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FIRS T MEASUREMENT S O F 4 STOKE S PARAMETER S WIT H TH E 6-M TELESCOP E

Yu . V . Glagolevskij , V . G . El ' kin , I . I . Romanyuk , V . G . Shtol ' Special Astrophysical Observatory, Stavropolskij Kraj , 357147, Nizhnij Arkhyz, USSR

ABSTRACT. First measurements of 4 Stokes parameters have been made 5200 A for two chemically peculiar in the wide depression at stare 53 Cam and 56 Tau using the 6-m telescope spectropolarimeter. Instrumental circular and linear polarization is shown to be insignificant.

The spectropolarimeter for measurements of 4 Stokes parameters is designed and constructed at SAO AS USSR. It is planned to be used for determination of a magnetic field structure of peculiar stars, and dipole orientation, for study of circular and linear polarization wide depressions within 4200 - 5200 A being characteristic for magnetic star spectra and also for other spectropolarimetric investigations of various objects in narrow (dozens of A) and wide (hundreds of A) spectral regions. The new device is based on the available hydrogen magnetometer (Shtol' et al., 1985). Adjustment and laboratory tests of the spectropolarimeter for narrow spectral regions were realized during 1987. This device is analogous to that described by Wolstencroft et al.(1983) with one electrooptical KDP crystal and two quarter-wave mica plates with optical axes oriented at 45° angle relative each other. When measuring Q and U Stokes parameters phase plates are introduced in turn; V-parameter is determined with both quarter-wave plates taken out from the optical scheme. The first observations of Serkovsky's (1979) standard stars with zero polarization made in January, 1988 showed that the degree of instrumental polarization in the Main Focus of the 6-m telescope is small : (v, q, u - normalized Stakes parameters).

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Our polarization degree p=3.14% for the supergiant HD 43384 (with large interstellar polarization) shows a good agreement with Serkovsky's data p=3.20%. We began to investigate circular and linear polarization in 5200 A for two chemically peculiar stars the wide depression at 56 Tau(it s circular polarization was suspected earlier,(se e Romanyuk, 1984), and 53 Can with a strong magnetic field. The spectral band width in our observations was 20 A, the light detector used was EMI-photomultiplier. Table 1 presents the investigation results of 53 Cam.

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The time of measuring of one of Stokes parameters at a given wavelength was 10 min on the average; the time necessary to change the wavelength or to measure another Stakes parameter was 10 s. The Fig. 1 shows the distribution of v, q , u - Stokes parameters along the depression profile at 5200A. Fig. la is taken from Maitzen's work (1976). It is seen that only the central part of the depression was investigated and a further study at a wider spectral range and with a higher accuracy is needed. The existence of weak (some hundredths of %) circular and 5200 A in the stars linear polarization in the depression at measured by us is highly probable. The device with two electrooptical crystals for quasisimultaneous measurements of all the four Stokes parameters is planned to be introduced in the nearest future.

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REFERENCES Nolstencroft, R. D., Cormack, W. A., Campbell, J. W., Smith, P. J.: 1983, Mon. Not. Roy. Astron. Soc , v. 205 , 23. Serkovsky, K.: 1979, In: Planets, Stars and Nebulae Studies by photopolarimetry , University of Arizona press, Tucson. Shtol', V. G., Bychkov, V. D., Vikul'ev, N. A., Georgiev, 0. Yu., Glagolevskij, Yu. V., Drabek, S. V., Naidenov, I. D., Romanyuk, I. I.: 1985, Astrofiz. Issled. (Izv. SAO) , v. 19,66 . Romanyuk, I. I.: 1984, In: Magnetic stars , Salaspils, p. 29. Haitzen, H.: 1976, In: Physics of Ap stars, IAU Coll No. 32 , Vienna, 233.