219 research outputs found
Density waves in the shearing sheet I. Swing amplification
The shearing sheet model of a galactic disk is studied anew. The theoretical
description of its dynamics is based on three building blocks: Stellar orbits,
which are described here in epicyclic approximation, the collisionless
Boltzmann equation determining the distribution function of stars in phase
space, and the Poisson equation in order to take account of the self-gravity of
the disk. Using these tools I develop a new formalism to describe perturbations
of the shearing sheet. Applying this to the unbounded shearing sheet model I
demonstrate again how the disturbances of the disk evolve always into `swing
amplified' density waves, i.e. spiral-arm like, shearing density enhancements,
which grow and decay while the wave crests swing by from leading to trailing
orientation. Several examples are given how such `swing amplification' events
are incited in the shearing sheet.Comment: small corrections, uses new A&A style fil
Kinematic Density Waves in Accretion Disks
When thin accretion disks around black holes are perturbed, the main
restoring force is gravity. If gas pressure, magnetic stresses, and radiation
pressure are neglected, the disk remains thin as long as orbits do not
intersect. Intersections would result in pressure forces which limit the growth
of perturbations. We find that a discrete set of perturbations is possible for
which orbits remain non-intersecting for arbitrarily long times. These modes
define a discrete set of frequencies. We classify all long-lived perturbations
for arbitrary potentials and show how their mode frequencies are related to
pattern speeds computed from the azimuthal and epicyclic frequencies. We show
that modes are concentrated near radii where the pattern speed has vanishing
radial derivative. We explore these modes around Kerr black holes as a possible
explanation for the high-frequency quasi-periodic oscillations of black hole
binaries such as GRO J1655-40. The long-lived modes are shown to coincide with
diskoseismic waves in the limit of small sound speed. While the waves have long
lifetime, they have the wrong frequencies to explain the pairs of
high-frequency quasi-periodic oscillations observed in black hole binaries.Comment: 28 pages, 6 figures; extended comparison with diskoseismology; added
reference to astro-ph/060368
Finite element modelling of perturbed stellar systems
I formulate a general finite element method (FEM) for self-gravitating
stellar systems. I split the configuration space to finite elements, and
express the potential and density functions over each element in terms of their
nodal values and suitable interpolating functions. General expressions are then
introduced for the Hamiltonian and phase space distribution functions of the
stars that visit a given element. Using the weighted residual form of Poisson's
equation, I derive the Galerkin projection of the perturbed collisionless
Boltzmann equation, and assemble the global evolutionary equations of nodal
distribution functions. The FEM is highly adaptable to all kinds of potential
and density profiles, and it can deal with density clumps and initially
non-axisymmetric systems. I use ring elements of non-uniform widths, choose
linear and quadratic interpolation functions in the radial direction, and apply
the FEM to the stability analysis of the cutout Mestel disc. I also integrate
the forced evolutionary equations and investigate the disturbances of a stable
stellar disc due to the gravitational field of a distant satellite galaxy. The
performance of the FEM and its prospects are discussed.Comment: 11 pages, three figures, accepted for publication by MNRA
Stationary perturbation configurations in a composite system of stellar and coplanarly magnetized gaseous singular isothermal discs
We construct aligned and unaligned stationary perturbation configurations in
a composite system of stellar and coplanarly magnetized gaseous singular
isothermal discs (SIDs) coupled by gravity. In comparison with SID problems
studied earlier, there exist three possible classes of stationary solutions
allowed by more dynamic freedoms. Our exact global perturbation solutions and
critical points are valuable for testing numerical magnetohydrodynamic codes.
For galactic applications, our model analysis contains more realistic elements
and offer useful insights for structures and dynamics of disc galaxies
consisting of stars and magnetized gas.Comment: 25 pages, 31 figures, accepted by Monthly Notices of Royal
Astronomical Society, style files include
Outline of the Unified Theory of Spiral and Bar-like Structures in Galaxies
This paper presents a new approach to studying galactic structures. They are
considered as the low-frequency normal modes in a disc of orbits precessing at
different angular speeds. Such a concept is an adequate alternative to the
commonly used approach of treating the disc as a set of individual stars
rotating at near-circular orbits around the centre. The problem of determining
the normal modes is reduced to a simple integral equation in the form of the
classical eigen-value problem, where the eigen-value is directly equal to the
pattern speed of the mode, . An examination of the general properties
of the basic integral equation shows that two types of solutions exist,
bar-like and spiral. The numerical solutions of both types are obtained. The
characteristic pattern speeds are of the order of the mean orbit precession
speed, although for the bar-modes can markedly exceed the maximum
precessing speed of orbits. It is shown that the bar-mode grows due to the
immediate action of its gravitational field on the stars at the resonance
regions. As for the spiral mode, its excitation is probably due to the inner
Lindblad resonance that can promote mode growth.Comment: 19 pages, 10 figures, 1 tabl
A new approach to the problem of modes in the Mestel disk
We examine the modes admitted by the Mestel disk, a disk with a globally flat
rotation curve. In contrast to previous analyses of this problem by Zang
(\cite{1976PhDT........26Z}) and Evans & Read (\cite{1998MNRAS.300...83E},
\cite{1998MNRAS.300..106E}), we approximate the orbits to obtain almost closed
expressions for the kernel of the integral equation governing the behaviour of
the modes. Otherwise we, like them, follow Kalnajs' programme to simultaneously
solve the Boltzmann and Poisson equations.
We investigate the modes admitted by both the self-consistent and a cut-out
Mestel disk, the difference being that in the latter, a part of the matter in
the disk is immobilised. This breaks the self-similarity and produces a
pronouncedly different picture, both technically and in terms of the disk
properties. The self-consistent disk is governed by a Cauchy integral equation,
the cut-out disk by an integral equation that can be treated as a Fredholm
equation of the second kind.
In general, our approximation reproduces the results of the previous works
remarkably well, yielding quantities mostly within 5% of the values reported by
Zang and Evans & Read and thus also the basic result that in a ``standard''
cut-out disk, only one-armed modes are unstable at the limit of axisymmetric
stability. In the self-consistent disk, relatively compact expressions for the
kernel allow an intuitive understanding of most of the properties of neutral
(non-rotating, non-growing) modes there. We finally show that self-consistent
Mestel disks do not admit growing or rotating modes in this sort of
stellar-dynamical analysis.Comment: 10 pages, 1 figure; accepted for publication in A&
Interplay between Stellar Spirals and the ISM in Galactic Disks
We propose a new dynamical picture of galactic stellar and gas spirals, based
on hydrodynamic simulations in a `live' stellar disk. We focus especially on
spiral structures excited in a isolated galactic disk without a stellar bar.
Using high-resolution, 3-dimensional N-body/SPH simulations, we found that the
spiral features of the gas in galactic disks are formed by essentially
different mechanisms from the galactic shock in stellar density waves. The
stellar spiral arms and the interstellar matter on average corotate in a
galactic potential at any radii. Unlike the stream motions in the galactic
shock, the interstellar matter flows into the local potential minima with
irregular motions. The flows converge to form dense gas clouds/filaments near
the bottom of the stellar spirals, whose global structures resemble dust-lanes
seen in late-type spiral galaxies. The stellar arms are non-steady; they are
wound and stretched by the galactic shear, and thus local densities of the arm
change on a time scale of ~ 100 Myrs, due to bifurcating or merging with other
arms. This makes the gas spirals associated with the stellar arms non-steady.
The association of dense gas clouds are eventually dissolved into inter-arm
regions with non-cirucular motions. Star clusters are formed from the cold,
dense gases, whose ages are less than ~30 Myrs, and they are roughly associated
with the background stellar arms without a clear spatial offset between gas
spiral arms and distribution of young stars.Comment: 13 pages, 12 figures, accepted by ApJ. Higher resolution of ms.pdf is
available at http://d.pr/Nvjk A targzipped Supplementary movies is available
at http://d.pr/TV6
Patterns in the Outer Parts of Galactic Disks
This paper describes test particle simulations of the response of the outer
parts of Galactic disks to barring and spiral structure. Simulations are
conducted for cold Mestel disks and warm quasi-exponential disks with
completely flat rotation curves, subjected to pure quadrupoles and logarithmic
spirals. Even though the starting velocity distributions are smooth, the
end-points of the bar simulations show bimodality and multi-peaked structures
at locations near the outer Lindblad resonance (OLR), although spirality can
make this smoother. The growth of a bar may cause the disk isophotes to become
boxy at the OLR, as stars accummulate particularly along the minor axis. The
growth of a bar is also accompanied by substantial heating of the disk stars
near the OLR. For the growth of a bar, the radial
velocity dispersion is typically quadrupled for initially cold disks (initial
\sigmau \kms), and typically doubled for disks with final \sigmau
\kms. Simulations performed of the growth and dissolution of bars
give very similar results, demonstrating that the heat once given to disk stars
is very difficult to remove.Comment: 14 pages, 19 figure
Dynamical friction force exerted on spherical bodies
We present a rigorous calculation of the dynamical friction force exerted on
a spherical massive perturber moving through an infinite homogenous system of
field stars. By calculating the shape and mass of the polarization cloud
induced by the perturber in the background system, which decelerates the motion
of the perturber, we recover Chandrasekhar's drag force law with a modified
Coulomb logarithm. As concrete examples we calculate the drag force exerted on
a Plummer sphere or a sphere with the density distribution of a Hernquist
profile. It is shown that the shape of the perturber affects only the exact
form of the Coulomb logarithm. The latter converges on small scales, because
encounters of the test and field stars with impact parameters less than the
size of the massive perturber become inefficient. We confirm this way earlier
results based on the impulse approximation of small angle scatterings.Comment: 5 pages, 2 figures, accepted in MNRA
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