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A SURFAC E MAGNETI C FIEL D STUD Y O F A p STA R

7 8 VIRGINI S

T. A. Ryabchikova, E. S. Davydova The Astronomical Council of the USSR Academy of Sciences, Pyatnitskaya Str., 48 , Moscow, USSR D. Z. Kolev Bulgarian Academy of Sciences, Department of Astronomy, Lenin Bul., 72 , Sofia, 1184, Bulgaria

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INTRODUCTIO N

Th e discovery of a strong variable surface magnetic field CrB, HD 126515 and 53 Cam (B ) in four Ap stars: HD 215441, (Preston, 1969a; Wolff and Wolff, 1970; Preston, 1970; Huchra, 1972) changed our knowledge about magnetic field geometry, formerly based only on effective field measurements (B ) . Unfortunately, for most of Ap stars the measurement of a full Zeeman pattern of a spectral line in unpolarized light is practically impossible because the very small splitting for B <= 5 kGs cannot be resolved with the accuracy of modern spectroscopy. Therefore, indirect methods of estimation of B were developed using the effect of a magnetic field on the intensity and width of spectral lines. On e of these methods, a photometric one (Cramer and Maeder, 1980) is based on the dependence of the 5200 depression on the magnetic field strength. Two other methods, spectroscopic ones, ar e based on the additional widening of spectral lines introduced by the magnetic field. Preston s method (Preston, 1971) makes it possible to estimate B by measuring the full widths of the spectral lines. This method requires high dispersion spectra. The method of the magnetic field estimation based on the magnetic intensification effect (Hensberge and De Loore, 1974; Ryabchikova an d Piskunov, 1984) (curve of growth method) permits us to use a lower dispersion. In this paper we present some results of an investigation of the variation with the rotation of the surface magnetic field for the Ap star 78 Vir using the curve of growth method.
S e $ S g

2. SPECTRAL DATA AND DATA REDUCTION We studied 14 spectra of 78 Vir obtained with the 2-m telescope in NAO BAS (9 spectra) by D. Kolev, with the 2.6-m telescope in Crimea (3 spectra) by T. Ryabchikova and with the 6-m telescope in SAO AS USSR (2 spectra) by I. Romanyuk. A dispersion of the spectra is 8-9 A/ee. The latter two spectra were obtained with the Zeeman analyzer. In order to obtain the equivalent width in the 40


unpolarized light the values of for both right and left-hand was increased polarization were summed. An accuracy of values by the averaging of equivalent width for the latter two spectra. Spectrograms were reduced on the microdensitometers MD-6 Joyce Loebl and 3CS Joyce Loebl with the help of the software developed in the Astronomical Council of USSR AS (Piskunov et al. , 1984).

3. AN ESTIMATION OF THE SURFACE MAGNETIC.FIELD OF 78 VIR A curve of growth method (Ryabchikova and Piskunov, 1984) was applied to estimate the value of the surface magnetic field BS . Only Fel and Fell lines were used. The theoretical curves of growth were calculated on the basis of Kurucz model with 9500 and log g = 4.0 (Kurucz et al. , 1974). Oscillator strengths were taken from Martin et al. (1987), and Lande factors Z from Beckers (1969). The classical damping constant was taken = 10 ycl, for Fell ones. Equivalent widths for Fel lines while obtained from the spectra are presented in Table 1. Phases were calculated according to ephemeris taken from Preston (1969). JD = 2434816.9 + 3.7220 E Estimates of the surface field S, values of the microturbulence parameter, and of the relative iron abundance are given in Table 2.

4. DISCUSSION In Fig. 1 the phase variation of the surface magnetic field BS (lb , c) of 78 Vir is compared with that of the effective field Be (1a) from Borra and Landstreet (1980) and Borra (1980a). One sees that both values vary practically in phase. Unfortunately, the absence of observations in the phase interval 0.2-0.55 , which corresponds to the minimum field, does not permit us to say whether the small phase shift is real. In general, the behaviour of BS for Fel and Fell lines is similar, although for the Fell lines the value of the field and the amplitude of variation are smaller (Fig. 1 b, c) . We noticed this fact earlier (Ryabchikova and Piskunov, 1984) for 78 Vir and for some other peculiar stars. Borra (1980b) has calculated magnetic field configurations for 78 Vir on the basis of the known effective field variations. According to his model 78 Vir has a dipole field with the surface field BS = 2.3 kGs near the phase 0.50. In this paper he also considered both the theoretical and observational evidence in favour of the conclusion that the maximum of S coincides with the narrower extremum of the Be photographic curve. According to observations by Preston (1969b) this extremum has a phase 0.00. Thus, the curve for the BS variations obtained from the Fel lines is in a good agreement both with the value of the surface field at maximum and with the phase variations predicted by Borra's model. Our estimate of BS at minimum seems to be low when compared to the 41


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model predictions. However, as we mentioned above the absence of observations in the phase interval 0.2 - 0.55 does not permit us to draw a firm conclusion on the reality of this discrepancy. Additional, observations in these phases are needed. The 2-m telescope of the NAO BAS seems to be quite suitable for this purpose.

5. CONCLUSION The method of surface magnetic field determination for the peculiar stars based on the magnetic intensification effect (the curve of growth method) makes it possible to study phase variations of the magnetic field B in those cases where the direct method based on the line splitting fails. It would be interesting to apply this method to the study of the surface magnetic field CVn. variations of other peculiar stars, primarily of
S

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Table 2. Measurements of the surface magnetic field, microturbulent velocity and iron abundance in the atmoiphere of 78

Vir.

REFERENCES Beckers, J. M.: 1969, A Table of Zeeman Muitiplets, Sacramento Peak Observatory, Phys. Sci. Research Papers, No. 371. Borra, E. F.: 1980a. Astrophys. J., v. 235, 911. Borra, E. F.: 1980b. Astrophys. J., v. 235, 915. Borra, E. F., Landstreet, J. D.: 1980, Astrophys. J. Suppi. Ser., v. 42 , 421. Cramer, N., Maeder, A.: Astron. Astrophys., v. 88 , 135. Hensberge, H., B e Loore, C : 1974, Astron. Astrophys., v . 3 7 , 3 6 7 . Huchra, J.: 1972, Astrophys. J., v. 174, 435. Kurucz, R. L., Peytreeann, E., Avrett,E.: 1974. Blanketed Model Atmospheres for Early-Type Stars. Smithsonian Inst. Washington, DC. Martin, G. A., Fuhr, J. R., Wiese, N. L.: 1987. NSRDS-NBS (preprint). Piskunov, N. E., Ptitsyn, D. A., Ryabchikova, T. A., Khokhlova, V. L.: 1984, Nauchnye Informatsii, Astr. Sovet Akademii Nauk SSSR, v. 54 , 45. Preston, G. N.: 1969a, Astrophys. J., v. 156, 967. Preston, G. N.: 1969b, Astrophys. J., v. 158, 243. Preston, G. N.: 1970, Astrophys. J., v. 160, 1059. Preston, G. N.: 1971, Astrophys. J., v. 164, 309. Ryabchikova, T. A., Piskunov, N. E.: 1984, In: Magnetic Stars, Eds. V. Khokhlova et al. , Salaspils, 27. Wolff, S . C , Wolff, R . J.: 1970, Astrophys. J., v . 1 6 0 , 1049.

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