861 research outputs found
Spin-up of the hyperon-softened accreting neutron stars
We study the spin-up of the accreting neutron stars with a realistic
hyperon-softened equation of state. Using precise 2-D calculations we study the
evolutionary tracks of accreting neutron stars in the angular-momentum -
frequency plane. In contrast to the case of spinning-down solitary
radio-pulsars, where a strong back-bending behavior has been observed, we do
not see back-bending phenomenon in the accretion-powered spinning-up case. We
conclude that in the case of accretion-driven spin-up the back-bending is
strongly suppressed by the mass-increase effect accompanying the
angular-momentum increase.Comment: 5 pages, 5 figures, accepted by Astronomy & Astrophysic
Signal of Quark Deconfinement in the Timing Structure of Pulsar Spin-Down
The conversion of nuclear matter to quark matter in the core of a rotating
neutron star alters its moment of inertia. Hence the epoch over which
conversion takes place will be signaled in the spin-down "signal_prl.tex" 581
lines, 22203 characters characteristics of pulsars. We find that an observable
called the braking index should be easily measurable during the transition
epoch and can have a value far removed (by orders of magnitude) from the
canonical value of three expected for magnetic dipole radiation, and may have
either sign. The duration of the transition epoch is governed by the slow loss
of angular momentum to radiation and is further prolonged by the reduction in
the moment of inertia caused by the phase change which can even introduce an
era of spin-up. We estimate that about one in a hundred pulsars may be passing
through this phase. The phenomenon is analogous to ``bachbending'' observed in
the moment of inertia of rotating nuclei observed in the 1970's, which also
signaled a change in internal structure with changing spin.Comment: 5 pages, 4 figures, Revtex. (May 12, 1997, submitted to PRL
Kaon condensation in the quark-meson coupling model and compact stars
The properties of neutron stars constituted of a crust of hadrons and an
internal part of hadrons and kaon condensate are calculated within the
quark-meson-coupling model. We have considered stars with nucleons only in the
hadron phase and also stars with hyperons as well. The results are compared
with the ones obtained from the non-linear Walecka model for the hadronic
phase.Comment: 10 pages, 6 figure
First Order Kaon Condensate
First order Bose condensation in asymmetric nuclear matter and in neutron
stars is studied, with particular reference to kaon condensation. We
demonstrate explicitly why the Maxwell construction fails to assure equilibrium
in multicomponent substances. Gibbs conditions and conservation laws require
that for phase equilibrium, the charge density must have opposite sign in the
two phases of isospin asymmetric nuclear matter. The mixed phase will therefore
form a Coulomb lattice with the rare phase occupying lattice sites in the
dominant phase. Moreover, the kaon condensed phase differs from the normal
phase, not by the mere presence of kaons in the first, but also by a difference
in the nucleon effective masses. The mixed phase region, which occupies a large
radial extent amounting to some kilometers in our model neutron stars, is thus
highly heterogeneous. It should be particularly interesting in connection with
the pulsar glitch phenomenon as well as transport properties.Comment: 25 pagees, 20 figures, Late
Stellar matter in the Quark-Meson-Coupling Model with neutrino trapping
The properties of hybrid stars formed by hadronic and quark matter in
-equilibrium are described by appropriate equations of state (EoS) in
the framework of the quark meson coupling (QMC) model. In the present work we
include the possibility of trapped neutrinos in the equation of state and
obtain the properties of the related hybrid stars. We use the quark meson
coupling model for the hadron matter and two possibilities for the quark matter
phase, namely, the unpaired quark phase and the color-flavor locked phase. The
differences are discussed and a comparison with other relativistic EoS is done.Comment: Reference added, accepted in PR
Kaon Condensation and Dynamical Nucleons in Neutron Stars
We discuss the nature of the kaon condensation phase transition. We find
several features which, if kaons condense in neutron stars, are not only
remarkable, but must surely effect such properties as superfluidity and
transport properties, which in turn are relevant to the glitch phenomenon and
cooling rates of neutron stars. The mixed phase, because of the extensive
pressure range that it spans, will occupy a broad radial extent in a neutron
star. This region is permeated with microscopic drops (and other
configurations) located at lattice sites of one phase immersed in the
background of the other phase. The electric charge on drops is opposite to that
of the background phase {\sl and} nucleons have a mass approximately a factor
two different depending on whether they are in the drops or the background
phase. A large part of the stellar interior has this highly non-homogeneous
structure.Comment: 5 pages, 6 figures, revtex. Physical Review Letters (accepted
Viscous damping of r-modes: Small amplitude instability
We study the viscous damping of r-modes of compact stars and analyze in
detail the regions where small amplitude modes are unstable to the emission of
gravitational radiation. We present general expressions for the viscous damping
times for arbitrary forms of interacting dense matter and derive general
semi-analytic results for the boundary of the instability region. These results
show that many aspects, like in particular the physically important minima of
the instability boundary, are surprisingly insensitive to detailed microscopic
properties of the considered form of matter. Our general expressions are
applied to the cases of hadronic stars, strange stars, and hybrid stars, and we
focus on equations of state that are compatible with the recent measurement of
a heavy compact star. We find that hybrid stars with a sufficiently small core
can "masquerade" as neutron stars and feature an instability region that is
indistinguishable from that of a neutron star, whereas neutron stars with a
core density high enough to allow direct Urca reactions feature a notch on the
right side of the instability region.Comment: 22 pages, 16 figures, published versio
Quark core impact on hybrid star cooling
In this paper we investigate the thermal evolution of hybrid stars, objects
composed of a quark matter core, enveloped by ordinary hadronic matter. Our
purpose is to investigate how important are the microscopic properties of the
quark core to the thermal evolution of the star. In order to do that we use a
simple MIT bag model for the quark core, and a relativistic mean field model
for the hadronic envelope. By choosing different values for the microscopic
parameters (bag constant, strange quark mass, strong coupling constant) we
obtain hybrid stars with different quark core properties. We also consider the
possibility of color superconductivity in the quark core. With this simple
approach, we have found a set of microscopic parameters that lead to a good
agreement with observed cooling neutron stars. Our results can be used to
obtain clues regarding the properties of the quark core in hybrid stars, and
can be used to refine more sophisticated models for the equation of state of
quark matter.Comment: 8 pages, 10 figures. Accepted for publication in Physical Review
Kaons production at finite temperature and baryon density in an effective relativistic mean field model
We investigate the kaons production at finite temperature and baryon density
by means of an effective relativistic mean-field model with the inclusion of
the full octet of baryons. Kaons are considered taking into account of an
effective chemical potential depending on the self-consistent interaction
between baryons. The obtained results are compared with a minimal coupling
scheme, calculated for different values of the anti-kaon optical potential.Comment: 3 pages, contribution presented to the International Conference on
Exotic Atoms and Related Topic
Rotating Neutron Stars in a Chiral SU(3) Model
We study the properties of rotating neutron stars within a generalized chiral
SU(3)-flavor model. The influence of the rotation on the inner structure and
the hyperon matter content of the star is discussed. We calculate the Kepler
frequency and moments of inertia of the neutron star sequences. An estimate for
the braking index of the associated pulsars is given.Comment: 14 pages, 9 figure
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