Growth of the MRI in Accretion Discs -- the Influence of Radiation Transport

TL;DR

This paper examines how radiative transport affects the growth of the magnetorotational instability in accretion discs, revealing that radiation can both suppress and enhance growth rates depending on mode orientation, with analytical and numerical insights.

Contribution

It introduces a dispersion relation incorporating radiative effects and analyzes their impact on MRI growth rates in accretion discs.

Findings

Vertical MRI modes have reduced growth rates due to radiation.

Non-vertical modes may experience enhanced growth rates.

Numerical simulations explore non-linear MRI evolution with radiation.

Abstract

In this paper we investigate the influence of radiative transport on the growth of the magnetorotational instability (MRI) in accretion discs. The analysis is performed by use of analytical and numerical means. We provide a general dispersion relation together with the corresponding eigenfunctions describing the growth rates of small disturbances on a homogeneous background shear flow. The dispersion relation includes compressibility and radiative effects in the flux-limited diffusion approximation. By introducing an effective speed of sound, all the effects of radiation transport can be subsumed into one single parameter. It can be shown that the growth rates of the vertical modes -- which are the fastest growing ones -- are reduced by radiative transport. For the case of non-vertical modes, the growth rates may instead be enhanced. We quantify the effects of compressibility and radiative diffusion on the growth rates for the gas-pressure dominated case. The analytical discussion is supplemented by numerical simulations, which are also used for a first investigation of the non-linear stage of the MRI in gas-pressure dominated accretion discs with radiation transport included.