Close Encounters of Stars with Stellar-mass Black Hole Binaries

TL;DR

This study uses 3-D hydrodynamics simulations to analyze how close encounters between stars and binary black holes affect the binary's orbit, spin, and merger timescale, revealing significant impacts from tidal disruption events.

Contribution

It provides the first detailed simulation-based analysis of star-BBH interactions, focusing on tidal disruption events and their effects on binary black hole evolution.

Findings

TDEs can significantly alter binary orbits, either widening or hardening them.

A single TDE can change the gravitational wave merger timescale by a factor of order unity.

Accretion rates during TDEs are highly super-Eddington with characteristic modulations.

Abstract

Many astrophysical environments, from star clusters and globular clusters to the disks of Active Galactic Nuclei, are characterized by frequent interactions between stars and the compact objects that they leave behind. Here, using a suite of $3-D$ hydrodynamics simulations, we explore the outcome of close interactions between $1M_{\odot}$ stars and binary black holes (BBHs) in the gravitational wave regime, resulting in a tidal disruption event (TDE) or a pure scattering, focusing on the accretion rates, the back reaction on the BH binary orbital parameters and the increase in the binary BH effective spin. We find that TDEs can make a significant impact on the binary orbit, which is often different from that of pure scattering. Binaries experiencing a prograde (retrograde) TDE tend to be widened (hardened) by up to $\simeq 20\%$. Initially circular binaries become more eccentric by $\lesssim 10\%$ by a prograde or retrograde TDE, whereas the eccentricity of initially eccentric binaries increases (decreases) by a retrograde (prograde) TDE by $\lesssim 5\%$. Overall a single TDE can generally result in changes of the gravitational wave-driven merger time scale by order unity. The accretion rates of both black holes are very highly super-Eddington, showing modulations (preferentially for retrograde TDEs) on a time scale of the orbital period, which can be a characteristic feature of BBH-driven TDEs. Prograde TDEs result in the effective spin parameter $\chi$ to vary by $\lesssim 0.02$ while $\chi\gtrsim -0.005$ for retrograde TDEs.