Impact of convection and resistivity on angular momentum transport in dwarf novae

Abstract

The eruptive cycles of dwarf novae (DN) are thought to be due to a thermal-viscous instability in the accretion disk surrounding the white dwarf (WD). This model has long been known to imply a stress to pressure ratio α~0.1 in outburst compared to α~ 0.01 in quiescence. Such an enhancement in $\alpha$ has recently been observed in simulations of turbulent transport driven by the magneto-rotational instability (MRI) when convection is present, without requiring a net magnetic flux. We independently recover this result by carrying out PLUTO MHD simulations of vertically stratified, radiative, shearing boxes with the thermodynamics and opacities appropriate to DN. The results are robust against the choice of vertical boundary conditions. The thermal equilibrium solutions found by the simulations trace the well-known S-curve in the density-temperature plane. We confirm that the high values of α~ 0.1 occur near the tip of the hot branch of the S-curve, where convection is active. However, we also present thermally-stable simulations at lower temperatures that have standard values of α~ 0.03 despite the presence of vigorous convection. We find no simple relationship between αand the strength of the convection, as measured by the ratio of convective to radiative flux. The cold branch is only very weakly ionized so, in the second part of this work, we studied the impact of non-ideal MHD effects on transport. We include resistivity in the simulations and find that the MRI-driven transport is quenched (α~ 0) below the critical density at which the magnetic Reynolds number R_m \leq 10^4. This is problematic as X-ray emission observed in quiescent systems requires ongoing accretion onto the WD. We verify that these X-rays cannot self-sustain MRI-driven turbulence by photo-ionizing the disk and discuss possible solutions to the issue of accretion in quiescence.

14 pages, 13 figures, 1 table; accepted for publication in A&A

References in corpus (6)

• Accretion disc viscosity: how big is alpha?
• Convection Causes Enhanced Magnetic Turbulence in Accretion Disks in Outburst
• The viscosity parameter alpha and the properties of accretion disc outbursts in close binaries
• Dwarf Nova Outbursts with Magnetorotational Turbulence
• Magnetic Turbulence and Thermodynamics in the Inner Region of Protoplanetary Discs
• Convective Quenching of Field Reversals in Accretion Disc Dynamos

Cited by in corpus (20)

• A characteristic optical variability timescale in astrophysical accretion disks
• Strong disc winds traced throughout outbursts in black-hole X-ray binaries
• Opacity Driven Convection and Variability in Accretion Disks around Supermassive Black Holes
• The impact of thermal winds on the outburst lightcurves of black hole X-ray binaries
• Models of Ultra-Luminous X-ray transient sources
• Investigating lack of accretion disk eccentricity growth in a global 3D MHD simulation of a superhump system
• Modelling rebrightenings, reflares, and echoes in dwarf nova outbursts
• Magnetic field transport in compact binaries
• Two-dimensional simulations of disks in close binaries: Simulating outburst cycles in cataclysmic variables
• Stratified Global MHD Models of Accretion Disks in Semi-Detached Binaries
• MRI turbulence in vertically stratified accretion discs at large magnetic Prandtl numbers
• The Role of Thermal Instability in Accretion Outbursts in High-Mass Stars
• Magnetohydrodynamic convection in accretion discs
• QPOs in compact binaries from small scale eruptions in an inner magnetized disk
• Self-similar structure of resistive ADAFs with outflow and large-scale magnetic field
• Analysis of accretion disc structure and stability using open code for vertical structure
• The youngest of hot jupiters in action: episodic accretion outbursts in Gaia20eae
• Resistive Hot Accretion Flows with Anisotropic Pressure
• Episodic accretion in high-mass star formation: An analysis of thermal instability for axially symmetric disks
• Spiral-wave-driven accretion in quiescent dwarf novæ