Spectropolarimetric Evidence for a Bipolar Flow in β Lyrae

Abstract

We present ultraviolet and visual spectropolarimetry of the interacting binary star β Lyrae, obtained with the Wisconsin Ultraviolet Photo-Polarimeter Experiment and the HPOL spectropolarimeter at Pine Bluff Observatory. Our observations span 3 years and cover the wavelength range from 1400 to 10500 A, with a resolution of 7.5–16 A. Detailed broad- and narrowband spectropolarimetric analysis allows us to begin to decompose the complex spectrum of β Lyr: by examining the polarization behavior of a line or continuum, we can determine which component scatters the light and, ultimately, from which component that light originates. After removing interstellar polarization from our data and rotating the results to the apparent intrinsic position angle of the system, we find that the polarization of the hydrogen Balmer and vacuum ultraviolet bump emission lines, as well as that of the near-UV continuum, remains mostly constant with phase and is oriented at 90° to the visible polarization, indicating that the scattering plane of the light in these three spectral components is perpendicular to the scattering plane of the visible light. We propose that the UV bump, Balmer emission, and near-UV continuum polarization is produced by electron scattering within a bipolar outflow in the β Lyr system. The intrinsic visible polarization of β Lyr shows eclipses that associate it with material near the accretion disk. We find conflicting evidence regarding both the origin of this visible light and the scattering surface that polarizes it: continuum evidence points toward the secondary object as the illuminator and the accretion disk edge as the scatterer, while line analysis suggests that light from the loser scatters off material between it and the disk. The presence of material away from the orbital plane may help resolve this contradiction.