Hysteresis and thermal limit cycles in MRI simulations of accretion discs

Latter, Henrik N.; Papaloizou, J. C. B.

doi:10.1111/j.1365-2966.2012.21748.x

Cited by 32 publications

(32 citation statements)

References 48 publications

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“…The results of these simulations may be relevant for understanding accretion state changes, transient bursts, and large observationally inferred accretion efficiencies for astrophysical disks. Indeed limit cycle behaviour reminiscent of dwarf novae eruptions could be produced with the inclusion of a temperature (and perhaps also surface density) dependent resistivity, following similar suggestions by Balbus & Lesaffre (2008), Lesaffre et al (2009), andPapaloizou (2012).…”

Section: Discussionsupporting

confidence: 63%

Global simulations of magnetorotational turbulence - III. Influence of field configuration and mass injection

Parkin

2014

Monthly Notices of the Royal Astronomical Society

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The stresses produced by magnetorotational turbulence can provide effective angular momentum transport in accretion disks. However, questions remain about the ability of simulated disks to reproduce observationally inferred stress-to-gas-pressure ratios. In this paper we present a set of high resolution global magnetohydrodynamic disk simulations which are initialised with different field configurations: purely toroidal, vertical field lines, and nested poloidal loops. A mass source term is included which allows the total disk mass to equilibrate in simulations with long run times, and also enables the impact of rapid mass injection to be explored. Notably different levels of angular momentum transport are observed during the early-time transient disk evolution. However, given sufficient time to relax, the different models evolve to a statistically similar quasi-steady state with a stress-to-gas-pressure ratio, α P ∼ 0.032 − 0.036. Such behaviour is anticipated based on consideration of mean magnetic field evolution subject to our adopted simulation boundary conditions. The indication from our results is that steady, isolated disks may be unable to maintain a large-scale magnetic field or produce values for the stress-to-gas-pressure ratio implied by some observations. Supplementary simulations exploring the influence of trapping magnetic field, injecting vertical field, and rapidly injecting additional mass into the disk show that large stresses can be induced by these mechanisms. In the first instance, a highly magnetized disk is produced with α P ∼ 0.21, whereas the latter cases lead to a transient burst of accretion with a peak α P ≃ 0.1 − 0.25. As a whole, the simulations highlight the common late-time evolution and characteristics of turbulent disks for which the magnetic field is allowed to evolve freely (i.e., without constraint/replenishment). In contrast, if the boundaries of the disk, the rate of injection of magnetic field, or the rate of mass replenishment are modified to mimic astrophysical disks, markedly different disk evolution occurs.

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Section: Discussionsupporting

confidence: 63%

Global simulations of magnetorotational turbulence - III. Influence of field configuration and mass injection

Parkin

2014

Monthly Notices of the Royal Astronomical Society

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“…Thus, the thermal stability of MRI unstable disks is uncertain, and can only be investigated with nonlinear radiation MHD simulations which capture the MHD turbulence, heating and cooling of the disk self-consistently. Evidence of dwarf novae thermal instability and associated limit cycle behavior have been found from MRI simulations (Latter and Papaloizou 2012).…”

Section: Introductionmentioning

confidence: 91%

On the Thermal Stability of Radiation-Dominated Accretion Disks

Jiang

Stone

Davis

2013

ApJ

127

238

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We study the long-term thermal stability of radiation dominated disks in which the vertical structure is determined self-consistently by the balance of heating due to dissipation of MHD turbulence driven by the magneto-rotational instability (MRI), and cooling due to radiation emitted at the photosphere. The calculations adopt the local shearing box approximation, and utilize the recently developed radiation transfer module in the Athena MHD code based on a variable Eddington tensor rather than an assumed local closure. After saturation of the MRI, in many cases the disk maintains a steady vertical structure for many thermal times. However, in every case in which the box size in the horizontal directions is at least one pressure scale height, fluctuations associated with MRI turbulence and dynamo action in the disk eventually trigger a thermal runaway which causes the disk to either expand or contract until the calculation must be terminated. During runaway, the dependence of the heating and cooling rates on total pressure satisfy the simplest criterion for classical thermal instability. We identify several physical reasons why the thermal runaway observed in our simulations differ from the standard α disk model, for example the advection of radiation contributes a non-negligible fraction to the vertical energy flux at the largest radiation pressure, most of the dissipation does not happen in the disk mid-plane, and the change of dissipation scale height with mid-plane pressure is slower than the change of density scale height. We discuss how and why our results differ from those published previously. Such thermal runaway behavior might have important implications for interpreting temporal variability in observed systems, but fully global simulations are required to study the saturated state before detailed predictions can be made.

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“…We cross-checked the numerical stability of the steady state solutions using a different code that does not include radial advection in the advection of energy. This code solves the usual mass and angular momentum conservation equations in a way similar to the full disk evolution code of Hameury et al (1998), but it uses a simplified vertical structure with powerlaw opacities, following Appendix A of Latter & Papaloizou (2012). In addition, the energy equation only takes into account viscous heating and radiative cooling, excluding radial energy transport terms which are present in the full disk evolution code.…”

Section: Wind-dominated Regionsmentioning

confidence: 99%

Magnetic wind-driven accretion in dwarf novae

Scepi

Dubus

Lesur

2019

A&A

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Context. Dwarf novae (DNe) and X-ray binaries exhibit outbursts thought to be due to a thermal-viscous instability in the accretion disk. The disk instability model (DIM) assumes that accretion is driven by turbulent transport, customarily attributed to the magnetorotational instability (MRI). However, recent results point out that MRI turbulence alone fails to reproduce the light curves of DNe. Aims. Our aim is to study the impact of wind-driven accretion on the light curves of DNe. Local and global simulations show that magneto-hydrodynamic winds are present when a magnetic field threads the disk, even for relatively high ratios of thermal pressure to magnetic pressure (β ≈ 10 5 ). These winds are very efficient in removing angular momentum but do not heat the disk, thus they do not behave as MRI-driven turbulence. Methods. We add the effect of wind-driven magnetic braking in the angular momentum equation of the DIM but neglect the mass loss due to the wind. We assume a fixed magnetic configuration: dipolar or constant with radius. We use prescriptions for the wind torque and the turbulent torque derived from shearing box simulations. Results. The wind torque enhances the accretion of matter, resulting in light curves that look like DNe outbursts when assuming a dipolar field with a moment µ ≈ 10 30 G cm 3 . In the region where the wind torque dominates the disk is cold and optically thin, and the accretion speed is super-sonic. The inner disk behaves as if truncated, leading to higher quiescent X-ray luminosities from the white dwarf boundary layer than expected with the standard DIM. The disk is stabilized if the wind-dominated region is large enough, potentially leading to "dark" disks that emitting little radiation. Conclusions. Wind-driven accretion can play a key role in shaping the light curves of DNe and X-ray binaries. Future studies will need to include the time evolution of the magnetic field threading the disk to fully assess its impact on the dynamics of the accretion flow.

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Hysteresis and thermal limit cycles in MRI simulations of accretion discs

Cited by 32 publications

References 48 publications

Global simulations of magnetorotational turbulence - III. Influence of field configuration and mass injection

Global simulations of magnetorotational turbulence - III. Influence of field configuration and mass injection

On the Thermal Stability of Radiation-Dominated Accretion Disks

Magnetic wind-driven accretion in dwarf novae

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