Nonlinear turbulent dynamo during gravitational collapse

TL;DR

This paper extends nonlinear turbulent dynamo theory to include gravitational compression effects, revealing how magnetic fields evolve during primordial gas collapse and highlighting the importance of reconnection diffusion in magnetic field amplification.

Contribution

It introduces a modified dynamo theory accounting for gravitational compression, explaining magnetic field behavior during star formation with new insights into flux-freezing violation.

Findings

Magnetic field growth rate is small in nonlinear dynamo.

Magnetic energy depends weakly on density during collapse.

Magnetic fields exhibit large correlation lengths and saturated energy levels.

Abstract

Via amplification by turbulent dynamo, magnetic fields can be potentially important for the formation of the first stars. To examine the dynamo behavior during the gravitational collapse of primordial gas, we extend the theory of nonlinear turbulent dynamo to include the effect of gravitational compression. The relative importance between dynamo and compression varies during contraction, with the transition from dynamo- to compression-dominated amplification of magnetic fields with the increase of density. In the nonlinear stage of magnetic field amplification with the scale-by-scale energy equipartition between turbulence and magnetic fields, reconnection diffusion of magnetic fields in ideal magnetohydrodynamic (MHD) turbulence becomes important. It causes the violation of flux-freezing condition and accounts for (a) the small growth rate of nonlinear dynamo, (b) the weak dependence of magnetic energy on density during contraction, (c) the saturated magnetic energy, and (d) the large correlation length of magnetic fields. The resulting magnetic field structure and the scaling of magnetic field strength with density are radically different from the expectations of flux-freezing.