Viscosity In Accretion Disks Research Articles

Accretion disc viscosity: a simple model for a magnetic dynamo

We develop here a simple physical model for the manner in which a magnetic dynamo might operate in an accretion disc and so provide an effective (magnetic) viscosity. In contrast to other dynamo models, the mechanism we discuss does not depend on the existence of some hydrodynamical small-scale turbulent flow hypothesized to be already present in a non-magnetic disc. Rather, the model we present depends on three well-established physical processes: the Parker instability, the Balbus–Hawley instability and magnetic field reconnection. The model gives rise to finite but non-stationary magnetic field configurations. For the set of parameters chosen here we find a time-averaged effective viscosity with Shakura–Sunyaev α-parameter around |$\alpha_\text {SS} \approx 0.4$|⁠.

The secondary outburst maximum of T Coronae Borealis - Implications for the physics of accretion disks

The hypothesis of Webbink (1976) that the accretion of a torus of matter onto a main-sequence star caused secondary maxima during the 1866 and 1946 eruptions of the recurrent nova T CrB is examined. Simple 1D calculations show that the accretion disk viscosity must increase with time to produce light curves resembling observations. It is inferred that the physical mechanism responsible for producing the viscous dissipation in the accretion disk has a fast growth rate and saturates to a viscosity parameter of order unity. The good agreement between parameters estimated from observations and those derived from a physical mechanism suggests that the growth rate instability identified by Balbus and Hawley (1991) is a promising source for accretion disk viscosity.

Neutron viscosity in accretion disks

Neutron viscosity is investigated as a possible mechanism for the dissipation of kinetic energy into luminosity in the innermost parts of accretion disks around compact objects. Simplified models are presented of the self-consistent, steady-state accretion flows in which viscosity is provided by neutron collisions with accreting ions. Ion temperatures are determined by balancing the heating of ions by viscous dissipation to their cooling by Coulomb collisions with the electrons, providing a self-consistent solution between neutron production and their impact on energy dissipation. The results indicate that neutrons can indeed provide the necessary dissipation to sustain the steady-state accretion of matter at rates of about 10 to the -8th solar mass/yr or less, and electron temperatures of about 100 keV-1 MeV. Neutrons thus present a promising way of modeling bright Galactic X-ray sources.

MHD turbulence and accretion disks

The viscosity in accretion disks derived from observations is many orders of magnitude larger than the molecular viscosity can account for. MHD turbulence may cause a drastic increase of effective viscosity. Using the cascade model we simulate magnetic turbulence in a differentially rotating disk. The magnetic field allows for an energy flow from small to larger length scales. The turbulent stress due to the combined action of turbulent velocity- and magnetic fields show large fluctuations around the mean. Bursts in the total turbulent energy are large contributors to the time averaged stress and thus to the effective viscosity. The simulations show that the shear force is sufficient to sustain the turbulence.

Observational constraints on viscosity in AGN accretion discs

La variabilite UV observee dans les spectres des noyaux actifs de galaxies peut etre causee par des instabilites du disque d'accretion et peut etre utilisee pour calculer la viscosite. La comparaison des echelles de temps thermiques avec les echelles de temps observees de la variabilite dans 10 galaxies Seyfert et 16 quasars indique des valeurs pour le parametre de viscosite α de l'ordre de 0.01 dans la plupart des cas

Spiral wave viscosity in self-gravitating accretion disks

view Abstract Citations (28) References (18) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Spiral Wave Viscosity in Self-gravitating Accretion Disks Anthony, D. M. ; Carlberg, R. G. Abstract A rotating, self-gravitating, disk of gas exhibits instabilities in the form of spiral waves. The waves transport angular momentum, giving the disk a source of internal viscosity. The mechanism of the waves is fairly well understood in linear theory, but the nonlinear consequences, such as the rate of angular momentum transport, depend on the amplitude of the spirals, making a numerical experiment useful. We describe N-body experiments of an accretion disk comprised of weakly self-gravitating particles in a background Keplerian potential. The heating effect of the spiral waves is offset by a radial drag force that keep the particles "cool." The numerical disk has an inner and outer boundary that respectively absorbs and emits angular momentum, similar to a viscosity measuring device or viscometer. The rate of angular momentum transfer dh/dt correlates with the self-gravity of the disk, f, such that dh/dt is proportional to f^7/2^. The rate of transport also shows a weak dependence on the boundary conditions of the disk. For a 1% self-gravitating disk, the implied accretion time scale, t_acc_ = h(dh/dt)^-1^, is 7 x 10^5^ rotation periods. The increased spiral activity in a 4% self-gravitating disk decreases the total accretion time to 4 x 10^3^ half-mass radius rotations. An imposed tidal force of a few percent only slightly enhances the viscosity. An autocorrelation analysis shows that the fundamental potential perturbations are the spiral wakes described by Julian and Toomre (1966). The peak overdensity of the spiral wakes is weakly related to the self-gravity, ~f^2/5^. The peak overdensity gradually increases with time, doubling in ~75 rotations, suggesting that the effective central mass of the wake is on the order of a few hundred particles. Applications to quasars and protoplanetary disks are briefly discussed. Publication: The Astrophysical Journal Pub Date: September 1988 DOI: 10.1086/166682 Bibcode: 1988ApJ...332..637A Keywords: Accretion Disks; Computational Astrophysics; Galactic Rotation; Gravitational Effects; Viscosity; Many Body Problem; Momentum Transfer; Protoplanets; Quasars; Astrophysics; GALAXIES: INTERNAL MOTIONS; GALAXIES: STRUCTURE full text sources ADS |

On convection-induced viscosity in accretion disks in cataclysmic variables

The vertical structure of accretion disks in cataclysmic variable systems is examined on the assumption that the convection is solely responsible for inducing the viscous heating. The viscous heating is distributed over the entire vertical structure so that purely radiative zones are able to carry a radial mass flux. The convective flux is computed by two different methods in separate calculations and the results from each method are compared. The values of the viscosity obtained are compared with those values inferred from the observed time scales of dwarf nova eruptions. Sequences of equilibrium solutions for constant radius and varying mass accretion rate are examined, the relations between viscosity and surface density are discussed in the context of recent models for the eruptions of dwarf novae.

On the magnetic viscosity in Keplerian accretion disks

view Abstract Citations (130) References (27) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS On the magnetic viscosity in Keplerian accretion disks Coroniti, F. V. Abstract The paper develops a model for the anomalous viscosity in accretion disks based on the hypothesis that the hydrodynamic turbulence within the disk takes the form of spatially localized magnetic flux cells. The local shear flow due to Keplerian differential rotation distorts the flux cell topology, converting shear flow energy into magnetic energy. In the radial diffusion approximation, the kinematic viscosity is estimated from the radial displacement and is shown to maximize at flux cell scale lengths for which the shear flow stopping and reconnection times are equal. Publication: The Astrophysical Journal Pub Date: March 1981 DOI: 10.1086/158739 Bibcode: 1981ApJ...244..587C Keywords: Kepler Laws; Magnetic Stars; Magnetohydrodynamic Turbulence; Stellar Mass Accretion; Stellar Winds; Viscosity; Fluid Dynamics; Magnetic Flux; Shear Flow; Stellar Magnetic Fields; Stellar Models; Transport Properties; Astrophysics full text sources ADS |

Viscosity in accretion discs and HD 101584.

Viscosity in accretion discs and HD 101584.

Magnetic viscosity in relativistic accretion disks

We formulate a detailed, self-consistent model of magnetic viscosity in an accretion disk around a rotating, Kerr black hole, in order to help clarify the nature of viscosity in the theory of accretion disks. The magnetic field is amplified by shear and dissipated by reconnection; in steady state a stable balance occurs, leading to a chaotic field configuration consisting of magnetic cells that reconnect with each other about once per orbit. We give detailed models of stationary disks based on the viscosity of this field. The observable properties of these models are similar to those of previous models based on less detailed models of viscosity. We include the relativistic equations governing the time-evolution of an accretion disk around a Kerr black hole, and some criteria for the existence of a radiation-pressure-dominated region or an optically thin region in a stationary disk.