Disk Formation in Magnetized Dense Cores with Turbulence and Ambipolar Diffusion

TL;DR

This study investigates how turbulence and ambipolar diffusion influence disk formation in magnetized dense cores, revealing their complementary roles in promoting disk growth and stability during star formation.

Contribution

It demonstrates that turbulence and ambipolar diffusion together facilitate larger, less magnetized disks, providing new insights into the physical processes of star and planet formation.

Findings

Turbulence warps but does not disrupt the pseudodisk structure.

Ambipolar diffusion reduces magnetic flux, aiding disk formation.

Disks inherit strong magnetic fields, challenging existing models.

Abstract

Disks are essential to the formation of both stars and planets, but how they form in magnetized molecular cloud cores remains debated. This work focuses on how the disk formation is affected by turbulence and ambipolar diffusion (AD), both separately and in combination, with an emphasis on the protostellar mass accretion phase of star formation. We find that a relatively strong, sonic turbulence on the core scale strongly warps but does not completely disrupt the well-known magnetically-induced flattened pseudodisk that dominates the inner protostellar accretion flow in the laminar case, in agreement with previous work. The turbulence enables the formation of a relatively large disk at early times with or without ambipolar diffusion, but such a disk remains strongly magnetized and does not persist to the end of our simulation unless a relatively strong ambipolar diffusion is also present. The AD-enabled disks in laminar simulations tend to fragment gravitationally. The disk fragmentation is suppressed by initial turbulence. The ambipolar diffusion facilitates the disk formation and survival by reducing the field strength in the circumstellar region through magnetic flux redistribution and by making the field lines there less pinched azimuthally, especially at late times. We conclude that turbulence and ambipolar diffusion complement each other in promoting disk formation. The disks formed in our simulations inherit a rather strong magnetic field from its parental core, with a typical plasma-$\beta$ of order a few tens or smaller, which is 2-3 orders of magnitude lower than the values commonly adopted in MHD simulations of protoplanetary disks. To resolve this potential tension, longer-term simulations of disk formation and evolution with increasingly more realistic physics are needed.