Disk Formation in Magnetized Clouds Enabled by the Hall Effect

TL;DR

This paper investigates how the Hall effect, a non-ideal MHD process, influences protostellar disk formation in magnetized clouds, showing it can enable disk formation and determine rotation direction.

Contribution

It is the first study to explore the Hall effect's role in star formation, demonstrating its ability to produce and control protostellar disks in magnetized environments.

Findings

Hall effect can spin up collapsing flows to form disks.

Disk rotation direction can be reversed by magnetic field orientation.

Disk formation occurs even without initial envelope rotation.

Abstract

Stars form in dense cores of molecular clouds that are observed to be significantly magnetized. A dynamically important magnetic field presents a significant obstacle to the formation of protostellar disks. Recent studies have shown that magnetic braking is strong enough to suppress the formation of rotationally supported disks in the ideal MHD limit. Whether non-ideal MHD effects can enable disk formation remains unsettled. We carry out a first study on how disk formation in magnetic clouds is modified by the Hall effect, the least explored of the three non-ideal MHD effects in star formation (the other two being ambipolar diffusion and Ohmic dissipation). For illustrative purposes, we consider a simplified problem of a non-self-gravitating, magnetized envelope collapsing onto a central protostar of fixed mass. We find that the Hall effect can spin up the inner part of the collapsing flow to Keplerian speed, producing a rotationally supported disk. The disk is generated through a Hall-induced magnetic torque. Disk formation occurs even when the envelope is initially non-rotating, provided that the Hall coefficient is large enough. When the magnetic field orientation is flipped, the direction of disk rotation is reversed as well. The implication is that the Hall effect can in principle produce both regularly rotating and counter-rotating disks around protostars. We conclude that the Hall effect is an important factor to consider in studying the angular momentum evolution of magnetized star formation in general and disk formation in particular.