Magnetic field and early evolution of circumstellar disks

TL;DR

This paper reviews how magnetic fields influence the formation and early evolution of circumstellar disks, highlighting mechanisms like magnetic braking and non-ideal effects that enable disk formation despite theoretical suppression.

Contribution

It emphasizes the role of non-ideal MHD effects, especially magnetic diffusion and the Hall effect, in resolving the magnetic braking problem during early disk formation.

Findings

Magnetic braking can suppress disk formation in ideal MHD models.

Non-ideal effects like magnetic diffusion facilitate early disk formation.

The Hall effect can provide sufficient angular momentum for T Tauri disks.

Abstract

The magnetic field plays a central role in the formation and evolution of circumstellar disks. The magnetic field connects the rapidly rotating central region with the outer envelope and extracts angular momentum from the central region during gravitational collapse of the cloud core. This process is known as magnetic braking. Both analytical and multidimensional simulations have shown that disk formation is strongly suppressed by magnetic braking in moderately magnetized cloud cores in the ideal magnetohydrodynamic limit. On the other hand, recent observations have provided growing evidence of a relatively large disk several tens of astronomical units in size existing in some Class 0 young stellar objects. This introduces a serious discrepancy between the theoretical study and observations. Various physical mechanisms have been proposed to solve the problem of catastrophic magnetic braking, such as misalignment between the magnetic field and the rotation axis, turbulence, and non-ideal effect. In this paper, we review the mechanism of magnetic braking, its effect on disk formation and early evolution, and the mechanisms that resolve the magnetic braking problem. In particular, we emphasize the importance of non-ideal effects. The combination of magnetic diffusion and thermal evolution during gravitational collapse provides a robust formation process for the circumstellar disk at the very early phase of protostar formation. The rotation induced by the Hall effect can supply a sufficient amount of angular momentum for typical circumstellar disks around T Tauri stars. By examining the combination of the suggested mechanisms, we conclude that the circumstellar disks commonly form in the very early phase of protostar formation.