Close Encounters of Wide Binaries Induced by the Galactic Tide: Implications for Stellar Mergers and Gravitational-Wave Sources

TL;DR

This paper investigates how Galactic tidal forces can induce extreme orbital oscillations in wide binaries, leading to stellar interactions, mergers, and gravitational-wave sources, which could explain observed astrophysical phenomena.

Contribution

It introduces a relativistic $N$-body simulation showing Galactic tides can cause wide binaries to undergo close encounters and mergers, a novel mechanism for stellar and compact object interactions.

Findings

Galactic tides induce large eccentricity oscillations in wide binaries.

Close passages can lead to stellar collisions or mergers.

Potential contribution to observed gravitational-wave events.

Abstract

A substantial fraction of stars can be found in wide binaries with projected separations between $\sim10^2$ and $10^5\,\rm AU$. In the standard lore of binary physics, these would evolve as effectively single stars that remotely orbit one another on stationary Keplerian ellipses. However, embedded in their Galactic environment their low binding energy makes them exceptionally prone to perturbations from the gravitational potential of the Milky Way and encounters with passing stars. Employing a fully relativistic $N$-body integration scheme, we study the impact of these perturbations on the orbital evolution of wide binaries along their trajectory through the Milky Way. Our analysis reveals that the torques exerted by the Galaxy can cause large-amplitude oscillations of the binary eccentricity to $1-e\lesssim10^{-8}$. As a consequence, the wide binary members pass close to each other at periapsis, which, depending on the type of binary, potentially leads to a mass transfer or collision of stars or to an inspiral and subsequent merger of compact remnants due to gravitational-wave radiation. Based on a simulation of $10^5$ wide binaries across the Galactic field, we find that this mechanism could significantly contribute to the rate of stellar collisions and binary black hole mergers as inferred from observations of Luminous Red Novae and gravitational-wave events by LIGO/Virgo/Kagra. We conclude that the dynamics of wide binaries, despite their large mean separation, can give rise to extreme interactions between stars and compact remnants.