34 research outputs found
Rotational Instabilities and Centrifugal Hangup
One interesting class of gravitational radiation sources includes rapidly
rotating astrophysical objects that encounter dynamical instabilities. We have
carried out a set of simulations of rotationally induced instabilities in
differentially rotating polytropes. An =1.5 polytrope with the Maclaurin
rotation law will encounter the =2 bar instability at .
Our results indicate that the remnant of this instability is a persistent
bar-like structure that emits a long-lived gravitational radiation signal.
Furthermore, dynamical instability is shown to occur in =3.33 polytropes
with the -constant rotation law at . In this case, the
dominant mode of instability is =1. Such instability may allow a
centrifugally-hung core to begin collapsing to neutron star densities on a
dynamical timescale. If it occurs in a supermassive star, it may produce
gravitational radiation detectable by LISA.Comment: 13 pages (includes 11 figures) and 1 separate jpeg figure; to appear
in Astrophysical Sources of Gravitational Radiation, AIP conference
proceedings, edited by Joan M. Centrell
Gravitational Radiation From and Instabilities in Compact Stars and Compact Binary Systems.
We have examined three types of compact astrophysical systems that are possible sources of detectable gravitational wave radiation (GWR): nonaxisymmetric pulsars; rapidly rotating compact stars undergoing the bar-mode instability; and coalescing compact binaries. Our analysis of nonaxisymmetric pulsars, based on the assumption that any equatorial asymmetries present in these objects were rotationally induced, indicates that nearby millisecond pulsars are generally better candidates for the detection of GWR than the Crab pulsar, which has been the object of an ongoing search for GWR (Tsubono 1991). Our finite difference hydrodynamics (FDH) simulation of an object encountering the rotationally induced bar-mode instability results in an ellipsoidal final configuration which, although gradually becoming more axisymmetric, persists for several orbits, continuously emitting GWR. We also have examined the stability and coalescence of equal mass binaries with polytropic, white dwarf (WD), and neutron star (NS) equations of state (EOS). In order for our explicit FDH code to be able to follow the coalescence of a binary system, it must proceed on a dynamical timescale. Hence, we began our investigation by performing FDH tests of the dynamical stability of individual models constructed along equilibrium sequences of binaries with the same total mass M\sb{T} and EOS but decreasing separation, in order to determine if any models on these sequences were unstable to merger on a dynamical timescale. Our simulations indicate that no points of instability exist on the WD EOS sequences with M\sb{T} =.500 M\sb{\odot} and 2.03 M\sb\odot or on the polytropic EOS sequences with polytropic indices n = 1.5 and 1.0. However, binary models on the n = 0.5 polytropic sequence and on two realistic NS EOS sequences were dynamically unstable to merger. Again using our FDH code, we followed the evolution of the binary with the minimum total energy and angular momentum on the n = 0.5 sequence through coalescence. At the end of the simulation, the ellipsoidal central object is encircled by spiral arms, ejected from the system during the merger, that have wrapped around on themselves and is continuing to emit low amplitude GWR
Millisecond Pulsars: Detectable Sources of Continuous Gravitational Waves?
Laboratory searches for the detection of gravitational waves have focused on
the detection of burst signals emitted during a supernova explosion, but have
not resulted in any confirmed detections. An alternative approach has been to
search for continuous wave (CW) gravitational radiation from the Crab pulsar.
In this paper, we examine the possibility of detecting CW gravitational
radiation from pulsars and show that nearby millisecond pulsars are generally
much better candidates. We show that the minimum strain h_c ~ 10E-26 that can
be detected by tuning an antenna to the frequency of the milli- second pulsar
PSR 1957+20, with presently available detector technology, is orders of
magnitude better than what has been accomplished so far by observing the Crab
pulsar, and within an order of magnitude of the maximum strain that may be
produced by it. In addition, we point out that there is likely to be a
population of rapidly rotating neutron stars (not necessarily radio pulsars) in
the solar neighborhood whose spindown evolution is driven by gravitational
radiation. We argue that the projected sensitivity of modern resonant detectors
is sufficient to detect the subset of this population that lies within 0.1 kpc
of the sun.Comment: 17 pages (including 2 Postscript figures), LaTeX file, uses AASTeX
macros, accepted for publication in the Astrophysical Journa
Constraint Likelihood analysis for a network of gravitational wave detectors
We propose a coherent method for the detection and reconstruction of
gravitational wave signals for a network of interferometric detectors. The
method is derived using the likelihood functional for unknown signal waveforms.
In the standard approach, the global maximum of the likelihood over the space
of waveforms is used as the detection statistic. We identify a problem with
this approach. In the case of an aligned pair of detectors, the detection
statistic depends on the cross-correlation between the detectors as expected,
but this dependence dissappears even for infinitesimally small misalignments.
We solve the problem by applying constraints on thelikelihood functional and
obtain a new class of statistics. The resulting method can be applied to the
data from a network consisting of any number of detectors with arbitrary
detector orientations. The method allows us reconstruction of the source
coordinates and the waveforms of two polarization components of a gravitational
wave. We study the performance of the method with numerical simulation and find
the reconstruction of the source coordinates to be more accurate than in the
standard approach.Comment: 13 pages, 6 figure
Three-dimensional adaptive evolution of gravitational waves in numerical relativity
Adaptive techniques are crucial for successful numerical modeling of
gravitational waves from astrophysical sources such as coalescing compact
binaries, since the radiation typically has wavelengths much larger than the
scale of the sources. We have carried out an important step toward this goal,
the evolution of weak gravitational waves using adaptive mesh refinement in the
Einstein equations. The 2-level adaptive simulation is compared with unigrid
runs at coarse and fine resolution, and is shown to track closely the features
of the fine grid run.Comment: REVTeX, 7 pages, including three figures; submitted to Physical
Review
The Origin and Kinematics of Cold Gas in Galactic Winds: Insight from Numerical Simulations
We study the origin of Na I absorbing gas in ultraluminous infrared galaxies
motivated by the recent observations by Martin of extremely superthermal
linewidths in this cool gas. We model the effects of repeated supernova
explosions driving supershells in the central regions of molecular disks with
M_d=10^10 M_\sun, using cylindrically symmetric gas dynamical simulations run
with ZEUS-3D. The shocked swept-up shells quickly cool and fragment by
Rayleigh-Taylor instability as they accelerate out of the dense, stratified
disks. The numerical resolution of the cooling and compression at the shock
fronts determines the peak shell density, and so the speed of Rayleigh-Taylor
fragmentation. We identify cooled shells and shell fragments as Na I absorbing
gas and study its kinematics. We find that simulations with a numerical
resolution of \le 0.2 pc produce multiple Rayleigh-Taylor fragmented shells in
a given line of sight. We suggest that the observed wide Na I absorption lines,
= 320 \pm 120 km s^-1 are produced by these multiple fragmented shells
traveling at different velocities. We also suggest that some shell fragments
can be accelerated above the observed average terminal velocity of 750 km s^-1
by the same energy-driven wind with an instantaneous starburst of \sim 10^9
M_\sun. The bulk of mass is traveling with the observed average shell velocity
330 \pm 100 km s^-1. Our results show that an energy-driven bubble causing
Rayleigh-Taylor instabilities can explain the kinematics of cool gas seen in
the Na I observations without invoking additional physics relying primarily on
momentum conservation, such as entrainment of gas by Kelvin-Helmholtz
instabilities, ram pressure driving of cold clouds by a hot wind, or radiation
pressure acting on dust. (abridged)Comment: 65 pages, 22 figures, accepted by Astrophys. J. Changes during
refereeing focused on context and comparison to observation
A Physical Model of Warped Galaxy Disks
Warped H I gas layers in the outer regions of spiral galaxies usually display
a noticeably twisted structure. This structure almost certainly arises
primarily as a result of differential precession in the H I disk as it settles
toward a preferred orientation in an underlying dark halo potential well that
is not spherically symmetric. In an attempt to better understand the structure
and evolution of these twisted, warped disk structures, we have adopted the
"twist-equation" formalism originally developed by Petterson (1977) to study
accretion onto compact objects. Utilizing more recent treatments of this
formalism, we have generalized the twist-equation to allow for the treatment of
non-Keplerian disks and from it have derived a steady-state structure of
twisted disks that develops from free precession in a nonspherical, logarithmic
halo potential. We have used this steady-state solution to produce H I maps of
five galaxies (M83, NGC 300, NGC 2841, NGC 5033, NGC5055), which match the
general features of the observed maps of these galaxies quite well. In
addition, the model provides an avenue through which the kinematical viscosity
of the H I disk and the quadrupole distortion of the dark halo in each galaxy
can be quantified. This generalized equation can also be used to examine the
time-evolutionary behavior of warped galaxy disks.Comment: 22 pages, 5 figures, to be published in The Astrophysical Journa
The Formation of Supermassive Black Holes and the Evolution of Supermassive Stars
The existence of supermassive black holes is supported by a growing body of
observations. Supermassive black holes and their formation events are likely
candidates for detection by proposed long-wavelength, space-based gravitational
wave interferometers like LISA. However, the nature of the progenitors of
supermassive black holes is rather uncertain. Supermassive black hole formation
scenarios that involve either the stellar dynamical evolution of dense clusters
or the hydrodynamical evolution of supermassive stars have been proposed. Each
of these formation scenarios is reviewed and the evolution of supermassive
stars is then examined in some detail. Supermassive stars that rotate uniformly
during their secular cooling phase will spin up to the mass-shedding limit and
eventually contract to the point of relativistic collapse. Supermassive stars
that rotate differentially as they cool will likely encounter the dynamical bar
mode instability prior to the onset of relativistic collapse. A supermassive
star that undergoes this bar distortion, prior to or during collapse, may be a
strong source of quasiperiodic, long-wavelength gravitational radiation.Comment: 6 pages, 1 figure; submitted to a Special Issue of Classical and
Quantum Gravity, Proceedings of the Third LISA Symposiu