The Radio Lifetime of Supernova Remnants and the D

astro-ph/940703112Jul1994SubmittedtotheAstrophysicalJournalTheRadioLifetimeofSupernovaRemnantsandtheDistributionofPulsarVelocitiesatBirthD.A.FrailandW.M.GossNationalRadioAstronomyObservatory,P.O.Box0,Socorro,NM87801andJ.B.Z.WhiteoakSchoolofPhysics,UniversityofSydney,Sydney,NSW2006,AustraliaABSTRACTWehavemadeVLAimagesoftheeldsaroundthreeyoungpulsarswhichhaveresultedinthediscoveryoftwonewsupernovaremnantsandconrmationofathird.Wearguethat,inatleasttwocasesandperhapsthethird,thepulsarsarephysicallyassociatedwiththesesupernovaremnants.Areviewofallknownyoungpulsarsshowsthatthemajorityareassociatedwithsupernovaremnants.Weshowthatthetypicaldensityoftheinterstellarmediumintowhichthesupernovaremnantsareevolvinghasadensityof0.2cm3insteadofthelowvalueof0.01cm3whichhadbeencalculatedfromotherstudies,andresultsinaconsiderablylongerradiolifetimeforsupernovaremnants.Boththemorphologyofthesupernovaremnantsandthelocationofthepulsarsimplythatmostoftheseyoungpulsarsarebornwithlargetransversevelocities(500kms1).Thishighvelocitymeaninthedistributionofpulsarvelocitiesappearstobeageneralpropertyofthepulsarpopulationatbirth,notseeninpropermotionstudies,duetoselectioneects.Weexploretheimplicationsofthisresultasitrelatestotheoriginofthesevelocitiesandthegalacticdistributionofpulsars.Highvelocitypulsarscanescapetheirsupernovaremnantinaveryshorttimescale,comparabletothelifetimeoftheremnantandmayevenplayaroleinextendingtheobservableradiolifetimeoftheremnant.AsignicantfractionwillbecapableofescapingthediskoftheGalaxy,producinganextendedhalopopulation.Subjectheadings:(ISM:)supernovaremnants,(stars:)pulsars:PSR164343,PSR170644,PSR1727331.INTRODUCTION{2{Youngpulsarsoerdirectinsightintosomeofthecentralquestionsconcerningtheoriginandevolutionofneutronstars.StatisticalstudiesofthepulsarpopulationasawholehaveestablishedbeyondadoubtthattheprogenitorsofpulsarsaremassivestarsandthattheirbirthplacesaretheOBassociations,densegasandspiralarmsfromwhichsuchstarsarise.Thisevidenceincludesthegalacticdistributionofpulsars(Lyne,Manchester&Taylor1985),theirkinematics(Lyne,Anderson&Salter1982),andtheirbirthrates(Narayan&Ostriker1990).However,despitetheinsightthathasbeengainedfromtheseindirectmethods,statisticalanalysiscanneverbeasubstitutefordirectobservationalevidence.LackinganyrecentsupernovaeinourourGalaxy,weturntoyoungpulsarsandtheirassociatedsupernovaremnants.Withamedianageof4106years(Taylor,Manchester&Lyne1993),theaveragepulsarispresentlywellawayfromitsbirthplace.FourmillionyearsisclosetothelifetimeofanOBassociationanditiswellinexcessoftheageexpectedforasupernovaremnantsurroundingthepulsar.Thusitisonlybystudyingtheenvironmentsaroundyoungpulsarsthatwecanhopetolearnmoreaboutthebirthplaceofneutronstars.Herewedeneyoungpulsars,somewhatarbitrarily,asthosepulsarswithcharacteristicagesc60,000years(=P/2_P,wherePisthepulsarperiodand_Pisitsperiodderivative).Throughthestudyofyoungpulsarsandtheirsupernovaremnantswecangaininformationabouttheperiods,magneticelds,andvelocitiesofpulsarsattheirbirth.Thesupernovaremnantprovidesanindependentageanddistanceestimateforthesystemandactsasaprobeoftheambientgas(density,llingfactor,pressure,etc)(Shull,Fesen&Saken1989).Theinteractionbetweentheenergeticwindofapulsarandthesurroundingmediumcangiverisetopulsarwindnebula{auniquediagnosticofthepulsarmagnetosphere(Wang,Li&Begelman1993,Bietenholz,Frail&Hankins1991).Thedicultyinpursuingsuchstudieshasbeenthesmallnumberofpulsar/supernovaremnantassociations(Helfand&Becker1984).Inthelastveyearsmajoradvanceshavebeenmadeontwofronts.HighfrequencyradiosurveyshavepenetratedfartherintotheplaneofourGalaxy(Cliftonetal.1992,Johnstonetal.1992)andrenewedradioimagingeorts(e.g.Caswelletal.1992,Frail&Kulkarni1991,Kassim&Weiler1990a)haveturnedupseveralnewassociations.Thereareperhapsasmanyas13supernovaremnantsinassociationwithyoungpulsars,andanother2in\adolescentpulsars(60kyrsc110kyrs).Recently,Johnstonetal.(1992),ina1.5GHzsurveyofthesouthernGalacticplane,foundveadditionalyoungpulsars.Thisbringsthetotalnumberofyoungpulsarsto18{stilllessthan3%ofthetotalpopulationofknownpulsars.Advancesinlowfrequencyradioimaginghavealsobeenneededinordertomakedeep,wide-eldimagestowardtheseyoungpulsars.Inparticular,thedevelopmentofpowerfulnewreductionandimagingsoftware(Cornwell1993,Cornwell,Uson&Haddad1992,Cornwell&Perley1992)attheVeryLargeArray(VLA)haspromptedthepresentstudytodoobservationsat90-cm(0.3GHz)and20-cm(1.4GHz)towardthreeofthesepulsarsinhopesofndingtheirputativesupernovaremnant.Theobservationsaredetailedinx2andwediscussourresultsinx3.Theimplicationsofourdetectionsarediscussedinx4.{3{2.OBSERVATIONSTheobservationsweremadewiththeVLAintheCnB(1993Jun.2)andDnC(1993Sept.14)hybridarraycongurations.Theeldstowardthreepulsarswereobserved(PSR164343,PSR170644,andPSR172733)at90cm(0.3GHz)and20cm(1.4GHz).The20-cmobservationswerecarriedoutinstandardcontinuummode.Two50MHzchannelsweremeasuredineachhandofcircularpolarizationat1385.1MHzand1464.9MHz.At90-cmtwoseparateIFbands,eachmeasuringbothhandsofcircularpol