Non-ideal magnetohydrodynamic simulations of the first star formation: the effect of ambipolar diffusion

TL;DR

This study uses non-ideal MHD simulations to explore how ambipolar diffusion influences magnetic field growth during the first star formation in the early universe, revealing that AD slightly suppresses dynamo effects but results in strong magnetic fields near protostars.

Contribution

It provides the first detailed analysis of ambipolar diffusion effects on magnetic field evolution during primordial star formation through comprehensive non-ideal MHD simulations.

Findings

Ambipolar diffusion slightly suppresses small-scale dynamo magnetic field growth.

Magnetic fields near protostars can reach 10^3-10^5 G, much stronger than in present-day star formation.

Magnetic fields influence inflow motions but do not launch winds in primordial conditions.

Abstract

In the present-day universe, magnetic fields play such essential roles in star formation as angular momentum transport and outflow driving, which control circumstellar disc formation/fragmentation and also the star formation efficiency. While only a much weaker field has been believed to exist in the early universe, recent theoretical studies find that strong fields can be generated by turbulent dynamo during the gravitational collapse. Here, we investigate the gravitational collapse of a cloud core ($\sim 10^{3}\ \rm cm^{-3}$) up to protostar formation ($\sim 10^{20}\ \rm cm^{-3}$) by non-ideal magnetohydrodynamics (MHD) simulations considering ambipolar diffusion (AD), the dominant non-ideal effects in the primordial-gas. We systematically study rotating cloud cores either with or without turbulence and permeated with uniform fields of different strengths. We find that AD can slightly suppress the field growth by dynamo especially on scales smaller than the Jeans-scale at the density range $10^{10}-10^{14}\ \rm cm^{-3}$, while we could not see the AD effect on the temperature evolution, since the AD heating rate is always smaller than compression heating. The inefficiency of AD makes the field as strong as $10^{3}-10^{5} \rm\ G$ near the formed protostar, much stronger than in the present-day cases, even in cases with initially weak fields. The magnetic field affects the inflow motion when amplified to the equipartition level with turbulence on the Jeans-scale, although disturbed fields do not launch winds. This might suggest that dynamo amplified fields have smaller impact on the dynamics in the later accretion phase than other processes such as ionisation feedback.