Gravitoturbulence in magnetised protostellar discs

TL;DR

This study explores how magnetic fields influence gravitational instability in protostellar disks, revealing a hot gravito-turbulent state with magnetic islands, and finds magnetic fields have limited impact on disk fragmentation.

Contribution

It provides the first detailed MHD simulation analysis of gravito-turbulence in magnetized protostellar disks, highlighting the role of magnetic fields in disk dynamics.

Findings

Discovered a hot gravito-turbulent state with high magnetic energy.

Identified long-lived magnetic islands or plasmoids.

Magnetic fields minimally affect disk fragmentation criteria.

Abstract

Gravitational instability (GI) features in several aspects of protostellar disk evolution, most notably in angular momentum transport, fragmentation, and the outbursts exemplified by FU Ori and EX Lupi systems. The outer regions of protostellar discs may also be coupled to magnetic fields, which could then modify the development of GI. To understand the basic elements of their interaction, we perform local 2D ideal and resistive MHD simulations with an imposed toroidal field. In the regime of moderate plasma beta, we find that the system supports a hot gravito-turbulent state, characterised by considerable magnetic energy and stress and a surprisingly large Toomre parameter $Q~10$. This result has potential implications for disk structure, vertical thickness, ionisation, etc. Our simulations also reveal the existence of long-lived and dense `magnetic islands' or plasmoids. Lastly, we find that the presence of a magnetic field has little impact on the fragmentation criterion of the disk. Though our focus is on protostellar disks, some of our results may be relevant for the outer radii of AGN.