# Massive binary star mergers in galactic nuclei: implications for blue stragglers, binary S-stars and gravitational waves

## TL;DR

This study uses high-precision simulations to explore how supermassive black holes influence the evolution and merger rates of massive binary stars in galactic nuclei, revealing implications for blue stragglers, G2-like objects, and gravitational wave sources.

## Contribution

It provides new insights into the fraction and rate of massive binary mergers near SMBHs, considering various initial conditions and physical effects, with implications for observable phenomena.

## Key findings

- Binary merger fraction is 4-15% regardless of initial conditions.
- Typical merger rate in Milky Way-like nuclei is 1.4×10^{-7} yr^{-1}.
- Merger products include blue stragglers, G2-like objects, and potential gravitational wave sources.

## Abstract

Galactic nuclei are often found to contain young stellar populations and, in most cases, a central supermassive black hole (SMBH). Most known massive stars are found in binaries or higher-multiplicity systems, and in a galactic nucleus the gravitational interaction with the SMBH can affect their long-term evolution. In this paper, we study the orbital evolution of stellar binaries near SMBHs using high precision $N$-body simulations, and including tidal forces and Post-Newtonian corrections to the motion. We focus on the Lidov-Kozai (LK) effect induced by the SMBH on massive star binaries. We investigate how the properties of the merging binaries change with varying the SMBH mass, the slope of the initial mass function, the distributions of the binary orbital parameters and the efficiency in energy dissipation in dissipative tides. We find that the fraction of merging massive binary stars is in the range $\sim 4\%$--$15\%$ regardless of the details of the initial distributions of masses and orbital elements. For a Milky Way-like nucleus, we find a typical rate of binary mergers $\Gamma\approx 1.4\times 10^{-7}$ yr$^{-1}$. The merger products of massive binaries can be rejuvenated blue-straggler stars, more massive than each of their original progenitors, and G2-like objects. Binary systems that survive the LK cycles can be source of X-rays and gravitational waves, observable with present and upcoming instruments.