On the cavity size in circumbinary discs

TL;DR

This study uses SPH simulations to explore how the size of cavities in circumbinary discs depends on binary and disc properties, revealing different regimes based on inclination and evolution.

Contribution

It provides new insights into cavity size evolution in circumbinary discs, including effects of inclination, eccentricity, and disc warping, using detailed SPH simulations.

Findings

Cavity size quickly reaches a dynamical timescale but is set on the viscous timescale.

Cavity sizes range from 2-5 times the binary semi-major axis, influenced by eccentricity and aspect ratio.

Inclination evolution leads to distinct cavity regimes: large, small, or intermediate sizes.

Abstract

How does the cavity size in circumbinary discs depend on disc and binary properties? We investigate by simulating disc cavities carved by binary companions using smoothed particle hydrodynamics (SPH). We find that a cavity is quickly opened on the dynamical time, while the cavity size is set on the viscous time. In agreement with previous findings, we find long term cavity sizes of 2-5 times the binary semi-major axis, increasing with eccentricity and decreasing with disc aspect ratio. When considering binaries inclined with respect to the disc we find three regimes: i) discs that evolve towards a coplanar orbit have a large cavity, slightly smaller than that of an initially coplanar disc; ii) discs that evolve towards a polar orbit by breaking have a small cavity, equal in size to that of an initially polar disc; iii) discs that evolve towards a polar orbit via warping have an intermediate-sized cavity. We find typical gas depletions inside the cavity of $\gtrsim 2$ orders of magnitude in surface density.