Dead Zone Accretion Flows in Protostellar Disks

TL;DR

This paper demonstrates that even in the dead zones of protostellar disks, magnetic fields enable laminar accretion flows toward the star, driven by large-scale magnetic field transfer and weak ionization.

Contribution

It reveals that dead zones are not entirely inactive but sustain magnetic coupling and accretion through large-scale magnetic fields and ionization processes.

Findings

Magnetic fields enable accretion flows in dead zones.

Ionization maintains magnetic coupling despite dust grain recombination.

Magneto-rotational turbulence generates net radial magnetic fields.

Abstract

Planets form inside protostellar disks in a dead zone where the electrical resistivity of the gas is too high for magnetic forces to drive turbulence. We show that much of the dead zone nevertheless is active and flows toward the star while smooth, large-scale magnetic fields transfer the orbital angular momentum radially outward. Stellar X-ray and radionuclide ionization sustain a weak coupling of the dead zone gas to the magnetic fields, despite the rapid recombination of free charges on dust grains. Net radial magnetic fields are generated in the magneto-rotational turbulence in the electrically conducting top and bottom surface layers of the disk, and reach the midplane by Ohmic diffusion. A toroidal component to the fields is produced near the midplane by the orbital shear. The process is similar to the magnetization of the Solar tachocline. The result is a laminar, magnetically-driven accretion flow in the region where the planets form.