Formation of Close Binaries through Massive Black Hole Perturbations and Chaotic Tides

TL;DR

This paper models how massive black holes can induce chaotic tidal interactions in binary systems, leading to the formation of close binaries and new pathways for hyper-velocity star production.

Contribution

It introduces a physical model incorporating orbital relaxation, tidal perturbations, and dynamical tides, revealing a new binary hardening mechanism near black holes.

Findings

Up to 50% of wide binaries avoid Hills breakup and become close binaries.

Chaotic tidal growth can rapidly harden binaries to semi-major axes less than 10 stellar radii.

Close binaries formed this way can produce hyper-velocity stars and relate to nuclear transients.

Abstract

Hills breakup of binary systems allows massive black holes (MBH) to produce hyper-velocity stars (HVSs) and tightly bound stars. The long timescale of orbital relaxation means that binaries must spend numerous orbits around the MBH before they are tidally broken apart. Repeated MBH tidal perturbations over multiple pericenter passages can perturb the binary inner orbit to high eccentricities, leading to strong tidal interactions between the stars. In this work, we develop a physical model of the MBH-binary system, taking into account outer orbital relaxation, MBH tidal perturbations, and tidal interactions between the binaries in the form of dynamical tides. We show that when the inner orbit reaches high eccentricities such that the pericenter radius is only a few times stellar radii ($R_*$), the stellar oscillation modes can grow chaotically and rapidly harden the binaries to semi-major axes $a_b\lesssim 10\,R_*$. We find that a significant fraction (up to 50%) of initially wide binaries that are in the empty loss-cone regime ($a_b\sim 1.0\,{\rm AU}$) do not undergo Hills breakup as wide binaries, but instead experience chaotic growth of tides and become close binaries. These tidally hardened binaries provide a new channel for the production of the fastest HVSs, and are connected to other nuclear transients such as repeating partial tidal disruption events and quasi-periodic eruptions.