Orbital Elements of the Symbiotic Star Z Andromedae from Optical Linear Polarization During the Quiescent Phase

We present low-resolution spectropolarimetry for the symbiotic star Z Andromedae at four different epochs during the quiescent phase. The linear polarization of the continuum showed a temporal variation; the difference between the maximum and the minimum is 0.3%-0.6% in Stokes q and is larger with shorter wavelengths. Applying scattering models to this variation, we found the variation in the continuum may be correlated with the orbital motion of the binary and estimated the orbital inclination angle i _c = 73° ± 14° and the orientation angle Ω_c = 80° ± 5°. We also confirmed that the intrinsic linear polarization of the Raman line λ683 varies with the orbital phase; from this modulation, the orbital elements were derived as i _r = 41° ± 8° and Ω_r = 82° ± 2°. The inclination derived from the continuum has a large error, and the value is larger by twice the error than the inclination angle value derived from the Raman line. The derived orientation, in contrast, is comparable with that derived from the Raman line. The possible inconsistency in the inclination may be due to the simplicity of our adopted model, or it may be caused by a bias effect due to the low quality of the observed continuum polarization data. An accurate estimation of the inclination from the continuum polarization could settle the question, but that estimation requires more frequent observations that cover at least more than a few orbital cycles during the quiescent phase when the observations are not interrupted by the activity of the hot component.