99 Herculis: Host to a Circumbinary Polar-ring Debris Disk

TL;DR

This paper presents Herschel images of a circumbinary debris disk around 99 Herculis, showing it is likely a polar ring stabilized by secular perturbations, which informs understanding of planetary system misalignments.

Contribution

It provides the first resolved images of a misaligned circumbinary debris disk and models its structure as a polar or near-polar ring, exploring its stability and formation scenarios.

Findings

The disk is misaligned with the binary plane.

A polar ring model explains the observed structure.

Formation likely involved stellar encounters or binary exchange.

Abstract

We present resolved Herschel images of a circumbinary debris disk in the 99 Herculis system. The primary is a late F-type star. The binary orbit is well characterised and we conclude that the disk is misaligned with the binary plane. Two different models can explain the observed structure. The first model is a ring of polar orbits that move in a plane perpendicular to the binary pericenter direction. We favour this interpretation because it includes the effect of secular perturbations and the disk can survive for Gyr timescales. The second model is a misaligned ring. Because there is an ambiguity in the orientation of the ring, which could be reflected in the sky plane, this ring either has near-polar orbits similar to the first model, or has a 30 degree misalignment. The misaligned ring, interpreted as the result of a recent collision, is shown to be implausible from constraints on the collisional and dynamical evolution. Because disk+star systems with separations similar to 99 Herculis should form coplanar, possible formation scenarios involve either a close stellar encounter or binary exchange in the presence of circumstellar and/or circumbinary disks. Discovery and characterisation of systems like 99 Herculis will help understand processes that result in planetary system misalignment around both single and multiple stars.