On the Role of Pseudodisk Warping and Reconnection in Protostellar Disk Formation in Turbulent Magnetized Cores

TL;DR

This paper investigates how turbulence and magnetic field misalignment facilitate the formation of protostellar disks by weakening magnetic braking through pseudodisk warping and reconnection, offering an alternative to magnetic reconnection explanations.

Contribution

It introduces a new interpretation that pseudodisk warping and magnetic decoupling-driven reconnection enable disk formation, unifying turbulence and misalignment effects.

Findings

Pseudodisk warping reduces magnetic flux in the disk region.

Turbulence and misalignment both promote disk formation.

Disks formed are thick and strongly magnetized.

Abstract

The formation of rotationally supported protostellar disks is suppressed in ideal MHD in non-turbulent cores with aligned magnetic field and rotation axis. A promising way to resolve this so-called "magnetic braking catastrophe" is through turbulence. The reason for the turbulence-enabled disk formation is usually attributed to the turbulence-induced magnetic reconnection, which is thought to reduce the magnetic flux accumulated in the disk-forming region. We advance an alternative interpretation, based on magnetic decoupling-triggered reconnection of severely pinched field lines close to the central protostar and turbulence-induced warping of the pseudodisk of Galli and Shu. Such reconnection weakens the central split magnetic monopole that lies at the heart of the magnetic braking catastrophe under flux freezing. We show, through idealized numerical experiments, that the pseudodisk can be strongly warped, but not completely destroyed, by a subsonic or sonic turbulence. The warping decreases the rates of angular momentum removal from the pseudodisk by both magnetic torque and outflow, making it easier to form a rotationally supported disk. More importantly, the warping of the pseudodisk out of the disk-forming, equatorial plane greatly reduces the amount of magnetic flux threading the circumstellar, disk-forming region, further promoting disk formation. The beneficial effects of pseudodisk warping can also be achieved by a misalignment between the magnetic field and rotation axis. These two mechanisms of disk formation, enabled by turbulence and field-rotation misalignment respectively, are thus unified. We find that the disks formed in turbulent magnetized cores are rather thick and significantly magnetized. Implications of these findings, particularly for the thick young disk inferred in L1527, are briefly discussed.