Growth and Dissipation of Be Star Discs in Misaligned Binary Systems

TL;DR

This study uses 3D simulations to explore how misaligned binary companions influence the growth, tilt, precession, and potential tearing of Be star discs, revealing complex dynamical behaviors and observational implications.

Contribution

It provides new insights into the dynamical evolution of Be star discs in misaligned binaries, including phenomena like disc tilting, precession, tearing, and Kozai-Lidov oscillations, through detailed simulations.

Findings

Discs tilt away from initial plane before settling after 40-50 orbits.

Disc precession occurs with periods of 20-50 orbits.

Disc tearing and Kozai-Lidov oscillations are observed.

Abstract

We use a three-dimensional smoothed particle hydrodynamics code to simulate growth and dissipation of Be star discs in systems where the binary orbit is misaligned with respect to the spin axis of the primary star. We investigate six different scenarios of varying orbital period and misalignment angle, feeding the disc at a constant rate for 100 orbital periods, and then letting the disc dissipate for 100 orbital periods. During the disc growth phase, we find that the binary companion tilts the disc away from its initial plane at the equator of the primary star before settling to a constant orientation after 40 to 50 orbital periods. While the mass-injection into the disc is ongoing, the tilting of the disc can cause material to reaccrete onto the primary star prematurely. Once disc dissipation begins, usually the disc precesses about the binary companion's orbital axis with precession periods ranging from 20 to 50 orbital periods. In special cases we detect phenomena of disc tearing, as well as Kozai-Lidov oscillations of the disc. These oscillations reach a maximum eccentricity of about 0.6, and a minimum inclination of about \ang{20} with respect to the binary's orbit. We also find the disc material to have highly eccentric orbits beyond the transition radius, where the disc changes from being dominated by viscous forces, to heavily controlled by the companion star, in contrast to its nearly circular motion inward of the transition radius. Finally, we offer predictions to how these changes will affect Be star observables.