Can non-ideal magnetohydrodynamics solve the magnetic braking catastrophe?

TL;DR

This study uses 3D non-ideal MHD simulations to explore if low ionisation fractions can resolve the magnetic braking catastrophe, revealing that the Hall effect's influence depends on magnetic field orientation, enabling disc formation.

Contribution

It demonstrates that the Hall effect, combined with ambipolar diffusion and Ohmic resistivity, can enable circumstellar disc formation under strong magnetic fields depending on field orientation.

Findings

Disc formation depends on magnetic field orientation due to the Hall effect.

Counter-rotating envelopes form around the first hydrostatic core.

Planet formation may depend on the magnetic field's polarity in molecular clouds.

Abstract

We investigate whether or not the low ionisation fractions in molecular cloud cores can solve the `magnetic braking catastrophe', where magnetic fields prevent the formation of circumstellar discs around young stars. We perform three-dimensional smoothed particle non-ideal magnetohydrodynamics (MHD) simulations of the gravitational collapse of one solar mass molecular cloud cores, incorporating the effects of ambipolar diffusion, Ohmic resistivity and the Hall effect alongside a self-consistent calculation of the ionisation chemistry assuming 0.1 micron grains. When including only ambipolar diffusion or Ohmic resistivity, discs do not form in the presence of strong magnetic fields, similar to the cases using ideal MHD. With the Hall effect included, disc formation depends on the direction of the magnetic field with respect to the rotation vector of the gas cloud. When the vectors are aligned, strong magnetic braking occurs and no disc is formed. When the vectors are anti-aligned, a disc with radius of 13AU can form even in strong magnetic when all three non-ideal terms are present, and a disc of 38 AU can form when only the Hall effect is present; in both cases, a counter-rotating envelope forms around the first hydrostatic core. For weaker, anti-aligned fields, the Hall effect produces massive discs comparable to those produced in the absence of magnetic fields, suggesting that planet formation via gravitational instability may depend on the sign of the magnetic field in the precursor molecular cloud core.