COMMISSION 27 OF THE I. A. U.
INFORMATION BULLETIN ON VARIABLE STARS
Number 1060
Konkoly Observatory
Budapest
1975 November 14
ABOUT THE INTRINSIC POLARIZATION OF AW UMa
Polarimetric observations of the W UMa-type eclipsing binary
AW UMa were made on five nights in March and April 1974. The
observations were carried out using the polarimeter and reduction
methods described by Piirola (1973, 1975). The polarimeter was
attached to the 60 cm Ritchey-Chretien telescope of the University
of Helsinki. With a 40 s effective integration at each of eight
position angles, the time for one observation was about 12 min. To
obtain as good signal-to-noise ratio as possible, no light filters
were used. The effective wavelength was about 4900 A.
In Fig. 1 the observed normalized Stokes parameters of linear
polarization, P_x and P_y, are plotted as a function of phase. Each
point represents a single observation. The changes in observed
polarization are very small and no clear systematic effects depending
on orbital phase can be found. When the Stokes parameters were plotted
against hour angle, to check for any systematic changes in
instrumental polarization, no correlation between observed polarization
and hour angle could be detected. Instrumental polarization
was found to be very small and stable also by observations of nearby
stars (Piirola 1975).
The mean values of the observed normalized Stokes parameters
with their standard errors are given in Table 1 for each night. It
can be seen that the mean values are practically zero, i.e. no
significant intrinsic polarization is present. Also interstellar
polarization is negligible, due to the short distance and high galactic
latitude of the object.
Table 1 also gives the standard deviations of the normalized
Stokes parameters around the mean values, sigma_x and sigma_y, and the
quadratic means of the internal standard errors, sigma_int. Internal
standard errors are calculated from the measurements made at the eight
position angles of the polarimeter. The deviations are not larger
than is expected from photon noise.
Table 1
Mean values of normalized Stokes parameters for AW UMa. 1 mean
value of P_x with standard error of the mean; 2 standard deviation
of P_x; 3 mean value of P_y with standard error of the mean;
4 standard deviation of P_y; 5 quadratic mean of internal standard
errors; 6 number of observations.
-------------------------------------------------------------------
Date 1 2 3 4 5 6
1974 P_x(%) sigmax(%) P_y(%) sigma_y(%) sigma_int(%) n
-------------------------------------------------------------------
Mar 09 -.002+-.007 +-.026 .016+-.006 +-.033+-.026 35
Mar 26 -.005 .009 .039-.005 .010 .037 .026 18
Apr 09 .008 .006 .022 .006 .007 .025 .019 12
Apr 11 .006 .006 .026 .019 .008 .033 .023 18
Apr 28 .013 .007 .023 .004 .008 .028 .022 14
mean .003+-.002 .011+-.004
-------------------------------------------------------------------
It is clear that the changes in polarization of AW UMa were
very small in the spring 1974, and no evidence for significant
gaseous streams or disks could be found. Thus we can state that
if the rather strong variable polarization of AW UMa observed by
Oshchepkov in the spring 1972 (Oshchepkov 1974) is real, the mechanism
producing the polarization has disappeared between the spring
1972 and the spring 1974.
V. PIIROLA
Observatory and
Astrophysics Laboratory
University of Helsinki
Acknowledgement
These observations are part of a work supported by Suomen
Kulttuurirahasto (Finnish Cultural Foundation).
References:
Oshchepkov, V.A. 1974, I.B.V.S. No. 884
Piirola, V., 1973, Astron.and Astrophys. 27, 383 [BIBCODE 1973A&A....27..383P ]
Piirola, V., 1975, Ann.Acad.Sci.Fennicae A VI, No. 418 [BIBCODE 1975ASFAS.418.....P ]
[FIGURE 1]
Fig. 1. The polarization parameters P_x and P_y, plotted as a function
of phase for AW UMa. The symbols refer to the following dates:
(filled circle) 1974 March 09, (circle) 1974 March 26, (triangle)
1974 April 09, x 1974 April 11, + 1974 April 28.