Averting the magnetic braking catastrophe on small scales: disk formation due to Ohmic dissipation

TL;DR

This study demonstrates that Ohmic dissipation can prevent magnetic braking from inhibiting disk formation around Class 0 objects, allowing small disks to form early despite magnetic effects.

Contribution

The paper shows that Ohmic dissipation enables the formation of small circumstellar disks during early star formation, even with magnetic braking present.

Findings

Disks form close to the second core with radii around 10 solar radii.

Ohmic dissipation renders magnetic braking ineffective within the first core.

Disk sizes can grow over time through additional processes like ambipolar diffusion.

Abstract

We perform axisymmetric resistive MHD calculations that demonstrate that centrifugal disks can indeed form around Class 0 objects despite magnetic braking. We follow the evolution of a prestellar core all the way to near-stellar densities and stellar radii. Under flux-freezing, the core is braked and disk formation is inhibited, while Ohmic dissipation renders magnetic braking ineffective within the first core. In agreement with observations that do not show evidence for large disks around Class 0 objects, the resultant disk forms in close proximity to the second core and has a radius of only $\approx 10~R_{\odot}$ early on. Disk formation does not require enhanced resistivity. We speculate that the disks can grow to the sizes observed around Class II stars over time under the influence of both Ohmic dissipation and ambipolar diffusion, as well as internal angular momentum redistribution.