1,726 research outputs found
On differentially dissipative dynamical systems
Dissipativity is an essential concept of systems theory. The paper provides
an extension of dissipativity, named differential dissipativity, by lifting
storage functions and supply rates to the tangent bundle. Differential
dissipativity is connected to incremental stability in the same way as
dissipativity is connected to stability. It leads to a natural formulation of
differential passivity when restricting to quadratic supply rates. The paper
also shows that the interconnection of differentially passive systems is
differentially passive, and provides preliminary examples of differentially
passive electrical systems
Angular Momentum Transfer and Lack of Fragmentation in Self-Gravitating Accretion Flows
Rapid inflows associated with early galaxy formation lead to the accumulation
of self-gravitating gas in the centers of proto-galaxies. Such gas
accumulations are prone to non-axisymmetric instabilities, as in the well-known
Maclaurin sequence of rotating ellipsoids, which are accompanied by a
catastrophic loss of angular momentum (J). Self-gravitating gas is also
intuitively associated with star formation. However, recent simulations of the
infall process display highly turbulent continuous flows. We propose that
J-transfer, which enables the inflow, also suppresses fragmentation.
Inefficient J loss by the gas leads to decay of turbulence, triggering global
instabilities and renewed turbulence driving. Flow regulated in this way is
stable against fragmentation, whilst staying close to the instability threshold
for bar formation -- thick self-gravitating disks are prone to global
instabilities before they become unstable locally. On smaller scales, the
fraction of gravitationally unstable matter swept up by shocks in such a flow
is a small and decreasing function of the Mach number. We conclude
counterintuitively that gas able to cool down to a small fraction of its virial
temperature will not fragment as it collapses. This provides a venue for
supermassive black holes to form via direct infall, without the intermediary
stage of forming a star cluster. Some black holes could have formed or grown in
massive halos at low redshifts. Thus the fragmentation is intimately related to
J redistribution within the system: it is less dependent on the molecular and
metal cooling but is conditioned by the ability of the flow to develop virial,
supersonic turbulence.Comment: 5 pp., 1 figures, to be published by the Astrophysical Journal
Letters. Minor corrections following the referee repor
The bar-mode instability in differentially rotating neutron stars: Simulations in full general relativity
We study the dynamical stability against bar-mode deformation of rapidly
spinning neutron stars with differential rotation. We perform fully
relativistic 3D simulations of compact stars with , where is
the total gravitational mass and the equatorial circumferential radius. We
adopt an adiabatic equation of state with adiabatic index . As in
Newtonian theory, we find that stars above a critical value of (where is the rotational kinetic energy and the gravitational
binding energy) are dynamically unstable to bar formation. For our adopted
choices of stellar compaction and rotation profile, the critical value of
is , only slightly smaller than the
well-known Newtonian value for incompressible Maclaurin spheroids.
The critical value depends only very weakly on the degree of differential
rotation for the moderate range we surveyed. All unstable stars form bars on a
dynamical timescale. Models with sufficiently large subsequently form
spiral arms and eject mass, driving the remnant to a dynamically stable state.
Models with moderately large do not develop spiral
arms or eject mass but adjust to form dynamically stable ellipsoidal-like
configurations. If the bar-mode instability is triggered in supernovae collapse
or binary neutron star mergers, it could be a strong and observable source of
gravitational waves. We determine characteristic wave amplitudes and
frequencies.Comment: 17 pages, accepted for publication in AP
Nonlinear Development of the Secular Bar-mode Instability in Rotating Neutron Stars
We have modelled the nonlinear development of the secular bar-mode
instability that is driven by gravitational radiation-reaction (GRR) forces in
rotating neutron stars. In the absence of any competing viscous effects, an
initially uniformly rotating, axisymmetric polytropic star with a ratio
of rotational to gravitational potential energy is driven by
GRR forces to a bar-like structure, as predicted by linear theory. The pattern
frequency of the bar slows to nearly zero, that is, the bar becomes almost
stationary as viewed from an inertial frame of reference as GRR removes energy
and angular momentum from the star. In this ``Dedekind-like'' state, rotational
energy is stored as motion of the fluid in highly noncircular orbits inside the
bar. However, in less than 10 dynamical times after its formation, the bar
loses its initially coherent structure as the ordered flow inside the bar is
disrupted by what appears to be a purely hydrodynamical, short-wavelength,
``shearing'' type instability. The gravitational waveforms generated by such an
event are determined, and an estimate of the detectability of these waves is
presented.Comment: 25 pages, 9 figures, accepted for publication in ApJ, refereed
version, updated, for quicktime movie, see
http://www.phys.lsu.edu/~ou/movie/fmode/new/fmode.b181.om4.2e5.mo
Gravito-inertial waves in a differentially rotating spherical shell
The gravito-inertial waves propagating over a shellular baroclinic flow
inside a rotating spherical shell are analysed using the Boussinesq
approximation. The wave properties are examined by computing paths of
characteristics in the non-dissipative limit, and by solving the full
dissipative eigenvalue problem using a high-resolution spectral method.
Gravito-inertial waves are found to obey a mixed-type second-order operator and
to be often focused around short-period attractors of characteristics or
trapped in a wedge formed by turning surfaces and boundaries. We also find
eigenmodes that show a weak dependence with respect to viscosity and heat
diffusion just like truly regular modes. Some axisymmetric modes are found
unstable and likely destabilized by baroclinic instabilities. Similarly, some
non-axisymmetric modes that meet a critical layer (or corotation resonance) can
turn unstable at sufficiently low diffusivities. In all cases, the instability
is driven by the differential rotation. For many modes of the spectrum, neat
power laws are found for the dependence of the damping rates with diffusion
coefficients, but the theoretical explanation for the exponent values remains
elusive in general. The eigenvalue spectrum turns out to be very rich and
complex, which lets us suppose an even richer and more complex spectrum for
rotating stars or planets that own a differential rotation driven by
baroclinicity.Comment: 33 pages, 14 figures, accepted for publication in Journal of Fluid
Mechanic
On the Formation and Evolution of Common Envelope Systems
We discuss the formation of a common envelope system following dynamically
unstable mass transfer in a close binary, and the subsequent dynamical
evolution and final fate of the envelope. We base our discussion on new
three-dimensional SPH calculations that we have performed for a close binary
system containing a red giant with a main-sequence
star companion. The initial parameters are chosen to model the formation of a
system resembling V~471~Tau, a typical progenitor of a cataclysmic variable
binary. In our highest-resolution calculation, we find evidence for a
corotating region of gas around the central binary. This is in agreement with
the theoretical model proposed by Meyer \& Meyer-Hofmeister (1979) for the
evolution of common envelope systems, in which this central corotating region
is coupled to the envelope through viscous angular momentum transport only. We
also find evidence that the envelope is convectively unstable, in which case
the viscous dissipation time could be as short as dynamical times,
leading to rapid ejection of the envelope. For V~471~Tau, our results, and the
observed parameters of the system, are entirely consistent with rapid envelope
ejection on a timescale yr and an efficiency parameter
.Comment: uses AAS latex macros v4, 36 pages with figures, submitted to ApJ,
complete postscript also available at http://ensor.mit.edu/~rasio/paper
A Survey on Continuous Time Computations
We provide an overview of theories of continuous time computation. These
theories allow us to understand both the hardness of questions related to
continuous time dynamical systems and the computational power of continuous
time analog models. We survey the existing models, summarizing results, and
point to relevant references in the literature
- …