Polar alignment of a circumbinary disc around a brown dwarf binary

TL;DR

This paper investigates how circumbinary discs around brown dwarf binaries can become polar aligned, identifying key parameters and confirming through simulations that such configurations are feasible and may lead to polar planet formation.

Contribution

The study provides analytical criteria and hydrodynamical simulations demonstrating the conditions for polar alignment of circumbinary discs around brown dwarf binaries, a novel insight into their evolution.

Findings

Critical tilt angle for polar alignment depends on binary eccentricity and angular momentum ratio.

Disc parameters like viscosity and aspect ratio influence alignment timescales.

Simulations show discs evolve towards polar states, supporting planet formation in such configurations.

Abstract

Inspired by recent observations suggesting that the retrograde precession of the brown dwarf binary 2M1510 AB is consistent with induction by a polar circumbinary planet, we investigate the formation of such planets by studying the evolution of a primordial misaligned circumbinary disc around a brown dwarf binary. Analytical calculations show that a critical tilt angle of $i_{\rm crit} \gtrsim 50^\circ$ for moderately eccentric binaries is needed for polar alignment of circumbinary discs in systems with low disc-to-binary angular momentum ratios. For higher ratios, this angle converges to the Kozai-Lidov instability threshold of $\sim 39^\circ$. We identify disc parameters, such as viscosity ($\alpha = 10^{-4}$) and aspect ratio ($H/r = 0.05$), that enable polar alignment within typical disc lifetimes. Notably, a circumbinary disc around a low-mass binary, such as a brown dwarf binary, will require more time to achieve polar alignment compared to higher-mass systems. A hydrodynamical simulation confirms that an initially inclined disc around a brown dwarf evolves towards a polar state, creating favorable conditions for polar planet formation. Using these results, we finish by placing 2M1510 AB into a wider context and speculate why such a polar circumbinary configuration has not been identified before.