Turbulence, Transport and Waves in Ohmic Dead Zones

TL;DR

This study uses local simulations to analyze how resistive dead zones in accretion disks affect turbulence, angular momentum transport, and wave excitation, revealing that thick dead zones suppress turbulence but support large kinetic energy.

Contribution

It provides new insights into the dynamics of Ohmic dead zones, showing their impact on turbulence, energy distribution, and wave phenomena in stratified accretion disks.

Findings

Reynolds stress decreases with dead zone thickness

Thick dead zones support large kinetic energy but with inefficient angular momentum transport

Oscillatory, low-frequency circulation patterns dominate in dead zones

Abstract

We use local numerical simulations to study a vertically stratified accretion disk with a resistive mid-plane that damps magnetohydrodynamic (MHD) turbulence. This is an idealized model for the dead zones that may be present at some radii in protoplanetary and dwarf novae disks. We vary the relative thickness of the dead and active zones to quantify how forced fluid motions in the dead zone change. We find that the residual Reynolds stress near the mid-plane decreases with increasing dead zone thickness, becoming negligible in cases where the active to dead mass ratio is less than a few percent. This implies that purely Ohmic dead zones would be vulnerable to episodic accretion outbursts via the mechanism of Martin & Lubow (2011). We show that even thick dead zones support a large amount of kinetic energy, but this energy is largely in fluid motions that are inefficient at angular momentum transport. Confirming results from Oishi & Mac Low (2009), the perturbed velocity field in the dead zone is dominated by an oscillatory, vertically extended circulation pattern with a low frequency compared to the orbital frequency. This disturbance has the properties predicted for the lowest order r mode in a hydrodynamic disk. We suggest that in a global disk similar excitations would lead to propagating waves, whose properties would vary with the thickness of the dead zone and the nature of the perturbations (isothermal or adiabatic). Flows with similar amplitudes would buckle settled particle layers and could reduce the efficiency of pebble accretion.