# Magnetic braking of supermassive stars through winds

## TL;DR

This paper explores how magnetic winds can remove angular momentum from supermassive stars, enabling their formation via accretion from a Keplerian disc, requiring magnetic fields of about 10 kG.

## Contribution

It demonstrates that magnetic coupling with stellar winds can facilitate supermassive star formation by angular momentum extraction, given sufficiently strong magnetic fields.

## Key findings

- A magnetic field of ~10 kG is needed at the stellar surface.
- Magnetic coupling can enable accretion from a Keplerian disc.
- Fields of this strength are consistent with primordial origins.

## Abstract

Supermassive stars (SMSs) are candidates for being progenitors of supermassive quasars at high redshifts. However, their formation process requires strong mechanisms that would be able to extract the angular momentum of the gas that the SMSs accrete. We investigate under which conditions the magnetic coupling between an accreting SMS and its winds can remove enough angular momentum for accretion to proceed from a Keplerian disc. We numerically computed the rotational properties of accreting SMSs that rotate at the $\Omega\Gamma$-limit and estimated the magnetic field that is required to maintain the rotation velocity at this limit using prescriptions from magnetohydrodynamical simulations of stellar winds. We find that a magnetic field of 10 kG at the stellar surface is required to satisfy the constraints on stellar rotation from the $\Omega\Gamma$-limit. Magnetic coupling between the envelope of SMSs and their winds could allow for SMS formation by accretion from a Keplerian disc, provided the magnetic field is at the upper end of present-day observed stellar fields. Such fields are consistent with primordial origins.