Numerical modelling of cold accretion discs in cataclysmic variables: the superhump phenomenon

Бисикало, Д. В.

doi:10.1080/10556790701306576

Astronomical & Astrophysical Transactions

2007

DOI: 10.1080/10556790701306576

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Numerical modelling of cold accretion discs in cataclysmic variables: the superhump phenomenon

Д. В. Бисикало

Abstract: The three-dimensional gas dynamic simulations of flow structure in binaries show the formation of the precessional spiral density wave in inner parts of a cold accretion disc. The precessional wave model is able to explain all types of observed superhump.

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Global MHD Simulations of Accretion Disks in Cataclysmic Variables. I. The Importance of Spiral Shocks

Stone

Zhu

2016

ApJ

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We present results from the first global 3D MHD simulations of accretion disks in Cataclysmic Variable (CV) systems in order to investigate the relative importance of angular momentum transport via turbulence driven by the magnetorotational instability (MRI) compared to that driven by spiral shock waves. Remarkably, we find that even with vigorous MRI turbulence, spiral shocks are an important component to the overall angular momentum budget, at least when temperatures in the disk are high (so that Mach numbers are low). In order to understand the excitation, propagation, and damping of spiral density waves in our simulations more carefully, we perform a series of 2D global hydrodynamical simulations with various equation of states and both with and without mass inflow via the Lagrangian point (L1). Compared with previous similar studies, we find the following new results. 1) Linear wave dispersion relation fits the pitch angles of spiral density waves very well. 2) We demonstrate explicitly that mass accretion is driven by the deposition of negative angular momentum carried by the waves when they dissipate in shocks. 3) Using Reynolds stress scaled by gas pressure to represent the effective angular momentum transport rate α ef f is not accurate when mass accretion is driven by nonaxisymmetric shocks. 4) Using the mass accretion rate measured in our simulations to directly measure α defined in standard thin-disk theory, we find 0.02 α ef f 0.05 for CV disks, consistent with observed values in quiescent states of dwarf novae (DNe). In this regime the disk may be too cool and neutral for the MRI to operate and spiral shocks are a possible accretion mechanism. However, we caution that our simulations use unrealistically low Mach numbers in this regime, and therefore future models with more realistic thermodynamics and non-ideal MHD are warranted.

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Global MHD Simulations of Accretion Disks in Cataclysmic Variables. I. The Importance of Spiral Shocks

Stone

Zhu

2016

ApJ

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Stratified Global MHD Models of Accretion Disks in Semidetached Binaries

Pjanka

Stone

2020

ApJ

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We present results of the first global magnetohydrodynamic simulations of accretion disks fed by Roche-lobe overflow, including vertical stratification, in order to investigate the roles of spiral shocks, magnetorotational instability (MRI), and the accretion stream in disk structure and evolution. Our models include a simple treatment of gas thermodynamics, with orbital Mach numbers at the inner edge of the disk M in of 5 and 10. We find mass accretion rates to vary considerably on all timescales, with only the Mach 5 model reaching a clear quasi-stationary state. For Mach 10, the model undergoes an outside-in, magnetically driven accretion event occurring on a timescale of ∼10 orbital periods of the binary. Both models exhibit spiral shocks inclined with respect to the binary plane, with their position and inclination changing rapidly. However, the time-averaged location of these shocks in the equatorial plane is well fit by simple linear models. MRI turbulence in the disk generates toroidal magnetic field patterns (butterfly diagrams) that are in some cases irregular, perhaps due to interaction with the spiral structure. While many of our results are in good agreement with local studies, we find some features (most notably those related to spiral shocks) can only be captured in global models such as studied here. Thus, while global studies remain computationally expensive—even as idealized models—they are essential (along with more sophisticated treatment of radiation transport and disk thermodynamics) for furthering our understanding of accretion in binary systems.

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Numerical modelling of cold accretion discs in cataclysmic variables: the superhump phenomenon

Abstract: The three-dimensional gas dynamic simulations of flow structure in binaries show the formation of the precessional spiral density wave in inner parts of a cold accretion disc. The precessional wave model is able to explain all types of observed superhump.

Cited by 2 publications

References 13 publications

Global MHD Simulations of Accretion Disks in Cataclysmic Variables. I. The Importance of Spiral Shocks

Global MHD Simulations of Accretion Disks in Cataclysmic Variables. I. The Importance of Spiral Shocks

Stratified Global MHD Models of Accretion Disks in Semidetached Binaries

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