Dissipation and Vertical Energy Transport in Radiation-Dominated Accretion Disks

TL;DR

This paper investigates how energy is transported and dissipated in radiation-dominated accretion disks, revealing the significant role of radiation advection and magnetic buoyancy in turbulence and energy dissipation.

Contribution

It demonstrates that radiation advection and buoyancy-driven magnetic fluctuations are key to energy transport and dissipation in radiation-supported accretion disks, challenging standard diffusive flux assumptions.

Findings

Radiation advection carries substantial energy flux inside the disk.

Radiative damping accounts for up to tens of percent of total dissipation.

Magnetic buoyancy influences both energy transport and magnetic field dynamics.

Abstract

Standard models of radiation supported accretion disks generally assume that diffusive radiation flux is solely responsible for vertical heat transport. This requires that heat must be generated at a critical rate per unit volume if the disk is to be in hydrostatic and thermal equilibrium. This raises the question of how heat is generated and how energy is transported in MHD turbulence. By analysis of a number of radiation/MHD stratified shearing-box simulations, we show that the divergence of the diffusive radiation flux is indeed capped at the critical rate, but deep inside the disk, substantial vertical energy flux is also carried by advection of radiation. Work done by radiation pressure is a significant part of the energy budget, and much of this work is dissipated later through damping by radiative diffusion. We show how this damping can be measured in the simulations, and identify its physical origins. Radiative damping accounts for as much as tens of percent of the total dissipation, and is the only realistic physical mechanism for dissipation of turbulence that can actually be resolved in numerical simulations of accretion disks. Buoyancy associated with dynamo-driven, highly magnetized, nearly-isobaric nonlinear slow magnetosonic fluctuations is responsible for the radiation advection flux, and also explains the persistent periodic magnetic upwelling seen at all values of the radiation to gas pressure ratio. The intimate connection between radiation advection and magnetic buoyancy is the first example we know of in astrophysics in which a dynamo has direct impact on the global energetics of a system.