Pulsating sdB Stars A New Approach to Probing their Interior(5)

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

InteriorsofsdBstars

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Figure2.EvolutionarytracksofEHBmodels.Modelsderivedfromthepost-helium ashcoreofa1.5M⊙.Alternatingtrackscanbedistinguishedbythedi erentlinetypes.Envelopemassesare0.00001,0.00004,0.00032,0.00122,0.00232,0.00322,0.00422M⊙,runningfromlowerlefttoupperright.DatafromKilkenny(2001).

3.AsteroseismologyProbes

NonradialpulsationsinsdBstarsprovideawindowintotheirinteriors,andmaytestthehypothesisthathorizontalbranchstarshaverapidlyrotatinginteriors.HereweillustratetheprocessbywhichwecandetectrapidrotationofthestellarinteriorusingcurrentlyobservedsdBVstars.Afuturepaperwilldescribeourasteroseismicresultsinmoredetailandapplytheanalysistoindividualpulsators.

Figure2showsanH–RdiagramthatincludestheknownsdBstarsandsampleevolutionarytracksfromoursetsofmodels.Giventheconvergenceofthetracksinthisplane,itisclearthatH–Rdiagrampositionaloneisinsu cienttojudgetheirevolutionarystate,withstarsthatareclosetotheZAEHBminglingwithstarsthathavedepletedheliumintheircores.ThelowestgravityEC14026starsfallsu cientlyfarfromtheZAEHBthattheycannotbecorehelium–burnersforcoremassesthatareconsistentwithsingle–starevolution.

Manyofthehigh–gravitypulsatingsdBstarsshowcomplexpulsa-tionsthatrequirealargenumberofperiodicities.ExamplesincludePG1047+003Kilkennyetal.(2001)andPG0014+067Brassardetal.(2001).Low nonradialmodeshavealimitedfrequencydistribution,meaningthatsomeofthepulsatorscannotbeunderstoodwithoutresortingtoeitherhighvaluesof ortorapidrotation.

Nonradialoscillationsarecharacterisedbytheeigenfrequencyσn mcorrespondingtoaspheroidalmodewithquantumnumbersn, ,andm.Thevalues andmrefertothesphericalharmonicY m.Thesign