855 research outputs found
Spin-Down of Neutron Stars and Compositional Transitions in the Cold Crustal Matter
Transitions of nuclear compositions in the crust of a neutron star induced by
stellar spin-down are evaluated at zero temperature. We construct a
compressible liquid-drop model for the energy of nuclei immersed in a neutron
gas, including pairing and shell correction terms, in reference to the known
properties of the ground state of matter above neutron drip density, . Recent experimental values and extrapolations of
nuclear masses are used for a description of matter at densities below neutron
drip. Changes in the pressure of matter in the crust due to the stellar
spin-down are calculated by taking into account the structure of the crust of a
slowly and uniformly rotating relativistic neutron star. If the initial
rotation period is ms, these changes cause nuclei, initially being in
the ground-state matter above a mass density of about , to absorb neutrons in the equatorial region where the matter
undergoes compression, and to emit them in the vicinity of the rotation axis
where the matter undergoes decompression. Heat generation by these processes is
found to have significant effects on the thermal evolution of old neutron stars
with low magnetic fields; the surface emission predicted from this heating is
compared with the observations of X-ray emission from millisecond
pulsars and is shown to be insufficient to explain the observed X-ray
luminosities.Comment: 32 pages, LaTeX, 11 Postscript figures. Accepted for publication in
Ap
Inertial modes in stratified rotating neutron stars : An evolutionary description
With (non-barotropic) equations of state valid even when the neutron, proton
and electron content of neutron star cores is not in beta equilibrium, we study
inertial and composition gravity modes of relativistic rotating neutron stars.
We solve the relativistic Euler equations in the time domain with a three
dimensional numerical code based on spectral methods, in the slow rotation,
relativistic Cowling and anelastic approximations. Principally, after a short
description of the gravity modes due to smooth composition gradients, we focus
our analysis on the question of how the inertial modes are affected by
non-barotropicity of the nuclear matter. In our study, the deviation with
respect to barotropicity results from the frozen composition of non-superfluid
matter composed of neutrons, protons and electrons, when beta equilibrium is
broken by millisecond oscillations. We show that already for moderatly fast
rotating stars the increasing coupling between polar and axial modes makes
those two cases less different than for very slowly rotating stars. In
addition, as we directly solve the Euler equations, without coupling only a few
number of spherical harmonics, we always found, for the models that we use, a
discrete spectrum for the inertial mode. Finally, we find that, for
non-barotropic stars, the frequency of this mode, which is our main focus,
decreases in a non-negligible way, whereas the time dependence of the energy
transfer between polar and axial modes is substantially different due to the
existence of low-frequencies gravity modes.Comment: 34 pages, 24 figures, published versio
Dynamical stability of strange quark stars
We show that the mass-radius
relation corresponding to the MIT bag models of strange quark matter
(SQM) and the models obtained by Day et al (1998) do not provide the necessary
and sufficient condition for dynamical stability for the equilibrium
configurations, since such configurations can not even fulfill the necessary
condition of hydrostatic equilibrium provided by the exterior Schwarzschild
solution. These findings will remain unaltered and can be extended to any other
sequence of pure SQM. This study explicitly show that although the strange
quark matter might exist in the state of zero pressure and temperature, but the
models of pure strange quark `stars' can not exist in the state of hydrostatic
equilibrium on the basis of General Relativity Theory. This study can affect
the results which are claiming that various objects like - RX J1856.5-3754, SAX
J1808.4-3658, 4U 1728-34, PSR 0943+10 etc. might be strange stars.Comment: 7 pages (including 6 tables and 1 figure) in MNRAS styl
In-medium enhancement of the modified Urca neutrino reaction rates
We calculate modified Urca neutrino emission rates in the dense nuclear
matter in neutron star cores. We find that these rates are strongly enhanced in
the beta-stable matter in regions of the core close to the direct Urca process
threshold. This enhancement can be tracked to the use of the in-medium nucleon
spectrum in the virtual nucleon propagator. We describe the in-medium nucleon
scattering in the non-relativistic Bruckner-Hartree-Fock framework taking into
account two-body as well as the effective three-body forces, although the
proposed enhancement does not rely on a particular way of the nucleon
interaction treatment. Finally we suggest a simple approximate expression for
the emissivity of the n-branch of the modified Urca process that can be used in
the neutron stars cooling simulations with any nucleon equation of state of
dense matter.Comment: 8 pages, 3 figures; accepted for publication in PLB. In v.2 misprint
in eq.(9) corrected and discussion of cooling curves expande
Gravitational waves from pulsations of neutron stars described by realistic Equations of State
In this work we discuss the time-evolution of nonspherical perturbations of a
nonrotating neutron star described by a realistic Equation of State (EOS). We
analyze 10 different EOS for a large sample of neutron star models. Various
kind of generic initial data are evolved and the corresponding gravitational
wave signals are computed. We focus on the dynamical excitation of fluid and
spacetime modes and extract the corresponding frequencies. We employ a
constrained numerical algorithm based on standard finite differencing schemes
which permits stable and long term evolutions. Our code provides accurate
waveforms and allows to capture, via Fourier analysis of the energy spectra,
the frequencies of the fluid modes with an accuracy comparable to that of
frequency domain calculations. The results we present here are useful for
provindig comparisons with simulations of nonlinear oscillations of (rotating)
neutron star models as well as testbeds for 3D nonlinear codes.Comment: 17 pages, 9 figures. Small changes. Version published in Phys. Rev.
Structure of the electrospheres of bare strange stars
We consider a thin ( fm) layer of electrons (the
electrosphere) at the quark surface of a bare strange star, taking into account
the surface effects at the boundary with the vacuum. The quark surface holds
the electron layer by an extremely strong electric field, generated in the
electrosphere to prevent the electrons from escaping to infinity by
counterbalancing the degeneracy and thermal pressure. Because of the surface
tension and depletion of quarks a very thin (a few fm) charged layer of
quarks forms at the surface of the star. The formation of this layer modifies
the structure of the electrosphere, by significantly changing the electric
field and the density of the electrons, in comparison with the case when the
surface effects are ignored. Some consequences of the modification of the
electrosphere structure on the properties of strange stars are briefly
discussed.Comment: 23 pages, 6 figures, to appear in Ap
Mass, radius, and composition of the outer crust of nonaccreting cold neutron stars
The properties and composition of the outer crust of nonaccreting cold
neutron stars are studied by applying the model of Baym, Pethick, and
Sutherland, which was extended by including higher order corrections of the
atomic binding, screening, exchange and zero-point energy. The most recent
experimental nuclear data from the atomic mass table of Audi, Wapstra, and
Thibault from 2003 is used. Extrapolation to the drip line is utilized by
various state-of-the-art theoretical nuclear models (finite range droplet,
relativistic nuclear field and non-relativistic Skyrme Hartree-Fock
parameterizations). The different nuclear models are compared with respect to
the mass and radius of the outer crust for different neutron star
configurations and the nuclear compositions of the outer crust.Comment: 5 pages, 2 figures, submitted to J. Phys. G, part of the proceedings
of the Nuclear Physics in Astrophysics III conference in Dresde
Nucleation of quark matter in neutron stars cores
We consider the general conditions of quark droplets formation in high
density neutron matter. The growth of the quark bubble (assumed to contain a
sufficiently large number of particles) can be described by means of a
Fokker-Planck equation. The dynamics of the nucleation essentially depends on
the physical properties of the medium it takes place. The conditions for quark
bubble formation are analyzed within the frameworks of both dissipative and
non-dissipative (with zero bulk and shear viscosity coefficients) approaches.
The conversion time of the neutron star to a quark star is obtained as a
function of the equation of state of the neutron matter and of the microscopic
parameters of the quark nuclei. As an application of the obtained formalism we
analyze the first order phase transition from neutron matter to quark matter in
rapidly rotating neutron stars cores, triggered by the gravitational energy
released during the spinning down of the neutron star. The endothermic
conversion process, via gravitational energy absorption, could take place, in a
very short time interval, of the order of few tens seconds, in a class of dense
compact objects, with very high magnetic fields, called magnetars.Comment: 31 pages, 2 figures, to appear in Ap
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