriginalformJanuary5 InstitutfirTheoretischePhysikundSternwarte,Olshausenstrasse,Physikzentrum,Kiel,Germany StewardObservatory,UniversityofArizona,Tucson,AZ87521,USA LunarandPlanetaryLaboratory,UniversityofArizona,Gould-SimpsonBuilding,Tucson,AZ85721,USA RH56NT and D.Wonnacottf systems Mon. Not.R.Astron.Soc.270,499-515(1994) DepartmentofPhysicsandAstronomy,UniversityLeicester,Road,LeicesterLEI7RH This isperhapsnotsurprising,astheyhavemostlybeen The majorityoftheknownwhitedwarfsareisolatedstars. trum ofanycompanionstartypeKorearlierwillcom- identified throughopticalbluestarcolourandpropermotion dwarfs inunresolvedbinarysystems.Forexample,thespec- exists awell-knowninherentbiasagainstdetectionofwhite undetectable atopticalwavelengths.Evenifthecompanion blue object.Indeed,wereitnotfortheproximitytoEarth pletely dominatethatofthewhitedwarf,rendering it surveys (e.g.Green,Schmidt&Liebert1986).Thusthere colours, therebypreventingthewhitedwarfappearingas a is anMstar,thecompositespectrumlikelytoredden 1 INTRODUCTION (UVS). (SAAO). ÍGuest ObserverattheSouthAfrican AstronomicalObservatory fGuest ObserverwiththeVoyager UltravioletSpectrometers satellite. * GuestObserverwiththeInternational UltravioletExplorer{IUE) © Royal Astronomical Society • Provided by the NASA Astrophysics Data System ABSTRACT A numberofnewnon-interactingbinarysystemscomprisingawhitedwarfplus the binaries,isdiscussed.Thesebinarysystemsareimportantrepresentativesof temperatures andgravitiesofthewhitedwarfs,comparetheiratmospheric normal stellarcompanionhavebeendiscoveredbytheROSATX-rayandEUVsky Key words:stars:atmospheres-binaries:generalwhitedwarfsultraviolet:stars tion ingeneral,andbinariesparticular. phases. Theirfurtherstudywillbeofgreatimportanceinunderstandingstellarevolu- emission isdetectedinseveralsystems,anditsnature,thelightoflikelyage ible toestimatehowmanyhotwhitedwarfsintotalresidebinarysystems.Coronal several possibleevolutionarypaths,includingcommon-envelopeandmass-transfer compositions withthepopulationofisolatedwhitedwarfs.Asaresult,itmaybeposs- surveys. Wediscusstheidentificationofninetheseobjects,determine X-rays: stars. 1 hidden whitedwarfcomponent. Anumberofsuchdis- through detectionofEUV and X-rayemissionfromthe tunity tosearchforhotwhite dwarfsinunresolvedbinaries white dwarfcomponent,wouldthenbecompletelyhidden. could notberesolved,andthevisiblelightfromSiriusB, of theSiriusbinarysystem(distance2.64pc)twostars types, fromA to M,havealreadybeenpresented inseveral coveries withcompanionscovering alargerangeofspectral and Poundsetal.1993,respectively) havegivenusanoppor- found intheIUEspectraofanumberstars-ÇCap Young 1970).Thespectralsignaturesofothershavebeen dwarf inthewell-studiedsystemV471Tauriwasrevealedas Several unresolvedbinarieswithhotwhitedwarfcom- a resultofitseclipsebytheKdwarfprimary(Nelson& HD 27483(Böhm-Vitense1993).However,asystematic Neff &McClure1984),4oOri(JohnsonAke1986)and (Böhm-Vitense 1980),56Peg(Schindleretal.1982;Stencel, panions havebeendiscoveredserendipitously.Thewhite ( 1989)revealednofurtherwhitedwarfcompanions. search ofthethen-existingIUEarchivebyShipman&Geczi The ROSATX-rayandEUV skysurveys(Triimper1992 19 94MNRAS.210. .4 99B et al.1993c;Hodgkin1993;Landsman,Simon& papers (e.g.Flemingetal.1991;Cooke1992;Barstow Therefore, inmostcases,thewhitedwarfisprobablyless binary systemswithcompanionsintherangeAtoK,exclud- Bergeron 1993;Tweedyetal.Wonnacott,Kellett& Approximately 120isolated,hotwhitedwarfshavebeen WHITE DWARFSINUNRESOLVEDBINARIES will refertothewhitedwarfassecondary,anditscom- we donot,ingeneral,knowthemassratioforthesesystems, the ROSATX-rayandEUVfluxesmaybeusedtodeter- tive temperaturesandgravitiesfromtheLymanlineprofiles panions byninesystems.Asrelativelyfewsuchsystemswere ing theDA+Mpre-CVsystemswhichwewillconsider Stickland 1993).Hereweconcentrateinparticularonthose detected ineithertheROSATX-rayorEUVall-skysurveys, panion starastheprimary. massive component.Forthepurposesofdifferentiation,we the companionstowhitedwarfsareallKorearlier. gen orcontainanyadditionalsourcesofopacity.Although mine whetherthewhitedwarfatmospheresarepurehydro- a systematicsurveyoftheseobjects,determiningtheireffec- from thewhitedwarf,whichmaybetoocool(T=23000K) In particular,theratioofWideFieldCamera(WFC) of theEUVandX-raypropertiesthesestarswithnormal logued asdegeneratestars(Flemingetal.1993).Comparison and morethan50percentofthesearepreviouslyuncata- and UVcontinuum.Knowingthetemperaturesgravities, Böhm-Vitense (1993),isanEUVsource(Poundsetal. ingly, theHyadestriplesystemHD27483,observedby separately. TheROSATdiscoverieshaveenlargedthetotal survey S2andSIfiltercountratestypicallyexceedsafactor soft spectra,whentheinterstellarHicolumnisrelativelylow. to bedetectedatthatdistance,ortheF6Vcompanions. already known,thisisahighlysignificantincrease.Interest- sample ofhotwhitedwarfswithunresolvedAtoKcom- stars andothersourcesshowsthattheyhaveverydistinctive 2 SELECTIONANDIDENTIFICATIONOF 500 MA.Barstowetal 3. Secondly,theEUVluminositiesofthesewhitedwarfsare 1993), butitisfarfromclearwhetherornottheemission Using IUEandVoyagerobservations,wehavecarriedout © Royal Astronomical Society • Provided by the NASA Astrophysics Data System o Table 1.Physicalparametersofthecompanionstars. RE2353-702 HD223816 F5IV/G0 9.9115-330 RE2300-070 HD217411 G5 9.887 RE2126+192 HR8210(IKPeg) A8m 6.0743-54 RE1925—566 G2-G810.6 100-153 RE1111-224 ßOrtA1V/A2III4.48 19-87 RE0515+324 HD33959CF4V7.95 89 RE0459-101 HR1608K0IV5.38 38-54 RE0044+093 BD+08102G10.0 66-132 RE No.Cat.NameSpectraltypeVmagnitude d est.(pc) RE0228-611 HD15638F3-F6V8.8 110-174 (1993); (4)Fleming etal.(1991);(5)Wonnacott,Kellett &Stickland(1993). Refs. (1)Barstowetal.(1993c);(2) Landsman,Simon&Bergeron(1993);(3)Hodgkinetal. be ausefuldiscriminator.Anadditionalmeansofconfirming However, thedistanceofanobjectmustbeknownforthisto generally greaterthaneventhemostactivelate-typestars. tional counter(PSPC)data.Forwhitedwarfs,allthephotons the presenceofawhitedwarfsourceistostudypulse- height spectrumoftheROSATposition-sensitivepropor- lie belowthe0.28-keVcarbonKedgeofcounterwin- extending tohigherenergies. dow. Incontrast,allotherobjectsgenerallyhavespectra binary natureisevidentincompositeopticalspectra,show- nature. ForwhitedwarfswithcompanionslaterthanK,their possible todiscriminatebetweenthetwostarsusingfar-UV panions ofearlierspectraltype,thewhitedwarfcomponent ing theHBalmerabsorptionseriestypicalofDAwhite survey data,furtherobservationsareneededtoconfirmits cannot bediscernedintheopticalspectrum.However,itis dwarfs andTiObandsfromtheMstarcompanion.Forcom- EUV/X-ray sources.Thereforeanysource calibration phaseoftheROSATmissiontoquantifyanyUV (see referencesinTables1and2). already beenidentifiedinthiswayusingIUEobservations leak inthefilters,demonstratethat,general,theyarenot spectra. Severalofthewhitedwarfsinthissamplehave their ownright,andfindingthosewithwhitedwarfcom- these onthegroundsofwhitedwarf-likeEUVcoloursand of companionwhichisresponsiblefortheEUV/X-rayemis- While itisclearthatanyAstar‘source’musthavesomekind apparently associatedwithanAstarisofimmediateinterest. those objectswherethecoloursarebiasedbyEUVflaresin chance associationswithcataloguedlate-typestars,and . Aftereliminatingfieldwhitedwarfswhichare panions ismoredifficult.Initially,weselectedasampleof the companion,theirvisualmagnitudesandreferencesto far intheROSATskysurvey,indicatingspectraltypeof tem HD33959C,publishedearlier(Hodgkinetal.1993). the S2filter,sevenstarswithspectraltypeinrangeFtoK sion, starsofspectraltypeF-Kcanbeintrinsicsourcesin appear tohavewhitedwarfcompanions,includingthesys- earlier publications.Onthebasisofthisinformation,an When alikelywhitedwarfsystemhasbeenfoundinthe Deep exposuresonbrightAstars,performedduringthe Table 1liststhenewwhitedwarfbinariesdiscoveredso References 1,2,5 1.3 1.4 1,2 1 19 94MNRAS.270. .4 99B the spectraltypeofprimaryisuncertain,arangepos- is amain-sequencestar.Iftheuncertaintyinspectraltype estimate ofthedistancetoeachsystemisprovided.Where MEASUREMENTS FROMIUEANDVOYAGER is notquantified,norangegiven. Apart fromverificationofthepresenceahotwhitedwarf DATA sible distancesisgiven,makingtheinitialassumptionthatit 3 TEMPERATUREANDGRAVITY perature andgravityofthewhitedwarfbyfitting companion, theIUEdatacanbeusedtoestimatetem- using theLymanainformationaloneisthatitcangive has beenwidelyused(e.g.Holberg,Wesemael&Basile continuum tosyntheticwhitedwarfspectra.Thistechnique observed LymanaprofileandtheuncontaminatedUV yields lowervaluesforTandloggof49000±2000K Lyman aprofileofthewhitedwarfHZ43are57500Kand perature andlogsurfacegravity(logg)inferredfromthe slightly ambiguousresults.Forexample,thebest-fittingtem- there isnopossibilityofusingtheBalmerlineprofilesfor 7.7 ±0.2,respectively(Napiwotzkietal.1993).Thelogof have obtainedobservationswiththeVoyager2ultraviolet measurement ofTandlogg.However,wherepossible,we 8.5, respectively.Incontrast,fittingtheBalmerlineprofiles IUE observationsusedinthispaperisgivenTable2. 1986) onisolatedwhitedwarfs.Onepotentialproblemwith work, istheregionspanningLyman/?lineat1026À. spectrometer (UVS),whichcoversthewavelengthrange from theLymanßprofileand thehigherlevelunresolved useful informationmaystillbe gainedregardingTandlogg Although thespectralresolutionofUVSis,at20À,sub- lines intheLymanseries.The logofVoyagerobservationsis included inTable2. stantially poorerthanthelow-dispersionIUEmode(«6Â), 500-1700 À.Ofparticularinterest,inthecontextofthis program xspec (Shaferetal.1991)tocompare synthetic dwarf inthesample,wehave usedthespectral-fitting Clearly, inbinarysystemssuchasthoseconsideredhere, To determinetemperatureand gravityforeachwhite © Royal Astronomical Society • Provided by the NASA Astrophysics Data System Table 2.LogofIUEandKoyagerobservations. ß Crt Name HD33959C HR1608 BD+08°102 HR8210 (IKPeg)45291 RE1925—566 HD15638 HD223816 45962 HD217411 46146 IUE SWPno. 42753 42752 42554 45960 45396 49048 46680 46678 91/268 92/007 92/290 92/234 92/218 92/145 92/145 92/008 92/311 93/303 Date 92/290 720 3,000 4.500 3,000 50(L), 150(S) 540 2.500 900 Exposure (s) 100(L), 300(S) 1,200 1,200 19-2 172 model atmospheresspanningatemperaturerangefrom lates achi-squaredstatisticforthefitbetweendataand for IUEdatadonotprovideanestimateoftheerrors spectra foragridoffullyline-blanketedLTEpure-hydrogen model, whichitthenattemptstominimizebyincremental involved indeterminingtheUVfluxagivenspectralbin. 20 000to100K,andloggfrom7.09.0.xspeccalcu- the errorsinIUEdata.Typically,theseareapproximately Hence wehaveusedthegeneralscatterinfluxvaluesfroma counting instrument,andfluxerrorsforeachchannelare steps inthefreeparameters.Thestandardextractiontools Holberg 1990). determined straightforwardlyfromthephoton-counting smooth curvepassingthroughtheIUEspectrumtorepresent account oftheuncertaintiesininstrumentcalibration(e.g. statistics andbackgroundsubtraction.Wealsoincorporatea Lyman aprofilessimultaneouslywithpartoftheUVcon- by theprimarystar.Inmostcases,Lymanaprofileis tinuum, outto~1400Ä,whichisgenerallyuncontaminated usually onlytheouterwingsoffeaturecanbeused.We 10 percentforeachobject.TheVoyagerUVSisaphoton- the absorptionprofilesbecause,ingeneral,thiswillbe but theircontributionissignificantlylowerthanthetypical is affected.Withhighercolumnsofafewx10cmthe do nottakeaccountofanypossibleinterstellarcomponentin significantly contaminatedbygeocoronalradiation,and 10 percentsystematicerrorintotheVoyagerdata,totake uncertainties intheobservedfluxes(~10percent).Unlike interstellar linebecomessaturatedandhasdiscerniblewings, negligible. Napiwotzkietal.(1993)notethat,withaninter- stellar columnof7x10cm'forHZ43,onlythelinecore contributes substantialfluxthroughoutthespectralrangeof A2). Onlyattheextremeshort-wavelength limitdoestheA all theotherbinaries,primaryinßCrtsystem trum carefully. star fluxapproachzero(Fig. la). Consequently,itisneces- IUE, becauseofitscomparatively earlyspectraltype(Al/ sary tosubtracttheAstarcomponent fromtheIUEspec- 2). Twospectra, SWP44752and 44753,were In ourTandloggdeterminationwefittheLyman/Land ß CrtwasobservedwithIUE on1992May24(seeTable Hot DAwhitedwarfsinbinarysystems501 V2 Obs.DateExposure(s) 92/336 93/054 92/350 93/052 92/355 22,590 23,581 32,071 63,127 23,834 19 94MNRAS.210. .4 99B CM ^ o LOm 502 M.A.Barstowetal. X ZlOl (VSLÜ0/sßj9) z © Royal Astronomical Society • Provided by the NASA Astrophysics Data System 2i 0L K) y x (vsüjo/s6j0)j s CM O y lO o ^ Q) LO d ° ? o > (D CD c en O > CD Ï.S ä it a H .2f d>> ^ cd § 1 ^ - PL- ^I 05. cd^ ^ oö fc- td^¡2 d S. 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© Royal Astronomical Society • Provided by the NASA Astrophysics Data System 19 94MNRAS.210. .4 99B uncertainties intheseanalyses.Table5liststhewhitedwarf ered here.Figs2(a)-(i)showthebest-fittingmodelsoverlying masses andradiiappropriatetothevaluesofloggconsid- the observedspectraforthoseparametersthatyieldan such asthoseofWood(1992),aresmallcomparedtothe & Salpeterradiiandvaluesfromevolutionarycalculations, binaries. ment withthedistancetoprimary.Innocaseistherea estimate ofthewhitedwarfdistancewhichisinclosestagree- concerned, itiscrediblethatallsevensystemsare,infact, contradiction betweenthedistancesderivedforprimary and secondary,implyingthat,asfarthefluxlevelsare Liebert (1992).Althoughoursampleissmall,lessthan1/10 white dwarfshasbeencarriedoutbyBergeron,Saffer& Bergeron, Saffer&Liebertdataitwouldappearthat,ina with eitherveryloworhighmass.Interestingly,fromthe the sizeoftheirs(129stars),generaldistributionis binary systemscommonlytransfermassbetweenthecom- which isscarcelysurprisinginlightofthefactthatclose tion ofthewhitedwarfsinbinariesislesspeaked,aresult not likely.Thismaybeanindicationthatthemassdistribu- group ofninestars,tofindtwoat0.3andone1.2M©is have temperaturesabove40000K,intheregimewhere temperature rangefromthelowestlimitfordetectableEUV ponents aspartoftheirnormalevolution.Thegroupspansa similar, withmostobjectsintherange0.5-0.7M©andfew indication ofhowthepresenceacompanionaffects white dwarfs(seeBarstowetal.1993b)mayprovidean group andthosestarsincludedinoursampleofisolated pure Hatmospheres.Acomparisonbetweenthebinary cooler andmightthereforebeexpectedtohavemoreorless atmospheres ofisolatedwhitedwarfs.Theremainderare and X-rayemissionuptoaround70000K.Threeobjects composition ofthewhitedwarfatmosphere. significant quantitiesofheavyelementsarepresentinthe X-RAY INSTRUMENTS X-ray fluxesasanindication ofthelevelphotospheric In anearlierpaperontheatmosphericcompositionofwhite dwarfs fromtheROSATsurvey,weusedEUVand OPACITY WITHTHEROSATEUVANDSOFT 4 DETERMINATIONOFATMOSPHERIC H atmosphere(Barstowetal. 1993b).Thisexerciseisonly opacity bycomparingthemwith predictedvaluesforapure carried outfor the binarywhitedwarfs. is knownand,withtheresults from Section3above,itcanbe possible oncethetemperature andgravityofthewhitedwarf A detailedstudyofthemassdistributionisolatedDA Table 5.Zero-temperaturelogg,massandradius. 9.0 8.5 8.2 8.0 7.5 7.0 logg © Royal Astronomical Society • Provided by the NASA Astrophysics Data System 0.83 0.68 0.15 0.55 0.30 Mass {Mq) 1.18 0.017 0.021 0.006 0.009 0.011 0.013 Radius (Ä©) -124 2 discussed extensivelyelsewhere(e.g.Barstowetal.1993b). PSPC atthefocalplanecoversenergyrange0.1-2.4keV Here wejustsummarizetheessentialdetails.Thesurveydata fully exploitedinpreviousstudiesofwhitedwarfs.Wehave (5.2-100 Â).TheWFCemployedtwofiltersduringthe cover threedistinctwavebands.TheX-raytelescopewiththe PSPC hasamodestenergyresolution,whichwehavenever PI andcoveringtheband56-83eV(150-220A).The exclusively duringthepointedphaseofmission,denoted Data havealsobeenobtainedwithanadditionalfilter,used (60-140 A),andtheS2band60-110eV(112-200A). survey -SIcoveringtheenergyrange90-200eV keV (44-100A)band,withinwhichweexpectallthewhite opted touseonlytheintegratedcountratein0.1-0.28 panion staraswellthewhitedwarf.Consequently,wehave dwarf fluxtolie.However,inthesebinariesitispossiblethat band. white dwarf.Table6liststhecountratesineachÆO&4T some X-rayorEUVemissionmayoriginatefromthecom- also notedthePSPCcountratesatphotonenergiesabove 0.4 keV,wherethereisnocontributionexpectedfromthe four examples(BD+08°102,HR1608,HD33959Cand the EUVandX-rayfluxineachofthesebinarysystems, identified awhitedwarfasbeingsignificantcontributorto HD 217411)thereismeasurablehigh-energyfluxattribut- white dwarfemissiondominates,isdifficulttoestimate. tribution fromtheprimarystaratlowerenergies,where EUV andX-raydatabecomesmorecomplex,asanycon- flux inthelowerPSPCband,andnothingmeasurable the whitedwarf.Thisisprobablyquitereasonableinmost the fluxinWFCandlowerenergyPSPCbandarisesfrom Initially, wehavemadetheassumptionthatmajorityof able totheprimarystar.Consequently,ananalysisof EUV bands(Barstowetal.1992). cases, asevenasystemwithanextremelyactiveprimarysuch observations. Thismodelgridisidenticaltothatusedfor as V471Taucontributesmuchlessthan20percenttothe corresponds tothesituationwhereHlayermassis determining Tandlogg,butisextendedtotheextradimen- g =9.0).Thereareseveralvariablesthatdeterminethe sion ofavariablehydrogenlayermass.ApureHatmosphere by oneofus(D.Koester,e.g.Koester1991)totheROSAT sufficiently thickthattheHeopacityisnegligible(e.g. interstellar Hi,Heiandncolumns,layermass a predicted EUVandX-rayfluxesforthemodels:7,log g, only havethreeorfourindependentdatapoints,weuse distance/radius-related normalizationconstant.Sincewe 5x10M© atlogg=7.5,and1.4x10'M© other informationtospecifysomeoftheparameters.7,log g from recentobservationsof the localinterstellarmedium make thesimplifyingassumptionthatHei/Hratio is and (R*ld)havealreadybeendeterminedfromthelUE/ gible absorptionfromHen.This appearstobereasonable cosmic, andthattheISMisnothighlyionized,yieldingnegli- (e.g. Kimble1993). determined from theTUEandVoyagerdata for allbutthree ROSAT datawithinthetemperature andgravitybounds Voyager dataandprimarydistances.Foreaseofanalysis,we The ROSATinstrumentsandskysurveyhavebeen It isimmediatelyapparentthat,althoughwehaveclearly We havefittedasetofstratifiedH+Hemodelscomputed We findthatitispossibleto fitstratifiedmodelstothe Hot DAwhitedwarfsinbinarysystems505 19 94MNRAS.210. .4 99B 506 M.A.Barstowetal. © Royal Astronomical Society • Provided by the NASA Astrophysics Data System Q 00 11 e~° ^ ËNO i o W II .2 II •S ^Il « ûÛX) T3 E-h T3 1 11 3 II^ _ o .2 toi) H g - t-i o b ^U ^ -St: ^ 2 § >, câ om U5 .O- g «.2 S m- C +toc J-î E^H cd (N cd ^ VO00 ° ^P; g ¿T® « ac O ^Ö0 S'“ m O CO ^ oo"^, &E^ X ^ s vh (Nr- ^ OrH , II o 00

Figure 2 - continued 19 94MNRAS.210. .4 99B 182 508 MA.Barstowetal H icolumnobtainedarelistedinTable7.Inaddition,Fig. distance andradiusinstrument effectiveareaaccording the observedcountrates,taking intoaccounttheeffectsof tions ofacanonicalpureHcompositionandanicolumn white dwarfsforeachofthesurveybandswithpredic- of thestarsinthissample.ThevaluesHlayermassand which hasbeendemonstrated tohavesignificantEUV/X-ray HZ 43,whichhasapureH atmosphere,andG191-B2B, al. 1993b).Fluxesarealso shown forthewhitedwarfs to theformulaweusedfor isolatedstars(seeBarstowet of 2.5x10cm.Thesefluxeshavebeencalculatedfrom opacity initsatmosphere (e.g.Barstowetal.1993a). 3, wealsocomparetherelativeemergentfluxesofall © Royal Astronomical Society • Provided by the NASA Astrophysics Data System 2 (2) (l ß Crt RE1925-566() HD15638 BD+08°102 Binary HR8210 HD33959 C HD223816 HD217411 HR1608 Table 6.ROSATPSPCandWFCcountrates. PSPC ratesnotcurrentlyavailableforthisobject. Correctedforpost-surveyinstrumentefficiencychanges. i) HD223816 1000 1200140016001800 PlCD 170 (50) 117 (40) 1 440 (21) 47(11) 563 (36) 2628 (40) 24 (7) S2 Count rate(1000s“) 587 (18) 714 (22) 258 (17) 1503 (51) Figure 2-continued Wavelength (Â) 619 (34) 63 (7) 25(6) 531 (27) SI 252 (15) 70 (17) 1172 (30) 15 (5) 126 (11) the observedEUVandX-rayfluxes.Withlimited require photosphericopacityinadditiontopureHexplain that thethreehottestwhitedwarfsinthisbinarysample whether thisopacityisHealoneorsomecombinationof The resultsoutlinedabovecan bediscussedbothasasample and heavyelements,asintheisolated DAs. spectral resolutionavailableitisnotpossibletodecide and asinterestingindividual objects.Inthissection,we 5 DISCUSSION:INDIVIDUAL STARS consider eachobject inturn.Then,Section 6,weconsider Combining theresultsofTable7andFig.3,itisapparent 442 (38) 6037 (139) 447 (36) 272 (28) 702 (48) 5361 (137) PSPC (0.1-0.4keV)(0.4-2.4keV) 1437 (58) 120 (17) 56 (15) 77 (17) 201 (24) 161 (22) 19 94MNRAS.210. .4 99B 182 Figure 3.X-ray/EUVcountrates,normalizedbythe1300-Àfluxes, instrumenteffectiveareasandpassbandstoyieldestimatesofthe bottom =PSPC(softband). emergent fluxforeachwhitedwarfinthesample.Opencircles, stars andcrossesrepresenttheS2,SIPSPCvalues,respectively.The dashed linescorrespondtothepredictedvaluesforapureHatmosphere withmodestHicolumn(2.5x10cm~)-top=S2,centreSIand how thesenewdiscoveriesmightaffectourviewofthe general whitedwarfpopulation. The spectraltype(nominally G) andluminosityclassofthe the possibledistance tothesystem,wehaveassumed thatthe primary ofBD+08°102are rather uncertain.Inestimating 5.1 BD+08°102 © Royal Astronomical Society • Provided by the NASA Astrophysics Data System ß Crt36,7507.5 Name Tl°gg Table 7.Best-fittingHlayermassesandcolumndensitiesforthesample. HD33959 C46,5508.0 HR1608 27,2608.0 HD15638 53,5008.2 BD+08°102 24,9007.5 RE1925—566 37,0008.0 HD217411 35,6008.2 HR8210 35,6009.0 HD223816 69,3008.0 14 4.9 9.5 23 (x 10” H mass(M)HIcolumnComment no fit 27 no fit no fit 180 1.4 Q 18 19 19 18 18 19 7.9 xlO 2.1 x10 2.1 x10 7.3 x10 1 x10 1.7 x10 that thestarisadwarfreasonable, sincethewhitedwarf analysis oftheIUEdataindicates thatthefurtherassumption spectral typecouldbeintherangefromGOtoKO.The or giantwouldhavetobemuch furtheraway.Thelowerlimit must becloserthan«80pc, and a10th-magnitudesubgiant ture andgravityofthewhite dwarfof«27500Kandlog of 66pctothedistanceplaces upperlimitsonthetempera- g«8.2, respectively. TheEUVemission from thewhite Hot DAwhitedwarfsinbinarysystems509 All ROSATdataconsistent All ROSATdataconsistent SI ratelowcomparedtoS2andPSPC Observed SIfluxhigherthanpureHprediction All ROSATdataconsistent PSPC excesscountrateat«30%level From S1+S2only,excesscountsinPSPC All ROSATdataconsistent Strong EUV/X-rayopacitypresent 19 94MNRAS.210. .4 99B -12 1-2 30l fraction (10-50percent)oftheX-rayemission.Theremain- consequently, logg>7.3,althoughTandarenot dwarf canbeeasilyexplainedintermsofapureHatmos- phere, butthetemperaturemustbeatleast24000Kand, completely. However,suchamodelcanonlyaccountfor 510 MA.Barstowetal the primarystarisclearlyquiteactive. der mustarisefromcoronalemissionintheGdwarf,as sufficiently wellconstrainedtoruleouttraceopacitysources already indicatedbythepresenceof0.4-2.4keVflux.Hence H andKemission,butthereisbarelyatraceofanyphoto- in Fig.4.NotonlydoestheprimaryexhibitverystrongCan Telescope (KittPeak,AZ)inthe3900-3990and yielding afluxof2x10ergcmsandluminosityin hard countratebyafactorof10“ergcm, this energybandcanbeobtainedbymultiplyingthePSPC the range(1-2)x10ergs~.Wehaveobtainedhigh(0.1-Â) sample. Anapproximateconversionfromcountstofluxin from theprimary,whichwouldimplyacoolerandless binary systemsarestrongX-raysources,andthevalueofL lines arerotationallybroadenedtothepointthatonly tion. Thestarisrotatingsorapidlythatthephotospheric resolution spectraofBD+08°102attheMcMathSolar white dwarf,thepreviousevolutionarystateofsystem the interactionofactive(possiblycontact)binarywith massive whitedwarfsecondary.Wecannotbeabsolutely Dupree 1984;Dempseyetal.1993). (cf. Fleming,Gioia&Maccacaro1989a).Theseactive quoted aboveistypicalforsuchsystems(Cruddace& spectroscopic signatureoftheRSCVnandWUMabinaries strongest aredetectableathighresolution.Thisisthe spheric absorptionlines,whichdoappearatlowerresolu- 6670-6740 Âwavelengthregions.Thesespectraareshown possible enhancementofcoronalactivitythroughEUV when thewhitedwarfwasonmainsequence,and this isauniqueobject.Furtherstudyofapparenttriple emission fromthewhitedwarf,asstatedabove.Inanyevent, certain aboutthis,sincetheEUVfluxisalsoconsistentwith irradiation bythewhitedwarf. EUV emissionthatwasdetectedbyROSATcomessolely system willbeofgreatinterest,toconsiderquestionssuchas Bergeron. Atthistemperature,theEUV/X-raydataindicate good agreementwiththeresultsofLandsman,Simon& temperature ~53500Kandlogg«8.2.Thesevaluesarein F6V. Accordingly,themostlikelydistancetosystem is F3IV-V, butLandsmanetal.(1993)find,onthebasisof HD 15638islistedintheMichiganSpectralCatalogueas temperature (seeBarstowetal. 1993b),andtheresultisalso man, Simon&Bergeron,based onanexaminationof that sometraceabsorbingmaterialispresent.Thisin that thewhitedwarfdoesnothaveapureHatmosphere,and this constraintontheIUE/Voyageranalysis,weobtain a the lowerboundofrangeincludedinTable1.Imposing rather sensitivetotheestimated temperature.Ifthewhite contrast withthepureHcomposition suggestedbyLands- 5.2 HD15638 so greatasinseveralisolated whitedwarfsofsimilar IUE long-wavelengthspectrum,thatitseemstobeclosest Einstein HRIdetectionofthe source,buttheopacityisnot x BD +08°102hasthelargest0.4-2.4keVX-rayfluxin Therefore itisquiteprobablethatalloftheX-rayand © Royal Astronomical Society • Provided by the NASA Astrophysics Data System Figure 4.High-resolution(0.1-Á)spectraofBD+08°102, flux scaleiscountswithanarbitrarynormalization. wavelength regions(a)3900-3990Âand(b)6670-6740Â.The obtained withtheMcMathSolarTelescope,covering tion andananswertothisquestion. needed toprovideamoreaccuratetemperaturedetermina- compatible withapureHatmosphere.AnEUVspectrum is dwarf wereafewthousanddegreescooler(andconsequently slightly moredistant),thentheROSATdatacouldbequite HR 1608(=63Eri,HD32008)isknowntobeasingle-lined perature andgravitylieatthe upper boundsoftherespective therein). Thebestestimatesof thedistance,coupledwith K0IV primary(seeLandsman etal.1993,andreferences IUE data,giveawhitedwarf temperatureintherange spectroscopic binaryhavingaperiodof903±5d,with a ROSAT X-rayandEUVcount ratesrequirethatthetem- 23 800-27260Kandlogg from 7.0to8.0.However,the 5.3 HR1608 19 94MNRAS.210. .4 99B 29- 29l The brightestbinarysystemintheROSATWFCcatalogue, that thesystemisasingle-linedspectroscopicbinarywith (Hodgkin etal.1993).Recentopticalworkhasdetermined HD 33959C,hasalreadybeenthesubjectofsomestudy wavelength spectrumoftheprimary(LWR11254)and gen. ranges, andthatthewhitedwarfatmosphereispurehydro- period of2.99d(Webbinketal.1993;P.B.Etzel&E.F. typical ofsuchastar(Micela,Maggio&Vaiana,1992). tion ofthesystemin0.4-2.4keVbandPSPC.The emission reversalsinthecoresofCanHandKlines an olddiscgiant,andaluminosityofthislevelwouldbe estimated X-rayluminosity(0.4-2.4keV,d=31pc)is the estimateddistancetosystemis~90pc.Thetem- suggest thatthestarisactive.Thisconfirmedbydetec- the ROSATX-rayandEUVdata.Inthisanalysis,wefind data are46550Kand8.0,respectively.Thisisingood Guinan, privatecommunication).ForanF4Vspectraltype, 5.4 HD33959C the systembeingcorrespondinglynearbyandprimaryof higher temperatureandgravitycanbeaccommodated,with to benolowerthanthevaluesquotedabove.However, that theROSATdataconstraintemperatureandgravity agreement withthetemperaturepreviouslydeterminedfrom perature andloggthusderivedfromtheIUEVoyager needed tosolvethisissue. phere ofthewhitedwarf,althoughwhethertheseareHe later spectraltypethanthatcatalogued.TheROSATdata cannot bedecidedbythesedata.AnEUVspectrumwill alone, viewedthroughathinHlayer,orheavierelements also indicatethattheremustbetraceabsorbersintheatmos- Algol system,butrecenthigh-resolutionspectratakenof between thestars,merelytostripouterenvelopefrom total massofthesystem(e.g.seeKing1988,equation1).An the twocomponentsfromobservedperiodandestimated The spectrapublishedinHodgkinetal.(1993),aswellour band werealsodetectedinthissystem.Atadistanceof HD 33959CattheMcMathSolarTelescopeshowthat dwarf progenitor.Alternatively,thebinarymaybeapost- envelope evolutionduringtheredgiantphaseofwhite F4 dwarfhasamass-1.3M,andlogg=8.0white higher resolutionMcMathspectra,indicatethatthe is higherthantypicalFstarluminosities(Schmittetal.1985). one component. evolution isnotbelievedtotransferalargeamountofmass argue againstthepost-Algolscenario.Common-envelope star isnotrotatingrapidly(t>sin/<10kms~^whichwould a mass«0.55M,givingseparation-10R.Hencethe X-ray source.Sowheredo the hardX-rayscomefrom? HD 33959Cprimaryisnotvery active,makingitanunlikely system hasprobablyundergoneaperiodofcommon- HD 33959A,aôScuti-type star, isalsoanunlikelyX-ray that HD33959Disacoronal X-raysource.Intheimages emitter. Atthemoment, reasonableexplanationis « 3x10ergs^HR1608isclassifiedbyEggen(1989)as published byHodgkin etal.(1993),this12th-magnitude star 0 o0 « 90pc,thiscorrespondstoanL7x10ergs~,which x Strong chromosphericMgnemissionintheIUElong- From Kepler’sthirdlawwecanestimatetheseparationof As canbeseenfromTable6,X-raysinthe0.4-2.4keV © Royal Astronomical Society • Provided by the NASA Astrophysics Data System = which isverynormalifHD33959DwereanMdwarf(cf. for theAlV/A2IIIrangeofpossiblespectraltypes,givinga As pointedoutbyFlemingetal.(1991),andreferences must betakentoconfirmthisinterpretation.However,since between -60and90pc.Itisquiteclearfromthedistances therein, thetrigonometricparallaxofßCrtismuchtoolarge Stocke etal.1991).Obviously,aspectrumofHD33959D appears quitered.Weestimatelog(/)-1.9forthisstar, implied bytheIUE/Voyagerdatathatadistanceaslow19 distance of19pccomparedtoaphotometricestimatelying white dwarfmodelfitwithoutfearofcontamination. the amountoffluxissmallcomparedwiththatwhite type, thewhitedwarfhasatemperatureinrange dwarf. Henceweconcludethattheparallaxdetermination pc isalsonotcompatiblewiththepresenceofwhite dwarf, wefeelconfidentinusingthe0.1-0.28keVflux with otherAstars(seeSection3),wouldfavourthelower must beinerror.Dependingontheprimary’struespectral 5.5 ßCrt lower EUVandX-rayfluxesthanwouldbeexpectedfora pair ofvalues.LikeHD15638and33959C,ßCrtalsohas classification, supportedbycomparisonoftheIUEspectrum between Heorheavierelementsasthecontributors. pure Hatmosphereand,onceagain,wecannotdiscriminate 36 500-40000Kandlogg=1.5-82.TheoriginalA2III find that7=31300-37000Kandlogg=7.0-8.0.X-ray but mightbeslightlyearlierorlater.Makingtheassumption The primarystarofthissystemisuncatalogued,andsoits that itliesonthemainsequence,weobtainapossibledis- African AstronomicalObservatory’s(SAAO)1.9-mtele- both havemasses>1M. that thissystemalsohasundergonecommon-envelope must belessthan45R.Thereforethereisthelikelihood the estimatedperiod.Theseparationofcomponents the totalsystemmass,itwillnothaveasignificanteffecton than allowedbytheIUE/Voyagergravitymeasurementsbut, data andtheestimatedmassesofcomponentsin be aresultofEUVemissionfrom thecoronaofprimary. that eventheEUVdataalone areincompatiblewithpureH tance rangingfrom«100to150pc.FromtheIUEdatawe of recentopticalspectrathisstar,obtainedontheSouth Landsman etal.1993;alsoseebelow)andSiriusBwhich dwarf mass(0.2-0.55M)comparedtoothersystemswithA .Thismayexplaintherelativelylowwhite evolution whenthewhitedwarfprogenitorevolvedoff as thediscrepancycanonlyamounttoabout10percentof assumed massforthewhitedwarfisuptoafactorof2higher count rateisalwaysmuchhigher thanexpected.Thiscould or H+Hemodelpredictions. Foranygivenmodel,theSI data arenotyetavailablefor this system.However,wefind suggests thatitisclosestinspectraltypetoHD217411(G5) scope (Figs5a-e),withdataforotherbinariesinthissample spectral typeandluminosityclassareunknown.Comparison system toplacealimitof<20.1dontheperiod.Their star companions,suchasHR8210(Wonnacottetal.1993; xv Such anideais supportedbythepresenceof strongemission 5.6 RE1925-566 0 0 0 Fleming etal.(1991)haveusedhistoricalradialvelocity Hot DAwhitedwarfsinbinarysystems511 19 94MNRAS.210. .4 99B 512 M.A.Barstowetal. © Royal Astronomical Society • Provided by the NASA Astrophysics Data System

3600 3700 3800 3900 4000 4100 4200 3600 3700 3800 3900 4000 4100 4200 Wavelength (Â) Wavelength (Â) Figure 5. Non-flux-calibrated optical spectra recorded using the RPCS on the SAAO 1.9-m telescope, (a) HD 15638 F6V, (b) HD 223816 F5IV/G0, (c) HD 219482 F7V/G0 ( standard), (d) HD 217411 G5, (e) RE 1925 - 566 G2-G8. 19 94MNRAS.210. .4 99B 18-2 Further studyofthesystemisrequiredatallwavelengths. explanation, RE1925-566wouldbespectacularlyactive. the otheractivesystemsinsample,and,ifthiswere reversals intheCaHandKlines,indicatingahighlevelof activity (Fig.4e).However,weseenoexcessSIfluxinanyof This systemhasawell-constraineddistance,allowingthe firming thehighmassofwhitedwarf.Interestingly,thisis temperature andgravitytobedeterminedwithahigher particular groupofstars.HR8210hasbeenknowntobe a the onlyexampleofahigh-masswhitedwarfinthis Landsman etal.(1993)andWonnacott(1993),con- respectively, areingoodagreementwiththeresultsof degree ofaccuracythaninmosttheothersystemsthis for ßCrt,coupledwiththefactthatprimariesareboth A between theperiodofHR8210andupperlimit had acommon-envelopeevolutionaryphase.Thesimilarity component separationof44R,thissystemmustalsohave sample. Valuesintherange32700-35600Kand8.5-90, 5.7 HR8210 genitor duringthecommon-envelope phase. particular, thedifferenceinmassoftwowhitedwarfs single-lined spectroscopicbinaryforsometime(Harper points toadifferenceinmasslossfromthewhitedwarfpro- systems isofgreatrelevancetobinaryevolutiontheory.In stars, isstriking.Thereforeadetailedcomparisonofthetwo perature andgravity mightbedetermined. g =9.0.Therelativelylow impliedcolumndensity sistent withapureHatmosphere atT=35600Kandlog date forEUVspectroscopy, from whichanaccuratetem- («7xl0cm) suggeststhat thiswouldbeagoodcandi- 1927), havinga21.7-dperiodandcircularorbit.With a 0 The ROSATX-rayandEUV dataarecompletelycon- © Royal Astronomical Society • Provided by the NASA Astrophysics Data System e) RE1925—566 Figure 5-continued Wavelength (Â) 29-1 2 from theprimary.Ifbinaryliesat87pc,X-ray for theabovewhitedwarfparameters.Sincesystemis value associatedwiththeunquantifieduncertaintyin primary isamain-sequencestar,weestimatethedistanceto The primaryofHD217411islistedasspectraltypeG5,but be aplausiblecounterparttoanX-raysource.TheSAAO typical foractiveGdwarfs.Thereisaproblemwiththis luminosity oftheprimaryis~5xl0ergs,which is giant lyingatcorrespondinglygreaterdistance. the IUEdataruleoutpossibilityofprimarybeinga the systemtobe87pc,butnotepossibleerrorinthis nothing elseisknownaboutthesystem.Assumingthat made withtheMcMathtelescope(0.5Â,cf.0.1Â),which position, orevennearby,whichisopticallybrightenoughto there isnootherobjectwithintheerrorcircleofWFC expect toseeiftheprimarywerecoronallyactive.However, cores oftheCanHandKlines(seeFig.5d),whichwemight interpretation intheabsenceofemissionreversals keV rangeisprobablycontaminatedbycoronalemission detected inthe0.4-2.4keVbandofPSPC,0.1-0.2 constraints onTand(R*/d)imposedthewhitedwarfby required toexplorethisproblem further. cores. Further,high-resolution studiesofthissystemare results inahigherthresholdfordetectionoftheemission observations haveafactor5poorerresolutionthanthose spectral type.However,forawhitedwarfatthisdistance,the The whitedwarfintheHD 223816systemisbyfarthe IUE datayieldT=35600andlogg=8.2.Furthermore,the 5.8 HD217411 hottest inthe sample. Theprimaryitselfis poorlystudied 5.9 HD223816 The ROSATdatadonotfitthestratifiedH+Hemodels Hot DAwhitedwarfsinbinarysystems513 19 94MNRAS.210. .4 99B the uncertaintiesassociatedwithnominalF5IV/G0 and, asinthecaseofHD217411,wedonothaveanyidea 514 MABarstowetal. primary iscertainlysomewhatlaterthanHD15638Aand which aretoosmallfortheprimarytobeasubgiantorgiant. significantly earlierthanHD217411A,sowebelievethatthe spectral type.Nevertheless,astheSAAOdatashow, log g=8.0.Atthistemperature,HD223816Bwouldbe For aGOVprimary,theestimateddistancetosystemis GO classificationismostaccurate.Furthermore,theIUEand could becoolerifthecorrectprimaryspectraltypewere among theveryhottestDAwhitedwarfsknown.However,it ways. First,assumingthatithasevolvedfromtheHe-rich, lower thanexpectedfromthepredictionsofapureHatmos- EUV andX-rayfluxistwoormoreordersofmagnitude illustrated inFig.3,whichshowsthat,unlikethoseother earlier thanGO,andthesystemcorrespondinglyfurther DO whitedwarfbranch,ifitisashot69300Ka with noevidenceofanyHe(Barstow,Holberg&Koester known tohaveanatmospherecontainingheavyelements phere. Indeed,fromthispointofviewHD223816Bisvery systems inthissamplewheresimilarproblemsarise,the stratified H+Hemodels.Thereasonforthisisclearly away (seeTable4). WHITE DWARFPOPULATION similar toG191-B2B,shownforcomparison,whichis A totalofninenewunresolvedhotwhitedwarfbinary with G191-B2B. presence ofthecompanion,providingausefulcomparison the levelofheavy-elementopacitymaybeinfluencedby may bespectroscopicallydetectableintheEUV.Secondly, recent arrivalontheDAcoolingsequenceandmaystillshow signs ofitsrecentevolutionintheformtraceHe,which vent thisbeinganunbiasedsample,andsoadetailed 6 DISCUSSION:IMPLICATIONSFORTHE less, apreliminarycomparisonofthesenumberswiththe systems havebeendiscoveredasaresultoftheROSATall- 115 pc,andthecorrespondingtemperatureis69300K total numberofwhitedwarfsdetectedandthefraction sky survey.Severalselectionmechanismsoperatewhichpre- known binariesinthatsampleisinteresting. statistical analysisisnotappropriateatthispoint.Neverthe- Voyager datapredictarangeofdistancesforthewhitedwarf 1994). HD223816isthereforeanexcitingsystemintwo the basisofEUV/X-raycolours.Approximately46out have chosenA,F,GandKcandidatespredominantlyon Pounds etal.1993),indicatingalowinterstellarcolumn. dwarfs withA-Kcompanionsinthesurvey,givingatotalof three knownunresolvedbinaries detectedbythesurvey column whitedwarfs,wemightexpectanother12orso ber isentirelyconsistentwith theestimatesfromoptical ing toabout20percentofthe surveypopulation.Thisnum- number ofhotwhitedwarfsin binariesis«25,correspond- resolved systems(SiriusB, HD 74389).Hencethetotal (V471Tau, GDI23,INCom), towhichcanbeaddedtwo about 20.Inadditiontothesenewdiscoveries,thereare Simply scalingoursamplebytheratiooflow-tohigh- surveys ofboth Bergetal.(1992;17percent forF-Mstars, 120 totalwhitedwarfdetectionshavesimilarcolours(see We areunabletofittheROSATddùdiusinggridof There isanimportanteffectthatmustbiasoursample.We © Royal Astronomical Society • Provided by the NASA Astrophysics Data System 3- mostly M)andwiththoseofGemmoetal.(1993;27percent or fouroutof15),implyingthatmost,ifnotall,oursample of nineobjectsaretruebinaries,notchancealignments.The confirmation ofthis,whereithasnotyetbeenconclusively of thesesystems. demonstrated, isthemostimportantnextstepinanalysis They estimateanupperlimittothespacedensityofwhite with whitedwarfcompanions,tobe2.2x10"starspc (GKM starsonly).Thislimitis«3.5timesthedensityof dwarfs inbinarysystems,basedonthenon-detectionofstars IUE low-dispersionarchivebyShipman&Geczi(1989). white dwarfsinbinariesisonly«0.2timesthenumberof isolated whitedwarfs.Sinceweestimatethatthenumberof isolated stars,thenumberofnewsystemswehavedis- lation arisesfromanassumptionthattheatmospheresof Geczi. covered doesnotconflictwiththeresultofShipman& isolated whitedwarfsarerepresentativeofthebinaries.The et al.1993b)showsthatthehotteststarscontainadditional extended toallDAsbyFlemingetal.(1993),intheiranalysis opacity sourcesintheformoftracemetals.Thisresultis follow thesametrend,thenourestimateofbinary of theirX-rayluminosityfunction.Ifthebinaryatmospheres through interactionwiththecompanion,binaryDAs ROSAT studyof30DAwhitedwarfatmospheres(Barstow generally containmoreabsorbingmaterial,wewillbe underestimated thenumberofbinaries.Alternatively,if fraction canbeappliedtothegeneralpopulation.If, function oftemperatureasdotheisolatedwhitedwarfs, pattern regardingthepresenceofheavyelementsasa detecting asmallerfractionofthepopulationandwillhave overestimate. Oursmallsampleseemstoshowthesame tive abundancesofheavyelementsindetail. atmospheres havelesscontamination,ournumberwillbean although wedonothaveenoughstarstocomparetherela- The ROSATsurveyhasdiscoveredanimportantgroupof X-ray emission)isundetectableinsomesystems,but is published dataonatleastsomeoftheobjects,that observations presentedhere,combinedwithpreviously clear fromtheresultsofIUE,VoyagerandROSAT 7 CONCLUSION periods rangefromafewdaystoseveralyears;magnetic unresolved binarysystemswithwhitedwarfcompanions.Itis reminiscent ofyoungrapidrotatorsinothers;somebinaries leading toaflatter distribution.Ifreal,thisshould notbetoo from isolatedexamples,despite beingmembersofbinary tion ofisolatedwhitedwarfs.However,thesenewly tion inthepast.Thenumberofobjectsthisgroupistoo activity intheprimary(inferredfromdetectionofcoronal sample isnotparticularlyhomogeneous.Knownbinary fraction oflow-andhigh-mass whitedwarfsthanexpected, discovered whitedwarfsdonotappeartobeverydifferent are likelytohavehadaperiodofcommon-envelopeevolu- pheres followsthesametemperature patternasforthe small foradetailedstatisticalcomparisonwiththepopula- isolated whitedwarfs.Themass distributionalsoseemstobe similar tothefieldpopulation, althoughtheremaybealarger systems. Thepresenceofopacity sourcesintheiratmos- This resultisofinterestinthelightsurvey A potentialproblemwiththisestimateofthebinarypopu- 19 94MNRAS.210. .4 99B 9 high levelsofcoronalactivity.Thefactthattheyareassoci- majority oftheprimarystarsinthesesystemsdisplayrather direction duringtheevolutionofsystem. surprising, giventhepossibilityofmasstransferineither interaction withacompanionwhich,inthesecases,mightbe main-sequence. Theymustthereforebefurtherexamplesof which makesitunlikelythattheyareyoung(<10yr)orpre- these systemsmighthavebeenRSCVnbinarieswhose the whitedwarfs.Itistemptingtospeculatethatsomeof older starswhoserotationalvelocitiesarereinforcedbytidal ated withwhitedwarfsplacesalowerlimitontheirages binary potentiallyallowsmoreaccuratedeterminationof tunity totesttheoriesofstellarevolution.Besidestheinter- primaries havenowevolvedintowhitedwarfs. than ispossibleforisolatedobjects.Forexample,theability physical stellarparameters(e.g.mass,radius,distanceetc.) esting possibilitiesofinteraction,theexistencestarsina constrain themassofwhitedwarfandspectraltype/ each componentofthebinarieshasalreadyallowedusto to estimateindependentlyandthencomparethedistancesof luminosity classoftheprimaryinsomecaseswithour these starsintheirobservationalprogrammes. will beamajorundertakingforevenoneobject,andsowe lities presentedhererequiresanarrayoffurtherobservations current data.Totakefulladvantageofthescientificpossibi- encourage theastronomicalcommunityasawholetoinclude at allwavelengthstolearnmoreaboutthesesystems.This ACKNOWLEDGMENTS NASA andNSF.JBHTAFaresupportedby MAB, MCMandDWaresupportedbytheSERC,UK.DK grants NAGW5-434andNAGW-3160,respectively.We acknowledges supportofthisworkthroughgrantsfrom We arealsoindebtedtoDrWayneLandsman,whomade whom thedatapresentedherecouldnothavebeenobtained. thank allthestaffofvariousspaceandground-based observatories -ROSAT,IUE,VoyagerandSAAO-without Darragh O’DonoghueandMatthewBurleigh. our IUEdatareduction.TheSAAOwerereducedby available toushisHR1608dataprioritsreleaseinthe REFERENCES IUE archiveandassistedusinsolvingatrickyproblemwith Barstow M.A.,SchmittJ.H.M.,ClemensC,PyeR,Denby Barstow M.A.etal,1993b,MNRAS,264,16 Barstow M.A.,FlemingT.FinleyD.S.,KoesterD.,Diamond Barstow M.A.,HodgkinS.T.,PyeJ.R,KingA.R.,FlemingT. 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