Dwarf Nova Outbursts with Magnetorotational Turbulence

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 $\sim 0.1-0.2$ in outburst compared to a low value $\sim 0.01$ 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.

19 pages, 14 figures, 2 tables, accepted for publication in MNRAS

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• Convective Quenching of Field Reversals in Accretion Disc Dynamos
• Extreme evaporation of planets in hot thermally unstable protoplanetary discs: the case of FU Ori
• Variability of the inner dead zone edge in 2D radiation hydrodynamic simulations
• Magnetic field transport in compact binaries
• Two-dimensional simulations of disks in close binaries: Simulating outburst cycles in cataclysmic variables
• The Role of Thermal Instability in Accretion Outbursts in High-Mass Stars
• Magnetohydrodynamic convection in accretion discs
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• Evolution of a Viscous Protoplanetary Disk with Convectively Unstable Regions. II. Accretion Regimes and Long-Term Dynamics
• Challenging the disk instability model: I -- The case of YZ LMi