On the fragmentation boundary in magnetised self-gravitating discs

TL;DR

This study uses 3D MHD simulations to explore how magnetic fields influence the fragmentation of self-gravitating discs, revealing conditions under which magnetic effects suppress or promote fragmentation and affecting fragment properties.

Contribution

It provides new insights into the magnetic regulation of disc fragmentation, showing magnetic fields can both suppress and enhance fragmentation depending on cooling times.

Findings

Magnetic fields suppress fragmentation when cooling time exceeds five dynamical times.

Magnetised discs produce more massive fragments due to magnetic pressure and Maxwell stresses.

Fragmentation boundary may occur at shorter cooling times than previously predicted.

Abstract

We investigate the role of magnetic fields in the fragmentation of self-gravitating discs using 3D global ideal magnetohydrodynamic simulations performed with the "phantom" smoothed particle hydrodynamics code. For initially toroidal fields, we find two regimes. In the first, where the cooling time is greater than five times the dynamical time, magnetic fields reduce spiral density wave amplitudes, which in turn suppresses fragmentation. This is the case even if the magnetic pressure is only a tenth of the thermal pressure. The second regime occurs when the cooling time is sufficiently short that magnetic fields cannot halt fragmentation. We find that magnetised discs produce more massive fragments, due to both the additional pressure exerted by the magnetic field, and the additional angular momentum transport induced by Maxwell stresses. The fragments are confined to a narrower range of initial semimajor axes than those in unmagnetised discs. The orbital eccentricity and inclination distributions of unmagnetised and magnetised disc fragments are similar. Our results suggest the fragmentation boundary could be at cooling times a factor of two lower than predicted by purely hydrodynamical models.