233 research outputs found
Stationary structures of irrotational binary systems -- models for close binary systems of compact stars
We propose a new numerical method to calculate irrotational binary systems
composed of compressible gaseous stars in Newtonian gravity. Assuming
irrotationality, i.e. vanishing of the vorticity vector everywhere in the star
in the inertial frame, we can introduce the velocity potential for the flow
field. Using this velocity potential we can derive a set of basic equations for
stationary states which consist of (i) the generalized Bernoulli equation, (ii)
the Poisson equation for the Newtonian gravitational potential and (iii) the
equation for the velocity potential with the Neumann type boundary condition.
We succeeded in developing a new code to compute numerically exact solutions to
these equations for the first time. Such irrotational configurations of binary
systems are appropriate models for realistic neutron star binaries composed of
inviscid gases, just prior to coalescence of two stars caused by emission of
gravitational waves. Accuracies of our numerical solutions are so high that we
can compute reliable models for fully deformed final stationary configurations
and hence determine the inner most stable circular orbit of binary neutron star
systems under the approximations of weak gravity and inviscid limit.Comment: 32 pages, 25 bitmapped ps files, to appear in ApJ supplemen
Pattern Matching and Discourse Processing in Information Extraction from Japanese Text
Information extraction is the task of automatically picking up information of
interest from an unconstrained text. Information of interest is usually
extracted in two steps. First, sentence level processing locates relevant
pieces of information scattered throughout the text; second, discourse
processing merges coreferential information to generate the output. In the
first step, pieces of information are locally identified without recognizing
any relationships among them. A key word search or simple pattern search can
achieve this purpose. The second step requires deeper knowledge in order to
understand relationships among separately identified pieces of information.
Previous information extraction systems focused on the first step, partly
because they were not required to link up each piece of information with other
pieces. To link the extracted pieces of information and map them onto a
structured output format, complex discourse processing is essential. This paper
reports on a Japanese information extraction system that merges information
using a pattern matcher and discourse processor. Evaluation results show a high
level of system performance which approaches human performance.Comment: See http://www.jair.org/ for any accompanying file
Dynamical instability of differentially rotating stars
We study the dynamical instability against bar-mode deformation of
differentially rotating stars. We performed numerical simulation and linear
perturbation analysis adopting polytropic equations of state with the
polytropic index . It is found that rotating stars of a high degree of
differential rotation are dynamically unstable even for the ratio of the
kinetic energy to the gravitational potential energy of .
Gravitational waves from the final nonaxisymmetric quasistationary states are
calculated in the quadrupole formula. For rotating stars of mass
and radius several 10 km, gravitational waves have frequency several 100 Hz and
effective amplitude at a distance of Mpc.Comment: 5 pages, 7 figures, accepted for publication in MNRA
Possible evolutionary transition from rapidly rotating neutron stars to strange stars due to spin-down
We present a scenario of formation of strange stars due to spin-down of {\it
rapidly rotating} neutron stars left after supernova explosions . By assuming a
process where the total baryon mass is conserved but the angular momentum is
lost due to emission of gravitational waves and/or the magnetic braking, we
find that the transition from rapidly rotating neutron stars to slowly rotating
strange stars is possible; a large amount of energy could
be released. The liberated energy might become a new energy source for a
delayed explosion of supernova. Furthermore, our scenario suggests that the
supernova associated with gamma-ray bursts could become candidates for targets
in the future observation of gravitational waves.Comment: 11 pages, 3 figures, Received November 5, 200
Maximal mass of uniformly rotating homogeneous stars in Einsteinian gravity
Using a multi domain spectral method, we investigate systematically the
general-relativistic model for axisymmetric uniformly rotating, homogeneous
fluid bodies generalizing the analytically known Maclaurin and Schwarzschild
solutions. Apart from the curves associated with these solutions and a further
curve of configurations that rotate at the mass shedding limit, two more curves
are found to border the corresponding two parameter set of solutions. One of
them is a Newtonian lens shaped sequence bifurcating from the Maclaurin
spheroid sequence, while the other one corresponds to highly relativistic
bodies with an infinite central pressure. The properties of the configuration
for which both the gravitational and the baryonic masses, moreover angular
velocity, angular momentum as well as polar red shift obtain their maximal
values are discussed in detail. In particular, by comparison with the static
Schwarzschild solution, we obtain an increase of 34.25% in the gravitational
mass. Moreover, we provide exemplarily a discussion of angular velocity and
gravitational mass on the entire solution class.Comment: 4 pages, 4 figures, 1 table, submitted to A&A, corrected eq. for W,
W' in 3.
Dynamical bar-mode instability of differentially rotating stars: Effects of equations of state and velocity profiles
As an extension of our previous work, we investigate the dynamical
instability against nonaxisymmetric bar-mode deformations of differentially
rotating stars in Newtonian gravity varying the equations of state and velocity
profiles. We performed the numerical simulation and the followup linear
stability analysis adopting polytropic equations of state with the polytropic
indices n=1, 3/2, and 5/2 and with two types of angular velocity profiles (the
so-called j-constant-like and Kepler-like laws). It is confirmed that rotating
stars of a high degree of differential rotation are dynamically unstable
against the bar-mode deformation, even for the ratio of the kinetic energy to
the gravitational potential energy of order 0.01. The criterion for
onset of the bar-mode dynamical instability depends weakly on the polytropic
index n and the angular velocity profile as long as the degree of differential
rotation is high. Gravitational waves from the final nonaxisymmetric
quasi-stationary states are calculated in the quadrupole formula. For
proto-neutron stars of mass , radius km and \beta \alt
0.1, such gravitational waves have the frequency of 600--1,400 Hz, and
the effective amplitude is larger than at a distance of about 100
Mpc irrespective of n and the angular velocity profile.Comment: 9 pages, 14 figures, accepted to MNRA
Gravitational waves from relativistic rotational core collapse
We present results from simulations of axisymmetric relativistic rotational
core collapse. The general relativistic hydrodynamic equations are formulated
in flux-conservative form and solved using a high-resolution shock-capturing
scheme. The Einstein equations are approximated with a conformally flat
3-metric. We use the quadrupole formula to extract waveforms of the
gravitational radiation emitted during the collapse. A comparison of our
results with those of Newtonian simulations shows that the wave amplitudes
agree within 30%. Surprisingly, in some cases, relativistic effects actually
diminish the amplitude of the gravitational wave signal. We further find that
the parameter range of models suffering multiple coherent bounces due to
centrifugal forces is considerably smaller than in Newtonian simulations.Comment: 4 pages, 3 figure
Black Holes Surrounded by Uniformly Rotating Rings
Highly accurate numerical solutions to the problem of Black Holes surrounded
by uniformly rotating rings in axially symmetric, stationary spacetimes are
presented. The numerical methods developed to handle the problem are discussed
in some detail. Related Newtonian problems are described and numerical results
provided, which show that configurations can reach an inner mass-shedding limit
as the mass of the central object increases. Exemplary results for the full
relativistic problem for rings of constant density are given and the
deformation of the event horizon due to the presence of the ring is
demonstrated. Finally, we provide an example of a system for which the angular
momentum of the central Black Hole divided by the square of its mass exceeds
one.Comment: 12 pages, 14 figures, revtex, v4: minor changes, Eq. (17) corrected,
corresponds to version in PR
Highly accurate calculation of rotating neutron stars
A new spectral code for constructing general-relativistic models of rapidly
rotating stars with an unprecedented accuracy is presented. As a first
application, we reexamine uniformly rotating homogeneous stars and compare our
results with those obtained by several previous codes. Moreover, representative
relativistic examples corresponding to highly flattened rotating bodies are
given.Comment: 4 pages, submitted to Astronomy & Astrophysic
- …