13,337 research outputs found
Gravitational waves from axisymmetrically oscillating neutron stars in general relativistic simulations
Gravitational waves from oscillating neutron stars in axial symmetry are
studied performing numerical simulations in full general relativity. Neutron
stars are modeled by a polytropic equation of state for simplicity. A
gauge-invariant wave extraction method as well as a quadrupole formula are
adopted for computation of gravitational waves. It is found that the
gauge-invariant variables systematically contain numerical errors generated
near the outer boundaries in the present axisymmetric computation. We clarify
their origin, and illustrate it possible to eliminate the dominant part of the
systematic errors. The best corrected waveforms for oscillating and rotating
stars currently contain errors of magnitude in the local wave
zone. Comparing the waveforms obtained by the gauge-invariant technique with
those by the quadrupole formula, it is shown that the quadrupole formula yields
approximate gravitational waveforms besides a systematic underestimation of the
amplitude of where and denote the mass and the radius of
neutron stars. However, the wave phase and modulation of the amplitude can be
computed accurately. This indicates that the quadrupole formula is a useful
tool for studying gravitational waves from rotating stellar core collapse to a
neutron star in fully general relativistic simulations. Properties of the
gravitational waveforms from the oscillating and rigidly rotating neutron stars
are also addressed paying attention to the oscillation associated with
fundamental modes
Axisymmetric general relativistic hydrodynamics: Long-term evolution of neutron stars and stellar collapse to neutron stars and black holes
We report a new implementation for axisymmetric simulation in full general
relativity. In this implementation, the Einstein equations are solved using the
Nakamura-Shibata formulation with the so-called cartoon method to impose an
axisymmetric boundary condition, and the general relativistic hydrodynamic
equations are solved using a high-resolution shock-capturing scheme based on an
approximate Riemann solver. As tests, we performed the following simulations:
(i) long-term evolution of non-rotating and rapidly rotating neutron stars,
(ii) long-term evolution of neutron stars of a high-amplitude damping
oscillation accompanied with shock formation, (iii) collapse of unstable
neutron stars to black holes, and (iv) stellar collapses to neutron stars. The
tests (i)--(iii) were carried out with the -law equation of state, and
the test (iv) with a more realistic parametric equation of state for
high-density matter. We found that this new implementation works very well: It
is possible to perform the simulations for stable neutron stars for more than
10 dynamical time scales, to capture strong shocks formed at stellar core
collapses, and to accurately compute the mass of black holes formed after the
collapse and subsequent accretion. In conclusion, this implementation is robust
enough to apply to astrophysical problems such as stellar core collapse of
massive stars to a neutron star and black hole, phase transition of a neutron
star to a high-density star, and accretion-induced collapse of a neutron star
to a black hole. The result for the first simulation of stellar core collapse
to a neutron star started from a realistic initial condition is also presented.Comment: 28 pages, to appear in PRD 67, 0440XX (2003
Thermodynamic properties of the one-dimensional Kondo insulators studied by the density matrix renormalization group method
Thermodynamic properties of the one-dimensional Kondo lattice model at
half-filling are studied by the density matrix renormalization group method
applied to the quantum transfer matrix. Spin susceptibility, charge
susceptibility, and specific heat are calculated down to T=0.1t for various
exchange constants. The obtained results clearly show crossover behavior from
the high temperature regime of nearly independent localized spins and
conduction electrons to the low temperature regime where the two degrees of
freedom couple strongly. The low temperature energy scales of the charge and
spin susceptibilities are determined and shown to be equal to the quasiparticle
gap and the spin gap, respectively, for weak exchange couplings.Comment: 4 pages, 3 Postscript figures, REVTeX, submitted to J. Phys. Soc. Jp
A relativistic formalism for computation of irrotational binary stars in quasi equilibrium states
We present relativistic hydrostatic equations for obtaining irrotational
binary neutron stars in quasi equilibrium states in 3+1 formalism. Equations
derived here are different from those previously given by Bonazzola,
Gourgoulhon, and Marck, and have a simpler and more tractable form for
computation in numerical relativity. We also present hydrostatic equations for
computation of equilibrium irrotational binary stars in first post-Newtonian
order.Comment: 5 pages, corrected eqs.(2.10), (2.11) and (3.1
Lorentz Covariance and the Dimensional Crossover of 2d-Antiferromagnets
We derive a lattice -function for the 2d-Antiferromagnetic Heisenberg
model, which allows the lattice interaction couplings of the nonperturbative
Quantum Monte Carlo vacuum to be related directly to the zero-temperature fixed
points of the nonlinear sigma model in the presence of strong interplanar and
spin anisotropies. In addition to the usual renormalization of the gapful
disordered state in the vicinity of the quantum critical point, we show that
this leads to a chiral doubling of the spectra of excited states
Black hole tidal problem in the Fermi normal coordinates
We derive a tidal potential for a self-gravitating fluid star orbiting Kerr
black hole along a timelike geodesic extending previous works by Fishbone and
Marck. In this paper, the tidal potential is calculated up to the third and
fourth-order terms in , where is the stellar radius and the
orbital separation, in the Fermi-normal coordinate system following the
framework developed by Manasse and Misner. The new formulation is applied for
determining the tidal disruption limit (Roche limit) of corotating Newtonian
stars in circular orbits moving on the equatorial plane of Kerr black holes. It
is demonstrated that the third and fourth-order terms quantitatively play an
important role in the Roche limit for close orbits with R/r \agt 0.1. It is
also indicated that the Roche limit of neutron stars orbiting a stellar-mass
black hole near the innermost stable circular orbit may depend sensitively on
the equation of state of the neutron star.Comment: Correct typo
Thermodynamics of doped Kondo insulator in one dimension: Finite Temperature DMRG Study
The finite-temperature density-matrix renormalization-group method is applied
to the one-dimensional Kondo lattice model near half filling to study its
thermodynamics. The spin and charge susceptibilities and entropy are calculated
down to T=0.03t. We find two crossover temperatures near half filling. The
higher crossover temperature continuously connects to the spin gap at half
filling, and the susceptibilities are suppressed around this temperature. At
low temperatures, the susceptibilities increase again with decreasing
temperature when doping is finite. We confirm that they finally approach to the
values obtained in the Tomonaga-Luttinger (TL) liquid ground state for several
parameters. The crossover temperature to the TL liquid is a new energy scale
determined by gapless excitations of the TL liquid. The transition from the
metallic phase to the insulating phase is accompanied by the vanishing of the
lower crossover temperature.Comment: 4 pages, 7 Postscript figures, REVTe
Possible explanation for star-crushing effect in binary neutron star simulations
A possible explanation is suggested for the controversial star-crushing
effect seen in numerical simulations of inspiraling neutron star binaries by
Wilson, Mathews and Marronetti (WMM). An apparently incorrect definition of
momentum density in the momentum constraint equation used by WMM gives rise to
a post-1-Newtonian error in the approximation scheme. We show by means of an
analytic, post-1-Newtonian calculation that this error causes an increase of
the stars' central densities which is of the order of several percent when the
stars are separated by a few stellar radii, in agreement with what is seen in
the simulations.Comment: 4 pages, 1 figure, uses revetx macros, minor revision
Mergers of binary stars: The ultimate heavy-ion experience
The mergers of black hole-neutron star binaries are calcuated using a
pseudo-general relativistic potential that incorporates post-Newtonian corrections. Both normal matter neutron stars and
self-bound strange quark matter stars are considered as black hole partners. As
long as the neutron stars are not too massive relative to the black hole mass,
orbital decay terminates in stable mass transfer rather than an actual merger.
For a normal neutron star, mass transfer results in a widening of the orbit but
the stable transfer ends before the minimum neutron star mass is reached. For a
strange star, mass transfer does not result in an appreciable enlargement of
the orbital separation, and the stable transfer continues until the strange
star essentially disappears. These differences might be observable through
their respective gravitational wave signatures.Comment: Contribution to QM04 proceedings. Submitted to Journal of Physics
Real Space Effective Interaction and Phase Transition in the Lowest Landau Level
The transition between the stripe state and the liquid state in a high
magnetic field is studied by the density-matrix renormalization-group (DMRG)
method. Systematic analysis on the ground state of two-dimensional electrons in
the lowest Landau level shows that the transition from the stripe state to the
liquid state at v=3/8 is caused by a reduction of repulsive interaction around
r=3. The same reduction of the interaction also stabilizes the incompressible
liquid states at v=1/3 and 2/5, which shows a similarity between the two liquid
states at v=3/8 and 1/3. It is also shown that the strong short-range
interaction around r=1 in the lowest Landau level makes qualitatively different
stripe correlations compared with that in higher Landau levels.Comment: 5 pages, to appear in J. Phys. Soc. Jpn. Vol.73, No.8 (2004
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