Pulsating sdB Stars A New Approach to Probing their Interior

Horizontal branch stars should show significant differential rotation with depth. Models that assume systematic angular momentum exchange in the convective envelope and local conservation of angular momentum in the core produce HB models that preserve a ra

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aPulsatingsdBStars:ANewApproachtoProbingtheirInteriorsStevenD.KawalerandShelbiR.HostlerDept.ofPhysicsandAstronomy,IowaStateUniversity,Ames,IA50011USAAbstract.Horizontalbranchstarsshouldshowsigni cantdi erentialrotationwithdepth.Modelsthatassumesystematicangularmomentumexchangeintheconvectiveen-velopeandlocalconservationofangularmomentuminthecoreproduceHBmodelsthatpreservearapidlyrotatingcore.Adirectprobeofcorerotationisavailable.ThenonradialpulsationsoftheEC14026starsfrequentlyshowrichpulsationspectra.Thustheirpulsationsprobetheinternalrotationofthesestars,andshouldshowthee ingmodelsofsdBstarsthatincludeangularmomentumevolution,weexplorethispossibilityandshowthatsomeofthesdBpulsatorsmayindeedhaverapidlyrotatingcores.1.IntroductionFirststudiedindetailbyR.Petersoninthemid1980s(Peterson1983,1985a,b),therapidrotationofsomehorizontalbranchstarsremainsanenigmaticfeatureofthisphaseofevolution.ObservationsbyBehretal.(2000a,b)showagapinthehorizontalbranchofM15andM13,withtwodistinctrotationratesoneitherside.Coolerthanabout11000K,HBstarsfrequentlyshowrapidrotation(vsini≈40km/s)whilenorotationratesabove10km/sappearinthehotterstars.Oneclassof eldstarsisclearlyrelatedtohorizontalbranchstars.ThesubdwarfB(sdB)starsareveryhot(Te 25,000K),andshowhighgravity(logg≈5.3 6.1).Modelsproposedtoexplaintheiroriginincludecommonenvelopebinaryevolution(eg.Sandquistetal.,2000,Mengeletal.,1976),variationsofmasslosse ciencyontheRGB(D’Cruzetal.,1996),andevenmassstrippingbyplanetarycompanions

(Soker&Harpaz,2000).Someproposedoriginscenariosmakeclearpredictionsabouttheirrotationalproperties;forexample,thosethatinvolvebinarymergersorcommonenvelopeevolutionshouldspinupthesdBproduct.ExtrapolationoftheresultsforPop.IIstarssuggeststhatsdBstarsshouldbeslowrotators,withvsini 10km/s.TheworkbyHeberetal.(1999)hasshownthatatleastonestar,PG1605+072,isamorerapidrotator–asexpectedbasedonanasteroseismicanalysisofthepulsationsbyKawaler(1998).

Sills&Pinsonneault(2000)(hereafterSP2K)exploredHBrotationandsingle-starevolutionwithdi erentassumptionsaboutinternalan- c2008KluwerAcademicPublishers.PrintedintheNetherlands.