719 research outputs found
Linear and non-linear theory of a parametric instability of hydrodynamic warps in Keplerian discs
We consider the stability of warping modes in Keplerian discs. We find them
to be parametrically unstable using two lines of attack, one based on
three-mode couplings and the other on Floquet theory. We confirm the existence
of the instability, and investigate its nonlinear development in three
dimensions, via numerical experiment. The most rapidly growing non-axisymmetric
disturbances are the most nearly axisymmetric (low m) ones. Finally, we offer a
simple, somewhat speculative model for the interaction of the parametric
instability with the warp. We apply this model to the masing disc in NGC 4258
and show that, provided the warp is not forced too strongly, parametric
instability can fix the amplitude of the warp.Comment: 14 pages, 6 figures, revised version with appendix added, to be
published in MNRA
HARM: A Numerical Scheme for General Relativistic Magnetohydrodynamics
We describe a conservative, shock-capturing scheme for evolving the equations
of general relativistic magnetohydrodynamics. The fluxes are calculated using
the Harten, Lax, and van Leer scheme. A variant of constrained transport,
proposed earlier by T\'oth, is used to maintain a divergence free magnetic
field. Only the covariant form of the metric in a coordinate basis is required
to specify the geometry. We describe code performance on a full suite of test
problems in both special and general relativity. On smooth flows we show that
it converges at second order. We conclude by showing some results from the
evolution of a magnetized torus near a rotating black hole.Comment: 38 pages, 18 figures, submitted to Ap
Impact of dimensionless numbers on the efficiency of MRI-induced turbulent transport
The magneto-rotational instability is presently the most promising source of
turbulent transport in accretion disks. However, some important issues still
need to be addressed to quantify the role of MRI in disks; in particular no
systematic investigation of the role of the physical dimensionless parameters
of the problem on the dimensionless transport has been undertaken yet. First,
we complete existing investigations on the field strength dependence by showing
that the transport in high magnetic pressure disks close to marginal stability
is highly time-dependent and surprisingly efficient. Second, we bring to light
a significant dependence of the global transport on the magnetic Prandtl
number, with for the explored range: and
( being in the range 0.25 to 0.5). We show that the
dimensionless transport is not correlated to the dimensionless linear growth
rate, contrarily to a largely held expectation. More generally, these results
stress the need to control dissipation processes in astrophysical simulations.Comment: 11 pages, 11 figures, accepted to MNRA
The Evolution of Protoplanetary Disks Around Millisecond Pulsars: The PSR 1257 +12 System
We model the evolution of protoplanetary disks surrounding millisecond
pulsars, using PSR 1257+12 as a test case. Initial conditions were chosen to
correspond to initial angular momenta expected for supernova-fallback disks and
disks formed from the tidal disruption of a companion star. Models were run
under two models for the viscous evolution of disks: fully viscous and layered
accretion disk models. Supernova-fallback disks result in a distribution of
solids confined to within 1-2 AU and produce the requisite material to form the
three known planets surrounding PSR 1257+12. Tidal disruption disks tend to
slightly underproduce solids interior to 1 AU, required for forming the pulsar
planets, while overproducing the amount of solids where no body, lunar mass or
greater, exists. Disks evolving under 'layered' accretion spread somewhat less
and deposit a higher column density of solids into the disk. In all cases,
circumpulsar gas dissipates on year timescales, making
formation of gas giant planets highly unlikely.Comment: 16 pages, 17 figures, Accepted for publication in The Astrophysical
Journal (September 20, 2007 issue
Linear coupling of modes in 2D radially stratified astrophysical discs
We investigate mode coupling in a two dimensional compressible disc with
radial stratification and differential rotation. We employ the global radial
scaling of linear perturbations and study the linear modes in the local
shearing sheet approximation. We employ a three-mode formalism and study the
vorticity (W), entropy (S) and compressional (P) modes and their coupling
properties. The system exhibits asymmetric three-mode coupling: these include
mutual coupling of S and P-modes, S and W-modes, and asymmetric coupling
between the W and P-modes. P-mode perturbations are able to generate potential
vorticity through indirect three-mode coupling. This process indicates that
compressional perturbations can lead to the development of vortical structures
and influence the dynamics of radially stratified hydrodynamic accretion and
protoplanetary discs.Comment: 10 pages, 10 figures, MNRAS (accepted
- …