Polar alignment of a dusty circumbinary disc -- II. Application to 99 Herculis

TL;DR

This study uses 3D simulations to show that dust rings form in polar-aligning circumbinary discs, potentially facilitating planetesimal formation and polar planet development around the 99 Herculis system.

Contribution

It demonstrates the formation and persistence of dust rings in polar-aligning circumbinary discs across various parameters using advanced simulations.

Findings

Dust rings form in polar-aligning circumbinary discs.

Dust traffic jams are long-lived and persist after alignment.

Streaming instability can grow rapidly in these environments.

Abstract

We investigate the formation of dust traffic jams in polar-aligning circumbinary discs. In our first paper, we found as the circumbinary disc evolves towards a polar configuration perpendicular to the binary orbital plane, the differential precession between the gas and dust components leads to multiple dust traffic jams. These dust traffic jams evolve to form a coherent dust ring. In part two, we use 3D smoothed particle hydrodynamical simulations of gas and dust to model an initially highly misaligned circumbinary disc around the 99 Herculis (99 Her) binary system. Our results reveal that the formation of these dust rings is observed across various disc parameters, including the disc aspect ratio, viscosity, surface density power law index, and temperature power law index. The dust traffic jams are long-lived and persist even when the disc is fully aligned polar. The midplane dust-to-gas ratio within the rings can surpass unity, which may be a favourable environment for planetesimal formation. Using 2D inviscid shearing box calculations with parameters from our 3D simulations, we find streaming instability modes with significant growth rates. The streaming instability growth timescale is less than the tilt oscillation timescale during the alignment process. Therefore, the dust ring will survive once the gas disc aligns polar, suggesting that the streaming instability may aid in forming polar planets around 99 Her.