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 $\beta$-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