arXiv subjects

R. Negreiros

Publications and source records attributed to R. Negreiros.

At least 19 recordsLinked to original sources

arXiv2023

Temperature and Strong Magnetic Field Effects in Dense Matter

We study consistently the effects of magnetic field on hot and dense matter. In particular, we look for differences that arise due to assumptions that reproduce the conditions produced in particle collisions or astrophysical scenarios, such as in the core of fully evolved neutron stars (beyond the protoneutron star stage). We assume the magnetic field to be either constant or follow a profile extracted from general relativity calculations of magnetars and make use of two realistic models that can consistently describe chiral symmetry restoration and deconfinement to quark matter, the Chiral Mean Field (CMF) and the Polyakov-loop extended Nambu-Jona-Lasinio (PNJL) models. We find that net isospin, net strangeness, and weak chemical equilibrium with leptons can considerably change the effects of temperature and magnetic fields on particle content and deconfinement in dense matter. We finish by discussing the possibility of experimentally detecting quark deconfinement in dense and/or hot matter and the possible role played by magnetic fields.

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arXiv2022

Compactness in the Thermal Evolution of Twin Stars

In this work, we study for the first time the thermal evolution of twin star pairs, i.e., stars that present the same mass but different radius and compactness. We collect available equations of state that give origin to a second branch of stable compact stars with quarks in their core. For each equation of state, we investigate the particle composition inside stars and how differently each twin evolves over time, which depends on the central density/pressure and consequent crossing of the threshold for the Urca cooling process. We find that, although the general stellar thermal evolution depends on mass and particle composition, withing one equation of state, only twin pairs that differ considerably on compactness can be clearly distinguished by how they cool down.

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arXiv2021

Effects of Magnetic Fields in Hot White Dwarfs

In this work, we study the effects of temperature on magnetic white dwarfs. We model their interior as a nuclei lattice surrounded by a relativistic free Fermi gas of electrons, accounting for effects from temperature, Landau levels and anomalous magnetic moment. We find that, at low densities (corresponding to the outer regions of star), both temperature and magnetic field effects play an important role in the calculation of microscopic thermodynamical quantities. To study macroscopic stellar structures within a general-relativistic approach, we solve numerically the coupled Einstein's-Maxwell's equations for fixed entropy per particle configurations and discuss how temperature affects stellar magnetic field profiles, masses and radii.

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arXiv2018

Could an hypercritical accretion be associated with the atypical magnetic-field behavior in RX J0822-4300?

Recent X-ray observations in the central compact remnant of Puppis A and other young pulsars have provided convincing evidence about the anti-magnetar nature of the so-called central compact objects (CCOs). The measured period derivative, corrected by the proper movement, provides exceptionally low superficial magnetic fields for such sources. Using the dipole radiation canonical model, the superficial magnetic field of the CCO (RX J0822-4300) in Puppis A was estimated to be $B\simeq 2.9 \times 10^{10}$ G. In this paper, we present a numerical model to interpret the unusual magnetic-field behavior in RX J0822-4300. Using the magnetohydrodynamic simulations during the hypercritical accretion phase, we propose that a variety of MeV neutrinos is created given evidence about the submergence of the magnetic field in the pulsar. We investigate the production, propagation, oscillations, and detection of MeV neutrinos from this source. The detection of neutrino events with their flavor ratios would be a compelling signature of the decreasing evolution of the magnetic field not only in RX J0822-4300 but also in CCO candidates.

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arXiv2018

A hypercritical accretion scenario in Central Compact Objects accompanied with an expected neutrino burst

The measurement of the period and period derivative, and the canonical model of dipole radiation have provided a method to estimate the low superficial magnetic fields in the so-called Central Compact Objects (CCOs). In the present work, a scenario is introduced in order to explain the magnetic behavior of such CCOs. Based on magnetohydrodynamic simulations of the post core-collapse supernova phase during the hypercritical accretion episode, we argue that the magnetic field of a newborn neutron star could have been early buried. During this phase, thermal neutrinos are created mainly by the pair annihilation, plasmon decay, photo-neutrino emission and other processes. We study the dynamics of these neutrinos in this environment and also estimate the number expected of the neutrino events with their flavor ratios on Earth. The neutrino burst is the only viable observable that could provide compelling evidence of the hypercritical phase and therefore, the hidden magnetic field mechanism as the most favorable scenario to explain the anomalous low magnetic fields estimated for CCOs.

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arXiv2017

Deconfinement to Quark Matter in Neutron Stars - The Influence of Strong Magnetic Fields

We use an extended version of the hadronic SU(3) non-linear realization of the sigma model that also includes quarks to study hybrid stars. Within this approach, the degrees of freedom change naturally as the temperature/density increases. Different prescriptions of charge neutrality, local and global, are tested and the influence of strong magnetic fields and the anomalous magnetic moment on the particle population is discussed.

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arXiv2016

Crust effects and the cooling relaxation time in highly magnetized neutron stars

We study the effects of high magnetic fields on the structure and on the geometry of the crust in neutron stars. We find that the crust geometry is substantially modified by the magnetic field inside the star. We build stationary and axis-symmetric magnetized stellar models by using well-known equations of state to describe the neutron star crust, namely the Skyrme model (Sky) for the inner crust and the Baym, Pethick, and Sutherland (BPS) equation of state for the outer crust. We show that the magnetic field has a dual role, contributing to the crust deformation via the electromagnetic interaction (manifested in this case as the Lorentz force) and by contributing to curvature due to the energy stored in it. We also study a direct consequence of the crust deformation due to the magnetic field: the thermal relaxation time. This quantity, which is of great importance to the thermal evolution of neutron stars is sensitive to the crust properties and, as such, we show that it may be strongly affected by the magnetic field.

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arXiv2016

Thermal evolution of hybrid stars within the framework of a nonlocal Nambu--Jona-Lasinio model

We study the thermal evolution of neutron stars containing deconfined quark matter in their core. Such objects are generally referred to as quark-hybrid stars. The confined hadronic matter in their core is described in the framework of non-linear relativistic nuclear field theory. For the quark phase we use a non-local extension of the SU(3) Nambu Jona-Lasinio model with vector interactions. The Gibbs condition is used to model phase equilibrium between confined hadronic matter and deconfined quark matter. Our study indicates that high-mass neutron stars may contain between 35 and 40 % deconfined quark-hybrid matter in their cores. Neutron stars with canonical masses of around $1.4\, M_\odot$ would not contain deconfined quark matter. The central proton fractions of the stars are found to be high, enabling them to cool rapidly. Very good agreement with the temperature evolution established for the neutron star in Cassiopeia A (Cas A) is obtained for one of our models (based on the popular NL3 nuclear parametrization), if the protons in the core of our stellar models are strongly paired, the repulsion among the quarks is mildly repulsive, and the mass of Cas A has a canonical value of $1.4\, M_\odot$.

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arXiv2015

Modelling Hybrid Stars in Quark-Hadron Approaches

The density in the core of neutron stars can reach values of about 5 to 10 times nuclear matter saturation density. It is, therefore, a natural assumption that hadrons may have dissolved into quarks under such conditions, forming a hybrid star. This star will have an outer region of hadronic matter and a core of quark matter or even a mixed state of hadrons and quarks. In order to investigate such phases, we discuss different model approaches that can be used in the study of compact stars as well as being applicable to a wider range of temperatures and densities. One major model ingredient, the role of quark interactions in the stability of massive hybrid stars is discussed. In this context, possible conflicts with lattice QCD simulations are investigated.

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arXiv2015

Reconciling Nuclear and Astrophysical Constraints

In view of new constraints put forth by recent observations and measurements in the realm of astrophysics and nuclear physics, we update the non-linear realization of the sigma model as to reflect such constraints. By doing this, we obtain new equations of state that may be used to describe neutron stars. Such equations of state are obtained by investigating different ways by which the vector mesons self-interact. Furthermore, we also investigate the role played by the delta mesons in the model. As a result, we are able to develop equations of state that are in better agreement with data, such as nuclear compressibility and slope of the symmetry energy at saturation, star masses, radii, and cooling profiles.

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arXiv2015

The role of strangeness in hybrid stars and possible observables

We study the effects of strangeness on the quark sector of a hybrid star equation of state. Since the model we use to describe quarks is the same as the one we use to describe hadrons, we can also study the effects of strangeness on the chiral symmetry restoration and deconfinement phase transitions (first order or crossover). Finally, we analyze combined effects of hyperons and quarks on global properties of hybrid stars, like mass, radius and cooling profiles. It is found that a large amount of strangeness in the core is related to the generation of twin-star solutions, which can have the same mass as the lower or zero strangeness counterpart, but with smaller radii.

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arXiv2014

Thermal evolution of neutron stars with global and local neutrality

Globally neutral neutron stars, obtained from the solution of the called Einstein-Maxwell-Thomas-Fermi equations that account for all the fundamental interactions, have been recently introduced. These configurations have a more general character than the ones obtained with the traditional Tolman-Oppenheimer-Volkoff, which impose the condition of local charge neutrality. The resulting configurations have a less massive and thinner crust, leading to a new mass-radius relation. Signatures of this new structure of the neutron star on the thermal evolution might be a potential test for this theory. We compute the cooling curves by integrating numerically the energy balance and transport equations in general relativity, for globally neutral neutron stars with crusts of different masses and sizes, according to this theory for different core-crust transition interfaces. We compare and contrast our study with known results for local charge neutrality. We found a new behavior for the relaxation time, depending upon the density at the base of the crust, $ρ_{\rm crust}$. In particular, we find that the traditional increase of the relaxation time with the crust thickness holds only for configurations whose density of the base of the crust is greater than $\approx 5\times 10^{13}$ g cm$^{-3}$. The reason for this is that neutron star crusts with very thin or absent inner crust have some neutrino emission process blocked which keep the crust hotter for longer times. Therefore, accurate observations of the thermal relaxation phase of neutron stars might give crucial information on the core-crust transition which may aid us in probing the inner composition/structure of these objects.

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arXiv2014

Dynamical instability of white dwarfs and breaking of spherical symmetry under the presence of extreme magnetic fields

Massive, highly magnetized white dwarfs with fields up to $10^9$ G have been observed and theoretically used for the description of a variety of astrophysical phenomena. Ultramagnetized white dwarfs with uniform interior fields up to $10^{18}$ G, have been recently purported to obey a new maximum mass limit, $M_{\rm max}\approx 2.58~M_\odot$, which largely overcomes the traditional Chandrasekhar value, $M_{\rm Ch}\approx 1.44~M_\odot$. Such a much larger limit would make these astrophysical objects viable candidates for the explanation of the superluminous population of type Ia supernovae. We show that several macro and micro physical aspects such as gravitational, dynamical stability, breaking of spherical symmetry, general relativity, inverse $β$-decay, and pycnonuclear fusion reactions are of most relevance for the self-consistent description of the structure and assessment of stability of these objects. It is shown in this work that the first family of magnetized white dwarfs indeed satisfy all the criteria of stability, while the ultramagnetized white dwarfs are very unlikely to exist in nature since they violate minimal requests of stability. Therefore, the canonical Chandrasekhar mass limit of white dwarfs has to be still applied.

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arXiv2013

Compact Stars - How Exotic Can They Be?

Strong interaction physics under extreme conditions of high temperature and/or density is of central interest in modern nuclear physics for experimentalists and theorists alike. In order to investigate such systems, model approaches that include hadrons and quarks in a unified approach, will be discussed. Special attention will be given to high-density matter as it occurs in neutron stars. Given the current observational limits for neutron star masses, the properties of hyperonic and hybrid stars will be determined. In this context especially the question of the extent, to which exotic particles like hyperons and quarks affect star masses, will be discussed.

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arXiv2013

Modeling Hybrid Stars in Quark-Hadron Approaches

The study of neutron stars, or more general compact stars, is a topic of central interest in nuclear astrophysics. Furthermore, neutron stars serve as the only physical systems whose properties can be used to infer information on cold and dense matter at several times nuclear saturation density. Therefore, neutron star physics is ideally suited to complement the studies of ultra-relativistic heavy-ion collisions that sample strongly interacting matter at high temperature and relatively small net baryon density. In general, in order to pin down or at least constrain the properties of dense matter, accurate measurements of neutron star properties like masses, radii, rotational frequency, and cooling behavior are needed. Here, in relatively recent times the reliable mass determination of the pulsar PSR J1614-2230 of $M = 1.97 \pm 0.04 M_\odot$ has introduced an important benchmark for modeling stars and strongly interacting matter. It puts constraints on the structure of compact stars and possible exotic phases in the core of the stars as will be discussed in this article. In order to investigate this point we will consider a model for star matter that includes hyperonic and quark degrees of freedom, and present results for compact star properties in the following.

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arXiv2013

Deconfinement to Quark Matter in Magnetars

We model magnetars as hybrid stars, which have a core of quark matter surrounded by hadronic matter. For this purpose, we use an extended version of the SU(3) non-linear realization of the sigma model in which the degrees of freedom change naturally from hadrons to quarks as the temperature/density increases. The presence of a variable magnetic field allows us to study in detail the influence of Landau quantization and the anomalous magnetic moment on the particle population of the star, more precisely on particles with different spin projections. This allows us to calculate the polarization of the system throughout different phases of the star, hadronic, quark and also a mixed phase.

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arXiv2013

Hybrid Stars in an SU(3) Parity Doublet Model

We apply an extended version of the SU(3) parity model, containing quark degrees of freedom, to study neutron stars. The model successfully reproduces the main thermodynamic features of QCD which allows us to describe the composition of dense matter. Chiral symmetry restoration is realized inside the star and the chiral partners of the baryons appear, their masses becoming degenerate. Furthermore, quark degrees of freedom appear in a transition to a deconfined state. Performing an investigation of the macroscopic properties of neutron stars, we show that observational constraints, like mass and thermal evolution, are satisfied and new predictions can be made.

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arXiv2012

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.

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