253 research outputs found
Linear Analysis of the Hall Effect in Protostellar Disks
The effects of Hall electromotive forces (HEMFs) on the linear stability of
protostellar disks are examined. Earlier work on this topic focused on axial
field and perturbation wavenumbers. Here we treat the problem more generally.
Both axisymmetric and nonaxisymmetric cases are investigated. Though seldom
explicitly included in calculations, HEMFs appear to be important whenever
Ohmic dissipation is. They allow for the appearance of electron whistler waves,
and since these have right-handed polarization, a helicity factor is also
introduced into the stability problem. This factor is the product of the
components of the angular velocity and magnetic field along the perturbation
wavenumber, and it is destabilizing when negative. Unless the field and angular
velocity are exactly aligned, it is always possible to find destabilizing
wavenumbers. HEMFs can destabilize any differential rotation law, even those
with angular velocity increasing outward. Regardless of the sign of the angular
velocity gradient, the maximum growth rate is always given in magnitude by the
local Oort A value of the disk, as in the standard magnetorotational
instability. The role of Hall EMFs may prove crucial to understanding how
turbulence is maintained in the ``low state'' of eruptive disk systems.Comment: 34 pages, 10 figures, AAS LaTEx, v.4.0. Submitted to Ap
Evolution of massive and magnetized protoplanetary disks
We present global 2D and 3D simulations of self-gravitating magnetized tori.
We used the 2D calculations to demonstrate that the properties of the MRI are
not affected by the presence of self-gravity: MHD turbulence and enhanced
angular momentum transport follow the linear growth of the instability. In 3D,
we have studied the interaction between an gravitational instability and
MHD turbulence. We found its strength to be significantly decreased by the
presence of the latter, showing that both instabilities strongly interact in
their non-linear phases. We discuss the consequences of these results.Comment: 8 pages, 7 figures, to appear in the proceedings of the XIXth IAP
colloquium "Extrasolar Planets: Today and Tomorrow" held in Paris, France,
2003, June 30 - July 4, ASP Conf. Serie
New composite models of partially ionized protoplanetary disks
We study an accretion disk in which three different regions may coexist: MHD
turbulent regions, dead zones and gravitationally unstable regions. Although
the dead zones are stable, there is some transport due to the Reynolds stress
associated with waves emitted from the turbulent layers. We model the transport
in each of the different regions by its own parameter, this being 10
to times smaller in dead zones than in active layers. In
gravitationally unstable regions, is determined by the fact that the
disk self-adjusts to a state of marginal stability. We construct steady-state
models of such disks. We find that for uniform mass flow, the disk has to be
more massive, hotter and thicker at the radii where there is a dead zone. In
disks in which the dead zone is very massive, gravitational instabilities are
present. Whether such models are realistic or not depends on whether
hydrodynamical fluctuations driven by the turbulent layers can penetrate all
the way inside the dead zone. This may be more easily achieved when the ratio
of the mass of the active layer to that of the dead zone is relatively large,
which in our models corresponds to in the dead zone being about 10% of
in the active layers. If the disk is at some stage of its evolution
not in steady-state, then the surface density will evolve toward the
steady-state solution. However, if in the dead zone is much smaller
than in the active zone, the timescale for the parts of the disk beyond a few
AU to reach steady-state may become longer than the disk lifetime. Steady-state
disks with dead zones are a more favorable environment for planet formation
than standard disks, since the dead zone is typically 10 times more massive
than a corresponding turbulent zone at the same location.Comment: 13 pages, 5 figures, accepted for publication in Ap
Evolution of self-gravitating magnetized disks. II- Interaction between MHD turbulence and gravitational instabilities
We present 3D magnetohydrodynamic (MHD) numerical simulations of the
evolution of self--gravitating and weakly magnetized disks with an adiabatic
equation of state. Such disks are subject to the development of both the
magnetorotational and gravitational instabilities, which transport angular
momentum outward. As in previous studies, our hydrodynamical simulations show
the growth of strong m=2 spiral structure. This spiral disturbance drives
matter toward the central object and disappears when the Toomre parameter Q has
increased well above unity. When a weak magnetic field is present as well, the
magnetorotational instability grows and leads to turbulence. In that case, the
strength of the gravitational stress tensor is lowered by a factor of about~2
compared to the hydrodynamical run and oscillates periodically, reaching very
small values at its minimum. We attribute this behavior to the presence of a
second spiral mode with higher pattern speed than the one which dominates in
the hydrodynamical simulations. It is apparently excited by the high frequency
motions associated with MHD turbulence. The nonlinear coupling between these
two spiral modes gives rise to a stress tensor that oscillates with a frequency
which is a combination of the frequencies of each of the modes. This
interaction between MHD turbulence and gravitational instabilities therefore
results in a smaller mass accretion rate onto the central object.Comment: 31 pages, 19 figures, accepted for publication in ApJ, animation
avalaible at http://www2.iap.fr/users/fromang/simu3d/simu3d.htm
How does disk gravity really influence type-I migration ?
We report an analytical expression for the locations of Lindblad resonances
induced by a perturbing protoplanet, including the effect of disk gravity.
Inner, outer and differential torques are found to be enhanced compared to
situations where a keplerian velocity field for the dynamics of both the disk
and the planet is assumed. Inward migration is strongly accelerated when the
disk gravity is only accounted for in the planet orbital motion. The addition
of disk self-gravity slows down the planet drift but not enough to stop it.Comment: 4 pages, accepted for publication in A&A Letter
Turbulence and angular momentum transport in a global accretion disk simulation
The global development of magnetohydrodynamic turbulence in an accretion disk
is studied within a simplified disk model that omits vertical stratification.
Starting with a weak vertical seed field, a saturated state is obtained after a
few tens of orbits in which the energy in the predominantly toroidal magnetic
field is still subthermal. The efficiency of angular momentum transport,
parameterized by the Shakura-Sunyaev alpha parameter, is of the order of 0.1.
The dominant contribution to alpha comes from magnetic stresses, which are
enhanced by the presence of weak net vertical fields. The power spectra of the
magnetic fields are flat or decline only slowly towards the largest scales
accessible in the calculation, suggesting that the viscosity arising from MHD
turbulence may not be a locally determined quantity. I discuss how these
results compare with observationally inferred values of alpha, and possible
implications for models of jet formation.Comment: ApJ Letters, in press. The paper and additional visualizations are
available at http://www.cita.utoronto.ca/~armitage/global_abs.htm
Wave Excitation in Disks Around Rotating Magnetic Stars
The accretion disk around a rotating magnetic star (neutron star, white dwarf
or T Tauri star) is subjected to periodic vertical magnetic forces from the
star, with the forcing frequency equal to the stellar spin frequency or twice
the spin frequency. This gives rise bending waves in the disk that may
influence the variabilities of the system. We study the excitation, propagation
and dissipation of these waves using a hydrodynamical model coupled with a
generic model description of the magnetic forces. The bending waves are
excited at the Lindblad/vertical resonance, and propagate either to larger
radii or inward toward the corotation resonance where dissipation takes place.
While the resonant torque is negligible compared to the accretion torque, the
wave nevertheless may reach appreciable amplitude and can cause or modulate
flux variabilities from the system. We discuss applications of our result to
the observed quasi-periodic oscillations from various systems, in particular
neutron star low-mass X-ray binaries.Comment: Small changes/clarifications. To be published in ApJ, Aug.20,2008
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Nonaxisymmetric Magnetorotational Instability in Proto-Neutron Stars
We investigate the stability of differentially rotating proto-neutron stars
(PNSs) with a toroidal magnetic field. Stability criteria for nonaxisymmetric
MHD instabilities are derived using a local linear analysis. PNSs are expected
to have much stronger radial shear in the rotation velocity compared to normal
stars. We find that nonaxisymmetric magnetorotational instability (NMRI) with a
large azimuthal wavenumber is dominant over the kink mode () in
differentially rotating PNSs. The growth rate of the NMRI is of the order of
the angular velocity which is faster than that of the kink-type
instability by several orders of magnitude. The stability criteria are
analogous to those of the axisymmetric magnetorotational instability with a
poloidal field, although the effects of leptonic gradients are considered in
our analysis. The NMRI can grow even in convectively stable layers if the
wavevectors of unstable modes are parallel to the restoring force by the
Brunt-V\"ais\"al\"a oscillation. The nonlinear evolution of NMRI could amplify
the magnetic fields and drive MHD turbulence in PNSs, which may lead to
enhancement of the neutrino luminosity.Comment: 24pages, 7figures, Accepted for publication in the Astrophysical
Journal (December 12, 2005
Zonal Flows and Long-Lived Axisymmetric Pressure Bumps in Magnetorotational Turbulence
We study the behavior of magnetorotational turbulence in shearing box
simulations with a radial and azimuthal extent up to ten scale heights. Maxwell
and Reynolds stresses are found to increase by more than a factor two when
increasing the box size beyond two scale heights in the radial direction.
Further increase of the box size has little or no effect on the statistical
properties of the turbulence. An inverse cascade excites magnetic field
structures at the largest scales of the box. The corresponding 10% variation in
the Maxwell stress launches a zonal flow of alternating sub- and
super-Keplerian velocity. This in turn generates a banded density structure in
geostrophic balance between pressure and Coriolis forces. We present a
simplified model for the appearance of zonal flows, in which stochastic forcing
by the magnetic tension on short time-scales creates zonal flow structures with
life-times of several tens of orbits. We experiment with various improved
shearing box algorithms to reduce the numerical diffusivity introduced by the
supersonic shear flow. While a standard finite difference advection scheme
shows signs of a suppression of turbulent activity near the edges of the box,
this problem is eliminated by a new method where the Keplerian shear advection
is advanced in time by interpolation in Fourier space.Comment: Accepted for publication in Ap
An Incoherent Dynamo in Accretion Disks
We use the mean-field dynamo equations to show that an incoherent alpha
effect in mirror-symmetric turbulence in a shearing flow can generate a large
scale, coherent magnetic field. We illustrate this effect with simulations of a
few simple systems. In accretion disks, this process can lead to axisymmetric
magnetic domains whose radial and vertical dimensions will be comparable to the
disk height. This process may be responsible for observations of dynamo
activity seen in simulations of dynamo-generated turbulence involving, for
example, the Balbus-Hawley instability. In this case the magnetic field
strength will saturate at times the ambient pressure in real
accretion disks. The resultant dimensionless viscosity will be of the same
order. In numerical simulations the azimuthal extent of the simulated annulus
should be substituted for . We compare the predictions of this model to
numerical simulations previously reported by Brandenburg et al. (1995). In a
radiation pressure dominated environment this estimate for viscosity should be
reduced by a factor of due to magnetic buoyancy.Comment: 23 pages, uses aaste
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