LP 400-22, A very low-mass and high-velocity white dwarf(3)

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