Binary dynamics near a massive black hole

TL;DR

This study models the evolution and disruption of binary stars near a massive black hole, revealing how their survival depends on distance and dynamical interactions, with implications for X-ray binaries and hyper-velocity stars.

Contribution

It provides a detailed Monte Carlo analysis of binary star dynamics near a massive black hole, highlighting new insights into binary survival rates and disruption mechanisms.

Findings

Binary fraction decreases closer to the MBH.

Binaries can only exist on eccentric orbits near the MBH.

Disruption mechanisms vary with proximity to the MBH.

Abstract

We analyze the dynamical evolution of binary stars that interact with a static background of single stars in the environment of a massive black hole (MBH). All stars are considered to be single mass, Newtonian point particles. We follow the evolution of the energy E and angular momentum J of the center of mass of the binaries with respect to the MBH, as well as their internal semi-major axis a, using a Monte Carlo method. For a system like the Galactic center, the main conclusions are the following: (1) The binary fraction can be of the order of a few percent outside 0.1 pc, but decreases quickly closer to the MBH. (2) Within ~0.1 pc, binaries can only exist on eccentric orbits with apocenters much further away from the MBH. (3) Far away from the MBH, loss-cone effects are the dominant mechanism that disrupts binaries with internal velocities close to the velocity dispersion. Closer to the MBH, three-body encounters are more effective in disrupting binaries. (4) The rate at which hard binaries become tighter is usually less than the rate at which a binary diffuses to orbits that are more bound to the MBH. (5) Binaries are typically disrupted before they experience an exchange interaction; as a result, the number of exchanges is less than one would estimate from a simple "nv\sigma estimate''. We give applications of our results to the formation of X-ray binaries near MBHs and to the production rates of hyper-velocity stars by intermediate mass MBHs.