1,083 research outputs found
A Numerical Study of Relativistic Fluid Collapse
We investigate the dynamics of self-gravitating, spherically-symmetric
distributions of fluid through numerical means. In particular, systems
involving neutron star models driven far from equilibrium in the strong-field
regime of general relativity are studied. Hydrostatic solutions of Einstein's
equations using a stiff, polytropic equation of state are used for the stellar
models. Many of the scenarios we examine involve highly-relativistic flows that
require improvements upon previously published numerical methods to simulate.
Here our particular focus is on the physical behavior of the coupled
fluid-gravitational system at the threshold of black hole formation--so-called
black hole critical phenomena. To investigate such phenomena starting from
conditions representing stable stars, we must drive the star far from its
initial stable configuration. We use one of two different mechanisms to do
this: setting the initial velocity profile of the star to be in-going, or
collapsing a shell of massless scalar field onto the star. Both of these
approaches give rise to a large range of dynamical scenarios that the star may
follow. These scenarios have been extensively surveyed by using different
initial star solutions, and by varying either the magnitude of the velocity
profile or the amplitude of the scalar field pulse. In addition to illuminating
the critical phenomena associated with the fluid collapse, the resulting phase
diagram of possible outcomes provides an approximate picture of the stability
of neutron stars to large, external perturbations that may occur in nature.Comment: 228 pages, 66 Postscript figures, Ph.D. Thesis, the University of
Texa s at Austin, uses utdiss2.sty v
Approximate black hole binary spacetime via asymptotic matching
We construct a fully analytic, general relativistic, nonspinning black hole
binary spacetime that approximately solves the vacuum Einstein equations
everywhere in space and time for black holes sufficiently well separated. The
metric is constructed by asymptotically matching perturbed Schwarzschild
metrics near each black hole to a two-body post-Newtonian metric far from them,
and a two-body post-Minkowskian metric farther still. Asymptotic matching is
done without linearizing about a particular time slice, and thus it is valid
dynamically and for all times, provided the binary is sufficiently well
separated. This approximate global metric can be used for long dynamical
evolutions of relativistic magnetohydrodynamical, circumbinary disks around
inspiraling supermassive black holes to study a variety of phenomena.Comment: 17 pages, 8 figures, 1 table. Appendix added to match published
versio
Dependence of inner accretion disk stress on parameters: the Schwarzschild case
We explore the parameter dependence of inner disk stress in black hole
accretion by contrasting the results of a number of simulations, all employing
3-d general relativistic MHD in a Schwarzschild spacetime. Five of these
simulations were performed with the intrinsically conservative code HARM3D,
which allows careful regulation of the disk aspect ratio, H/R; our simulations
span a range in H/R from 0.06 to 0.17. We contrast these simulations with two
previously reported simulations in a Schwarzschild spacetime in order to
investigate possible dependence of the inner disk stress on magnetic topology.
In all cases, much care was devoted to technical issues: ensuring adequate
resolution and azimuthal extent, and averaging only over those time-periods
when the accretion flow is in approximate inflow equilibrium. We find that the
time-averaged radial-dependence of fluid-frame electromagnetic stress is almost
completely independent of both disk thickness and poloidal magnetic topology.
It rises smoothly inward at all radii (exhibiting no feature associated with
the ISCO) until just outside the event horizon, where the stress plummets to
zero. Reynolds stress can also be significant near the ISCO and in the plunging
region; the magnitude of this stress, however, depends on both disk thickness
and magnetic topology. The two stresses combine to make the net angular
momentum accreted per unit rest-mass 7-15% less than the angular momentum of
the ISCO.Comment: Accepted for publication in ApJ, 52 pages, 38 figures, AASTEX.
High-resolution versions can be found at the following links:
http://ccrg.rit.edu/~scn/papers/schwarzstress.ps,
http://ccrg.rit.edu/~scn/papers/schwarzstress.pd
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