Gravito-turbulence and dynamo in poorly ionised protostellar discs. I. Zero-net-flux case

TL;DR

This study investigates how gravitational instability in poorly ionised protoplanetary discs can generate magnetic fields through a dynamo process, using advanced simulations to explore the effects of ambipolar diffusion.

Contribution

It demonstrates the operation of a dynamo driven by gravitational instability in poorly ionised discs, highlighting its dependence on ambipolar diffusion strength and robustness across simulation parameters.

Findings

Dynamo operates over a wide range of ambipolar Elsasser numbers.

Magnetic energy reaches near thermal energy levels in certain regimes.

Magnetic field generation is linked to spiral density waves and differential rotation.

Abstract

In their early stages, protoplanetary discs are sufficiently massive to undergo gravitational instability (GI). This instability is thought to be involved in mass accretion, planet formation via gas fragmentation, the generation of spiral density waves, and outbursts. A key and very recent area of research is the interaction between the GI and magnetic fields in young protoplanetary discs, in particular whether this instability is able to sustain a magnetic field via a dynamo. We conduct three-dimensional, stratified shearing-box simulations using two independent codes, PLUTO and Athena++, to characterise the GI dynamo in poorly ionised protostellar discs subject to ambipolar diffusion. We find that the dynamo operates across a large range of ambipolar Elssaser number Am (which characterises the strength of ambipolar diffusion) and is particularly strong in the regime Am=10-100, with typical magnetic to thermal energy ratios of order unity. The dynamo is only weakly dependent on resolution (at least for Am <100), box size, and cooling law. The magnetic field is produced by the combination of differential rotation and large-scale vertical roll motions associated with spiral density waves. Our results have direct implications for the dynamo process in young protoplanetary discs and possibly some regions of AGN discs.