Turbulent Stresses in Local Simulations of Radiation-Dominated Accretion Disks, and the Possibility of the LIghtman-Eardley Instability

TL;DR

This study uses radiation-MHD simulations to explore the relationship between stress and surface density in radiation-dominated accretion disks, revealing conditions that could lead to the Lightman-Eardley instability.

Contribution

First explicit physics-based mapping of the stress-surface density relation in local radiation-dominated accretion disk simulations, highlighting potential instability conditions.

Findings

Stress is proportional to total thermal pressure in equilibrium.

Stress decreases with surface density in radiation pressure dominance.

Potential for Lightman-Eardley instability under certain conditions.

Abstract

We present the results of a series of radiation-MHD simulations of a local patch of an accretion disk, with fixed vertical gravity profile but with different surface mass densities and a broad range of radiation to gas pressure ratios. Each simulation achieves a thermal equilibrium that lasts for many cooling times. After averaging over times long compared to a cooling time, we find that the vertically integrated stress is approximately proportional to the vertically-averaged total thermal (gas plus radiation) pressure. We map out--for the first time on the basis of explicit physics--the thermal equilibrium relation between stress and surface density: the stress decreases (increases) with increasing surface mass density when the simulation is radiation (gas) pressure dominated. The dependence of stress on surface mass density in the radiation pressure dominated regime suggests the possibility of a Lightman-Eardley inflow instability, but global simulations or shearing box simulations with much wider radial boxes will be necessary to confirm this and determine its nonlinear behavior.