Exponentially amplified magnetic field eliminates disk fragmentation around the Population III protostar

TL;DR

This study demonstrates that exponential amplification of primordial magnetic fields can prevent disk fragmentation around Population III protostars, influencing their formation and the initial mass function.

Contribution

It introduces a novel simulation approach showing how amplified magnetic fields suppress disk fragmentation in early star formation.

Findings

Magnetic fields amplify from pico- to kilo-gauss within 10 years after protostar formation.

Magnetic braking prevents disk fragmentation in magnetized models.

Amplified magnetic fields eliminate the formation of multiple protostars in the disk.

Abstract

One critical remaining issue to unclear the initial mass function of the first (Population III) stars is the final fate of secondary protostars formed in the accretion disk, specifically whether they merge or survive. We focus on the magnetic effects on the first star formation under the cosmological magnetic field. We perform a suite of ideal magnetohydrodynamic simulations until 1000 years after the first protostar formation. Instead of the sink particle technique, we employ a stiff equation of state approach to represent the magnetic field structure connecting to protostars. Ten years after the first protostar formation in the cloud initialized with $B_0 = 10^{-20}$ G at $n_0 = 10^4\,{\rm cm^{-3}}$, the magnetic field strength around protostars amplifies from pico- to kilo-gauss, which is the same strength as the present-day star. The magnetic field rapidly winds up since the gas in the vicinity of the protostar ($\leq\!10$ au) has undergone several tens orbital rotations in the first decade after protostar formation. As the mass accretion progresses, the vital magnetic field region extends outward, and the magnetic braking eliminates fragmentation of the disk that would form in the unmagnetized model. On the other hand, assuming a gas cloud with small angular momentum, this amplification might not work because the rotation would be slower. However, disk fragmentation would not occur in that case. We conclude that the exponential amplification of the cosmological magnetic field strength, about $10^{-18}$ G, eliminates disk fragmentation around the Population III protostars.