Discs in misaligned binary systems

TL;DR

This study uses SPH simulations to explore how circumprimary discs in misaligned binary systems precess and change alignment, revealing the limitations of the rigid-disc approximation and identifying both alignment and anti-alignment processes.

Contribution

It demonstrates the applicability and limitations of the rigid-disc approximation in modeling precession and alignment in misaligned binary systems.

Findings

Rigid-disc approximation accurately models precession.

Alignment involves fluid interactions, not captured by the approximation.

Systems near 90° misalignment tend to become more aligned.

Abstract

We perform SPH simulations to study precession and changes in alignment between the circumprimary disc and the binary orbit in misaligned binary systems. We find that the precession process can be described by the rigid-disc approximation, where the disc is considered as a rigid body interacting with the binary companion only gravitationally. Precession also causes change in alignment between the rotational axis of the disc and the spin axis of the primary star. This type of alignment is of great important for explaining the origin of spin-orbit misaligned planetary systems. However, we find that the rigid-disc approximation fails to describe changes in alignment between the disc and the binary orbit. This is because the alignment process is a consequence of interactions that involve the fluidity of the disc, such as the tidal interaction and the encounter interaction. Furthermore, simulation results show that there are not only alignment processes, which bring the components towards alignment, but also anti-alignment processes, which tend to misalign the components. The alignment process dominates in systems with misalignment angle near 90 degrees, while the anti-alignment process dominates in systems with the misalignment angle near 0 or 180 degrees. This means that highly misaligned systems will become more aligned but slightly misaligned systems will become more misaligned.