COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 1681 Konkoly Observatory Budapest 1979 October 3 INTRINSIC POLARIZATION IN XZ CEPHEI Polarimetric observations of XZ Cephei were carried out during and after primary eclipses from October 1970 to October 1972 with the 80 cm telescope of the Haute Provence Observatory. Two different one-channel polarimeters were used: - the first one, PI, described by Martel and Martel (1964) and later modified (Martel, 1971) was operate up to June 1971; - the second one, PII, operated in October 1972 only (see also Chevreton et al, 1977). The filters used are approximately the standard U,B and V filters, their inverse effective wavelengths being respectively 1/lambda= 2.74, 2.33 and 1.87 micro m^-1. The determination of the amount of linear polarization, P, and of the orientation of the electric vector, Theta, is made according to the scheme described in Martel and Martel (1964) and adopted so as to take account of the light variation with time. Orbital phases were calculated according to the ephemeris Min I =J.D. 2426033.52+5.0972155d*E given by Woodward (1943) and adopted by Kukarkin (1969). The instrumental polarization parameters were determined using our polarimetric measurements on unpolarized standard stars. From Chevreton et al. (1977) there results a negligible instrumental polarization for any filter, using PII in October, 1972. About the interstellar polarization our measurements on the comparison stars used (BD +65d1774 and +65d1778) and the data available in the literature for other stars did not allow us to deduce the right parameters in the immediate vicinity of XZ Cephei, as it appears clearly from Table I. Table I The interstellar polarization and absorption in a large neighbourhood of XZ Cephei. The colour excess and distance modulus for BD +65d1774 were determined on the basis of our indices B-V=0.50 and U-B=-0.50 and the spectral type B2 II given by Hill and Lynas-Gray (1977). Star name l_II b_II 3E_B-V m-M P% Theta d Source HD 213087 109 +6.4 1.95 9.3 0.97 23 a 0.78 33 b HD 213405 109.2 6.4 2.25 9.3 0.97 2 a 0.60 9 b HD 213481 109.5 7.4 2.13 10.9 1.95 27 d (BD +65d1774) 2.38 31 e 2.39 29 f HD 213571 111.9 10.6 - - 0.28 174 a HD 213832 109.8 7.5 - - 0.44 11 d (BD +65d1778) 1.14 54 e HD 215371 109.2 6.1 - - 0.74 132 a HD 216014 110.9 5.3 1.77 8.6 1.89 38 a 1.61 58 b HD 216228 111.1 6.2 0.01 2.4 0.01 108 c (iota Cep) BD +63d1907 111.5 4.3 3.33 12.6 3.69 40 b ----------------------------------------------------- Sources: a) Hall, 1958 (lambda_eff= 4500A) Hiltner, 1956 (lambda_eff= 5400A); c) Behr, 1959 (lambda_eff=4600A); d), e) and f) This work (lambda_eff=3650, 4300 and 5350A). Table II The journal of observations Hel.J.D. Orbital (p+-DeltaP)% (theta+-DeltaTheta)d 2440000 Date phase 1/lambda_eff Delta m 866.4545 06 X 70 0.0073 2.74 3.83+-0.30 65.2+-1.91.50 .4550 .0074 3.60 0.27 62.3 1.8 1.51 .5386 .0238 3.68 0.19 69.3 1.2 1.40 .5391 .0239 3.73 0.27 73.0 1.7 1.39 .5424 .0246 3.37 0.16 75.1 1.1 1.37 .5430 .0247 3.79 0.23 71.1 1.4 1.37 913.2958 22 XI 70 .1969 3.25 0.17 74.5 1.3 0.58 .2964 .1970 3.49 0.15 70.8 1.0 0.59 1131.5221 28 VI 71 .0098 4.65 0.19 70.0 1.1 1.53 861.3296 01 X 70 .0019 2.33 5.89 0.21 72.6 0.9 1.27 .3301 .0020 5.84 0.13 71.2 0.6 1.28 866.4444 06 X 70 .0054 4.29 0.10 73.1 0.6 1.29 .4449 .0055 4.15 0.13 72.6 0.8 1.29 .4491 .0063 4.92 0.24 70.1 1.2 1.29 .4496 .0064 4.80 0.23 70.5 1.2 1.28 .5193 .0201 4.01 0.21 84.0 1.3 1.22 .5199 .0202 3.80 0.46 82.8 3.0 1.21 .5245 .0210 4.98 0.21 69.5 1.1 1.20 912.3868 21 XI 70 .0186 3.91 0.27 71.0 1.8 1.31 .4059 .0224 4.89 0.51 71.5 2.6 1.27 913.3162 22 XI 70 0.2009 2.33 3.88+-0.05 74.3+-0.6 0.44 .3166 .2010 3.84 0.18 77.4 1.2 0.44 .3485 .2073 3.85 0.25 74.3 1.6 0.41 .3490 .2074 3.85 0.29 76.6 1.9 0.40 914.3601 23 XI 70 .4057 3.60 0.09 71.9 0.6 0.51 .3606 .4058 3.56 0.09 72.5 0.6 0.51 915.3310 24 XI 70 .5962 5.95 0.30 68.9 1.4 0.53 917.4054 26 XI 70 .0032 4.19 0.45 84.7 2.4 1131.4799 28 VI 71 .0015 4,29 0.23 69.8 1.4 1.28 .4804 .0016 4.29 0.26 71.6 1,5 1.27 .5082 .0070 4.49 0.18 75.1 1.0 1.29 .5088 .0071 4.22 0.05 78.1 0.5 1.29 1600.4483 .0063 2.95 0.22 84.5 2.1 1.27 .4609 .0088 3.54 0.64 69.0 9.9 1.33 913.3382 22 XI 70 .2052 1.87 4.59 0.13 74.3 1.0 0.13 .3386 .2053 4.69 0.15 73.5 1.0 0.13 1131.4655 28 VI 71 .9987 4.29 0.04 75.2 0.2 0.97 .4660 .9988 4.40 0.04 74.6 0.3 0.97 Table II gives the journal of observations, in columns 1) the heliocentric Julian day, 2) the date, 3) the orbital phase phi of the system, 4) the effective wavenumber of the passband in inverse microns, 5) the intrinsic plus interstellar percentage of polarization P and the standard error DeltaP, 6) The electric vector orientation Theta and the standard error DeltaTheta, 7) the difference of magnitudes Deltam=m_*-m(BD +65d1774)^*, Figure 1 presents for B filter the percentage P of polarization (intrinsic plus interstellar) and the orientation Theta of the electric vector as functions of phase from the time of primary minimum to phi=0.0205. The vertical bars correspond to twice the standard errors. We represented by a horizontal dotted line the approximate mean value of P and Theta_B outside eclipse for phi=0.205, i.e. P_B=3.86 % and Theta_B=76d. These values could be reasonably considered as a measure of the interstellar polarization at XZ Cephei, under the condition that no part of this polarization effect is due to scattering in an external envelope (we shall examine this point later.). In a general way, no correlation of P or Theta with phi is present, due to the poor distribution of our measurements with time. In particular: * Table II exhibits only some of the Deltam values obtained from our polarimetric records. [FIGURE 1] Figure 1. The polarization amount P_B and the orientation of the electric vector Theta_B (1/lambda=2.33 micron^-1) around and immediately after the primary minimum of XZ Cephei. - From the comparison of P_B values on 1970 October 01, 1970 November 26 and 1971 June 28 at phi=0.0016, it follows that variations of P from cycle to cycle may exist, which could reveal the presence of non permanent circumstellar matter. We must notice that no corresponding variations of Theta occur in the above cases, except the peak value Theta_B = 84.7d on 1970 November 26. Then, the enormous flow of matter necessary to produce an increase of 2% in the percentage of polarization should be highly concentrated in the equatorial plane so as to let Theta unchanged. - On 1970 October 06, we notice the occurrence of large instantaneous variations of P_B and Theta_B values, especially around phi=0.0205, and on the contrary the near constancy of P_U on the same date from phi=0.0073 to phi=0.0247. If real, these variations could originate only from very violent ejection of matter, inasmuch as the geometrical configuration remains the same. An identical conclusion holds on 19 70 November 21. - The peak values P_B=5.89% and 5.84% on 1970 October 01 and P_B=5.95% on 1970 November 24 are unhappily not related to simultaneous corresponding peak values in U or V colours. In U colour only one peak value is registrated, P_U=4.65% on 1971 June 28 and has to be compared to P_U=3.83% and 3.60% on 1970 October 06 at phi=0.0073. - Finally, it is interesting to note that during the orbital cycle from 1970 November 21 to 1970 November 26, two very different values of P_B are recorded at phi=0.4 and phi=0.6, i.e. for two identical geometrical configurations of the system (under the condition of a circular orbit) apart a reflection in a plane through the line of sight and perpendicular to the orbital plane. Very few data are available about the physical parameters of the system. Only one spectrum is known: O 9.5 from Roman (1956). The O character of the composite spectrum was born out owing to the U-B and B-V indices deduced from our polarimetric records for a number of values of phi representative of the entire orbital period. All the corresponding points in the ((B-V)_0,(U-B)_0),diagram are scattered exactly around the reddening path for O stars (slope 0.72) in a circle of radius 0.06 centered on B-V =+0.80 and U-B=-0.33 (Let us mention that for BD +65d1774 our value B-V=+0.50 is in good agreement with the value B-V=+0.55 given by Hill and Lynas-Gray (1977)). There results a high value of reddening A_v=3.3 magnitudes which would give a distance r=280 pc, on the basis of the absolute magnitudes M_1=-1.2 and M_2=-1.1 adopted by Dworak (1975). Indeed, this value of r is in serious disagreement with the photometric parallax pi"= 0.0011+-0.0005 calculated by Dworak with the above values for M_1 and M_2. Instead, we shall adopt M = -4.5 which is probably not so far from the true value for a system consisting of an O 9.5 primary component and a somewhat cooler secondary component. This would give a distance r=920 pc: in this case too; the major part of the absorption would be of circumstellar origin, as shown from inspection of the (r,E_y) correspondence of Fitzgerald (1968). From our polarimetric measurements, we suspected some variations of the instrinsic polarization of XZ Cephei. This preliminary study has to be supported by new extensive polarimetric and spectroscopic investigations. M. SAUTE and M. T. MARTEL Observatoire de Lyon, Observatoire de Haute Provence France References: Behr, A. 1959, Veroff.Gottingen Sternw. 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