Binary-disk interaction: Gap-Opening criteria

TL;DR

This paper develops an analytic criterion for gap formation in circumbinary disks caused by equal mass binaries, supported by SPH simulations, and draws analogies with planetary migration types.

Contribution

It introduces a new gravitational interaction-based gap-opening criterion for equal mass binaries in circumbinary disks, validated by simulations and consistent with existing literature.

Findings

The analytic criterion accurately predicts gap formation thresholds.

Simulations show a clear separation between gap and no-gap regimes.

The study draws parallels between binary-disk interactions and planetary migration types.

Abstract

We study the interaction of an equal mass binary with an isothermal circumbinary disk motivated by the theoretical and observational evidence of the formation of massive black holes binaries surrounded by gas, after a major merger of gas-rich galaxies. We focus on the torques that the binary produces on the disk and how the exchange of angular momentum can drive the formation of a gap on it. We propose that the angular momentum exchange between the binary and the disk is through the gravitational interaction of the binary and a (tidally formed) global non-axisymmetric perturbation in the disk. Using this gravitational interaction we derive an analytic criterion for the formation of a gap in the disk that can be expressed on the structural parameters h/a and M(< r)/M_{bin}. Using SPH simulations we show that the simulations where the binary opens a gap in the disk and the simulations where the disk does not have a gap are distributed in two well separate regions. Our analytic gap-opening criterion predicts a shape of the threshold between this two regions that is consistent with our simulations and the other ones in the literature. We propose an analogy between the regime without (with) a gap in the disk and the Type I (Type II) migration that is observed in simulations of planet-disk interaction (binaries with extreme mass ratios), emphasizing that the interaction that drives the formation of a gap on the disk is different in the regime that we analyze (comparable mass binary).