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Th e longitudina l magneti c fiel d o f th e A p sta r H D 4710 3
V.G . Elkin , G.A . Wade
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Specia l Astrophysica l Observator y o f th e Russia n AS , Nizhni j Arkhy z 357147 , Russi a Astronom y Department , Universit y o f Wester n Ontario , London , Canada , N6 A 3K 7

1

Introductio n

A magneti c fiel d i n th e A p sta r H D 4710 3 ha s recentl y bee n detecte d b y Babe l e t al . (1995) . Usin g th e ELODI E spectrograp h an d a cross-correlatio n technique , the y discovere d magneticall y spli t lines i n th e spectru m o f thi s sta r fro m whic h the y inferre d a mea n magneti c fiel d modulu s o f abou t 17.5 kG . Whe n w e compar e th e fiel d modulu s o f H D 4710 3 wit h th e result s o f Mathy s e t al . (1996) , w e fin d tha t th e magneti c fiel d o f thi s sta r i s on e o f th e stronges t eve r studied . B y obtainin g mea n longitudina l magneti c fiel d measurement s w e hop e t o begi n modelin g th e magneti c fiel d geometr y o f thi s ver y interestin g object , an d t o obtai n constraint s o n th e physic s whic h operate s i n suc h a n uncommon , strong-fiel d environment .

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Observation s

W e hav e obtaine d eigh t Zeema n spectr a o f H D 4710 3 usin g th e Mai n Stella r Spectrograp h an d th e circula r polarizatio n analyze r (Bychko v e t al. , 1988) installe d o n th e 6 m telescope . W e employe d th e secon d camera , wit h th e 530 X 580 pixe l CC D (Borisenk o e t al. , 1991 ) a s th e detector . Th e spectr a obtaine d cove r th e tw o wavelengt h region s ~ 608 3 -- 620 5 an d ~ 628 6 -- 640 8 . Th e spectra l resolutio n i s approximatel y 0.4 5 A , an d th e signal-to-nois e rati o obtaine d i s typicall y ~ 100 . Reductio n o f th e 2-dimensiona l CC D image s wa s performe d usin g th e DEC H packag e develope d b y Galazutdino v (1992) . Th e longitudina l magneti c fiel d B wa s determine d from th e Zeema n shift betwee n th e left an d righ t circula r polarizatio n spectr a for a numbe r o f spectra l lines . T o measur e th e centr e o f gravit y o f eac h spectra l lin e w e employe d th e programm e VECVI Z (Vlasyuk , 1994) .
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Radia l velocit y

W e determine d radia l velocitie s (RV ) fro m S i I I 634 7 an d 6371 , th e stronges t line s i n ou r spectra . Thes e result s ar e show n i n Tabl e 1 . Radia l velocitie s wer e calculate d usin g th e intensit y spectru m compute d from th e averag e o f th e left an d righ t circula r polarizatio n spectra . Th e radia l velocitie s ar e somewha t lowe r tha n thos e foun d b y Babe l e t al . (1995) , an d appea r t o b e slightl y variable . Th e precisio n o f ou r radia l velocitie s i s aroun d 0.5 k m s .
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Magneti c fiel d
spectr s fro m o f ou r d righ t a of HD -2. 7 k G spectr a circula r 4710 3 sho w a variabl e longitudina l fiel d wit h a negativ e polarity . Th e t o -4. 4 kG . Thes e measurement s ar e als o presente d i n Tabl e 1 . Whil e th e i s no t particularl y high , th e fiel d i s quit e stron g an d th e shift s betwee n polarizatio n spectra l lines ar e evident . Thi s i s illustrate d i n Fig . 1 , wher e 106

Ou r eigh t fiel d varie resolutio n th e left an


THE LONGITUDINAL MAGNETIC FIELD OF THE AP STAR HD 47103

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w e sho w par t o f on e Zeema n spectru m containin g S i I I 6371 . Th e asymmetr y o f th e appear s t o b e real , a s i s als o apparen t i n anothe r spectru m obtaine d i n thi s region . an d tw o component s (67 < splittin g o f S i I I 637 1 ha s a simpl e structure : tw o (1000)) . Fro m th e shift betwee n th e tw o component s w e ca n calculat e th e valu e o f Fro m four spectr a w e obtai n a mea n field modulu s B = 1 1 0.9 kG .
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lin e i n Fig . 1 Th e Zeema n 1000 > 133 surfac e field.

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Rotationa l perio d

Ther e ar e n o previou s dat a concernin g th e rotationa l perio d o f thi s star . Babe l e t al . (1995 ) propose d tha t perio d shoul d b e mor e tha n 3 3 days . I n Fig. 2 w e sho w th e relatio n betwee n mea n field modulu s an d perio d for a numbe r o f cool A p stars , fro m th e lis t compile d b y Glagolevski j e t al . (1986 ) an d als o take n fro m pape r b y Mathy s e t al . (1996) . Onl y severa l star s hav e mea n field modul i greate r tha n 1 0 kG , an d all o f the m hav e period s shorte r tha n 1 0 days . A smal l grou p o f star s wit h perio d mor e 3 0 day s hav e mea n surfac e fields nea r 1 0 kG . O n thi s (admittedl y no t ver y significant ) evidenc e w e ca n speculat e tha t perio d o f H D 4710 3 i s als o shorte r tha n 1 0 day s o r mor e tha n 3 0 days . A perio d searc h o f ou r longitudina l field dat a indicate s a numbe r o f possibl e periods . W e ar e unabl e t o selec t amon g th e mos t significan t period . Clearl y mor e dat a ar e necessar y t o determin e th e rotationa l perio d o f thi s star .

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Discussio n
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Ou r lower valu e o f B i n compariso n wit h th e result s o f Babe l e t al . (1995 ) coul d b e connecte d wit h th e low resolutio n o f ou r spectra . Thi s difference ma y als o b e du e t o th e fac t tha t th e magneti c quantit y whic h Babe l e t al . (1995 ) calculat e fro m thei r spectr a i s poorl y defined (sinc e the y emplo y a blen d o f man y line s wit h man y different Land e factors) . A final possibilit y i s that , sinc e ou r measurement s o f B an d th e measuremen t o f Babe l e t al . (1995 ) wer e acquire d a t different times , an d likely a t different phases , w e expec t t o find a difference betwee n them . W e observe d H D 4710 3 durin g a four-mont h span , an d i t i s clea r tha t w e hav e observe d th e sta r nea r th e negativ e magneti c pole . A s th e sta r ha s low v sin i = 4. 4 k m s (Babe l e t al. , 1995) , i t i s quit e possibl e tha t th e lin e o f sigh t i s no t far from th e rotationa l axis . I f thi s i s th e case , w e woul d no t expec t t o see a chang e i n th e sig n o f th e longitudina l field wit h phase .
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Acknowledgements .

Thi s wor k wa s partl y supporte d b y RFB R gran t 94-02-06584 .


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ELKIN,

WADE


THE

L O N G IT U D IN A L

M A G N E T IC

F IE L D

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THE

AP

STAR

HD

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