## 3. ResultsData reduction and analysis were done by the help of software
developed at the MPIA Heidelberg (see Köhler 1997). Data
reduction consisted of excluding specklegrams with large noise,
subtraction of the mean sky from each remaining specklegram, flat
field correction, bad pixel correction and baseline subtraction. For
the resulting frames a 2D FFT was applied. Finally, point source
calibrated Fourier modulus (visibility) and phase (from bispectrum
analysis) were averaged for each data cube. K band visibilities of the
Z CMa image at 4 different analyzer orientations (averaged over all
cubes) are shown in Fig. 1. Although close to the cut-off
frequency, the plots of the visibility for the four polarizer settings
show that the fringe pattern typical for a binary star was clearly
detected. This allowed the application of a fitting algorithm to
reproduce visibility and phase images by adjusting a binary model in
the Fourier plane. Thus, the polarization derived from this fit has to
be considered as the net (i.e. spatially unresolved) polarization of
each component. Visibility and phase images were reproduced by
adjusting a binary model in the Fourier plane. By this, separation and
position angle of the binary and the contribution of each component to
the total luminosity of the system were extracted. Afterwards we were
able to derive the degree and position angle
of the linear polarization of the binary
components A and B on the basis of the intensities
, measured at the four polarizer settings and
the intensity ratios of the components (deduced
before): The total intensities were calculated using the requirement that and .
Since the data cubes for each polarizer setting were obtained sequentially, we cannot rule out variations of the seeing conditions. In order to minimize this effect, we used a novel statistical approach to derive the polarization properties and to assess their formal errors. The polarization of the Z CMa binary was calculated for all possible combinations of the measurements at 4 analyzer orientations. For 7 sets of data we got combinations. Polarization was deduced from the resulting distributions (see Fig. 2). Noise biasing of the polarization degree was corrected using the analytical formula (Wardle & Kronberg 1974) where is the standard deviation of the observed polarization .
For the Z CMa binary a separation of and a position angle of were measured for the observational epoch (average over the measurements at all analyzer orientations). These results are in a fairly good agreement with those from speckle observations mentioned before. The trend of an increasing position angle with time due to the binary orbit should be checked by future observations. © European Southern Observatory (ESO) 1998 Online publication: June 2, 1998 |