Investigating Kozai-Lidov Oscillations and Disc Tearing in Be Star Discs

TL;DR

This study uses simulations and radiative transfer modeling to explore how misaligned binary systems influence Be star discs, revealing phenomena like Kozai-Lidov oscillations, disc tearing, and observable signatures in polarization and interferometry.

Contribution

It extends previous simulations by analyzing different mass ratios and viscosities, and links disc dynamics to observable features, enhancing understanding of Be star disc behavior in binary systems.

Findings

Kozai-Lidov oscillations occur at mass ratio 0.5 but not at 0.1.

Higher viscosity dampens disc oscillations and tearing.

Observable signatures include oscillating polarization angles and interferometric visibility changes.

Abstract

Recent simulations of Be stars in misaligned binary systems have revealed that misalignment between the disc and binary orbit can cause the disc to undergo Kozai-Lidov (KL) oscillations or disc-tearing. We build on our previous suite of three-dimensional smoothed particle hydrodynamics simulations of equal-mass systems by simulating eight new misaligned Be star binary systems, with mass-ratios of 0.1 and 0.5, or equal-mass systems with varying viscosity. We find the same phenomena occur as previously for mass ratios of 0.5, while the mass ratio of 0.1 does not cause KL oscillations or disc-tearing for the parameters examined. With increased viscosity in our equal-mass simulations, we show that these phenomena and other oscillations are damped out and do not occur. We also briefly compare two viscosity prescriptions and find they can produce the same qualitative disc evolution. Next, we use the radiative transfer code HDUST to predict observable trends of a KL oscillation, and show how the observables oscillate in sync with disc inclination and cause large changes in the polarization position angle. Our models generate highly complex line profiles, including triple-peak profiles that are known to occur in Be stars. The mapping between the SPH simulations and these triple-peak features gives us hints as to where they originate. Finally, we construct interferometric predictions of how a gap in the disc, produced by KL oscillations or disc-tearing, perturbs the visibility versus baseline curve at multiple wavelengths, and can cause large changes to the differential phase profile across an emission line.