Misaligned circumbinary discs around unequal-mass eccentric binaries: alignment, morphology, and binary accretion variability

TL;DR

This study uses 3D hydrodynamical simulations to explore how initial tilt and mass ratio influence the evolution, accretion variability, and morphology of misaligned circumbinary discs around eccentric binaries.

Contribution

It reveals the effects of initial tilt and mass ratio on accretion patterns, disc morphology, and mass loss in misaligned circumbinary discs, advancing understanding of their evolution.

Findings

Discs near polar and coplanar retrograde favor primary star accretion.

Accretion ratio varies non-monotonically with mass ratio.

Polar discs exhibit the lowest mass loss rates.

Abstract

Binary systems are ubiquitous in the Universe and often host circumbinary discs that are misaligned with the binary orbital plane. Such misalignments can affect disc evolution and binary accretion variability. We here present 3D hydrodynamical simulations of circumbinary discs with initial tilts $i_0$ from $0^\circ$ to $180^\circ$, around eccentric binaries with secondary-to-primary mass ratios of $0.11-0.67$. We find that both the initial tilt and mass ratio can affect the long-term accretion variability in our simulations. Discs evolving towards polar and coplanar retrograde generally favour accretion onto the primary star, while discs evolving towards coplanar prograde generally favour accretion onto the secondary. We find preferential accretion ratio $\eta=\langle\dot{M_2}\rangle/\langle\dot{M_\mathrm{b}}\rangle$ to be a non-monotonic function of the mass ratio. For discs close to coplanar prograde alignment, $\eta$ increases with decreasing mass ratio, whereas for discs with $30^\circ \le i_0 \le 135^\circ$, $\eta$ decreases for smaller mass ratios. Polar discs show the lowest mass loss rates, slightly lower than those of coplanar prograde discs, while retrograde discs lose mass faster than their prograde counterparts. Discs that undergo strong warping or breaking experience rapid mass loss. Our findings provide insights into observed circumbinary discs and have implications for circumbinary planet formation.