Turbulent and wind-driven accretion in dwarf novae threaded by a large scale magnetic field

Abstract

Dwarf novae (DNe) are accreting white dwarfs that show eruptions due to a thermal-viscous instability in the accretion disk. The outburst timescales constrain $α$, the ratio of the viscous stress to the thermal pressure, and so the mechanism of angular momentum transport. The eruptive state has $α\approx0.1$ while the quiescent state has $α\approx0.03$. Turbulent transport due to the magneto-rotational instability (MRI) is generally considered to be the source of angular momentum transport in DNe. Here, we perform 3D local magnetohydrodynamic (MHD) shearing box simulations including vertical stratification, radiative transfer and a net constant vertical magnetic flux to investigate how transport changes between the outburst and quiescent states of DNe. We find that a constant $B_z$ provides a higher $α$ in quiescence than in outburst, in opposition to what is expected. Including resistivity quenches MRI turbulence in quiescence, suppressing transport, unless the magnetic field is high enough, which again leads to $α\approx0.1$. A major difference between simulations with a net poloidal flux and simulations without is that angular momentum transport in the former is shared between turbulent and wind-driven transport. We find that wind-driven transport dominates in quiescence even for low magnetic fields $\sim 1$ G. This can have a major impact on observational signatures since wind-driven transport does not heat the disk. Furthermore, wind transport cannot be reduced to an $α$ prescription. We provide fits for $α$ and the wind torque with $β$, ratio of thermal to magnetic pressure. We conclude that the evolution of the thermal-viscous instability, and its consequences on the outburst cycles of CVs, needs to be seriously revised to take into account that most of the accretion energy may be carried away by a wind instead of being locally dissipated.