Convection Enhances Magnetic Turbulence in AM CVn Accretion Disks

TL;DR

This study uses advanced simulations to show that convection significantly boosts magnetic turbulence in helium-rich AM CVn accretion disks, revealing key differences from hydrogen-rich dwarf novae.

Contribution

It demonstrates that convection enhances MRI-driven turbulence in AM CVn disks and identifies differences from dwarf novae, including persistent convection and the validity of ideal MHD.

Findings

Convection enhances stress-to-pressure ratio $eta$ near the upper branch of the S-curve.

AM CVn disks exhibit persistent convection during outburst, unlike dwarf novae.

Convective enhancement of MRI is anticorrelated with mean molecular weight.

Abstract

We present the results of local, vertically stratified, radiation magnetohydrodynamic shearing box simulations of magnetorotational instability (MRI) turbulence for a (hydrogen poor) composition applicable to accretion disks in AM CVn type systems. Many of these accreting white dwarf systems are helium analogues of dwarf novae (DNe). We utilize frequency-integrated opacity and equation of state tables appropriate for this regime to accurately portray the relevant thermodynamics. We find bistability of thermal equilibria in the effective temperature, surface mass density plane typically associated with disk instabilities. Along this equilibrium curve (i.e. the S-curve) we find that the stress to thermal pressure ratio $\alpha$ varied with peak values of $\sim 0.15$ near the tip of the upper branch. Similar to DNe, we found enhancement of $\alpha$ near the tip of the upper branch caused by convection; this increase in $\alpha$ occurred despite our choice of zero net vertical magnetic flux. Two notable differences we find between DN and AM CVn accretion disk simulations are that AM CVn disks are capable of exhibiting persistent convection in outburst, and ideal MHD is valid throughout quiescence for AM CVns. In contrast, DNe simulations only show intermittent convection, and non-ideal MHD effects are likely important in quiescence. By combining our previous work with these new results, we also find that convective enhancement of the MRI is anticorrelated with mean molecular weight.