\documentstyle[twoside, epsf]{article}

\input ibvs2.sty

\begin{document}

\IBVShead{4934}{5 August 2000}

\IBVStitletl{A Probable Variation in the Polarization of The Early-Type}{ Eclipsing Binary System XZ C\lowercase{ep}}

\IBVSauth{Kondoh, Masahiro; Nakamura, Yasuhisa}

\IBVSinsto{Astronomical Institute, Department of Science Education,
Faculty of Education, Fukushima University,\\ \indent Kanayagawa 1,
Fukushima, Japan, e-mail:{\tt kondoh@cosmos.educ.fukushima-u.ac.jp}}

\SIMBADobjAlias{XZ Cep}{HIP111257}
\IBVStyp{ O9.5+B1 }
\IBVSkey{early-type, eclipsing binary, polarization}
\IBVSabs{The early-type eclipsing binary system XZ Cep are observed}
\IBVSabs{spectro-polarimetrically.}
\IBVSabs{We report a probable detection of temporal variations in the} 
\IBVSabs{polarization.}

\begintext

XZ Cep (BD +66\deg 1512) is an eclipsing and double-lined spectroscopic
binary system, consisting of late O and early B stars.  The large
reddening in its color (Saute \& Martel, 1979) suggests the existence
of a large amount of interstellar matter in the direction.  A recent
photometric study of this binary with the $UBV$ photoelectric system
was done by Antokhina \& Kumsiashvili (1991), which yielded a
semidetached configuration.  Their light-curves were analyzed again by
Harries et al.\ (1997), who also obtained a new radial-velocity curve
of this system.  Harries et al.\ confirmed that the system is a
semidetached one and the cooler, less massive component is filling its
critical Roche lobe.

Kreiner et al.\ (1990) performed a period study of the system, 
and derived a new photometric ephemeris as follows.
$$
{\rm Min \; I = HJD}\; 2426033.421 + 5\fday0972531 \times E.
$$
Harries et al.\ (1997) employed this and we also use it here.

Saute \& Martel (1979) made polarimetric observations (mainly in $B$
filter) and reported that very large and rapid changes in the
polarization of XZ Cep were detected in 1970--1971.  They did not
mention the cause of these variations explicitly, but commented that if
the variations were real, they could originate only from very violent
ejection of matter.  There has been no polarimetric report on XZ Cep
since then.

We observed this object with a low resolution spectropolarimeter
(referred to as HBS, Kawabata et al., 1999), mounted at the 36-inch
reflector at Dodaira Observatory of the National Astronomical
Observatory of Japan.  In the observations, we used a diaphragm of
$17\arcm~\phi$, which yields a spectral resolution of about 100
\AA\ (limited by the seeing at Dodaira).  On twelve nights from
November 1998 to December 1999, we observed the object at various
orbital phases and got around $10$ sets of data at each night.  To
reduce the observed data, the standard reduction software for HBS was
used (Kawabata et al., 1999).  The nightly mean value of the
polarization is shown in Table 1.

First, to see the wavelength dependence of polarization as precisely as
possible, the Stokes parameters are binned in wavelength to a constant
photon noise of 0.01\%.  The typical resolution goes down to about 300
\AA\ after this procedure.  The polarization spectra are displayed
in Fig.~1.

\begin{center}
\normalsize
Table 1: The journal of polarimetric observations.  The meaning of the
symbols are as follows. $p$: polarization degree, $\theta$:
polarization angle, $q, u$: Stokes parameters.  The subscript $B$ means
synthetic $B$ filter of the standard Johnson system.
\vskip 2mm
\begin{tabular}{lccccc} \hline
Date       & Mid.\ phase & $p_B$ (\%)         & $\theta_B$ (\deg)   & $q_B$ (\%) & $u_B$ (\%)\\ \hline
1998 Nov.\ 24 & 0.93       & 4.240${}\pm{}$0.064 & 73.45${}\pm{}$0.43 & $-$3.552${}\pm{}$0.065 & +2.315${}\pm{}$0.064 \\
1998 Nov.\ 25 & 0.10       & 4.227${}\pm{}$0.043 & 73.20${}\pm{}$0.29 & $-$3.521${}\pm{}$0.042 & +2.339${}\pm{}$0.044 \\
1998 Dec.\ 29 & 0.77       & 4.143${}\pm{}$0.041 & 73.01${}\pm{}$0.28 & $-$3.435${}\pm{}$0.041 & +2.316${}\pm{}$0.040 \\
1998 Dec.\ 30 & 0.98       & 4.242${}\pm{}$0.057 & 73.00${}\pm{}$0.39 & $-$3.517${}\pm{}$0.057 & +2.372${}\pm{}$0.057 \\
1999 Jan.\ 1  & 0.35       & 4.160${}\pm{}$0.055 & 73.30${}\pm{}$0.38 & $-$3.473${}\pm{}$0.055 & +2.290${}\pm{}$0.055 \\
1999 Jan.\ 2  & 0.55       & 4.216${}\pm{}$0.054 & 72.78${}\pm{}$0.37 & $-$3.477${}\pm{}$0.054 & +2.384${}\pm{}$0.054 \\
1999 Nov.\ 13 & 0.36       & 4.175${}\pm{}$0.026 & 73.13${}\pm{}$0.17 & $-$3.472${}\pm{}$0.026 & +2.319${}\pm{}$0.025 \\
1999 Nov.\ 16 & 0.95       & 4.160${}\pm{}$0.030 & 72.99${}\pm{}$0.21 & $-$3.448${}\pm{}$0.030 & +2.327${}\pm{}$0.030 \\
1999 Dec.\ 16 & 0.83       & 4.091${}\pm{}$0.017 & 73.09${}\pm{}$0.12 & $-$3.399${}\pm{}$0.017 & +2.277${}\pm{}$0.017 \\
1999 Dec.\ 19 & 0.42       & 4.173${}\pm{}$0.046 & 72.87${}\pm{}$0.32 & $-$3.450${}\pm{}$0.045 & +2.349${}\pm{}$0.047 \\
1999 Dec.\ 20 & 0.62       & 4.125${}\pm{}$0.039 & 74.11${}\pm{}$0.27 & $-$3.507${}\pm{}$0.039 & +2.172${}\pm{}$0.038 \\
1999 Dec.\ 21 & 0.80       & 4.243${}\pm{}$0.015 & 74.12${}\pm{}$0.10 & $-$3.607${}\pm{}$0.015 & +2.234${}\pm{}$0.015 \\
\hline
\end{tabular}
\bigskip
\end{center}

As seen in Fig.~1, XZ Cep shows large polarization degree up to 4.4\%.
The value had been almost constant throughout our observations at the
whole wavelength.  Furthermore, the feature of polarization degree
versus wavelength is quite consistent with an empirical formula of
interstellar polarization derived by Serkowski et al.\ (1975).
Therefore, the interstellar polarization must be dominant in the
observed polarization of XZ Cep.  In general, the polarization angle
does not depend on wavelength for interstellar polarization.  But in
our case, the polarization angle is not constant.  This
slight change (or rotation) of polarization angle versus wavelength may
be caused by the existence of two or more interstellar clouds with
different properties which lie between XZ Cep and us.

\IBVSfig{8cm}{4934-f1.eps}{Dependence of the polarization on
wavelength.\\ Left: polarization degree versus wavelength.  Right:
polarization angle versus wavelength.}

Almost all polarization angles are between 73--76\deg, except those on
December 20 and 21, 1999.  The polarization angles shifted by $1 \sim
2$\deg as a whole on these two days.  It is noticed that the values of
polarization degrees on these two days in Fig.~1 are also changed
slightly, especially in blue wavelengths on Dec.~21.

We also observed polarized standard stars for the calibration.  On
Dec.~21, 1999 we observed the strongly polarized standard star HD 26433
after the observation of XZ Cep.  The observation of this standard star
was repeated on Nov.~4, 1999.  The result for HD 26433 is shown in
Table~2.  The difference between the data for HD~26433 on these two days
is roughly within the observational error.  On the other hand, the
difference, e.g.\ between the data on Dec.~16 and 21, is ten times
larger than the error ($\sim 0.015$) for XZ Cep.  It should be noticed
that the data on these nights have smaller errors than those of the
other days because of the fine weather conditions.  Hence, we conclude
that these variations of polarization of XZ Cep are not instrumental,
but intrinsic.  We also mention that these variations do not depend on
the orbital phase, because the data on Dec.~21 and Dec.~16 are
different from each other, though the orbital phase of XZ Cep on
Dec.~21 is close to that on Dec.~16.

\begin{center}
\normalsize
Table 2:
Observed data for HD 26433, a strongly polarized standard star
\vskip 2mm
\begin{tabular}{lcccc} \hline
Date       & $p_B$ (\%)        & $\theta_B$ (\deg)    & $q_B$ (\%)& $u_B$ (\%)\\ \hline
1999 Nov.\ 4 & 5.083${}\pm{}$0.026 & 135.23${}\pm{}$0.15 & +0.041${}\pm{}$0.026 & $-$5.083${}\pm{}$0.026 \\
1999 Dec.\ 21 & 5.062${}\pm{}$0.017 & 135.88${}\pm{}$0.09 & +0.156${}\pm{}$0.016 & $-$5.060${}\pm{}$0.017 \\
\hline
\end{tabular}
\bigskip
\end{center}

\IBVSfig{10cm}{4934-f2.eps}{Polarization behaviors on the $q$--$u$ plane.
Left-up: $B$ filter; right-up: $V$ filter;\\
left-down: $R_c$ filter; right-down: $I_c$ filter.}

Next, to see the dependence of variation on the wavelength, we applied
the synthetic standard Johnson--Cousins $BVR_cI_c$ filters (Bessell
1990).  Fig.~2 shows the values of Stokes parameter on Dec.~20 and
Dec.~21 and the mean value of the other days for $B,V,R_c,I_c$ on the
$q$--$u$ plane.  The Stokes parameter on Dec.~21 is different from that
of Dec.~20 for each color.

The directions of the Stokes vector from the mean point to the point of
Dec.~20 on the $q$--$u$ plane are almost same in all filters.  The
Stokes vector from the point of Dec.~20 to that of Dec.~21 rotated
compared to the vector from the mean point to the point of Dec.~20,
except in the $I_c$ filter.  In the $B$ filter, the vector is almost
perpendicular to the one in the $I_c$.  The angles of the rotation of
the vectors are larger in shorter wavelength.

We detected a probable variation in the polarization of XZ Cep on
Dec.~20 and 21, 1999, though its nature is not clear now.  As the
allocated telescope time to us for HBS observations ended on Dec.~21,
we could not follow these variations in the whole season.  We suggest here
that the event happened around Dec.~20, 1999.

There are some reports on the temporal and irregular variations in the
polarizations of early-type close binary systems (for example, U Cep,
Piirola, 1980).  Almost all variations of polarization reported so far
(including the report of Saute \& Martel, 1979) have short time scales
(a few minutes up to a few tens of minutes).  The variations reported
here has a longer time scale (order of a few hours) compared with
them.  In the case of U Cep, a strong increase of polarization was
observed in late 1975 (Piirola, 1980).  Piirola suggested that this was
caused by mass-transfer events which occurred in U Cep in late 1974 and
1975.  And it was also reported that a period increase of U Cep
occurred in late 1974 (Olson et al., 1981).

We appreciate useful discussions with K.S. Kawabata.

\references

Antokhina, E.A. \&  Kumsiashvili, M.I., 1991, {\it Sov.\ Astron.}, {\bf 35}, 504

Bessell, M.S., 1990, {\it PASP}, {\bf 102}, 1181

Harries, T.J., Hilditch, R. W., \& Hill, G., 1997, {\it MNRAS}, {\bf 285}, 277

Kawabata, K.S., Okazaki, A., Akitaya, H., Hirakata, N., Hirata, R., Ikeda, Y., Kondoh, M., Masuda, S., \& Seki, M., 1999, {\it PASP}, {\bf 111}, 898

Kreiner, J M., Kumsiashvili, M.I., \& Tremko, J., 1990, {\it Bull.\ Astron.\ Inst.\ Czech.}, {\bf 41}, 51

Olson, E.C., Hall, D.S., Louth, H., Markworth, N.L., Piirola, V., \& Crawford, R.C., 1981, {\it PASP}, {\bf 93}, 464

Piirola, A., 1980, {\it A\&A}, {\bf 90}, 48

Saute, M. \& Martel, M.T., 1979, {\it IBVS}, No.\,1681

Serkowski, K., Mathewson, D.S., \& Ford, V.L., 1975, {\it ApJ}, {\bf 196}, 261

\end{document}



