1988ApJ. . .335. .953R 6 -1 (Holberg, Wesemael,andBasile1986),EXOSATEUV models tohighertemperatures.Themorerecentacquisitionof known. Analyzingbroadbandphotometry,Koester,Schulz companion ofamain-sequenceKdwarf.Subsequentobserva- below themainsurveylimit,wasdeemedsufficientlyunusual during theLowellpropermotionsurvey(Giclas,Burnham, narrow absorptionlinesduetoNv,Civ,andSiiv(Bruhweiler paper. High-resolutionIUEdatashowthepresenceofweak, revealed severalpeculiaritiesapartfromthosediscussedin this accessible tohigh-resolutionobservationsandthesehave evolved throughtheplanetarynebulastage. indentifying thestarasawhitedwarfratherthanhot sub- with analysisoftheLyaprofilegivingT~62,250±3520K note thatthisresultisbasedonextrapolatingfromtheir and Weidemann(1979)determinedtheeffectivetemperatureas showed thatthisstarisamongthehottestDAwhitedwarfs tions byEggen&Greenstein(1967)and(1969) to becataloged,inthiscaseasabluecommonpropermotion and Thomas1972)which,althoughhavingapropermotion dwarf. Assuchthecoolingtimeisi<5x10yr(Koester and Holberg etal.calculatethegravityaslogg=7.55±0.35, tories leadstoanupwardrevisioninthetemperatureestimate, observations atshorterwavelengthsfromsatelliteobserva- observations indicatingT~65,000K(Jordanetal1987). 56,800 ±3300Kandcalculatedloggas5.95,althoughthey Schoenberner 1986),andthusthestarhasonlyrecently The AstrophysicalJournal,335:953-961,1988December15 © 1988.TheAmericanAstronomicalSociety.Allrightsreserved.PrintedinU.S.A. (Dupree andRaymond1982). InG191-B2Bthesefeatureshave were foundinseveralotherDA whitedwarfs,notablyFeige24 a radialvelocityof18±4.1 kmsascomparedwiththe and Kondo1981).Similarhigh-excitationlinessubsequently Technology andtheCarnegieInstitution ofWashington. eff c Palomar Observatorywhichisjointly ownedbytheCaliforniaInstitueof eff 1 G191-B2B (orEG247)isoneofthemanystarsdiscovered With arelativelybrightapparentmagnitude,G191-B2B is Basedonobservationsobtainedwith the60inch(1.5m)telescopeat _1 -1 photospheric origin.Thelowredshiftimpliesamassof0.45Mforthishotwhitedwarf,althoughtheuncer- planetary nebularemnantorlocalionizationoftheinterstellarmedium.Ourdeterminationsystemic four days,rulingoutline-doublinginaclosebinaryasanexplanation.Theobservedemissioncannotbedue emission inthecoreofHaline.Ourobservationsshowlittlevariationlineprofileoveraperiod Subject headings:stars:emission-line—individual(G191-B2B)whitedwarfs tainties intheeffectivetemperatureandparallaxpermitmasses(1o)range0.29to0.60M. and thehigh-excitationspeciesobservedinultravioletareredshiftedby19±3kms,suggestinga velocity, usingthecompanionreddwarfG191-B2A,is5±2kmsandshowsthatbothHaemission to anearbyreddwarfcompanion,whiletheabsenceofanyspatiallyextendedemissionarguesagainsteither 0 0 © American Astronomical Society • Provided by the NASA Astrophysics Data System We haveobtainedhigh-resolutionspectraofG191-B2B,thehottestknownDAwhitedwarf,whichreveal 1 DETECTION OFH-ALPHAEMISSIONINTHEHOTWHITEDWARFG191-B2B I. INTRODUCTION Palomar Observatory,CaliforniaInstituteofTechnology Received 1988March16;acceptedMay27 Dartmouth College Gary Wegner ABSTRACT Neill Reid AND 953 -1 -1 _ 1 usually assumedtobehelium but,asJordanetal.(1987)point the presenceofabsorption at 2<200Â.Theabsorberis among theknownhotwhite dwarfsweredetected,implying the EinsteinObservatoryHRI instrument.Infactfewstars dwarfs arestrongX-rayemitters totheextentthatallsuch Cañizares (1986)haveshownthatpurehydrogenDAwhite the softX-rayspectrumofG191-B2B.Petre,Shipman, and gested thatthehigh-excitationlinesinG191-B2Baredue to companion. Finally,DupreeandRaymond(1983)have sug- wind fromtheMdwarf.G191-B2B,however,lacksaclose metals couldbeaccretedfrommaterialexpelledthe M (1983) suggestedthatinthecaseofFeige24,wherethere are stars withT>30,000Kand d<100pcweredetectableby the hotwhitedwarfionizinglocalinterstellarmedium. this hypothesissuggestingthatthelinesoriginateinastellar dwarf companion.SionandStarrfield(1984)havedeveloped ionized linesintheultravioletspectrumofG191-B2Barered- (Trimble andGreenstein1972). high-velocity lineswellseparatedfromtheinterstellarlines, the their origininthestellarphotosphere.DupreeandRaymond cussed furtherin§III,wehaveredeterminedtheheliocentric force issufficienttosupporttracesofC,N,andOinhydrogen- shifted withrespecttothesystemicvelocity,andprobablyhave velocity ofG191-B2Aas5±2kmsandfindthatthehighly a stellarwindismoredifficultproblem.However,asdis- helium-rich atmospheres.Expellingthematterfromstarin rich envelopes,whilehigherabundancescouldbeexpectedin expelled asalow-velocitywindandthattheobservedabsorp- leads totransportofmetalsthesurfacewheretheyare Vauclair andGreenstein(1979)suggeststhattheradiative analysis byFontaineandMichaud(1979)Yauclair, tion lineshavetheirorigininanexpandingshell.Theoretical that radiativeaccelerationinthewhitedwarfphotosphere lines andwiththe34kmsvelocityascribedtoG191-B2B 5 ±kmsradialvelocityoftheinterstellarlowerionization eff It ispossiblethatthesemetalsalsoplayaroleinexplaining Bruhweiler andKondo(1981,1982,1983)haveproposed 1988ApJ. . .335. .953R 1 -3 separation ofadjacentorders. airmass of1.06. 28 andDecember1.OneachoccasionG191-B2Bwasatan sisted of2400sexposuresobtainedon1987November27and the Palomar60inch(1.5m)telescope.Thelatterspectracon- this starathigherresolutionusingtheechellespectrographon structure inthecoreofHaline.Wethereforereobserved ion Kdwarf)wereobtainedontheHale200inch(5.1m)tele- Our spectra,witharesolutionof~1.3Â,appearedtoshow scope aspartofalargersurveyreddwarf-whitedwarfpairs. in §111. tions andwediscusstheirinterpretationtheimplications Greenstein (1972).Thefollowingsectionpresentsourobserva- companion reddwarfandderivearadialvelocityof~5km s“, ratherthanthe34kms~publishedbyTrimbleand emission inthecoreofHaline.Wehavealsoobserved copy ofG191-B2Bwhichshowthatthisstarexhibitsnarrow having athinouterlayerofhydrogen. derived asimilartemperatureandinterpretG191-B2Bas factor of10higherthanHZ43(T-57,000K),thisabun- the opticalspectrum.Inaddition,Vennesetal(1988)have dance ismarginallyconsistentwiththeabsenceofHenlinesin derive avalueofHe/H~5x10.Althoughmorethan atmosphere analysisofEUVandX-raydata,Jordanetal helium abundanceinthesestarscanbederivedfrommodel has asoftEUVspectrumresemblingthatofG191-B2B.The 24, whichalsoexhibitshigh-excitationlinesintheultraviolet, out, Siiv,CandNvcouldalsopartiallycontribute.Feige 954 eff Fig. 1.Across-cutthroughtheechelle spectrumofG191-B2B.Hafallsinorder50,locatedatpixel160,while Hßisinorder69,atpixel350.Notethedistinct Our initialobservationsofG191-B2B(andthecompan- We haverecentlyobtainedhigh-resolutionopticalspectros- © American Astronomical Society • Provided by the NASA Astrophysics Data System a) DataReduction II. OBSERVATIONS REID ANDWEGNER pixel no. Balmer linesinthecurrentpaper,wehaveextractedonly shows. Sincewearemainlyconcernedwiththehydrogen individual ordersarestillwellseparatedatHa,asFigure1 order separationdecreaseswithincreasingwavelength,butthe the “eraseline”overscanobtainedwitheachdataframe.The data inthestandardmanner,subtractingbiaslevelusing calibrating spectrumwasobtainedwhileBS8733closeto the zenithtomatchG191-B2observations. dimensional polynomialcalculatedandthedatarebinned.The BS8733)beingusedtodefineatemplate,two- tion routinesinFIGARO,withabrightstar(inthiscasetheB2 involves straighteningtheorderstopermittheireasyextrac- tion. Thesecorrectionweremadeusingthes-distortioncorrec- tral ordersonthedetector.Thusfirststageinreduction inch telescope.Theechelleopticaldesignleadstocurvedspec- at Caltech,andonthemicrovaxcomputerPalomar60 package onDEIMOS,theastronomydepartmentVAX11/780 provide somespatialinformation. use ofslitlengthsupto6"andthe2"x5"useddoes tions. However,theorderseparationissufficienttoallow echelle formatobviouslydoesnotpermitlong-slitobserva- length calibrationspectra,of3.5pixels,or~0.55Â,atHa.The width 2",givingaresolution,measuredfromHe-Arwave- FWHM), butourobservationswereobtainedusingaslitof TI CCDdetectorleadstoaneffectiveshortwavelengthlimitof cross dispersergivesapossiblewavelengthcoverageoffrom is describedindetailbyMcCarthy(1985).Theuseofaprism ~4000 Â.Theinstrumentalresolutionis~20,000(2pixels ~ 3200to8000Â,buttheresponseof800xformat Once thespectrawerestraightened,wereducedCCD Our datareductionwascarriedoutusingtheFIGARO The echellespectrographmountedonthe60inchtelescope Vol. 335 1988ApJ. . .335. .953R case. relevant ordersfromthecalibratedspectra.The5"slitprovides sufficient spatialcoveragetoallowskysubtractionineach No. 2,1988 dome. Comparingtheindividualflatfieldobservationsshowed using atungstenlamptoilluminatediffusingscreenonthe no significantdifferencesamongthesixframes,sowehave compensates forpixeltovariationsinsensitivityandpro- by rowtoallowforintensityvariationalongtheslit. summed thesedata,giving-50,000countsperpixelatHaand vides azero-ordercorrectionfortheblazefunctioneach straightened usingthesametemplateandthennormalizedrow the advantageofasmoothfluxdistributionand,mostimpor- spectra ontoanapproximaterelativefluxscale.Thisstarhas order. WehaveusedobservationsofvanMaanen2tosetour width ofHaabsorptionand20mAonHßinvMa2.)We tant, nodetectableBalmerabsorption.(Ourobservations extracted thevariousordersasdescribedabove,smoothed allow ustoset3aupperlimitsof30mAontheequivalent data usinga40pixelrunningmean(omittingbadcolumns), must beobtainedwiththespectrographofftelescope. and usedthenormalized,smoothedspectrumtosetG191- not mountedinthespectrographassembly,andspectra using athoriumlamp.Unfortunately,thiscalibrationlampis B2B observationsonarelativefluxscale. to monitorthezeropointofwavelengthscale,andour However, He-Ne-Arspectra(andnightskylines)canbeused communication) tosettheextractedorderonalinearwave- wavelength calibration(calculatedbyKentBudge,private than 0.1pixels(0.02ÂatHa).Wehavethereforeusedasingle observations indicatethatvariationsduetoflexureareless length scale,establishingthezeropointusingHe-Ne-Ar -15,000 countsperpixelat4500Â.Thesummedflatfieldwas known velocity.ComparingthewavelengthsofNaD(San spectra, nightskylines,andobservationsofstandardstars rms residualsareonly0.03Â. Diego) nightskylinesinthethreeobservationsofEG247, plicated (Fig.2a),anemission-likefeatureisclearlyevidentin and HßlinesinG191-B2B.Whilethelineappearsuncom- At thestartofeachnightweobtainedflatfieldobservations, Dividing thestellarspectrabyfinalnormalizedflatfield The wavelengthcalibrationfortheechelledataisdefined Figure 2presentsourfullyreducedobservationsoftheHa © American Astronomical Society • Provided by the NASA Astrophysics Data System b) SpectralLineProfiles col. (5)givesthewidthofemission featureasmeasuredatthe“continuum”level col. 4givesthefullwidthathalf-maximum estimatedbyeyefromthespectra;and described inthetext;col.(3)gives measuredequivalentwidthinmilliangstroms; G191-B2A, theintensityweightedmean wherethe“continuum”isdefinedas adopted forthecalculationofequivalent width(seetext). 1987 Nov2808456563.33101.81.8 1.9 1987 Nov2708536563.2695.31.5 1.9 1987 Dec110156563.2773.81.2 2.2 1987 Nov270846 6562.98 7321.42 (1)(2)(3)(4)(5) (6)^ Col. (2)givesthewavelengthofHa emissioninG191-B2Bandabsorption > Date (UT)(Â)(mÂ) WHITE DWARFG191-B2B Ha ObservationsofG191-B2B Time LeEWFWHMFWZI Ha inG191-B2A TABLE l 20 1 presence ofabrightmoonaccountsforboththelowersignal- residual effectsoftheblazefunctionechelle.)Figure2b profile, suchasthe“shoulder”at4880ÂinHß,reflect no morethan2a.Thelarge-scaleasymmetriesintheline gests thatweakemissionmaybepresent,butthesignificanceis the coreofHaline.(CloseinspectionHßsug- to-noise andtheresidualsolarHaabsorptioninspectrum shows thattheemissionispresentonallthreenights—the spectrum. sky spectrum.ThereisnoevidenceofanyHaemissioninthis spatial resolution,andFigure2cshowstheNovember28night obtained onDecember1.Asnotedabovewehavelimited line isrelativelyweak,butalsobecausetheemissionfillsin width oftheemissionineachspectrum(Table1).Obviously ment canalsoleadtosystematicdifferencesbetweenthe “continuum” levelof3300-4000counts,theuncertaintiesdue non-LTE coreoftheunderlyingabsorption.Witha the equivalentwidthisratheruncertain,notonlybecause estimated thedepthofabsorptionbyextrapolatingcore equivalent widthisprobablyatleast150mÂ.Foradistanceof emission. Allowingfortheunderlyingabsorption,true be consistentwithnovariationintheequivalentwidthofHa to photonstatisticsare-20%,whilethe“continuum”place- profile oneithersideoftheemission,whilelatteris at half-maximumand“zerointensity.”Intheformercasewe in thelineemission. 45 parsecsthiscorrespondstoatotalfluxof1.0x10watts spectra. Withthisinmind,weconsidertheresultsTable1to spectra. Thustheemissionisresolvedwithadeconvolved mental resolutionis0.55ÂasmeasuredfromtheHe-Ne-Ar between thesetwoestimates.Againthereisnoevidencefor difference inwavelengthbetweenthecuspswhereemission variation amongthespectra.Aswenotedabove,instru- and absorptionprofilesmeet.ThetrueFWHMislikelytolie km s“. expected toproducelittleeffectonthevisiblespectrumatthis et aVs(1980)purehydrogenmodelatmospheres.WhileCj191- tions ofEG247againstthelineprofilespredictedbyWesemael FWHM of—1.4Â,implyingavelocitydispersion—±30 He/H ratio(Petre,Shipman,andCañizares1986).Figure3a B2B ishelium-richforaDA,themixedcomposition We havemeasuredthecentralwavelengthandequivalent Table 1alsogivesourestimateoftheemission-linefullwidth Finally, wehavematchedourechellespectrographobserva- 955 enPC coLO LO PO 'o ft 00 CO00

6530 6540 6550 6560 6570 6580 6590 6600 wavelength Fig. 2b Fig. 2.—Observations of G191-B2B. (a) the Hß profile, the sum of the three observations; (b) the individual Ha profiles; (c) the night-sky spectrum immediately adjacent to G191-B2B. Note the absence of Ha emission. All spectra have been reduced to a relative flux scale as described in the text, using the flat field data and the observations of van Maanen 2. 956

© American Astronomical Society • Provided by the NASA Astrophysics Data System 1988ApJ. . .335. .953R plots ourHaobservationsagainsttheWesemaeletal50,000K (< 1%)core-reversal—substantiallysmallerthantheobserved models atthelowertemperature.Theformershowsaslight  and6627-6637plotboththeLTEnon-LTE the datatofitmodelsinwavelengthranges6510-6520 and 70,000Kmodels,bothwithlogg=8.0.Wehavescaled files. Althoughtheblazefunctioncomplicatesadetailedcom- emission—which mayariseinthelineprofileinterpolation parison, thelowertemperaturemodelappearstogivea routine. Figure3bshowsthetheoreticalandobservedHßpro- temperature of~62,000KderivedfromtheEUVdata.More- somewhat betterfittothedata,notwithstandingeffective gravity oflogg=7.55weexpectnarrowerlineprofilesthan in a pointwereturntoin§Hid.NotethatifG191-B2Bhas a over, bothlineshavebroadercoresthanpredictedbytheory— the logg=8.0models. consider theaccuracyofastrometricmeasurements for G191-B2B—in particularthedistance.Thisstarisincluded in the USNOparallaxprogram (USNO122,Harringtonand Dahn 1979),wheretheabsolute trigonometricparallaxisgiven while thepropermotionismeasured as as Before discussingthespectroscopicobservations,webriefly ti =0"0219±0.0026,M 8.48±0.26, © American Astronomical Society v g =0'/0876±0.0017, 0=175?2±1?2. a) TheDistancetoG191-B2B in. DISCUSSION WHITE DWARFG191-B2B Provided bythe NASA Astrophysics Data System Fig. 2c wavelength (1985) haspointedoutthatthetrigonometricparallaxof G191-B2B placesthestar-1.6magabovewhitedwarf M =6.76fortheKdwarfcompanion,G191-B2A.Greenstein degenerate starsequenceisnotwelldefinedatsuchhightem- sequence inthe[M,(G—R)]colormagnitudediagram. peratures. Furthermore,thephotometricparallaxofG191- However, G191-B2Bhasa(G—R)colorof—0.79andthe ing discussionweshallassumethatG191-B2AandG191-B2B in goodagreementwiththetrigonometricresult.Infollow- B2A [determinedfromthe(B—V)color]is0"024(M=6.95), separation betweenthetwostarsisatleast2200astronomical are companionsatadistanceof45parsecs.Atthisthe units. material inthelocalinterstellarmedium;(4)andemissionfrom doubling inabinarysystem;(2)chromosphericemissionfrom morphology wouldbethatG191-B2Bisadouble-linedspec- emission. However,analternativeexplanationfortheobserved the whitedwarfphotosphere. a closereddwarfcompanion;(3)gaseousemissionfrom covered anexampleofthistype ofbinarysystem—L870-2.If dwarfs. Saffer,Liebert,andOlszewski (1988)haverecentlydis- troscopic binarywithboth componentsbeingDAwhite estimates.However, itseemsunlikelythattheHa discrepancy betweenthephotometric andtrigonometric G191-B2B wereabinary,this wouldexplainatleastpartofthe v profile inG191-B2B canbeexplainedin this wayfortwo v v The trigonometricparallaxgivesanabsolutemagnitudeof We considerfourpossibleoriginsoftheHaprofile:(1)core We havereferredtothefeatureincoreofHaline as b) PossibleHypothesesontheOriginofHolFeature 957 1988ApJ. . .335. .953R non-LTE models,again scaledtoacontinuumlevelof1.0. Thespectrawerematchedtothemodels forthewavelengthrange4845^1850 Âand4875-4885Á. offset by0.1forclarityandthespectra werescaledtofitinthewavelengthrange6510-6520Âand6627-6637 Á.(b)Hßprofiles.Inthiscaseweshowonlythe exhibits aslightcore-reversal,rather thanthesharpabsorptionofnon-LTEmodel.Inbothcasescontinuum levelis1.0—thehighertemperaturemodel (u) Hamatchedtothe50,000Kand 70,000K,logg=8.0models.WeshowboththeLTEandnon-LTEmodels forthelowertemperaturecase—theLTEmodel Fig. 3.Acomparisonbetweenour observations ofG191-B2BandtheBalmerlineprofilesfrompurehydrogen modelstabulatedbyWesemaeletal.(1980). © American Astronomical Society • Provided by the NASA Astrophysics Data System Fig. 3a 958 1988ApJ. . .335. .953R 1 4 -2 -3 energy distribution,andthedetectionoftwolinecores, reasons. First,theabsenceofanyirregularitiesinspectral . Thisseemsunlikelygiventheshortcoolingtimes requires thatthetwostarshavesimilartemperaturesand expect toseevelocityvariationsandlineprofilechangesover s~—that is,anorbitalsemimajoraxisofatmost~0.1AU separation is—4Â,implyinganorbitalvelocityof~100km shown inFigure2basthesuperpositionoftwocores, of hotwhitedwarfs.Second,ifweinterprettheHaspectra and aperiodoflessthan~11days.Obviously,onewould could ariseinalow-massdMecompanion.However,infrared none areobserved. the fourdayscoveredbyourobservations.AsTable1shows, continuum inlate-typemainsequencestars,implyingamagni- tude differenceofAR-1-2magbetweenEG247andthe Ha emissionpeaksatalevelfrom2-5timeshigherthanthe observations ruleoutthispossibility.Typically,chromospheric (R —K)colorofatleast3magandapredictedK~9 hypothetical companion.GivenM~8.5forthewhitedwarf, for thesystem.Thusreddwarf-whitedwarfbinaryHZ9, this impliesM~10forthecompanion,correspondingtoan which hasdetectableBalmeremissionfromthechromosphere Zuckerman andE.Becklin(privatecommunication)obtained JHK photometryofG191-B2BandfindthatithasaKmag of theMdwarf,hasa(V—K)4.1mag(Probst1983).B. presence ofanunresolvedmain-sequencecompanion. medium orfromaremnantplanetarynebula.Durisen,Save- vations isgaseousemissioneitherfromthelocalinterstellar interstellar mediumandproducesmallStrömgrenspheresand, dwarfs havesufficientshortwavelengthfluxtoionizethelocal doff, andVanHorn(1976)originallysuggestedthathotwhite that thehighlyionizedlinesobservedinultravioletorig- as notedin§I,DupreeandRaymond(1983)havesuggested R 12.81 ±0.03[i.e.,(V—K)~1.03],whichclearlyrulesoutthe R inate withinthisionizedregion.Indeed,Reynolds(1987)has detected faintemissionaroundthehot(T~75,000K) PG01084-101.However,wehavealreadynotedthat (±2.2 x10").Thehydrogencolumndensitytoward Ha emissionisundetectedintheskyimmediatelyadjacentto G191-B2B—implying anextentof<±1",oronly±45AU G191-B2 is~0.01cm(BruhweilerandKondo1982),giving culate theexpectedradiusofanHnregion,usingReynolds formula (2);assumingaluminosityofL/L—3thisyields an averagevolumedensityof-0.006cm.Thuswecancal- that thefluxdistributioncanbeapproximatedbyablackbody G191-B2B hasstrongabsorptionbelow-250Â,butthesofter at shortwavelengths,whereasEUVobservationsshow that spectrum reducestheexpectedradiusbyafactorof10atmost. eff detection) ontheequivalentwidthofnebular[O m] Finally, ourobservationssetanupperlimitof10m (3 a white dwarf. remnant planetarynebulaintheimmediatevicinityof the ^4959,5007 emissionlinesinG191-B2B,arguingagainst a plained emissionlines.Balmer emissionhasbeenobservedin the magneticwhitedwarfGD 356(GreensteinandMcCarthy 0 ture expectedfromZeemansplitting northecircularpolariz- ation observedinmostmagnetic whitedwarfs(Angel,Borra, — 54pc—thatis,encompassingtheSun.Reynoldsassumes and Landstreet, 1974).Furthermore,Sion,Liebert, andWese- 1985). However,G191-B2Bneither exhibitsthetripletstruc- Discarding thebinarywhitedwarfhypothesis,Haemission A furtherpossibilityappearinginconsistentwithourobser- G191-B2B isnottheonlydegenerate starthatshowsunex- © American Astronomical Society • Provided by the NASA Astrophysics Data System WHITE DWARFG191-B2B -1 1 -1 _1 _1 -1 1 _1 _1 _ 1 1 1 mael (1985)havereportedemissionreversalintheHen4686 1 (1987) andMendezhaveshownthatinthemoremassive line inthespectrumofDOstarPG1034+001.Kudritzki central starsinplanetarynebulaelinessuchasHen4686 related withthestellarmass.Thelast-mentionedemissionis thought tooriginateinastellarwind.Itispossiblethatweare switch fromabsorptiontoemission,withthelinestrengthcor- vations ofseveralotherhotwhitedwarfsandOBsubdwarfs. more detailinafuturepaper(WegnerandReid, While noneoftheformershowedemission,several observing ananalogousprocessinthishotDAwhitedwarf. subdwarfs—notably BD+284211—haveemissionnotonlyat menon isnotpeculiartoG191-B2B,andmayevenbearela- preparation). Forthepresentwemerelynotethatpheno- Ha, butalsoatHß.Weshalldiscusstheseobservationsin tively commonoccurrenceamongthehottersubluminous stars. vious section,weareleftwiththepossibilitythatemission arises atornearthestellarphotosphere.Ifso,weexpectto length oftheHaemissionis6563.29±0.04Â,where observe agravitationalredshift.Themeanmeasuredwave- ments inTable1andprobablyoverestimatestheaccuracyof quoted uncertaintyisthermsscatterofthreemeasure- the mean.Thiscorrespondstoaheliocentricvelocityof22±2 from theradialvelocityofmain-sequencecompanionstar. km s.Thespacevelocityofthewhitedwarfcanbeestimated c) TheRadialVelocityandGravitationalRedshiftofG191-B2B Trimble andGreenstein(1972)quoteV=34kms. However, wederive7±4kmsfromourechelledata,and trograph ontheHale200inch.Infact,TrimbleandGreen- stein valueappearstohavebeenacopyingerror,sincethe original measurementofthephotographicspectrumyielded One ofus(I.N.R.)hasremeasuredthiscoudéplate,deriving 12 ±5kmsfromspectraobtainedwiththedoublespec- compares withF=22±2kmsfortheHaemissionand and doublespectrographobservations,weconcludethatthe two measurementsandcombiningtheresultwithechelle F =1±3kms(J.L.Greenstein,privatecommunication). rad radial velocityoftheG191-B2systemis5±2kms.This F =4.8±2.1kms(from25Feilines).Averagingthelast expanding shellandisconsistentwithagravitationalredshift. inferred byBruhweilerandKondo.Thisclearlyrulesout an ultraviolet. central wavelengthismeasuredas6563.45±0.1Â,ora red- We canusethebroadabsorptioncoreofHalinetoobtain a redshiftof-13kms,ratherthanthe16blueshift shifted by17±4kmswhiletheultravioletmetallineshave an independentmeasurementoftheEinsteinredshift. The metallic linesandtheHaemission originateclosetothephoto- the HaemittingandUVmetalliclinesystem.Theagreement the measurementuncertaintiesthisagreeswithvelocity of shift (withrespecttoG191-B2A)of21.5±4.5kms.Given we findagravitationalredshift of19±4kmsforG191- among thesethreeredshiftestimatessuggeststhatboththe UV measurement ofgravitational redshiftsforseveralotherDA sphere ofthewhitedwarf.Combining thethreeobservations B2B. F =18±4kms“fortheNv,Siiv,andCivlinesin r r r r Since thesubmissionofthispaperwehaveobtainedobser- Excluding thefirstthreehypothesesdiscussedinpre- With thisrevisedsystemicvelocitytheHaemissionisred- Recent advancesindetector technologyhaveallowedthe 959 1988ApJ. . .335. .953R 1 -1 _ 1 1 .1985,Pub.A.S.P.,97,827. tional velocity. Greenstein, J.L.1969,Ap.J.,158,281. Giclas, H.L.,Burnham,R.,andThomas, N.C.1972,LowellObs.Bull,No.58. observed intheultravioletarguefurtheragainstahighrota- specialized geometry.Finally,thenarrowhigh-excitationlines wavelength unlesstheemissionisconfinedtoasmallareanear is notconsistentwiththenarrowlineprofileandinvariant the rotationalpole.Whilenotimpossible,thisisarather emission linewithintheabsorptioncore—thenrapidrotation Fontaine, G.,andMichaud,G.1979, Ap. J.,231,826. G191-B2B shouldbeanexampleofsuchastronglylayered escape oftheEUVandsoftX-rayphotons.Theyfind that which producestheDAvisiblespectrum,butpermits the dwarfs andhaveconcludedthatthehotDAwhite librium ofhydrogenandheliumdiffusionsettlingforwhite to thephotosphere—assuggestedbycentralpositionof stratified atmosphereswithathinouterlayerofhydrogen 60 ±10kms’).Moreover,iftheHaemissionoriginatesclose than 50kms(EG15at±20andEG184 However, onlytwoofthe15DAwhitedwarfsobservedby period ofatmost15minutes(forR=0.018andi90°). correspond toat;siniof~80kms.Thisimpliesrotation .1983,Ap.J.(Letters),275,L71. Pilachowski andMilkey(1987)havevelocities,vsini,ofmore half-maximum of~4ÂtheabsorptioncoreHawould .1982,Ap.J.,259,232. It ispossiblethatthisreflectsrapidrotation—thefullwidth erate cores. 0.29 to0.60M.Allowingfortheobservationaluncertainties, Eggen, O.J.,andGreenstein,J.L.1967, Ap.J.,150,927. Durisen, R.H.,Savedoff,M.R,and VanHorn,H.M.1976,Ap.J.(Letters), .1983,Ap.J.,269,657. ter (1961)zero-temperaturerelationforeitherHeorCdegen- to 0.018Rand2.13.5L,giving,intheextreme,massesof , the1alimitsonradiusandluminosityare0.011 Dupree, A.K.,andRaymond,J.C.1982,Ap.(Letters),263,L63. the averagevaluesareconsistentwithHamadaandSalpe- Bruhweiler, F.C,andKondo,Y.1981,Ap.J.(Letters),248,L123. Angel, J.R.R,Borra,E.F.,andLandstreet,D.1981,Ap.Suppl,45,474. Allowing foruncertaintiesinthetemperature,redshift,and where Kisthegravitationalredshift,amassof0.40M. 960 and, using distance wehaveassumedforthestar.Assumingatemperature 0"023, whichcorrespondstoM=3.62(orL2.6L)atthe of 62,250K(Holbergetal.)leadstoaradiusR~0.013 clusters (SionandGuinan1983;Koester1987;Wegner white dwarfs—eitherdegeneratesinbinarysystemsoropen ometric magnitudeofM=3.69usingaparallaxtt spectrum. Koester,Schulz,andWeidemann(1979)quoteabol- e 50 kms~,placingG191-B2Batthelowerendofobserved Reid 1987andinpreparation).Thevelocitiesrangefrom20to 0 q0 0 bot0 0 bol 206, L149. Vennes etal.(1988)havecalculatedmodelsfortheequi- We havecommentedonthewidthofBalmerlinecores. e) ComparisonwithLayeredAtmosphereModels © American Astronomical Society • Provided by the NASA Astrophysics Data System d) PossibilityofRotation REID ANDWEGNER REFERENCES 3 1 partially supportedbygrantAST85-15219fromtheNational fitted fromcommentsbythereferee,J.Liebert.Thiswork was Science Foundation. G191-B2A. Thepresentationoftheresultsinthispaperbene- for accesstohisrecordsandphotographicspectrum of Koester, D.,Schulz,H.,andWeidemann, V.1979,Astr.Ap.,76,262. Koester, D.,andSchoenbemer,D.1986, Astr.Ap.,154,125. Jordan, S.,Koester,D.,Wulf-Mathies, C.,andBrunner,H.1987,Astr.Ap.,185, Jordan, S.,andKoester,D.,1986,Astr. Ap.Suppl,65,367. spectrograph andindatareductionwethankJ.Greenstein —give1crlimitsof0.29to0.60M. Koester, D.,andJordan,S.1984,Astr. Ap.,65,367. Koester, D.1987,Ap.J.,322,854. Greenstein, J.L.,andMcCarthy,K.1985,Ap.J.,289,732. electron densityisunlikelytoexceedthelattervalue. Holberg, J.B.,Wesemael,F.,andBasile, J.1986,Ap.J.,306,629. Harrington, R.S.,andDahn,C.1979,A.J.,85,454. Hamada, T.,andSalpeter,E.1961,Ap.J.,134,683. Greenstein, J.L.,Boksenberg,A.,Carswell,R.andShortridge,K.1977, Ap. although theobservationaluncertainties—particularlyin proposed byBruhweilerandKondo(1982).Theobservedred- Thus evenwiththeuncertaintiesinobservedFWHM, shift leadstoamassestimateof0.40MforG191-B2B, near thestellarphotosphere,ratherthaninexpandingshell of electrondensity,withaFWHM1.65Aatlogn=16. that boththeHaandhighlyionizedspeciesarelocated stellar .Thelinewidthrisessharplyasafunction tent withtheemissionlinebeingproducedneartopof value ofnoccursinthe60,000Kmodelatt~10”,consis- absorption core.Takentogether,theseobservationssuggest s’. Moreover,theemissionlineiscenteredinhydrogen iv linesobservedinIUEspectrahavearedshiftof~19km for theheliocentricradialvelocityofcompanionKdwarf Â, inagreementwithourlimitontheobservedvalue.This line, anelectrondensityoflogn=15.5givesaFWHM1.4 from Vidal,Cooper,andSmith(1973),wefindthatfortheHa star showsthatboththeHaemissionandNv,Civ,Si narrow emissioninthecoreofHaline.Ourrevisedvalue K, log0=8model,whichisnear40,000K.Then,usingresults boundary temperatureofWesemaeleta/.’s(1980)7¡=60,000 is Starkbroadened.Asanestimateoftemperature,wetakethe DA whitedwarfG191-B2B(EG247)revealingthepresenceof estimate theelectrondensity,n,ifemissioncoreofHa (1986) showthatundersomecircumstances,temperatures reversals givingrisetoemissioncoresintheBalmerlinescan 0 occur. 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