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/