Magnetic fields during primordial star formation

TL;DR

This paper investigates the amplification of magnetic fields during primordial star formation, combining observational constraints, theoretical models, and magneto-hydrodynamical simulations to understand their role in early star and disk evolution.

Contribution

It provides the first comprehensive simulation-based analysis confirming small-scale dynamo action and magnetic field amplification during primordial cloud collapse.

Findings

Magnetic fields can be significantly amplified by turbulence during collapse.

The magnetic power spectrum follows the Kazantsev slope, indicating small-scale dynamo activity.

Potential impact on protostellar disks includes altered morphology and reduced accretion rates.

Abstract

Recent FERMI observations provide a lower limit of 10^{-15} G for the magnetic field strength in the intergalactic medium (IGM). This is consistent with theoretical expectations based on the Biermann battery effect, which predicts such IGM fields already at redshifts z~10. During gravitational collapse, such magnetic fields can be amplified by compression and by turbulence, giving rise to the small-scale dynamo. On scales below the Jeans length, the eddy turnover timescale is much shorter than the free-fall timescale, so that saturation can be reached during collapse. This scenario has been tested and confirmed with magneto-hydrodynamical simulations following the collapse of a turbulent, weakly magnetized cloud. Based on a spectral analysis, we confirm that turbulence is injected on the Jeans scale. For the power spectrum of the magnetic field, we obtain the Kazantsev slope which is characteristic for the small-scale dynamo. A calculation of the critical length scales for ambipolar diffusion and Ohmic dissipation shows that these scales are always small enough to allow significant amplification of the magnetic field by small-scale eddies. We discuss potential implications for the protostellar accretion disk, with particular focus on the magneto-rotational instability, which may change the morphology of the disk and reduce the accretion rate by a factor of a few.