1,174 research outputs found
Dynamical Bar-Mode Instability in Differentially Rotating Magnetized Neutron Stars
This paper presents a numerical study over a wide parameter space of the
likelihood of the dynamical bar-mode instability in differentially rotating
magnetized neutron stars. The innovative aspect of this study is the
incorporation of magnetic fields in such a context, which have thus far been
neglected in the purely hydrodynamical simulations available in the literature.
The investigation uses the Cosmos++ code which allows us to perform three
dimensional simulations on a cylindrical grid at high resolution. A sample of
Newtonian magneto-hydrodynamical simulations starting from a set of models
previously analyzed by other authors without magnetic fields has been
performed, providing estimates of the effects of magnetic fields on the
dynamical bar-mode deformation of rotating neutron stars. Overall, our results
suggest that the effect of magnetic fields are not likely to be very
significant in realistic configurations. Only in the most extreme cases are the
magnetic fields able to suppress growth of the bar mode.Comment: 12 pages, 16 figures. References added and minor edits made to match
published versio
Radiation recoil from highly distorted black holes
We present results from numerical evolutions of single black holes distorted
by axisymmetric, but equatorially asymmetric, gravitational (Brill) waves. Net
radiated energies, apparent horizon embeddings, and recoil velocities are shown
for a range of Brill wave parameters, including both even and odd parity
distortions of Schwarzschild black holes. We find that a wave packet initially
concentrated on the black hole throat, a likely model also for highly
asymmetric stellar collapse and late stage binary mergers, can generate a
maximum recoil velocity of about 150 (23) km/sec for even (odd) parity
perturbations, significantly less than that required to eject black holes from
galactic cores.Comment: 15 pages, 8 figure
Warped Entanglement Entropy
We study the applicability of the covariant holographic entanglement entropy
proposal to asymptotically warped AdS spacetimes with an SL(2,R) x U(1)
isometry. We begin by applying the proposal to locally AdS backgrounds
which are written as a real-line fibration over AdS. We then perturb away
from this geometry by considering a warping parameter to get an
asymptotically warped AdS spacetime and compute the dual entanglement
entropy perturbatively in . We find that for large separation in the
fiber coordinate, the entanglement entropy can be computed to all orders in
and takes the universal form appropriate for two-dimensional CFTs. The
warping-dependent central charge thus identified exactly agrees with previous
calculations in the literature. Performing the same perturbative calculations
for the warped BTZ black hole again gives universal two-dimensional CFT
answers, with the left-moving and right-moving temperatures appearing
appropriately in the result.Comment: 25 pages plus appendices; v2 references added, discussions clarified
and equations sharpene
Grassmann Matrix Quantum Mechanics
We explore quantum mechanical theories whose fundamental degrees of freedom
are rectangular matrices with Grassmann valued matrix elements. We study
particular models where the low energy sector can be described in terms of a
bosonic Hermitian matrix quantum mechanics. We describe the classical curved
phase space that emerges in the low energy sector. The phase space lives on a
compact Kahler manifold parameterized by a complex matrix, of the type
discovered some time ago by Berezin. The emergence of a semiclassical bosonic
matrix quantum mechanics at low energies requires that the original Grassmann
matrices be in the long rectangular limit. We discuss possible holographic
interpretations of such matrix models which, by construction, are endowed with
a finite dimensional Hilbert space.Comment: 25 pages + appendice
Cosmic Microwave Background Anisotropies from the Rees-Sciama Effect in Universes
We investigate the imprint of nonlinear matter condensations on the Cosmic
Microwave Background (CMB) in cold dark matter (CDM) model
universes. We consider simulation domains ranging from Mpc to
Mpc in size. We concentrate on the secondary temperature
anisotropies induced by time varying gravitational potentials occurring after
decoupling. Specifically, we investigate the importance of the Rees-Sciama
effect due to: (1) intrinsic changes in the gravitational potential of forming,
nonlinear structures, (2) proper motion of nonlinear structures, and (3) late
time decay of gravitational potential perturbations in open universes. CMB
temperature anisotropies are obtained by numerically evolving matter
inhomogeneities and CMB photons from an early, linear epoch () to the
present, nonlinear epoch . We test the dependence and relative
importance of these secondary temperature anisotropies as a function of the
scale of the underlying matter (voids, superclusters) and as a function of
. The results of the models are compared to a
similarly executed simulation. We find that in low density
models all three sources of anisotropy could be relevant and reach levels of
. In particular, we find that for at
large scales, secondary temperature anisotropies are dominated by the decaying
potential.Comment: 20 pages + 7 figures + 4 plates, self-expanding uuencoded compressed
tar archive of postscript file
Cosmological Hydrodynamics with Multi-Species Chemistry and Nonequilibrium Ionization and Cooling
We have developed a method of solving for multi-species chemical reaction
flows in non--equilibrium and self--consistently with the hydrodynamic
equations in an expanding FLRW universe. The method is based on a backward
differencing scheme for the required stability when solving stiff sets of
equations and is designed to be efficient for three-dimensional calculations
without sacrificing accuracy. In all, 28 kinetic reactions are solved including
both collisional and radiative processes for the following nine separate
species: H, H+, He, He+, He++, H-, H2+, H2, and e-. The method identifies those
reactions (involving H- and H2+) ocurring on the shortest time scales,
decoupling them from the rest of the network and imposing equilibrium
concentrations to good accuracy over typical cosmological dynamical times.
Several tests of our code are presented, including radiative shock waves,
cosmological sheets, conservation constraints, and fully three-dimensional
simulations of CDM cosmological evolutions in which we compare our method to
results obtained when the packaged routine LSODAR is substituted for our
algorithms.Comment: Latex and postscript, 24 pages, with 6 figures. The paper is also
available at http://zeus.ncsa.uiuc.edu:8080/~abel/PGas/bib.html Submitted to
New Astronom
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