On Hydrodynamic Motions in Dead Zones

TL;DR

This study uses 3D simulations to analyze fluid motions in dead zones of accretion disks, revealing axisymmetric wave modes that influence gravitational stability but do not form vortices.

Contribution

It identifies a specific axisymmetric wave mode in dead zones and examines its properties, advancing understanding of fluid dynamics in stratified accretion disks.

Findings

No coherent vortices found in dead zones.

Axisymmetric wave mode identified and characterized.

Wave mode may impact gravitational stability of dust layers.

Abstract

We investigate fluid motions near the midplane of vertically stratified accretion disks with highly resistive midplanes. In such disks, the magnetorotational instability drives turbulence in thin layers surrounding a resistive, stable dead zone. The turbulent layers in turn drive motions in the dead zone. We examine the properties of these motions using three-dimensional, stratified, local, shearing-box, non-ideal, magnetohydrodynamical simulations. Although the turbulence in the active zones provides a source of vorticity to the midplane, no evidence for coherent vortices is found in our simulations. It appears that this is because of strong vertical oscillations in the dead zone. By analyzing time series of azimuthally-averaged flow quantities, we identify an axisymmetric wave mode particular to models with dead zones. This mode is reduced in amplitude, but not suppressed entirely, by changing the equation of state from isothermal to ideal. These waves are too low-frequency to affect sedimentation of dust to the midplane, but may have significance for the gravitational stability of the resulting midplane dust layers.