Characterize the accessibility of minidisk spin states

Characterize the conditions under which the retrograde circumbinary-binary system realizes the both-retrograde, both-prograde, or mixed minidisk spin states, and determine whether the both-prograde and both-retrograde states are equally accessible or whether one is preferred under realistic astrophysical conditions.

Background

The simulations identify three persistent minidisk configurations for equal-mass binaries: both minidisks retrograde, both prograde, or one prograde and one retrograde. The realized state depends on the disk’s initial conditions, but the initial-condition space is not systematically surveyed. The mixed state is also insufficiently characterized because computational costs prevented a complete eccentricity sweep at Mach 40.

A central unresolved issue is whether these states occur with comparable frequency in realistic systems. Resolving this requires simulations spanning binary mass ratios, disk Mach numbers, and initial disk conditions, with the goal of determining the prevalence of each state and its associated orbital evolution.

References

The initial conditions of the disk are the primary factor determining which spin state is realised, although we have not explored this systematically. The mixed $\uparrow\downarrow$ state also remains uncharacterised, due to the computational demands of eccentricity sweeps at Mach 40. Fully mapping the conditions under which each state is common (and the associated orbital evolution) is one of the most astrophysically relevant questions arising from this work. Dedicated simulations spanning a range of mass ratios, Mach numbers, and initial disk conditions would clarify whether the $\uparrow\uparrow$ and $\downarrow\downarrow$ states are equally accessible, or whether one is preferred under realistic astrophysical conditions.

— Electromagnetic Emission and Orbital Evolution of Eccentric Supermassive Black Hole Binaries in Retrograde Disks  (2609.09314 - ONeill et al., 8 Sep 2026) in Section 5.4, Outlook