Thermal equilibrium curves of accretion disks driven by magnetorotational instability

TL;DR

This paper reviews first-principles thermal equilibrium curves for MRI-driven accretion disks, especially in dwarf novae, obtained through radiation magnetohydrodynamics simulations, highlighting the physical origin of disk instabilities without using parameterized viscosity.

Contribution

It presents a novel approach to deriving thermal equilibrium curves from first principles using radiation MHD simulations, avoiding the traditional alpha-viscosity parameterization.

Findings

Reproduces S-shaped equilibrium loci without prescribed alpha viscosity.

Provides insights into the physical origin of angular-momentum transport.

Discusses stability of radiation-dominated accretion flows.

Abstract

Analogous to the HR diagram for stars, the thermal equilibrium curve encodes the thermodynamics of accretion disks by expressing the local balance between heating -- primarily via viscous dissipation -- and cooling -- typically through radiative transfer. These curves are commonly plotted as surface density versus effective temperature. When an S-shaped locus appears, local annuli become bistable, and limit-cycle oscillations arise when the external mass-transfer rate falls within an unstable band. This behavior underpins the disk instability model for recurring outbursts in cataclysmic variables. This paper reviews first-principles thermal equilibrium curves for accretion disks driven by magnetorotational instability (MRI), with emphasis on dwarf novae. Unlike the parameterized $\alpha$-viscosity approach, the curves are obtained by solving the governing equations with radiation magnetohydrodynamics simulations, thereby reproducing S-shaped loci without prescribing $\alpha$. The disk instability in dwarf-nova systems and the physical origin of angular-momentum transport (shear stresses) are also briefly reviewed. Notes on the stability of radiation-dominated accretion flows are included in the Appendix.