Effect of Magnetic Braking on the Circumstellar Disk Formation in a Strongly Magnetized Cloud

TL;DR

This study uses resistive magnetohydrodynamics simulations to explore how magnetic braking influences the formation and growth of circumstellar disks in strongly magnetized clouds, revealing conditions under which large disks can form.

Contribution

It demonstrates that despite magnetic braking, circumstellar disks larger than 100 AU can form in strongly magnetized environments, highlighting the role of envelope depletion and outflow dynamics.

Findings

Early disk size limited to ~10 AU due to magnetic braking.

Rapid disk growth occurs after envelope depletion, exceeding 100 AU.

Magnetic braking is ineffective when the disk becomes more massive than the envelope.

Abstract

Using resistive magnetohydrodynamics simulation, we investigate circumstellar disk formation in a strongly magnetized cloud. As the initial state, an isolated cloud core embedded in a low-density interstellar medium with a uniform magnetic field is adopted. The cloud evolution is calculated until almost all gas inside the initial cloud falls onto either the circumstellar disk or a protostar, and a part of the gas is ejected into the interstellar medium by the protostellar outflow driven by the circumstellar disk. In the early main accretion phase, the disk size is limited to \sim 10 AU because the angular momentum of the circumstellar disk is effectively transferred by both magnetic braking and the protostellar outflow. In the later main accretion phase, however, the circumstellar disk grows rapidly and exceeds 100 AU by the end of the main accretion phase. This rapid growth of the circumstellar disk is caused by the depletion of the infalling envelope, while magnetic braking is effective when the infalling envelope is more massive than the circumstellar disk. The infalling envelope cannot brake the circumstellar disk when the latter is more massive than the former. In addition, the protostellar outflow weakens and disappears in the later main accretion phase, because the outflow is powered by gas accretion onto the circumstellar disk. Although the circumstellar disk formed in a magnetized cloud is considerably smaller than that in an unmagnetized cloud, a circumstellar disk exceeding 100 AU can form even in a strongly magnetized cloud.