Magnetic-Field-Induced Inspiral of Binaries with Circumbinary Disk: Black Hole and Protostellar Systems

TL;DR

This study uses 3D MHD simulations to show magnetic fields can drive orbital decay in binary systems, offering a new pathway for black hole mergers and binary star formation.

Contribution

It introduces a magnetic mechanism for orbital decay in binaries, applicable to black holes and stars, advancing understanding of their evolution.

Findings

Magnetic processes efficiently transport angular momentum, causing orbital decay.

Purely hydrodynamical models show orbital expansion, contrasting with magnetic models.

Decay rate depends on initial magnetic field strength, reaching up to 0.7% per orbit.

Abstract

The orbital decay of binary systems is a critical process for understanding the evolution of massive binary black holes (MBBHs) and binary star formation. Performing high-resolution three-dimensional magnetohydrodynamic (MHD) simulations, we investigate a binary system that accretes gas from an infalling envelope analogous to the collapse of molecular cloud cores in the context of binary star formation. Our simulations reveal the presence of outflows/jets launched from both the circumstellar (mini) disks and the circumbinary disk (CBD). The magneto-rotational instability is also excited within the CBD. These magnetic processes efficiently transport angular momentum in the gas surrounding the binary and thereby drive orbital decay, while a purely hydrodynamical model exhibits orbital expansion. The decay rate reaches $\sim 0.3-0.7\%$ per orbital period, depending on the initial magnetic field strength. By appropriately scaling these numerical results, we propose a new mechanism for MBBHs mergers within a Hubble time, overcoming the bottlenecks encountered at separations near the final parsec scales. Additionally, we present a formation scenario for close twin binary star systems, emphasizing the significant role of magnetic processes in driving orbital evolution across various astrophysical systems.