45 research outputs found
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
Nonlinear evolution of the r-modes in neutron stars.
The evolution of a neutron-star -mode driven unstable by gravitational radiation is studied here using numerical solutions of the full nonlinear fluid equations. The dimensionless amplitude of the mode grows to order unity before strong shocks develop which quickly damp the mode. In this simulation the star loses about of its initial angular momentum and of its rotational kinetic energy before the mode is damped. The nonlinear evolution causes the fluid to develop strong differential rotation which is concentrated near the surface and poles of the star
Further Evidence for an Elliptical Instability in Rotating Fluid Bars and Ellipsoidal Stars
Using a three-dimensional nonlinear hydrodynamic code, we examine the
dynamical stability of more than twenty self-gravitating, compressible,
ellipsoidal fluid configurations that initially have the same velocity
structure as Riemann S-type ellipsoids. Our focus is on ``adjoint''
configurations, in which internal fluid motions dominate over the collective
spin of the ellipsoidal figure; Dedekind-like configurations are among this
group. We find that, although some models are stable and some are moderately
unstable, the majority are violently unstable toward the development of ,
, and higher-order azimuthal distortions that destroy the coherent,
bar-like structure of the initial ellipsoidal configuration on a dynamical time
scale.
The parameter regime over which our models are found to be unstable generally
corresponds with the regime over which incompressible Riemann S-type ellipsoids
have been found to be susceptible to an elliptical strain instability
\citep{LL96}. We therefore suspect that an elliptical instability is
responsible for the destruction of our compressible analogs of Riemann
ellipsoids. The existence of the elliptical instability raises concerns
regarding the final fate of neutron stars that encounter the secular bar-mode
instability and regarding the spectrum of gravitational waves that will be
radiated from such systems.Comment: 28 pages, submitted to ApJ, quicktime movies are available at:
http://www.phys.lsu.edu/~ou/movie/s_type/index.htm
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
The Stability of Double White Dwarf Binaries Undergoing Direct Impact Accretion
We present numerical simulations of dynamically unstable mass transfer in a
double white dwarf binary with initial mass ratio, q = 0.4. The binary
components are approximated as polytropes of index n = 3/2 and the initially
synchronously rotating, semi-detached equilibrium binary is evolved
hydrodynamically with the gravitational potential being computed through the
solution of Poisson's equation. Upon initiating deep contact in our baseline
simulation, the mass transfer rate grows by more than an order of magnitude
over approximately ten orbits, as would be expected for dynamically unstable
mass transfer. However, the mass transfer rate then reaches a peak value, the
binary expands and the mass transfer event subsides. The binary must therefore
have crossed the critical mass ratio for stability against dynamical mass
transfer. Despite the initial loss of orbital angular momentum into the spin of
the accreting star, we find that the accretor's spin saturates and angular
momentum is returned to the orbit more efficiently than has been previously
suspected for binaries in the direct impact accretion mode. To explore this
surprising result, we directly measure the critical mass ratio for stability by
imposing artificial angular momentum loss at various rates to drive the binary
to an equilibrium mass transfer rate. For one of these driven evolutions, we
attain equilibrium mass transfer and deduce that effectively q_crit has evolved
to approximately 2/3. Despite the absence of a fully developed disk, tidal
interactions appear effective in returning excess spin angular momentum to the
orbit.Comment: 27 pages, 6 figures. Please see
http://www.phys.lsu.edu/faculty/tohline/astroph/mftd07/ for animations and
full resolution figures. Accepted for publication in the Astrophysical
Journa
One-armed Spiral Instability in a Low T/|W| Postbounce Supernova Core
A three-dimensional, Newtonian hydrodynamic technique is used to follow the
postbounce phase of a stellar core collapse event. For realistic initial data
we have employed post core-bounce snapshots of the iron core of a 20 solar mass
star. The models exhibit strong differential rotation but have centrally
condensed density stratifications. We demonstrate for the first time that such
postbounce cores are subject to a so-called low-T/|W| nonaxisymmetric
instability and, in particular, can become dynamically unstable to an m=1 -
dominated spiral mode at T/|W| ~ 0.08. We calculate the gravitational wave
emission by the instability and find that the emitted waves may be detectable
by current and future GW observatories from anywhere in the Milky Way.Comment: 4 pages, 4 figures, final, accepted (ApJL) versio
Constructing Synchronously Rotating Double White Dwarf Binaries
We have developed a self-consistent-field technique similar to the one
described by Hachisu, Eriguchi, & Nomoto (1986b) that can be used to construct
detailed force-balanced models of synchronously rotating, double white dwarf
(DWD) binaries that have a wide range of total masses, mass ratios, and
separations. In addition to providing a computational tool that can be used to
provide quiet initial starts for dynamical studies of the onset of mass
transfer in DWD systems, we show that this SCF technique can be used to
construct model sequences that mimic the last portion of the detached inspiral
phase of DWD binary evolutions, and semi-detached model sequences that mimic a
phase of conservative mass transfer.Comment: 51 pages, 10 figures, submitted for publication in Astrophysical
Journal Supplement Serie