Magnetic Braking and Protostellar Disk Formation: Ambipolar Diffusion

TL;DR

This paper investigates whether ambipolar diffusion can weaken magnetic braking enough to allow protostellar disk formation, concluding it is insufficient alone and other effects or late-stage processes are necessary.

Contribution

The study demonstrates that ambipolar diffusion alone does not enable disk formation under realistic conditions, highlighting the need for additional non-ideal MHD effects.

Findings

Ambipolar diffusion does not significantly weaken magnetic braking.

Disk formation requires either very weak magnetic fields or low ionization rates.

Additional processes like Ohmic dissipation or Hall effect are likely necessary.

Abstract

It is established that the formation of rotationally supported disks during the main accretion phase of star formation is suppressed by a moderately strong magnetic field in the ideal MHD limit. Non-ideal MHD effects are expected to weaken the magnetic braking, perhaps allowing the disk to reappear. We concentrate on one such effect, ambipolar diffusion, which enables the field lines to slip relative to the bulk neutral matter. We find that the slippage does not sufficiently weaken the braking to allow rotationally supported disks to form for realistic levels of cloud magnetization and cosmic ray ionization rate; in some cases, the magnetic braking is even enhanced. Only in dense cores with both exceptionally weak fields and unreasonably low ionization rate do such disks start to form in our simulations. We conclude that additional processes, such as Ohmic dissipation or Hall effect, are needed to enable disk formation. Alternatively, the disk may form at late times when the massive envelope that anchors the magnetic brake is dissipated, perhaps by a protostellar wind.