Dwarf Nova Outbursts with Magnetorotational Turbulence

TL;DR

This paper integrates MRI turbulence simulations into the Disc Instability Model to better reproduce dwarf nova outburst light curves, revealing successes and challenges such as reflares not typically observed.

Contribution

It introduces MRI-based variations of the alpha parameter into the Disc Instability Model, linking turbulence simulations with observational light curve features.

Findings

MRI-based models reproduce outburst durations and amplitudes

Models show reflares during decay, unlike typical observations

Highlights issues in quiescence physics in current models

Abstract

The phenomenological Disc Instability Model has been successful in reproducing the observed light curves of dwarf nova outbursts by invoking an enhanced Shakura-Sunyaev $\alpha$ parameter $\sim0.1-0.2$ in outburst compared to a low value $\sim0.01$ in quiescence. Recent thermodynamically consistent simulations of magnetorotational (MRI) turbulence with appropriate opacities and equation of state for dwarf nova accretion discs have found that thermal convection enhances $\alpha$ in discs in outburst, but only near the hydrogen ionization transition. At higher temperatures, convection no longer exists and $\alpha$ returns to the low value comparable to that in quiescence. In order to check whether this enhancement near the hydrogen ionization transition is sufficient to reproduce observed light curves, we incorporate this MRI-based variation in $\alpha$ into the Disc Instability Model, as well as simulation-based models of turbulent dissipation and convective transport. These MRI-based models can successfully reproduce observed outburst and quiescence durations, as well as outburst amplitudes, albeit with different parameters from the standard Disc Instability Models. The MRI-based model lightcurves exhibit reflares in the decay from outburst, which are not generally observed in dwarf novae. However, we highlight the problematic aspects of the quiescence physics in the Disc Instability Model and MRI simulations that are responsible for this behavior.