Modelling the periodical variations in multiband polarisation and photometry for discs of binary Be stars

Abstract

The tidal interaction of a Be star with a binary companion forms two spiral arms that cause orbital modulation of the Be disc structure. The aim of this work is to identify observables in which this modulation is apparent. The structure of a Be disc in a coplanar circular binary system is computed with a smoothed-particle hydrodynamics code, and a radiation transfer code calculates the spectral energy distribution. Line depolarisation was confirmed, with polarisation profiles nearly reverse to emission-line profiles. The continuum flux maximizes for pole-on discs, but photometric variability maximizes for edge-on discs. The linear polarisation exhibits one or two maxima per orbital cycle. While polarisation variability in visible passbands is important only at low inclinations, infrared bands may demonstrate high orbital variability even at large inclinations. More evident is the modulation in the polarisation angle (PA) for low inclinations. The latter can be used to track azimuthal asymmetries for pole-on discs, where the spectroscopic variability in the violet-to-red (V/R) emission-component ratio disappears. PA reversals coincide with phases where V/R=1, tracking lines of sight directed towards regions where the approaching and receding arms overlap. Continuum flux and polarisation are mostly in phase for neighbouring wavelength regions. It is suggested that studies of non-symmetric discs distorted by tidal forces from a secondary star may be used to study disc variabilities of other origins.

arXiv comment20 pages, 19 figures; accepted for publication in MNRAS

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• Optically thin circumstellar medium in $\beta$ Lyr A system
• The frequency of Kozai-Lidov disc oscillation driven giant outbursts in Be/X-ray binaries
• Spiral Density Enhancements in Be Binary Systems
• Visible and near-infrared spectro-interferometric analysis of the edge-on Be star $o$ Aquarii
• High-spatial-resolution simulations of Be star disks in binary systems: I. Structure and kinematics of coplanar disks