Disk Formation Enabled by Enhanced Resistivity

TL;DR

This paper investigates how enhanced resistivity levels can facilitate the formation of rotationally supported disks in magnetized star-forming cores, overcoming magnetic braking that typically prevents disk formation.

Contribution

It demonstrates that classical microscopic resistivity is insufficient and identifies a significantly higher resistivity threshold needed for disk formation.

Findings

A resistivity of about 10^{19} cm^2/s enables 100 AU Keplerian disks.

Classical resistivity values are too low to allow disk formation.

Enhanced resistivity levels are necessary, but their natural occurrence remains uncertain.

Abstract

Disk formation in magnetized cloud cores is hindered by magnetic braking. Previous work has shown that for realistic levels of core magnetization, the magnetic field suppresses the formation of rotationally supported disks during the protostellar mass accretion phase of low-mass star formation both in the ideal MHD limit and in the presence of ambipolar diffusion for typical rates of cosmic ray ionization. Additional effects, such as ohmic dissipation, the Hall effect, and protostellar outflow, are needed to weaken the magnetic braking and enable the formation of persistent, rotationally supported, protostellar disks. In this paper, we first demonstrate that the classic microscopic resistivity is not large enough to enable disk formation by itself. We then experiment with a set of enhanced values for the resistivity in the range $\eta=10^{17}$--$10^{22}$ cm^2/s. We find that a value of order $10^{19}$ cm^2/s is needed to enable the formation of a 100 AU-scale Keplerian disk; the value depends somewhat on the degree of core magnetization. The required resistivity is a few orders of magnitude larger than the classic microscopic values. Whether it can be achieved naturally during protostellar collapse remains to be determined.