11 research outputs found
On the Shear Instability in Relativistic Neutron Stars
We present new results on instabilities in rapidly and differentially
rotating neutron stars. We model the stars in full general relativity and
describe the stellar matter adopting a cold realistic equation of state based
on the unified SLy prescription. We provide evidence that rapidly and
differentially rotating stars that are below the expected threshold for the
dynamical bar-mode instability, beta_c = T/|W| ~ 0.25, do nevertheless develop
a shear instability on a dynamical timescale and for a wide range of values of
beta. This class of instability, which has so far been found only for small
values of beta and with very small growth rates, is therefore more generic than
previously found and potentially more effective in producing strong sources of
gravitational waves. Overall, our findings support the phenomenological
predictions made by Watts, Andersson and Jones on the nature of the low-T/|W|.Comment: 20 pages; accepted to the Classical and Quantum Gravity special issue
for MICRA200
Shell sources as a probe of relativistic effects in neutron star models
A perturbing shell is introduced as a device for studying the excitation of
fluid motions in relativistic stellar models. We show that this approach allows
a reasonably clean separation of radiation from the shell and from fluid
motions in the star, and provides broad flexibility in the location and
timescale of perturbations driving the fluid motions. With this model we
compare the relativistic and Newtonian results for the generation of even
parity gravitational waves from constant density models. Our results suggest
that relativistic effects will not be important in computations of the
gravitational emission except possibly in the case of excitation of the neutron
star on very short time scales.Comment: 16 pages LaTeX with 6 eps figures; submitted to Phys. Rev.
Gravitational waves from a test particle scattered by a neutron star: Axial mode case
Using a metric perturbation method, we study gravitational waves from a test
particle scattered by a spherically symmetric relativistic star. We calculate
the energy spectrum and the waveform of gravitational waves for axial modes.
Since metric perturbations in axial modes do not couple to the matter fluid of
the star, emitted waves for a normal neutron star show only one peak in the
spectrum, which corresponds to the orbital frequency at the turning point,
where the gravitational field is strongest. However, for an ultracompact star
(the radius ), another type of resonant periodic peak appears in
the spectrum. This is just because of an excitation by a scattered particle of
axial quasinormal modes, which were found by Chandrasekhar and Ferrari. This
excitation comes from the existence of the potential minimum inside of a star.
We also find for an ultracompact star many small periodic peaks at the
frequency region beyond the maximum of the potential, which would be due to a
resonance of two waves reflected by two potential barriers (Regge-Wheeler type
and one at the center of the star). Such resonant peaks appear neither for a
normal neutron star nor for a Schwarzschild black hole. Consequently, even if
we analyze the energy spectrum of gravitational waves only for axial modes, it
would be possible to distinguish between an ultracompact star and a normal
neutron star (or a Schwarzschild black hole).Comment: 21 pages, revtex, 11 figures are attached with eps files Accepted to
Phys. Rev.
Non-axisymmetric instability and fragmentation of general relativistic quasi-toroidal stars
In a recent publication, we have demonstrated that differentially rotating stars admit new channels of black hole formation via fragmentation instabilities. Since a higher order instability of this kind could potentially transform a differentially rotating supermassive star into a multiple black hole system embedded in a massive accretion disk, we investigate the dependence of the instability on parameters of the equilibrium model. We find that many of the models constructed exhibit non-axisymmetric instabilities with corotation points, even for low values of T/|W|, which lead to a fission of the stars into one, two or three fragments, depending on the initial perturbation.
At least in the models selected here, an m=1 mode becomes unstable at lower values of T/|W|, which would seem to favor a scenario where one black hole with a massive accretion disk forms. In this case, we have gained evidence that low values of compactness of the initial model can lead to a stabilization of the resulting fragment, thus preventing black hole formation in this scenario