Swirling secrets in stars' planet-forming discs

By better understanding the nature of gases, scientists hope to learn more about how particles interact with each other and coagulate to ultimately form planets. The challenge is to develop correct models for the structure of the discs, describing how density and temperature change with distance from the star.

Assumptions must also be made as to the strength of the magnetic field that is present and the ionisation structure of the disc. Finding where the temperature might not be high enough to remove electrons from atoms and molecules is of importance in determining where turbulence will be more vigorous.

The challenge addressed within the EU-funded project HALLDISCS (Hall dominated turbulence in protoplanetary discs) was related to a technical issue regarding magnetohydrodynamics (MHD) simulations. The existing algorithms were not able to capture the nature of the Hall effect.

In plasmas composed of neutral molecules, ions and electrons, the velocity difference between positively and negatively charged species gives rise to the Hall effect. In addition, Ohmic dissipation is caused by collisions between electrons and neutrals and ambipolar diffusion by collisions between ions and neutrals.

The HALLDISCS team performed 3D simulations that included all three non-ideal MHD effects to investigate the role of the Hall effect on disc gas dynamics. The Hall effect revived 'dead' zones by producing a magnetic field together with a considerable stress throughout the disc mid-plane.

Specifically, the plasma flow in the mid-plane was found to be generally laminar, suggesting that the rate with which dust settles is high. These results call into question contemporary models of layered accretion and demonstrate that the Hall effect must be considered to obtain even qualitatively correct results.

By comparing observations with the theoretical predictions, HALLDISCS scientists hope to be able to verify their understanding of how disc accretion works in the next few years.