92 research outputs found
Disk Instabilities and Cooling Fronts
Accretion disk outbursts, and their subsequent decline, offer a unique
opportunity to constrain the physics of angular momentum transport in hot
accretion disks. Recent work has centered on the claim by Cannizzo et al. that
the exponential decay of luminosity following an outburst in black hole
accretion disk systems is only consistent with a particular form for the
dimensionless viscosity, . This result can be
understood in terms of a simple model of the evolution of cooling fronts in
accretion disks. In particular, the cooling front speed during decline is , where denotes the position of the
cooling front, and the exact value of depends on the hot state opacity,
(although generally ). Setting this speed proportional to
constrains the functional form of in the hot phase of the disk, which
sets it apart from previous arguments based on the relative durations of
outburst and quiescence. However, it remains uncertain how well we know the
exponent . In addition, more work is needed to clarify the role of
irradiation in these systems and its effect on the cooling front evolution.Comment: 10 pages, uses aipproc.st
Fast Magnetic Reconnection and Spontaneous Stochasticity
Magnetic field-lines in astrophysical plasmas are expected to be frozen-in at
scales larger than the ion gyroradius. The rapid reconnection of magnetic flux
structures with dimensions vastly larger than the gyroradius requires a
breakdown in the standard Alfv\'en flux-freezing law. We attribute this
breakdown to ubiquitous MHD plasma turbulence with power-law scaling ranges of
velocity and magnetic energy spectra. Lagrangian particle trajectories in such
environments become "spontaneously stochastic", so that infinitely-many
magnetic field-lines are advected to each point and must be averaged to obtain
the resultant magnetic field. The relative distance between initial magnetic
field lines which arrive to the same final point depends upon the properties of
two-particle turbulent dispersion. We develop predictions based on the
phenomenological Goldreich & Sridhar theory of strong MHD turbulence and on
weak MHD turbulence theory. We recover the predictions of the Lazarian &
Vishniac theory for the reconnection rate of large-scale magnetic structures.
Lazarian & Vishniac also invoked "spontaneous stochasticity", but of the
field-lines rather than of the Lagrangian trajectories. More recent theories of
fast magnetic reconnection appeal to microscopic plasma processes that lead to
additional terms in the generalized Ohm's law, such as the collisionless Hall
term. We estimate quantitatively the effect of such processes on the
inertial-range turbulence dynamics and find them to be negligible in most
astrophysical environments. For example, the predictions of the
Lazarian-Vishniac theory are unchanged in Hall MHD turbulence with an extended
inertial range, whenever the ion skin depth is much smaller than the
turbulent integral length or injection-scale Comment: 31 pages, 5 figure
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Gravitational Instabilities In A Protoplanetary Disk Including The Effects Of Magnetic-Fields
We investigate the gravitational instability of a thin, Keplerian protoplanetary disk including the effects of a largely azimuthal magnetic field. The model follows that of our previous work (Noh, Vishniac, & Cochran 1991) except for the inclusion of a magnetic field. The disk is assumed to consist of neutral and ionized gas and neutral dust which are coupled by gravity and friction. The growth rates and eigenfunctions are calculated numerically using nonaxisymmetric linear perturbation methods. The results show that the growth rate has a maximum at some intermediate azimuthal number m, but for each value of m it is reduced relative to the unmagnetized case. The effects of the magnetic field appear more strongly on small scales. As the strength of the equilibrium magnetic field increases the growth rates decrease, and the maximum instability occurs at a lower value of m due to the increasing magnetic pressure. The response of each component to the magnetic field is discussed using the behavior of the eigenfunctions in the radial direction. With the inclusion of the magnetic field, the effects of the ionization fraction and friction on the growth rates also appear to be important for high m modes. Increasing the ionization fraction or the friction suppresses instability, but only slightly changes the maximally unstable azimuthal scales. The enhanced growth rates due to a dust component for which thermal pressure is negligible are somewhat reduced by the inclusion of a magnetic field. The effects of different boundary conditions (reflecting and transmitting) on the growth rates are also shown.NASA NAGW 2418Astronom
The distribution of dark matter, galaxies, and the intergalactic medium in a cold dark matter dominated universe
The evolution and distribution of galaxies and the intergalactic medium (IGM) have been studied, along with collisionless dark matter in a Universe dominated by cold dark matter. The Einstein-deSitter universe with omega sub 0 = 1 and h = 0.5 was considered (here h = H sub 0 bar 100/kms/Mpc and H sub 0 is the present value of the Hubble constant). It is assumed that initially dark matter composes 90 pct and baryonic matter composes 10 pct of total mass, and that the primordial baryonic matter is comprised of H and He, with the abundance of He equal to 10 pct of H by number. Galaxies are allowed to form out of the IGM, if the total density and baryonic density satisfy an overdensity criterion. Subsequently, the newly formed galaxies release 10 to the 60th ergs of energy into the IGM over a period of 10 to the 8th years. Calculations have been performed with 32 to the 3rd dark matter particles and 32 to the 3rd cells in a cube with comoving side length L = 9.6/h Mpc. Dark matter particles and galaxies have been followed with an N-body code, while the IGM has been followed with a fluid code
Conserving Local Magnetic Helicity in Numerical Simulations
Magnetic helicity is robustly conserved in systems with large magnetic
Reynolds numbers, including most systems of astrophysical interest. This plays
a major role in suppressing the kinematic large scale dynamo and driving the
large scale dynamo through the magnetic helicity flux. Numerical simulations of
astrophysical systems typically lack sufficient resolution to enforce global
magnetic helicity over several dynamical times. Errors in the internal
distribution of magnetic helicity are equally serious and possibly larger. Here
we propose an algorithm for enforcing strict local conservation of magnetic
helicity in the Coulomb gauge in numerical simulations.Comment: Comments are welcom
The Growth Rate of Tidally Excited Waves in Accretion Disks
Accretion disks in close binary systems are subject to a tidally driven
parametric instability which leads to the growth of internal waves near the
outer edges of such disks (Goodman 1993). These waves are important in
understanding the torque exerted on a disk by tidal forces and may play a role
in the structure of the disk at small radii. Here we calculate the growth rate
of this instability, including the effects of vertical structure and fluid
compressibility. We find growth rates which are only slightly different from
Goodman's original results, except that near the vertical resonance radius the
growth rate can have an extremely broad and strong peak when the disk is stably
stratified in the vertical direction. Higher order modes, in the sense of
increasing number of vertical nodes, have similar growth rates. Our results
differ from a previous calculation along these lines by Lubow et al. (1993).
The difference is mostly due to their neglect of radial gradients in the
tidally distorted streamlines.Comment: LaTeX (aastex
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