Magnetically driven accretion in protoplanetary discs

TL;DR

This study uses non-ideal MHD simulations to analyze magnetically driven accretion in protoplanetary discs, revealing a Hall effect-induced bi-modality and turbulence patterns that depend on magnetic field orientation and chemical conditions.

Contribution

It provides new insights into how the Hall effect influences accretion and turbulence in protoplanetary discs, highlighting the complexity and variability of these processes.

Findings

Hall effect causes bi-modality in accretion at 1-10 au.

Presence of turbulence in outer disc independent of magnetic field orientation.

Identification of Hall-mediated bi-modality extending to 10 au.

Abstract

We characterize magnetically driven accretion at radii between 1 au and 100 au in protoplanetary discs, using a series of local non-ideal magnetohydrodynamic (MHD) simulations. The simulations assume a Minimum Mass Solar Nebula (MMSN) disc that is threaded by a net vertical magnetic field of specified strength. Confirming previous results, we find that the Hall effect has only a modest impact on accretion at 30 au, and essentially none at 100 au. At 1-10 au the Hall effect introduces a pronounced bi-modality in the accretion process, with vertical magnetic fields aligned to the disc rotation supporting a strong laminar Maxwell stress that is absent if the field is anti-aligned. In the anti-aligned case, we instead find evidence for bursts of turbulent stress at 5-10 au, which we tentatively identify with the non-axisymmetric Hall-shear instability. The presence or absence of these bursts depends upon the details of the adopted chemical model, which suggests that appreciable regions of actual protoplanetary discs might lie close to the borderline between laminar and turbulent behaviour. Given the number of important control parameters that have already been identified in MHD models, quantitative predictions for disc structure in terms of only radius and accretion rate appear to be difficult. Instead, we identify robust qualitative tests of magnetically driven accretion. These include the presence of turbulence in the outer disc, independent of the orientation of the vertical magnetic fields, and a Hall-mediated bi-modality in turbulent properties extending from the region of thermal ionization to 10 au.