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"Stellar magnetism"

International Conference Proceedings, 1992 , 57-59 .

OBSERVATIO N O F FOU R STOKE S PARAMETER S I N TH E CONTINUU M O F HE-RIC H STA R HD3777 6

I.I . ROMANYUK , V.G . EL'KIN , V.G . SHTOL ' Specia l Astro-physical Observator y o f the Russia n AS , Nizhni j Arkhy z 357147 , Russi a

ABSTRACT . Observations of the He-rich star 777 6 on the 6 m telescope have registered constant linear polarization (p = 0.44 %, 0=38° ) within

AA 4580-481 0 A in August - October 1991. No circular polarization is found within the measurement errors. This may be explained under the assumption that emission polarization of this star has interstellar or circumstellar

origin.

Th e helium-ric h

sta r HD3777 6 i s on e o f magneti c stars , whos e quadrupol e field

componen t i s stronge r than dipol e (Thompson and Landstreet , 1985) . Bohlende r and Landstree t (1990 ) hav e reported o n possibl e existenc e o f a fiel d o f ~6 0 kG s o n the surfac e o f this star. Fro m the Zeema n splittin g o f spectra l lines Kopylov a e t al . (1991 ) hav e determin ed, that the existenc e of a comple x structur e magneti c fiel d S reachin g ~8 0 kG s is real. Sinc e a t suc h stron g field s magneto-optica l effect s ar e possible , bot h i n the star continuu m an d i n its spectra l lines, w e decide d t o investigat e spectropolarimet ricall y the continuu m and to carr y out a detaile d investigatio n of lines . In particular , of grea t interest is the questio n on polarizatio n variabilit y (if it is detected ) wit h a perio d of magneti c and photometri c changes , correspondin g to the rotatio n perio d o f HD3777 6 P=l. 58 (Thompson and Landstreet , 1985) . Kem p an d Woltenscrof t (1974) hav e investigated the linear polarizatio n i n the con tinuum of severa l CP star s and detecte d in some of them variabilit y of th e degre e and angl e o f polarization . Th e author s o f the abov e mentione d pape r believe , that the transversa l Zeema n ef fect cause d the appearanc e of the prope r variabl e linear polarization . In thi s paper w e presente d the measuremen t result s o f 4 Stoke s parameter s i n the continuu m obtaine d
d

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on the 6 m telescope .

In Augus t - Octobe r 1991 we had a possibilit y to perfor m seve -

ral observation s o f 4 Stoke s parameter s i n the continuu m o f HD3777 6 withi n the wave lengt h interva l AA 458 0 - 481 0 A wit h the prim e focu s spectropolarimete r of the 6 m telescop e (Glagolevski j e t al. , 1988) . Regula r observation s of nonpolarize d star s show, instrumenta l percent . 4 Stoke s parameter s o f the standard star HD792 7 wit h stron g interstella r polarizatio n wer e measure d o n the same night s a s HD37776 . Bastie n e t al . linear that in ou r investigation s the

linear and circula r polarizatio n doe s no t excee d som e hundredth s o f a

(1988 ) pre -

sented a list of the followin g parameter s of the linear polarizatio n for thi s star: Ou r measurement s yielde d the mea n the degre e of p and angl e polarization : p=3.11%±0.26% , the measuremen t errors . Thus , valu e o f

(in the abov e mentione d spectra l interval ) of the linear =94°5±2°2 . N o circula r polarizatio n wa s foun d withi n betwee n th e standar d value s

n o difference s ar e found

and our measurement s of p and

, the degre e of instrumenta l transformatio n of the li -

nea r polarizatio n into the circula r on e is insignificant. Le t u s retur n t o the observation s o f the peculia r sta r HD37776 . result s of its polarizatio n ar e presente d in Tabl e 1: V-parameter . characterizin g circula r polarization . p=(q normalize d Stoke s parameter s and zation. Th e measuremen t

v=V/ I is the normalize d Stoke s
2

+u )

2

1/ 2

, wher e q=Q/I , u=U/ I -

=l/2*arctg(u/q ) - characteristic s o f linear polari -

Th e dat e and U T o f the middl e o f the exposur e a s wel l a s its duratio n i n mi -

nute s ar e als o presente d in Tabl e 1. Tabl e 1.

Thus , as a resul t of our measurements ,

it wa s found,

that the emissio n of HD3777 6

i s linearl y polarized . Withi n the measuremen t error s the degre e o f the linear polari zatio n is constant , p=0.44% , and at a wavelengt h of about 470 0 A the degre e of polarizatio n =38°. The numbe r of observation s variations . is insufficien t to judge Th e circula r polarizatio n

and the angl e

abou t the realit y o f the polarizatio n angl e doe s no t excee d the measuremen t errors .

Sinc e the perio d o f magneti c and spectral

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variation s of HD3777 6 equal s 1. 58, then, apparently , the absenc e of linea r polariza tio n variabilit y i s rather indicativ e o f its interstella r o r circumstella r origin . A s fo r polarizatio n magnitud e i n individual lines, w e performe d th e firs t serie s o f ob servation s only , and althoug h Zeema n effect i s wel l pronounce d i n hydroge n lines, furthe r investigation s ar e needed.

d

REFERENCE S

Bastie n P., Drisse n L. , Menar d F. , Moffa t A.F.J., Rober t C. , St.Loui s N. : 1988,Astron.J.,95,900. Thompso n I.., Landstree t J.D.: 1985, Astrophys.J. , 289, No.1 , 9. Bohlende r D., Landstree t J.D.,: 1990, Science News, 27 January .

Kopylov a F.G., Glagolevski j Yu.V. , Romanyu k 1.1.: 1991 , A Peculiar Newsletter, No. 21 , 27 . Glagolevski j Yu.V. , El'ki n V.G. , Romanyu k I. I. , Shtol ' V.G. : 1988 , in: Magnetic stars, Proceed , of Intern. Meeting , eds. : Glagolevski j Yu.V. , Kopylo v I.M., Lenin grad , Nauka , 22 . Kem p J.C. , Wolstencrof t R.D. : 1974, in: Planet, Stars and Nebulae Studied by Spectropolarimetry, Tucson , Arizona , 988 .

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