436 research outputs found
Constraining phases of quark matter with studies of r-mode damping in neutron stars
The r-mode instability in rotating compact stars is used to constrain the
phase of matter at high density. The color-flavor-locked phase with kaon
condensation (CFL-K0) and without (CFL) is considered in the temperature range
10^8K < T <10^{11} K. While the bulk viscosity in either phase is only
effective at damping the r-mode at temperatures T > 10^{11} K, the shear
viscosity in the CFL-K0 phase is the only effective damping agent all the way
down to temperatures T > 10^8 K characteristic of cooling neutron stars.
However, it cannot keep the star from becoming unstable to gravitational wave
emission for rotation frequencies f ~ 56-11 Hz at T ~ 10^8-10^9 K. Stars
composed almost entirely of CFL or CFL-K0 matter are ruled out by observation
of rapidly rotating neutron stars, indicating that dissipation at the
quark-hadron interface or nuclear crust interface must play a key role in
damping the instability.Comment: 8 pages, 2 figure
Innermost stable circular orbits around magnetized rotating massive stars
In 1998, Shibata and Sasaki [Phys. Rev. D 58, 104011 (1998)] presented an
approximate analytical formula for the radius of the innermost stable circular
orbit (ISCO) of a neutral test particle around a massive, rotating and deformed
source. In the present paper, we generalize their expression by including the
magnetic dipole moment. We show that our approximate analytical formulas are
accurate enough by comparing them with the six-parametric exact solution
calculated by Pach\'on et. al. [Phys. Rev. D 73, 104038 (2006)] along with the
numerical data presented by Berti and Stergioulas [MNRAS 350, 1416 (2004)] for
realistic neutron stars. As a main result, we find that in general, the radius
at ISCO exhibits a decreasing behavior with increasing magnetic field. However,
for magnetic fields below 100GT the variation of the radius at ISCO is
negligible and hence the non-magnetized approximate expression can be used. In
addition, we derive approximate analytical formulas for angular velocity,
energy and angular momentum of the test particle at ISCO.Comment: 8 pages, 3 figure
The runaway instability in general relativistic accretion disks
When an accretion disk falls prey to the runaway instability, a large portion
of its mass is devoured by the black hole within a few dynamical times. Despite
decades of effort, it is still unclear under what conditions such an
instability can occur. The technically most advanced relativistic simulations
to date were unable to find a clear sign for the onset of the instability. In
this work, we present three-dimensional relativistic hydrodynamics simulations
of accretion disks around black holes in dynamical space-time. We focus on the
configurations that are expected to be particularly prone to the development of
this instability. We demonstrate, for the first time, that the fully
self-consistent general relativistic evolution does indeed produce a runaway
instability.Comment: 5 pages, 3 figures, minor corrections to match published version in
MNRAS, +link to animatio
R-mode oscillations of rapidly rotating barotropic stars in general relativity: Analysis by the relativistic Cowling approximation
We develop a numerical scheme for obtaining the -mode oscillations of
rapidly rotating relativistic stars. In the present scheme, we neglect all
metric perturbations and only take account of the dynamics of the fluid in the
background spacetime of the unperturbed star (the relativistic Cowling
approximation). We also assume the star is barotropic, i.e. neutrally stable
against convection under the assumption of adiabatic oscillations. Our
numerical scheme is based on the Yoshida-Eriguchi formulation for the analysis
of the general relativistic f-mode oscillations in the Cowling approximation
and a general relativistic generalization of the
Karino-Yoshida-Yoshida-Eriguchi's numerical scheme for obtaining oscillations
of rapidly rotating Newtonian stars. By this new numerical scheme, frequencies
of the -mode oscillations are obtained as functions of the ratio of the
rotational energy to the absolute value of the gravitational energy
along sequences of polytropic equilibrium stars whose ratio of the pressure to
the total energy density at the center of the star and the polytropic index are
kept constant. It is found that the dimensionless oscillation frequency
is a linearly decreasing function of , where
and are the oscillation frequency and the angular velocity of the star
measured in an inertial frame at spatial infinity. We also find that
oscillation frequencies of the -modes are highly dependent on the
relativistic factor of the star as already found in previous studies in
which the slow rotation approximation has been used. Here and denote
the mass and radius of the star, respectively.Comment: 9 pages, 6 figure
Quark core formation in spinning-down pulsars
Pulsars spin-down due to magnetic torque reducing its radius and increasing
the central energy density. Some pulsar which are born with central densities
close to the critical value of quark deconfinement may undergo a phase
transition and structural re-arrengement. This process may excite oscillation
modes and emmit gravitational waves. We determine the rate of quark core
formation in neutron stars using a realistic population synthesis code.Comment: Proceedings of the 2nd International Workshop on Astronomy and
Relativistic Astrophysics, to appear in IJMP
Electron-positron energy deposition rate from neutrino pair annihilation in the equatorial plane of rapidly rotating neutron and quark stars
The neutrino-antineutrino annihilation into electron-positron pairs near the
surface of compact general relativistic stars could play an important role in
supernova explosions, neutron star collapse, or for close neutron star binaries
near their last stable orbit. General relativistic effects increase the energy
deposition rates due to the annihilation process. We investigate the deposition
of energy and momentum due to the annihilations of neutrinos and antineutrinos
in the equatorial plane of the rapidly rotating neutron and quark stars,
respectively. We analyze the influence of general relativistic effects, and we
obtain the general relativistic corrections to the energy and momentum
deposition rates for arbitrary stationary and axisymmetric space-times. We
obtain the energy and momentum deposition rates for several classes of rapidly
rotating neutron stars, described by different equations of state of the
neutron matter, and for quark stars, described by the MIT bag model equation of
state and in the CFL (Color-Flavor-Locked) phase, respectively. Compared to the
Newtonian calculations, rotation and general relativistic effects increase the
total annihilation rate measured by an observer at infinity. The differences in
the equations of state for neutron and quark matter also have important effects
on the spatial distribution of the energy deposition rate by
neutrino-antineutrino annihilation.Comment: 29 pages, 6 figures, accepted for publication in MNRA
Jacobi-like bar mode instability of relativistic rotating bodies
We perform some numerical study of the secular triaxial instability of
rigidly rotating homogeneous fluid bodies in general relativity. In the
Newtonian limit, this instability arises at the bifurcation point between the
Maclaurin and Jacobi sequences. It can be driven in astrophysical systems by
viscous dissipation. We locate the onset of instability along several constant
baryon mass sequences of uniformly rotating axisymmetric bodies for compaction
parameter . We find that general relativity weakens the Jacobi
like bar mode instability, but the stabilizing effect is not very strong.
According to our analysis the critical value of the ratio of the kinetic energy
to the absolute value of the gravitational potential energy for compaction parameter as high as 0.275 is only 30% higher than the
Newtonian value. The critical value of the eccentricity depends very weakly on
the degree of relativity and for is only 2% larger than the
Newtonian value at the onset for the secular bar mode instability. We compare
our numerical results with recent analytical investigations based on the
post-Newtonian expansion.Comment: 15 pages, 8 figures, submitted to Phys. Rev.
Perturbative approach to the structure of rapidly rotating neutron stars
We construct models of rotating stars using the perturbative approach
introduced by J. Hartle in 1967, and a set of equations of state proposed to
model hadronic interactions in the inner core of neutron stars. We integrate
the equations of stellar structure to third order in the angular velocity and
show, comparing our results to those obtained with fully non linear codes, to
what extent third order corrections are needed to accurately reproduce the
moment of inertia of a star which rotates at rates comparable to that of the
fastest isolated pulsars.Comment: 17 pages, 5 figures, minor changes to match version accepted by Phys.
Rev.
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