Growth and dissipation of Be star discs in misaligned binary systems

Abstract

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 40–50 orbital periods. While the mass-injection into the disc is ongoing, the tilting of the disc can cause material to reaccrete on to 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 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 inwards of the transition radius. Finally, we offer predictions to how these changes will affect Be star observables.