Self-organisation in protoplanetary disks: global, non-stratified Hall-MHD simulations

TL;DR

This study uses 3D Hall-MHD simulations to explore how the Hall effect influences the self-organization of protoplanetary disks, revealing the formation of zonal flows and vortices that may explain observed disk structures.

Contribution

It demonstrates that the Hall effect can induce self-organized structures like zonal flows and vortices in global MRI-unstable disk models, without the need for planetary influences.

Findings

Transition from turbulence to organized flows with increased Hall effect

Formation of long-lived magnetized vortices depending on magnetic flux and Hall intensity

Structures can trap dust particles, potentially explaining observed disk features

Abstract

Recent observations revealed organised structures in protoplanetary disks, such as axisymmetric rings or horseshoe concen- trations evocative of large-scale vortices. These structures are often interpreted as the result of planet-disc interactions. However, these disks are also known to be unstable to the magneto-rotational instability (MRI) which is believed to be one of the dominant angular momentum transport mechanism in these objects. It is therefore natural to ask if the MRI itself could produce these structures without invoking planets. The nonlinear evolution of the MRI is strongly affected by the low ionisation fraction in protoplanetary disks. The Hall effect in particular, which is dominant in dense and weakly ionised parts of these objects, has been shown to spontaneously drive self- organising flows in shearing box simulations. Here, we investigate the behaviour of global MRI-unstable disc models dominated by the Hall effect and characterise their dynamics. We perform 3D unstratified Hall-MHD simulations of keplerian disks for a broad range of Hall, ohmic and ambipolar Elsasser numbers. We confirm the transition from a turbulent to an organised state as the intensity of the Hall effect is increased. We observe the formation of zonal flows, their number depending on the available magnetic flux and on the intensity of the Hall effect. For intermediate Hall intensity, the flow self-organises into long-lived magnetised vortices. Neither the addition of a toroidal field nor ohmic or ambipolar diffusion drastically change this picture in the range of parameters we have explored. The ability of these structures to trap dust particles in this configuration is demonstrated. We conclude that Hall-MRI driven organisation is a plausible scenario which could explain some of the structures found in recent observations.