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Some comment s on evolutio n of magneti c fields of CP stars
Yu.V . Glagolevskij , G.A . Chountono v Special Astrophysical Observatory of th e Russian AS, Nizhnij Arkhyz 369167, Russia

Theses
1 . A comprehensiv e stud y o f th e magneti c fields o f uppe r mai n sequenc e C P star s an d thei r progeni tors , emissio n Herbi g stars , ha s show n tha t th e mag neti c fields an d chemica l anomalie s i n C P star s origi nat e a t th e momen t youn g Ae/B e star s arriv e o n th e zero-ag e line , whe n th e gaseous-dus t envelope s sur roundin g the m dissipat e an d accretio n ceases . I t i s th e ZAM S tha t i s th e plac e magneti c C P star s for m at . 2 . Nea r th e ZAM S th e atmosphere s stabilize , th e emerge d magneti c field stabilize s i n additio n th e up per part s o f th e stella r atmospheres , condition s for diffusion o f chemica l element s tha t bring s abou t th e observe d chemica l anomalie s arise . 3 . Th e stud y o f th e rat e o f C P star s ami d mai n sequenc e star s ha s show n tha t th e smalle r v sin i th e greate r th e proportio n o f C P stars . So , magneti c star s accoun t for 30% , a t a n averag e level o f 10-15% , amongs t th e star s wit h vsin i = 0-1 0 km/s . Thi s re latio n i s contrar y t o th e on e expecte d i n th e cas e th e magneti c field i s dynam o generated . Th e dynam o magneti c field mus t lea d t o a field magnitud e propor tional t o th e rotatio n velocity , an d i t mus t b e nul l i n star s wit h minimu m velocities . Th e assumptio n tha t the observe d relatio n i s th e resul t o f subsequen t mag netic brakin g wit h th e magneti c field involve d ma y lead t o mos t improbabl e law s o f braking . 4 . Th e dependenc e o f th e averag e magneti c field o n vsin i confirm s th e conclusio n draw n i n poin t 3 . This dependenc e show s tha t practicall y al l star s wit h stron g magneti c fields fall withi n th e rang e v si n i = 3 0 - 40km/s , whil e afte r vsin i = 40km/ s onl y star s with wea k magneti c fields , mos t frequentl y wit h fields weaker tha n th e norma l detectio n threshold . Thi s boundar y i s sufficientl y clear-cut . Takin g int o accoun t point 3 i t i s inferre d tha t whethe r th e reli c magneti c fiel d i s preserve d depend s strongl y o n th e sta r rota tion velocity . 5 . Magneti c field measurement s i n youn g emis sion Herbi g stars , 1 5 % o f whic h late r becom e mag netic C P stars , hav e show n tha t typica l stron g dipola r magneti c fields ar e absen t i n them . 6 . Th e result s give n i n point s 3 an d 4 illustrat e tha t th e slo w rotatio n velocitie s o f magneti c C P star s resul t no t fro m magneti c brakin g bu t rathe r fro m th e

fac t tha t i t i s th e slo w rotator s tha t conserv e th e reli c magneti c field better . 7 . Th e magneti c field s o f C P star s ar e no t gen erate d i n th e convectiv e core , neithe r the n ar e the y drive n t o th e surface . Thi s assumptio n i s verifie d b y th e fac t tha t alon g th e whol e mai n sequence , C P star s for m exactl y o n th e ZAMS , an d th e mos t massiv e o f the m o n th e ZAM S ar e tw o order s o f magnitud e younge r tha n th e mos t low-mas s stars . Takin g int o accoun t tha t i t take s th e fiel d a lon g tim e t o ris e t o th e surface , on e migh t expec t th e fiel d i n thes e star s t o appea r a t late r phase s tha n i n low-mas s stars . 8 . A s a consequenc e o f th e present-da y poin t o f vie w tha t th e protostar s an d emissio n Herbi g star s d o no t underg o th e convec wit h masse s M > 1.5 tiv e phas e o f evolutio n throughou t th e whol e lifetime , differen t author s assum e tha t th e magneti c field s ar e relic . Thi s assumptio n i s corroborate d b y inference s mad e i n point s 3, 4 an d 7 . Despit e th e failur e t o revea l stron g dipola r magneti c field s i n them , on e ca n clai m tha t stron g magneti c field s d o exis t i n th e subsurfac e layer . Havin g a relativel y lo w spee d o f emergenc e an d undergoin g th e distructiv e actio n o f mas s o f accre tion , a magneti c fiel d canno t appea r o n th e entir e surface . Afte r th e en d o f accretio n an d stabilizatio n o f th e surfac e layers , th e magneti c fiel d rise s t o th e surfac e unimpede d (se e p.2) . 9 . A compariso n o f th e behaviou r o f vsini for norma l an d C P star s a s the y evolv e fro m th e ZAM S t o th e uppe r boundar y o f th e mai n sequenc e ha s show n tha t th e rati o o f thei r vsini i s preserve d withi n th e errors . Thi s suggest s tha t C P star s d o no t underg o additiona l braking . I n s o fa r a s ther e i s n o magneti c brakin g a t th e phas e o f emissio n Herbi g star s eithe r (se e p.5) , ther e remain s th e onl y possibilit y for th e magneti c fiel d t o b e involve d i n th e brakin g a t th e earlies t stage s o f thei r formation , prio r t o th e "birth line " o f Herbi g stars . 10 . Th e propertie s o f C P star s describe d i n point s 3 an d 4 giv e ris e t o th e assumptio n tha t on e o f th e mai n condition s for retainin g o f th e fiel d i s th e low rotatio n velocit y (th e magneti c brakin g i s likel y t o b e o f mino r importance) . 11 . A t low velocities , a s on e ma y suppose , n o differentia l rotatio n occurs , whic h woul d hav e converte d a poloida l initia l fiel d int o a torroida l fiel d an d le d t o othe r instabilities . I n tande m wit h th e effect o f am © Specia l Astrophysica l Observator y o f th e Russia n AS , 200 0


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bipola r diffusion , th e absenc e o f differentia l rotatio n i s possibl y th e secon d principa l conditio n o f conser vatio n o f th e poloida l fiel d i n magneti c stars . I t i s impossibl e t o detec t a toroida l fiel d b y mean s o f th e classica l Zeema n procedure . Th e immediat e tas k i s a searc h for sign s o f toroida l field s i n stars . 12. Investigation s o f emissio n Herbi g star s demon strat e tha t man y properties , for instance , th e loca l outflows , canno t b e accounte d for withou t assumin g tha t loca l magneti c field s ar e involved . I t i s no t im probabl e tha t i n particula r portion s o f th e loca l mag neti c fields condition s aris e tha t favou r th e emergenc e o f magneti c line s o f force . Th e proble m consist s i n searchin g an d detectin g suc h cases . 13 . Th e fractio n o f star s wit h measure d magneti c

field s ha s bee n foun d t o decreas e monotonicall y wit h growin g mass . So , th e shar e o f He-stron g star s wit h a magneti c fiel d i s 40% , whil e tha t o f magneti c SrCrE u star s i s a s larg e a s 9 0 % . I t implie s tha t th e magneti c fiel d formatio n tim e i n massiv e star s i s rathe r lon g a s compare d t o th e lifetim e o f star s o n th e mai n se quence . Thi s fac t togethe r wit h th e result s o f mod elin g allow s u s t o infe r tha t th e appearanc e o f a magneti c fiel d lag s behin d th e generatio n o f chemi ca l anomalies , an d th e fiel d ha s initiall y a comple x configuration . A s a result , th e measure d fiel d ma y initiall y b e weak . Acknowledgements . Th e work was supporte d throug h grant s o f RFB R an d "Astronomy" .