Rodrigo Negreiros

Electrically charged strange quark stars

The possible existence of compact stars made of absolutely stable strange quark matter--referred to as strange stars--was pointed out by Witten almost a quarter of a century ago. One of the most amazing features of such objects concerns the possible existence of ultrastrong electric fields on their surfaces, which, for ordinary strange matter, is around 10{sup 18} V/cm. If strange matter forms a color superconductor, as expected for such matter, the strength of the electric field may increase to values that exceed 10{sup 19} V/cm. The energy density associated with such huge electric fields is on the same order of magnitude as the energy density of strange matter itself, which, as shown in this paper, alters the masses and radii of strange quark stars at the 15% and 5% levels, respectively. Such mass increases facilitate the interpretation of massive compact stars, with masses of around 2M{sub {center_dot}}, as strange quark stars.

Hybrid stars in a strong magnetic field

We study the effects of high magnetic fields on the particle population and equation of state of hybrid stars using an extended hadronic and quark SU(3) non-linear realization of the sigma model. In this model the degrees of freedom change naturally from hadrons to quarks as the density and/or temperature increases. The effects of high magnetic fields and anomalous magnetic moment are visible in the macroscopic properties of the star, such as mass, adiabatic index, moment of inertia, and cooling curves. Moreover, at the same time that the magnetic fields become high enough to modify those properties, they make the star anisotropic.

Neutron Star Interiors and the Equation of State of Superdense Matter

There has been much recent progress in our understanding of quark matter, culminating in the discovery that if such matter exists in the cores of neutron stars it ought to be in a color superconducting state. This paper explores the impact of superconducting quark matter on the properties (e.g., masses, radii, surface gravity, photon emission) of compact stars.

ULTRA-DENSE NEUTRON STAR MATTER, STRANGE QUARK STARS, AND THE NUCLEAR EQUATION OF STATE

With central densities way above the density of atomic nuclei, neutron stars contain matter in one of the densest forms found in the universe. Depending of the density reached in the cores of neutron stars, they may contain stable phases of exotic matter found nowhere else in space. This article gives a brief overview of the phases of ultradense matter predicted to exist deep inside neutron stars and discusses the equation of state (EoS) associated with such matter.

Impact of rotation-driven particle repopulation on the thermal evolution of pulsars

Abstract Driven by the loss of energy, isolated rotating neutron stars (pulsars) are gradually slowing down to lower frequencies, which increases the tremendous compression of the matter inside of them. This increase in compression changes both the global properties of rotating neutron stars as well as their hadronic core compositions. Both effects may register themselves observationally in the thermal evolution of such stars, as demonstrated in this Letter. The rotation-driven particle process which we consider here is the direct Urca (DU) process, which is known to become operative in neutron stars if the number of protons in the stellar core exceeds a critical limit of around 11% to 15%. We find that neutron stars spinning down from moderately high rotation rates of a few hundred Hertz may be creating just the right conditions where the DU process becomes operative, leading to an observable effect (enhanced cooling) in the temperature evolution of such neutron stars. As it turns out, the rotation-driven DU process could explain the unusual temperature evolution observed for the neutron star in Cas A, provided the mass of this neutron star lies in the range of 1.5 to 1.9 M ⊙ and its rotational frequency at birth was between 40 (400 Hz) and 70% (800 Hz) of the Kepler (mass shedding) frequency, respectively.

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 the microscopic properties of the quark core are to the thermal evolution of the star. In order to do that we use a simple Massachusetts Institute of Technology (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 those of observed cooling neutron stars. Our results can be used to obtain clues regarding the properties of the quark core in hybrid stars and to refine more sophisticated models for the equation of state of quark matter.

Meissner effect and vortex expulsion in color-superconducting quark stars, and its role for reheating of magnetars

Compact stars made of quark matter, rather than confined hadronic matter, are expected to form a color superconductor. This superconductor ought to be threaded with rotational-vortex lines, within which the stars interior magnetic field is at least partially confined. The vortices (and thus magnetic flux) would be expelled from the star during stellar spin-down, leading to magnetic reconnection at the surface of the star and the prolific production of thermal energy. In this paper, we show that this energy release can reheat quark stars to exceptionally high temperatures, such as observed for soft gamma repeaters, anomalous x-ray pulsars, and x-ray dim isolated neutron stars. Moreover, our numerical investigations of the temperature evolution, spin-down rate, and magnetic field behavior of such superconducting quark stars suggest that soft gamma repeaters, anomalous x-ray pulsars, and x-ray dim isolated neutron stars may be linked ancestrally. Finally, we discuss the possibility of a time delay before the star enters the color-superconducting phase, which can be used to estimate the density at which quarks deconfine. From observations, we find this density to be of the order of 5 times that of nuclear saturation.

Thermal evolution of neutron stars in two dimensions

Department of Physics, San Diego State University, 5500 Campanile Drive, San Diego, California 92182, USA(Received 3 February 2012; published 9 May 2012)There are manyfactors that contribute to the breaking of the spherical symmetry of a neutron star. Mostnotable are rotation, magnetic fields, and/or accretion of matter from companion stars. All thesephenomena influence the macroscopic structures of neutron stars, but also impact their microscopiccompositions. The purpose of this paper is to investigate the cooling of rotationally deformed, two-dimensional (2D) neutron stars in the framework of general relativity theory, with the ultimate goal ofbetter understanding the impact of 2D effects on the thermal evolution of such objects. The equations thatgovern the thermal evolution of rotating neutron stars are presented in this paper. The cooling of neutronstars with different frequencies is computed self-consistently by combining a fully general relativistic 2Drotation codewith a general relativistic 2D cooling code. We show that rotation can significantly influencethe thermal evolution of rotating neutron stars. Among the major new aspects are the appearances of hotspots onthe poles,and anincrease of the thermalcoupling times between thecore andthe crustof rotatingneutron stars. We show that this increase is independent of the microscopic properties of the stellar core,but depends only on the frequency of the star.

On the Transport Properties of a Quark-Hadron Coulomb Lattice in the Cores of Neutron Stars

Already more that 40 years ago, it has been suggested that because of the enormous mass densities in the cores of neutron stars, the hadrons in the centers of neutron stars may undergo a phase transition to deconfined quark matter. In this picture, neutron stars could contain cores made of pure (up, down, strange) quark matter which are surrounded by a mixed phase of quarks and hadrons. More than that, because of the competition between the Coulomb and the surface energies associated with the positively charged regions of nuclear matter and negatively charged regions of quark matter, the mixed phase may develop geometrical structures, similarly to what is expected of the sub-nuclear liquid-gas phase transition. In this paper we restrict ourselves to considering the formation of rare phase blobs in the mixed quark-hadron phase. The influence of rare phase blobs on the thermal and transport properties of neutron star matter is investigated. The total specific heat,

The Effects of Charge On The Structure of Strange Stars

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