Spectropolarimetry and nebular geometry of the symbiotic star HBV 475.

We present and discuss spectropolarimetry of the Raman scattered emission features at λ6825 and λ7082 of the symbiotic nova HBV 475. Meaningful interpretation in relation to the orbital phase requires an improved ephemeris and we first review available photometry and deduce a new period of 956.5d and minimum epoch at JD 2444890. The flux of the λ6825 line varies with photometric phase by about a factor 2.5. The profile normally is triple-peaked with the blue component showing the largest orbital amplitude. The polarization in the Raman lines depends strongly on the wavelength. At quadrature, it is about 19% and 10% in the blue and red line wings of λ6825 respectively with polarization angles perpendicular to each other. The percentage polarization in all parts of the profile exhibits phase-locked variations with highest values at quadrature and values close to zero at conjunction. The polarization angle shows hardly any rotation as expected for eclipsing systems. The inclination of the system is found to be i=86+/-2° and the position angle of the orbital plane is 11°. Polarization measurements are sensitive indicators of the symmetries prevailing in a scattering arrangement. In HBV 475, the Raman scattering geometry has an up-down symmetry with respect to the orbital plane. Near conjunction, a weak line polarization perpendicular to the orbital plane indicates that the scattering geometry is not rotationally symmetric with respect to the binary axis. The observations are compatible with a spherically expanding wind from the cool giant which has a density enhancement in the orbital plane. We have retrieved an HST image of HBV 475 from the archive which shows extended [OIII]λ5007 emission. The main nebulosity is aligned with the orbital plane and has a size of ~0.4".