On the possible turbulence mechanism in accretion disks in nonmagnetic binary stars (original) (raw)
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Turbulence in Accretion Disks of Non-Magnetic Binary Stars
Proceedings of XI Multifrequency Behaviour of High Energy Cosmic Sources Workshop — PoS(MULTIF15)
As well known the Keplerian disks are stable against generation of turbulence. At the other side the astrophysical accretion disks reveal intensive turbulence process which can be identified via relatively high accretion rates. One of the challenging astrophysical problems is the searching for possible instabilities leading to the turbulization of gas-dynamic disks. There are some features that make accretion disks non-keplerian in gas-dynamical flows in binary stellar systems. In particular, the non-symmetric gravitational force causes formation of a precessional wave in the disk. The wave characterized by strong density and velocity gradients. Linear analysis of perturbations in such disks showed that they are subject of small-scale radial instability with growth rate about tenths and hundredths of orbital period of the binary. If the variation of velocity on the perturbation wavelength scale is about or higher than the sound speed, the perturbations become unstable. This instability may lead to growth of the turbulence in one period of the disk. We performed calculations of two disks with different values of the secondary-to-primary mass ratio of binary components, q = 0.05 and q = 0.27, and the same separation values. The characteristics of turbulence correspond to observations, namely the Shakura-Sunyaev parameter α ∼ 0.01. The high-q model showed twice more value of α than the low-q model.
2015
As well known the Keplerian disks are stable against generation of turbulence. At the other side the astrophysical accretion disks reveal intensive turbulence process which can be identified via relatively high accretion rates. One of the challenging astrophysical problems is the searching for possible instabilities leading to the turbulization of gas-dynamic disks. There are some features that make accretion disks non-keplerian in gas-dynamical flows in binary stellar systems. In particular, the non-symmetric gravitational force causes formation of a precessional wave in the disk. The wave characterized by strong density and velocity gradients. Linear analysis of perturbations in such disks showed that they are subject of small-scale radial instability with growth rate about tenths and hundredths of orbital period of the binary. If the variation of velocity on the perturbation wavelength scale is about or higher than the sound speed, the perturbations become unstable. This instabil...
Excitation of turbulence in accretion disks of binary stars by non-linear perturbations
Astronomy Reports, 2017
Accretion disks in binary systems can experience hydrodynamic impact at inner as well as outer edges. The first case is typical for protoplanetary disks around young T Tau stars. The second one is typical for circumstellar disks in close binaries. As a result of such an impact, perturbations with different scales and amplitudes are excited in the disk. We investigated the nonlinear evolution of perturbations of a finite, but small amplitude, at the background of sub-Keplerian flow. Nonlinear effects at the front of perturbations lead to the formation of a shock wave, namely the discontinuity of the density and radial velocity. At this, the tangential flow in the neighborhood of the shock becomes equivalent to the flow in in the boundary layer. Instability of the tangential flow further leads to turbulization of the disk. Characteristics of the turbulence depend on perturbation parameters, but α-parameter of Shakura-Sunyaev does not exceed ∼ 0.1.
The collective mode and turbulent viscosity in accretion discs
Physics Letters A, 2003
The existence of a spiral-vortex structure is revealed by a numerical simulation of the dynamics of an accretion disc in close binary stars. This structure is not related to the tidal influence of a companion star. It is a density wave containing a one-armed spiral and an anticyclonic vortex. The formation of the structure is caused by a hydrodynamical instability. The latter results in a disc turbulence with a turbulent viscosity coefficient ν 0.035 Ωh 2 (h is a semithickness of the disc). This value is in accordance with both the value of a numerical viscosity in presented calculations and the results of observations. The period of the density wave rotation is in agreement with the typical periods of light curve variations observed in cataclysmic binary stars.
Instability, turbulence, and enhanced transport in accretion disks
Reviews of Modern Physics, 1998
Recent years have witnessed dramatic progress in our understanding of how turbulence arises and transports angular momentum in astrophysical accretion disks. The key conceptual point has its origins in work dating from the 1950s, but its implications have been fully understood only in the last several years: the combination of a subthermal magnetic field (any nonpathological configuration will do) and outwardly decreasing differential rotation rapidly generates magnetohydrodynamic (MHD) turbulence via a remarkably simple linear instability. The result is a greatly enhanced effective viscosity, the origin of which had been a long-standing problem. The MHD nature of disk turbulence has linked two broad domains of magnetized fluid research: accretion theory and dynamos. The understanding that weak magnetic fields are not merely passively acted upon by turbulence, but actively generate it, means that the assumptions of classical dynamo theory break down in disks. Paralleling the new conceptual understanding has been the development of powerful numerical MHD codes. These have taught us that disks truly are turbulent, transporting angular momentum at greatly enhanced rates. We have also learned, however, that not all forms of disk turbulence do this. Purely hydrodynamic turbulence, when it is imposed, simply causes fluctuations without a significant increase in transport. The interplay between numerical simulation and analytic arguments has been particularly fruitful in accretion disk theory and is a major focus of this article. The authors conclude with a summary of what is now known of disk turbulence and mention some knotty outstanding questions (e.g., what is the physics behind nonlinear field saturation?) for which we may soon begin to develop answers. [S0034-6861(98)00501-7] CONTENTS 42 5. The evolution of an initially random field 42 6. Shear vs vorticity 43 7. Density stratification 44 C. MHD simulations: a summary 45 VI. Accretion Disk Dynamos 45 A. The dynamo-electric machine 45 B. A brief review of mean-field dynamo theory 46 C. Mean-field theory and nonlinear evolution of the magnetorotational instability 47 D. Saturation 49 VII. Summary 50 Acknowledgments 50 References 51
Global Aspects of Elliptical Instability in Tidally Distorted Accretion Disks
The Astrophysical Journal, 1996
Tidally distorted accretion disks in binary star systems are subject to a local hydrodynamic instability which excites m = 1 internal waves. This instability is three dimensional and approximately incompressible. We study the global aspects of this local instability using equations derived under the shearing sheet approximation, where the e ects of the azimuthal variation along distorted orbital trajectories are included in source terms which oscillate with local orbital phase. Linear analyses show that the excitation of the instability is essentially local, i.e. insensitive to radial boundary conditions. The region of rapid growth feeds waves into the region of slow or negligible growth, allowing the instability to become global. The global growth rate depends the maximum local growth rate, the size of the rapid growth region, and the local group velocity. We present an empirical expression for the global growth rate. We note that the local nature of the instability allows the excitation of waves with m 6 = 1 when the local growth rate is large.
Evolution of a Mode of Oscillation within Turbulent Accretion Disks
The Astrophysical Journal
We investigate the effects of subsonic turbulence on a normal mode of oscillation [a possible origin of the high-frequency quasi-periodic oscillations (HFQPOs) within some black hole accretion disks]. We consider perturbations of a time-dependent background (steady state disk plus turbulence), obtaining an oscillator equation with stochastic damping, (mildly) nonlinear restoring, and stochastic driving forces. The (long-term) mean values of our turbulent functions vanish. In particular, turbulence does not damp the oscillation modes, so 'turbulent viscosity' is not operative. However, the frequency components of the turbulent driving force near that of the mode can produce significant changes in the amplitude of the mode. Even with an additional (phenomenological constant) source of damping, this leads to an eventual 'blowout' (onset of effects of nonlinearity) if the turbulence is sufficiently strong or the damping constant is sufficiently small. The infrequent large increases in the energy of the mode could be related to the observed low duty cycles of the HFQPOs. The width of the peak in the power spectral density (PSD) is proportional to the amount of nonlinearity. A comparison with observed continuum PSDs indicates the conditions required for visibility of the mode.
ON THE DYNAMICS AND EVOLUTION OF GRAVITATIONAL INSTABILITY-DOMINATED DISKS
The Astrophysical Journal, 2010
We derive the evolution equations describing a thin axisymmetric disk of gas and stars with an arbitrary rotation curve that is kept in a state of marginal gravitational instability and energy equilibrium due to the balance between energy released by accretion and energy lost due to decay of turbulence. Rather than adopt a parameterized α prescription, we instead use the condition of marginal gravitational instability to self-consistently determine the position-and time-dependent transport rates. We show that there is a steady-state configuration for disks dominated by gravitational instability, and that this steady state persists even when star formation is taken into account if the accretion rate is sufficiently large. For disks in this state we analytically determine the velocity dispersion, surface density, and rates of mass and angular momentum transport as a function of the gas mass fraction, the rotation curve, and the rate of external accretion onto the disk edge. We show that disks that are initially out of steady state will evolve into it on the viscous timescale of the disk, which is comparable to the orbital period if the accretion rate is high. Finally, we discuss the implications of these results for the structure of disks in a broad range of environments, including high redshift galaxies, the outer gaseous disks of local galaxies, and accretion disks around protostars.
Instability of counter-rotating stellar disks
Journal of Physics: Conference Series
We use an N-body simulation, constructed using GADGET-2, to investigate an accretion flow onto an astrophysical disk that is in the opposite sense to the disk's rotation. In order to separate dynamics intrinsic to the counter-rotating flow from the impact of the flow onto the disk, we consider an initial condition in which the counter-rotating flow is in an annular region immediately exterior the main portion of the astrophysical disk. Such counter-rotating flows are seen in systems such as NGC 4826 (known as the "Evil Eye Galaxy"). Interaction between the rotating and counter-rotating components is due to two-stream instability in the boundary region. A multi-armed spiral density wave is excited in the astrophysical disk and a density distribution with high azimuthal mode number is excited in the counter-rotating flow. Density fluctuations in the counter-rotating flow aggregate into larger clumps and some of the material in the counter-rotating flow is scattered to large radii. Accretion flow processes such as this are increasingly seen to be of importance in the evolution of multi-component galactic disks.