Bridging the gap: disk formation in the Class 0 phase with ambipolar diffusion and Ohmic dissipation

TL;DR

This study demonstrates that non-ideal magnetohydrodynamics effects like ambipolar diffusion and Ohmic dissipation enable the formation of small, early-stage disks during star formation, overcoming magnetic braking issues seen in ideal MHD simulations.

Contribution

It introduces a detailed chemical and physical model showing how magnetic field dissipation allows early disk formation in the Class 0 phase of star formation.

Findings

A small (~10 R_Sun) disk forms during early star formation.

Magnetic diffusion shocks and core profiles are analytically understood.

Disk remains undetectable by ALMA in early stages.

Abstract

Context: Ideal MHD simulations have revealed catastrophic magnetic braking (MB) in the protostellar phase, which prevents the formation of a centrifugal disk around a nascent protostar. Aims: We determine if non-ideal MHD, including the effects of ambipolar diffusion and Ohmic dissipation determined from a detailed chemical network model, allows for disk formation at the earliest stages of star formation (SF). Methods: We employ the axisymmetric thin-disk approximation in order to resolve a dynamic range of 9 orders of magnitude in length and 16 in density, while also calculating partial ionization using up to 19 species in a detailed chemical equilibrium model. MB is applied using a steady-state approximation, and a barotropic relation is used to capture the thermal evolution. Results: We resolve the formation of the first and second cores, with expansion waves at the periphery of each, a magnetic diffusion shock, and prestellar infall profiles at larger radii. Power-law profiles in each region can be understood analytically. After the formation of the second core, centrifugal support rises rapidly and a low-mass disk of radius ~10 R_Sun is formed, when the second core has mass ~0.001 M_Sun. The mass-to-flux ratio is ~10,000 times the critical value in the central region. Conclusions: A small centrifugal disk can form in the earliest stage of SF, due to a shut-off of MB caused by magnetic field dissipation in the first core region. There is enough angular momentum loss to allow the second collapse to occur directly, and a low-mass stellar core to form with a surrounding disk. The disk mass and size will depend upon how the angular momentum transport mechanisms within the disk can keep up with mass infall onto the disk. We estimate that the disk will remain <~10 AU, undetectable even by ALMA, in the early Class 0 phase.