The formation of high-mass binary star systems

TL;DR

This paper presents a semi-analytic model showing how magnetic fields can facilitate the formation of close, high-mass binary star systems from molecular clouds by reducing angular momentum during accretion.

Contribution

It introduces a novel semi-analytic approach incorporating magnetic braking to explain the formation of close, high-mass binaries from large molecular clouds.

Findings

Magnetic fields of about 100 μG enable formation of >25 M☉ binaries.

Clouds over 1000 M☉ and 0.5 pc radius can produce high-mass close binaries.

Magnetic braking reduces angular momentum, aiding in close binary formation.

Abstract

We develop a semi-analytic model to investigate how accretion onto wide low-mass binary stars can result in a close high-mass binary system. The key ingredient is to allow mass accretion while limiting the gain in angular momentum. We envision this process as being regulated by an external magnetic field during infall. Molecular clouds are made to collapse spherically with material either accreting onto the stars or settling in a disk. Our aim is to determine what initial conditions are needed for the resulting binary to be both massive and close. Whether material accretes, and what happens to the binary separation as a result, depends on the relative size of its specific angular momentum, compared to the specific angular momentum of the binary. When we add a magnetic field we are introducing a torque to the system which is capable of stripping the molecular cloud of some of its angular momentum, and consequently easing the formation of high-mass binaries. Our results suggest that clouds in excess of 1000 M$_\odot$ and radii of 0.5 pc or larger, can easily form binary systems with masses in excess of 25 M$_\odot$ and separations of order 10 R$_\odot$ with magnetic fields of order 100 {\mu}G (mass-to-flux ratios of order 5).