LP 400-22, A very low-mass and high-velocity white dwarf

We report the identification of LP 400-22 (WD 2234+222) as a very low-mass and high-velocity white dwarf. The ultraviolet GALEX and optical photometric colors and a spectral line analysis of LP 400-22 show this star to have an effective temperature of 1108

DraftversionFebruary5,2008

APreprinttypesetusingLTEXstyleemulateapjv.6/22/04

LP400-22,AVERYLOW-MASSANDHIGH-VELOCITYWHITEDWARF

ˇAstronomick´yu´stav,AVCR,Friˇcova298,25165Ondˇrejov,CzechRepublic

AdelaKawka

St´ephaneVennes,TerryD.Oswalt,J.AllynSmith1

DepartmentofPhysicsandSpaceSciences,FloridaInstituteofTechnology,

150WUniversityBlvd.,Melbourne,FL32901-6975

arXiv:astro-ph/0604470v3 5 May 2006

and

NicoleM.Silvestri

DepartmentofAstronomy,UniversityofWashington,Box351580,Seattle,WA98195

DraftversionFebruary5,2008

ABSTRACT

Wereporttheidenti cationofLP400-22(WD2234+222)asaverylow-massandhigh-velocitywhitedwarf.TheultravioletGALEXandopticalphotometriccolorsandaspectrallineanalysisofLP400-22showthisstartohaveane ectivetemperatureof11080±140Kandasurfacegravityoflogg=6.32±0.08.Therefore,thisisaheliumcorewhitedwarfwithamassof0.17M⊙.Thetangentialvelocityofthiswhitedwarfis414±43kms 1,makingitoneofthefastestmovingwhitedwarfsknown.Wediscussprobableevolutionaryscenariosforthisremarkableobject.Subjectheadings:stars:atmospheres—whitedwarfs—stars:individual(LP400-22)

1.INTRODUCTION

Thevastmajorityofwhitedwarfsevolvefromnor-malmain-sequencestarsfollowingnormalevolutionaryprocesses.However,ultramassive(>1.1M⊙)andinfra-massive(<0.40M⊙)whitedwarfsrequirespecialevolu-tionarypaths.Theformationoflow-massheliumwhitedwarfs(MWD 0.4M⊙)hasbeenshowntobetheresultofclosebinaryevolution&Tutukovandref-erencestherein).Indeed,theGalaxyisnotoldenoughfortheseobjectstohaveformedthroughsinglestarevolu-tion.Thegeneralevolutionaryscenariofortheformationoflow-massheliumwhitedwarfsisthatthecompanionstrippedthewhitedwarfofitsenvelopebeforecomplet-ingitsredgiantevolution(KippenhahnetRecently,severalverylowmasswhitedwarfs(MWD 0.2M⊙)havebeendiscoveredascompanionstopul-sars(vanKerkwijketal.Theorbitalperiodsvaryfromafewhourstoseveralyears.ThemassesofsomeofthesewhitedwarfsmaybedeterminedfromtheShapirodelayofradiopulsesprovidedthatthesystemisnearlyedgeon2005).Forexample,Jacobyetal.deducedamassof0.20M⊙forthecompanionofPSRJ1909 3744,andobtainedaspec-trumwhichcon rmed,atleastqualitatively,thepres-enceofalowmassDAwhitedwarf.Inaddition,themassesofthecompanionstoPSRJ1012+5307andPSRJ1911 5958weredeterminedspectroscopicallytobe0.16M⊙(vanKerkwijketal.Callananetal.1998)and0.18M⊙respectively.Finally,severallow-masswhitedwarfintheSloanDigitalSkySurvey2004).Liebertetal.(2004)analyzedthebrighteststarinthe

Electronicaddress:kawka@sunstel.asu.cas.cz

Electronicaddress:svennes@ t.edu,toswalt@ t.edu,

1VisitingAstronomer,KPNO/NOAO,whichisoperatedbyAURA,undercooperativeagreementwiththeNSF.Electronicaddress:nms@astro.washington.edu

sample,SDSSJ123410.37 022802.9andshowedthatithasamassof～0.18M⊙andthatitdoesnothaveanobviousneutronstarcompanion.

Inthispaper,wereporttheidenti cationofahighvelocitywhitedwarfwithaverylowmass,LP400-22(WD2234+2222,NLTT54331).Ourphotometricandspectroscopicobservationsarepresentedin§2.1and2.2respectively.Wederivethestellarparametersin§3anddiscussourresultsin§4.

2.OBSERVATIONS

LP400-22wasspectroscopicallyidenti edasawhitedwarfaspartofasurveyofcommon-propermo-tionbinarieswithsuspectedwhitedwarfcomponents(Oswaltetal.Weobtainedadditionalhigh-resolutionopticalspectra(Silvestriaswellasnewopticalphotometryaspartofthesameproject.Morerecently,LP400-22wasobservedduringtheGalaxyEvolutionExplorer(GALEX)all-skysurvey.

2.1.Photometry

TheBVRIphotometryforLP400-21/22wereob-tainedwiththe2.1mtelescopeatKPNOon1995July5UT.ATek1KCCD(with24µmpixels)operatingattheCassegrainfocuswasused,providing0′′.305perpixel

′

anda5.2 eldofview.ThedataforLP400-21/22wereobtainedunderphotometricconditions.StandardstarsforthisprogramwerechosenfromLandoltWeobtainedultraviolet(UV)photometryfromGALEXAll-SkySurvey3.GALEXprovidesphotometryintwobands,FUVandNUV,whicharebasedontheABsystemOke&Gunn1983).ThebandwidthofFUVis1344to1786 Awithane ective

wavelengthof1528A.ThebandwidthofNUVis1771to

jasmith@astro. t.edu Awithane ectivewavelengthof2271 A.

23

OnlineatAvailablefromhttp://galex.stsci.edu/GR1/

We report the identification of LP 400-22 (WD 2234+222) as a very low-mass and high-velocity white dwarf. The ultraviolet GALEX and optical photometric colors and a spectral line analysis of LP 400-22 show this star to have an effective temperature of 1108

2Kawkaet

al.

Fig.1.—TheenergydistributionofLP400-22combiningallavailable11000Kdataandlogcomparedg=6.50to(seeour§3).

H-richmodelspectrumatTe =TABLE1Photometry

BandLP400-22LP400-21FUVa18·NUVa18.18.38.18±19±0.09mag±00..0805magmag······BV171818R17.338.14±±0.04mag17.742±··0···

.025magmagI

17.219±00..025021magmag17..202210±±00..023024magmag

15.14.177.933±340±0±0.0.021.023023

magmag

aThemeanofthesevaluesareusedinthispaper.

Table1presentstheopticalandultravioletphotometryandFigure1showstheenergydistributioncomparedtoasyntheticspectrum.

2.2.Spectroscopy

Weobtainedalow-resolutionspectrumofLP400-22usingtheR-Cspectrographattachedtothe4mtele-scopeatKittPeakNationalObservatory(KPNO)on1988October6.TheBL250grating(158lines/mm)wasusedtoobtainaspectralrangeof3500to6200 Adispersionof5.52 witha

Aperpixelandaresolutionof14 A.

LP400-22wasre-observedusingtheDualImagingSpectrogram(DIS)attachedtothe3.5mtelescopeattheApachePointObservatory(APO)on2001July10andOctober14.The1200lines/mmgratingwasusedtoobtainaspectralrangeof3800to4600 Awithadisper-sionof1.6 Aperpixel,andthe830.8lines/mmgratingwasusedtoobtainaspectralrangeof6180to7210 withadispersionof1.3 Aperpixel.A1′′

A

.5slitwasusedtoobtainaspectralresolutionof～2 Aintheblueand～2.6 Ainthered.

3.DETERMININGTHEPARAMETERS

InouranalysisofLP400-22,weusedagridofcom-putedpurehydrogenLTEplaneparallelmodels(see

Kawka&Vennes(2006)andreferencesthereinforde-tails).ThegridofmodelsextendfromT16000K(instepsof1000K),from18000e =7000toto32000K(instepsof2000K)andfrom36000to84000K(instepsof4000K)atlogg=6.0to9.5(instepsof0.25dex).Allourloggvaluesareincgs.Wealsopreparedcorrespond-inggridsofsyntheticspectra,oneofwhichincludesthee ectofLyαsatellites(Allard&Koester1992),andtheotherexcludesthate ect.

3.1.Photometry

Usingourspectralgrid,wehavecalculatedsyntheticoptical(BVRI)andultraviolet(FUV/NUV)colors.Figure2showstheobservedphotometriccolors(V FUVversusFUV NUVandB VversusV R)ofLP400-22comparedtosyntheticwhitedwarfandmain-sequencecolors.WeusedKuruczsyntheticspectra(Kurucz1993)tocalculateourmain-sequencecolors.IntheUV-opticaldiagram(V FUV/FUV NUV)ofFigure2weshowtwosetsofWDsyntheticcolors.Thegridshowninblackincludesthee ectofLyαsatel-lites(Allard&Koester1992)ascomparedtothegridingreenwhichexcludesthem.Acomparisonofthetwogridsshowsthesigni cante ectthattheLyαsatelliteshaveontheUVcolorsatTe <paringtheUV-opticalphotometryofLP400-22tothewhitedwarfgrid,alowsurfacegravitylogg～6andane ectivetem-peratureof～11000Kisimplied.Theopticaldiagram(B V/V R)inFigure2con rmsthewhitedwarftemperatureof11000Kandthelowsurfacegravity.However,whencomparingthephotometrytomain-sequencecolors,aA3VspectraltypeisimpliedintheopticalandaB8Vspectraltypeintheultraviolet.There-fore,thedataareincompatiblewithmain-sequencecol-ors.OpticalandUVcolorsareusefultodistinguishwhitedwarfsfrommain-sequencestars.

3.2.Spectroscopy

TheBalmerlinesofLP400-22wereanalyzedinall

threeavailablespectrausingaχ2minimizationtech-nique.Thequoteduncertaintiesarestatisticalonly(1σ).TheBalmerlines(HβtoH9)intheKPNOspectrumwere ttedwithmodelspectrawhich weresmoothedtotheinstrumentalresolutionof14A,toobtainT±350Kandlogg=6.48±0.27.Forthee =11000twohigh-resolutionAPOspectrawe ttedHαandHγtoH9withmodelspectra,toobtainT.46e =11060±180Kand11160±250K,andlogg=6±0.13and6.22±0.10.ThesyntheticspectrausedintheanalysisoftheAPOspectraweresmoothedwith agaussianpro letotheinstrumentalresolutionof2A.Notethatthedisprep-ancyinthesurfacegravitiesfromthe2APOspectraaremostlikelytheresultofuncertaintiesinthe uxcal-ibrationaroundthehigherBalmerlines.TheBalmerline toftheKPNOspectrumisshowninFigure3.Thesemeasurementsclearlycon rmthatLP400-22isawhitedwarfwithalowsurfacegravity.ThecalculatedweightedaverageofthetemperatureandsurfacegravityisTe =11080±140Kandlogg=6.32±0.08.

Weusedtheevolutionarytracksforhelium-corewhitedwarfsofAlthausetal.(2001)andSerenellietal.(2001)todetermineamassof0.17±0.01M⊙andacoolingageof5±1×108years.Notethatthecoolingageofthewhitedwarfissensitivetothemassofthehydrogen

We report the identification of LP 400-22 (WD 2234+222) as a very low-mass and high-velocity white dwarf. The ultraviolet GALEX and optical photometric colors and a spectral line analysis of LP 400-22 show this star to have an effective temperature of 1108

Low-masshigh-velocityWD

3

Fig.2.—GALEXV-FUVversusFUV-NUV(left)andopticalB-VversusV-R(right)diagramsshowingthepositionofLP400-22comparedtotheDAwhitedwarfsequenceandthemain-sequence(inred).Thee ectivetemperaturesareindicatedinunitsof1000Kandthelogg=6.0,7.0,8.0and9.0(frombottomtotop).Intheopticaldiagramlogg=6.0isindicatedbytheblueline.IntheUV-opticaldiagram,thegridshowninblackincludestheLyαsatelliteandthegridingreendoes

not.

Fig.3.—Spectral toftheBalmerlines(HβtoH9)oftheKPNOspectrumofLP400-22.

envelopeleftbeforeenteringthe nalcoolingtrack(seeAlthausetal.2001,andreferencestherein).ResidualH-burninginathickH-envelopecausesthewhitedwarftocoolslowerascomparedtoawhitedwarfwithathinH-envelope.

Thetemperatureof11080KplacesLP400-22neartheblueedgeoftheZZCetiinstabilitystrip(Gianninasetal.2005).Giventhelackoftimecoverageinourdata,wecannotstatewhetherthestarisvariableornot.Time-seriesphotometryisrequiredtoexplorevariabilityandplaceconstraintsontheblueedgeoftheinstabilitystripatthelow-massrange.

4.DISCUSSION

LP400-22waslistedintheNewLuytenTwo-Tenths(NLTT)catalog(Luyten1979)tohaveacommonpropermotioncompanion(LP400-21)338′′away.Recently,Salim&Gould(2003)haverevisedthecoordinatesandpropermotionsofmoststarsintheNLTTcatalogbycross-correlatingthecatalogwiththeTwoMicronAllSkySurvey(2MASS)andtheUSNO-Acatalogs.How-everLP400-22wasnotdetectedin2MASS,andthere-

foretheyrelistedLuyten’smeasurementofthepropermotion.TheylistedthepropermotionofLP400-22tobeµα=0′′.1950±0′′.0200yr 1andµδ=0′′.0563±0′′.0200 1

yr.ForLP400-21theymeasuredapropermotionofµα=0′′.2158±0′′.0055yr 1andµδ=0′′.0283±0′′.0055 1

yr.Thesepropermotionmeasurementsagreewithin2σandonthisbasisthetwostarsappeartobeacommonpropermotionbinary.However,similarmea-surementswerereportedbyLepine&Shara(2005),i.e.,µα=0′′.198yr 1andµδ=0′′.053yr 1forLP400-22,andµα=0′′.228yr 1µδ=0′′.020yr 1forLP400-21.ThequoteduncertaintiesintheLepine&Shara(2005)mea-surementsare～0′′.007yr 1and,therefore,thediverg-ingpropermotionsofthetwostarsappearstoexcludeaphysicalassociation.Anotherwaytocheckwhetherthestarsareaphysicalbinaryistodeterminethedistanceofeachstar.

Toestimatethedistance,wecalculatedanabsolutemagnitudeofMV=9.1±0.2magforLP400-22andadistancemodulus(V MV)=8.2mag.Thisplacesthewhitedwarfatadistanceof430±45pc.NotethattheGalacticextinctionforthisobjectislowanditse ectwasnotincluded.Silvestrietal.(2005)classi edLP400-21adM4.5e,andusingtheMV/V IrelationfromReid&Gizis(1997)weestimatetheabsolutemagnitudeofthereddwarfas12.7mag.TheapparentVmagnitudeforLP400-21isV=17.177±0.021mag,and,therefore,thereddwarfisatadistanceof～80pc.Reid&Gizis(1997)noteascatterofvaluesabouttherelationwithσ=0.46.EvenifweconsiderLP400-21attheextremaofthisdispersion,itwouldplaceitatadistanceofonly～100pc.ThelargedistancediscrepancymakesLP400-22/21acoincidentalpairratherthenawidebinaryashasbeenthoughtbasedontheirpropermotionalone.

Thelargedistanceandhigh-propermotionofLP400-22implyalargetangentialvelocityof414±43kms 1.Onlyafewwhitedwarfsareknowntohavevtan>350kms 1,withmostofthesehavinghalospaceveloc-ities(Bergeronetal.2005).InordertoobtaintheU,V,WspacevelocitycomponentsforLP400-22,wemeasuredtheradialvelocityofthewhitedwarfusing

We report the identification of LP 400-22 (WD 2234+222) as a very low-mass and high-velocity white dwarf. The ultraviolet GALEX and optical photometric colors and a spectral line analysis of LP 400-22 show this star to have an effective temperature of 1108

4

TABLE2

LP400-22Parameters

Parameter

E ectiveTemperatureSurfaceGravityMassMV

Distance

ProperMotionKinematics

Measurement

11080±140K6.32±0.080.17±0.01M⊙9.1±0.2mag430±45pcµ=0′′.203yr 1,0′′.205yr 1

θ=73.9,75.0

U= 388±43kms 1V= 81±22kms 1W= 83±22kms 1

Kawkaetal.

measurementsagreewithinerrorbars.However,aseriesofradialvelocitymeasurementsshouldbeobtainedtoestablishwhetherornotLP400-22isinaclosebinarysystem.

YetanotherpossibilityfortheoriginofLP400-22isthatitmayhaveoncebeeninaclosedouble-degeneratebinary,wherethecompanionhasgonethroughasuper-novaeventthatdisruptedthebinarylosingtheremnantofthedonorstarwithahigh-velocityandalowmass(Hansen2003).

5.SUMMARY

Reference

111112,32,3111

References.—(1)Thiswork;(2)Luyten(1979);Salim&Gould(2003);(3)Lepine&Shara(2005)

HαintheAPOhigh-dispersionspectratoobtainahe-liocentricvalueof 50±20kms 1,whichisdi erentthanthevelocitymeasuredforthereddwarf(7.3kms 1)bySilvestrietal.(2002).WecalculatedU,V,WforLP400-22usingJohnson&Soderblom(1987)toobtainU= 388±43,V= 81±22,W= 83±22kms 1.Thesevelocitycomponentsdonotagreewitheitherdiskorhalopopulations(Chiba&Beers2000)andsuggestadi erentoriginforitspeculiarmotion.TheGalacticorbitforLP400-22shouldbecalculated.

MostwhitedwarfswithM<0.2M⊙arecompanionstopulsars.WesearchedforradiosourcesinthevicinityofLP400-22usingVizieR4,andthenearestwasthatofthegalaxyKUG2234+223～7.5′away.Therefore,ifLP400-22isacompaniontoaneutronstar,thenitisprobablyadeadpulsar.AnotherpossibilityisthatLP400-22hasaverycoolcompanion,whichshouldbedetectableasinfraredexcess.However,LP400-22wasnotdetectedby2MASS.Thetwohigh-dispersionvelocity

4

WehavedemonstratedthatLP400-22isahigh-velocitywhitedwarfwithaverylowmass(M=0.17M⊙)andatemperatureof11080K.Table2sum-marizesthepropertiesofLP400-22.Sincewhitedwarfswithmassesbelow0.4M⊙musthavebeenformedinclosebinarysystems,radialvelocitymeasurementsandinfraredphotometryarerequiredtodeterminewhetherLP400-22hasaclosecompanion.Ontheotherhand,alackofradialvelocityvariationswouldindicatethatLP400-22lostitsclosemassivecompanionfollowingatypeIasupernovaevent.

ThisresearchissupportedinpartbyaNASA/GALEX

ˇgrant(NNG05GE33G).A.K.issupportedbyGACR

205/05/P186.T.D.O.acknowledgessupportfromtheNSF(AST0206115).J.A.S.wassupportedonaNASAGSRPTrainingGrant,NGT-51086.N.M.S.acknowl-edgessupportfromaNASAGSRPgrant(NST200415)andNSFAST02-05875.WethankPaulHintzenforac-quiringtheoriginalKPNOspectrumofLP400-22.ThisresearchhasmadeuseoftheVizieRcatalogueaccesstool,CDS,Strasbourg,France.BasedonobservationsobtainedwiththeAPO3.5mtelescope,whichisownedandoperatedbytheAstrophysicalResearchConsortium.

http://vizier.u-strasbg.fr/viz-bin/VizieR

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