2,417 research outputs found
The response of a turbulent accretion disc to an imposed epicyclic shearing motion
We excite an epicyclic motion, whose amplitude depends on the vertical
position, , in a simulation of a turbulent accretion disc. An epicyclic
motion of this kind may be caused by a warping of the disc. By studying how the
epicyclic motion decays we can obtain information about the interaction between
the warp and the disc turbulence. A high amplitude epicyclic motion decays
first by exciting inertial waves through a parametric instability, but its
subsequent exponential damping may be reproduced by a turbulent viscosity. We
estimate the effective viscosity parameter, , pertaining to
such a vertical shear. We also gain new information on the properties of the
disc turbulence in general, and measure the usual viscosity parameter,
, pertaining to a horizontal (Keplerian) shear. We find that,
as is often assumed in theoretical studies, is approximately
equal to and both are much less than unity, for the field
strengths achieved in our local box calculations of turbulence. In view of the
smallness () of and we conclude
that for the timescale for diffusion
or damping of a warp is much shorter than the usual viscous timescale. Finally,
we review the astrophysical implications.Comment: 12 pages, 18 figures, MNRAS accepte
Magnetohydrodynamic turbulence in warped accretion discs
Warped, precessing accretion discs appear in a range of astrophysical
systems, for instance the X-ray binary Her X-1 and in the active nucleus of
NGC4258. In a warped accretion disc there are horizontal pressure gradients
that drive an epicyclic motion. We have studied the interaction of this
epicyclic motion with the magnetohydrodynamic turbulence in numerical
simulations. We find that the turbulent stress acting on the epicyclic motion
is comparable in size to the stress that drives the accretion, however an
important ingredient in the damping of the epicyclic motion is its parametric
decay into inertial waves.Comment: to appear in the proceedings of the 20th Texas Symposium on
Relativistic Astrophysics, J. C. Wheeler & H. Martel (eds.
The Shape of an Accretion Disc in a Misaligned Black Hole Binary
We model the overall shape of an accretion disc in a semi-detached binary
system in which mass is transfered on to a spinning black hole the spin axis of
which is misaligned with the orbital rotation axis. We assume the disc is in a
steady state. Its outer regions are subject to differential precession caused
by tidal torques of the companion star. These tend to align the outer parts of
the disc with the orbital plane. Its inner regions are subject to differential
precession caused by the Lense-Thirring effect. These tend to align the inner
parts of the disc with the spin of the black hole. We give full numerical
solutions for the shape of the disc for some particular disc parameters. We
then show how an analytic approximation to these solutions can be obtained for
the case when the disc surface density varies as a power law with radius. These
analytic solutions for the shape of the disc are reasonably accurate even for
large misalignments and can be simply applied for general disc parameters. They
are particularly useful when the numerical solutions would be slow.Comment: Accepted for publication in MNRA
Fundamental length in quantum theories with PT-symmetric Hamiltonians II: The case of quantum graphs
Manifestly non-Hermitian quantum graphs with real spectra are introduced and
shown tractable as a new class of phenomenological models with several
appealing descriptive properties. For illustrative purposes, just equilateral
star-graphs are considered here in detail, with non-Hermiticities introduced by
interactions attached to the vertices. The facilitated feasibility of the
analysis of their spectra is achieved via their systematic approximative
Runge-Kutta-inspired reduction to star-shaped discrete lattices. The resulting
bound-state spectra are found real in a discretization-independent interval of
couplings. This conclusion is reinterpreted as the existence of a hidden
Hermiticity of our models, i.e., as the standard and manifest Hermiticity of
the underlying Hamiltonian in one of less usual, {\em ad hoc} representations
of the Hilbert space of states in which the inner product is local
(at ) or increasingly nonlocal (at ). Explicit examples of
these (of course, Hamiltonian-dependent) hermitizing inner products are offered
in closed form. In this way each initial quantum graph is assigned a menu of
optional, non-equivalent standard probabilistic interpretations exhibiting a
controlled, tunable nonlocality.Comment: 33 pp., 6 figure
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
Interaction of the solar wind with Venus
Two topics related to the interaction of the solar wind with Venus are considered. First, a short review of the experimental evidence with particular attention to plasma measurements carried out on Mariner-5 and Mariner-10 is given. Secondly, the results of some recent theoretical work on the interaction of the solar wind with the ionosphere of Venus are summarized
Aligning spinning black holes and accretion discs
We consider the alignment torque between a spinning black hole and an
accretion disc whose angular momenta are misaligned. This situation must hold
initially in almost all gas accretion events on to supermassive black holes,
and may occur in binaries where the black hole receives a natal supernova kick.
We show that the torque always acts to align the hole's spin with the total
angular momentum without changing its magnitude. The torque acts dissipatively
on the disc, reducing its angular momentum, and aligning it with the hole if
and only if the angle theta between the angular momenta J_d of the disc and J_h
of the hole satisfies the inequality cos theta > -J_d / 2 J_h. If this
condition fails, which requires both theta > pi/2 and J_d < 2 J_h, the disc
counteraligns.Comment: MNRAS, in pres
Viscous Effects on the Interaction between the Coplanar Decretion Disc and the Neutron Star in Be/X-Ray Binaries
We study the viscous effects on the interaction between the coplanar Be-star
disc and the neutron star in Be/X-ray binaries, using a three-dimensional,
smoothed particle hydrodynamics code. For simplicity, we assume the Be disc to
be isothermal at the temperature of half the stellar effective temperature. In
order to mimic the gas ejection process from the Be star, we inject particles
with the Keplerian rotation velocity at a radius just outside the star. Both Be
star and neutron star are treated as point masses. We find that the Be-star
disc is effectively truncated if the Shakura-Sunyaev viscosity parameter
alpha_SS >> 1, which confirms the previous semi-analytical result. In the
truncated disc, the material decreted from the Be star accumulates, so that the
disc becomes denser more rapidly than if around an isolated Be star. The
resonant truncation of the Be disc results in a significant reduction of the
amount of gas captured by the neutron star and a strong dependence of the mass
capture rate on the orbital phase. We also find that an eccentric mode is
excited in the Be disc through direct driving due to a one-armed bar potential
of the binary. The strength of the mode becomes greater in the case of a
smaller viscosity. In a high-resolution simulation with alpha_SS=0.1, the
eccentric mode is found to precess in a prograde sense. The mass capture rate
by the neutron star modulates as the mode precesses.Comment: 15 pages, including 20 figures and 1 table, accepted for publication
in MNRA
An alpha theory of time-dependent warped accretion discs
The non-linear fluid dynamics of a warped accretion disc was investigated in
an earlier paper by developing a theory of fully non-linear bending waves in a
thin, viscous disc. That analysis is here extended to take proper account of
thermal and radiative effects by solving an energy equation that includes
viscous dissipation and radiative transport. The problem is reduced to simple
one-dimensional evolutionary equations for mass and angular momentum, expressed
in physical units and suitable for direct application. This result constitutes
a logical generalization of the alpha theory of Shakura & Sunyaev to the case
of a time-dependent warped accretion disc. The local thermal-viscous stability
of such a disc is also investigated.Comment: 16 pages, 3 figures, to be published in MNRA
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