Review of Thanatology in Protoplanetary Discs:
The combined influence of Ohmic, Hall, and ambipolar diffusion on dead zones (Lesur, Kunz & Fromang 2014)
Author: Gregory Hunt
Protoplanetary discs are cool and dense, their gas is composed of neutrals, ions and electron, and in the presence of a background magnetic field this becomes magnetised plasma. Due to the fact that this is normally poorly ionised, the plasma in protoplanetary discs, is therefore subject to three non-ideal magnetohydrodynamic (MHD) effects; Ohmic dissipation, ambipolar diffusion and Hall effect. Following the discovery of the magnetorotional instability (MRI) (Balbus & Hawley 1991,1998) the possibility of turbulence magnetically driven in protoplanetary disc as a process to transport angular momentum has been an important question to answer. Ohmic dissipation is caused by collisions between the electrons and neutrals, ambipolar diffusion is due to the collisions between electrons and neutrals and the Hall effect by the velocity difference between the positively and negatively charged species. All of these act to decouple the gas from the magnetic field and therefore reduced the effect MRI. To properly understand the effect MRI has in formation and evolution of protoplanetary it is important to understand the non-ideal MHD effects in the plasma and the effect on MRI, this is the aim of the paper discussed here.
The paper discussed in this essay focuses on the combined influence of the Ohmic, Hall and ambipolar diffusion on the magnetically “dead zones” in the midplane of the disc, this is the first time that all three of these non-ideal MHD effects have been accounted in a numerical simulation of the MRI. The “dead zone” is the region where the disc is not sufficiently ionised (ie the ionisation factor is less than 10-12) and therefore MRI does not operate. Previous studies have focused on Ohmic dissipation on a stratified disc, which lead to the picture of layered acceration in which the dead zone in the midplane is located between magnetically active surface layers (Gammie 1996 and Sano & Miyama 1999).
More recent works have combined the Ohmic dissipation and ambipolar diffusion to show that they can cause the midplane and surface layers to become dormant (Bai & Stone 2013 and Simon et al. 2013) and the measured acceration rates require torques due to outflows which are magnetically driven. A recent publication, Kunz & Lesur 2013 showed that the vertical magnetic field can organizes itself into axisymmetric zonal structures in Hall dominated magnetorotational turbulence, this lead to the rate of angular momentum transport to become very small, this result was based on a simple unstratified model for a proplanetary disc so one of the questions this paper asks is whether this results applies in a more realistic disc with Ohmic dissipation and amipolar diffusion included.
Discussion of Methods
To study the evolution of the system a shearing-box approximation...