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Constança Providência

Publications and source records attributed to Constança Providência.

At least 145 records · Page 8Linked to original sources

Nuclear matter EOS with light clusters within the mean-field approximation

The crust of a neutron star is essentially determined by the low-density region ($ρ<ρ_0\approx0.15-0.16\unit{fm}^{-3}$) of the equation of state. At the bottom of the inner crust, where the density is $ρ\lesssim0.1ρ_0$, the formation of light clusters in nuclear matter will be energetically favorable at finite temperature. At very low densities and moderate temperatures, the few body correlations are expected to become important and light nuclei like deuterons, tritons, helions and $α$-particles will form. Due to Pauli blocking, these clusters will dissolve at higher densities $ρ\gtrsim 0.1ρ_0$. The presence of these clusters influences the cooling process and quantities, such as the neutrino emissivity and gravitational waves emission. The dissolution density of these light clusters, treated as point-like particles, will be studied within the Relativistic Mean Field approximation. In particular, the dependence of the dissolution density on the clusters-meson couplings is studied.

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Phase structure of hadronic and Polyakov-loop extended NJL model at finite isospin density

It is believed that there exists a rich phase structure of quantum chromodynamics (QCD) at finite temperature and baryon density, namely, the deconfinement process from hadron gas to quark-gluon plasma, the transition from chiral symmetry breaking phase to the symmetry restoration phase, and the color superconductivity at low temperature and high baryon density. In the present work we study the hadron-quark phase transition by investigating the binodal surface and extending it to finite temperature in order to mimic the QCD phase diagram. In order to obtain these conditions we use different models for the two possible phases, namely the quark and hadron phases. The phase separation boundary (binodal) is determined by the Gibbs criteria for phase equilibrium. The boundaries of the mixed phase and the related critical points for symmetric and asymmetric matter are obtained. Isospin effects appear to be rather significant. The critical endpoint (CEP) and the phase structure are also studied in the Polyakov-loop extended Nambu-Jona-Lasinio model.

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Imprint of the symmetry energy on the inner crust and strangeness content of neutron stars

In this work we study the effect of the symmetry energy on several properties of neutron stars. First, we discuss its effect on the density, proton fraction and pressure of the neutron star crust-core transition. We show that whereas the first two quantities present a clear correlation with the slope parameter $L$ of the symmetry energy, no satisfactory correlation is seen between the transition pressure and $L$. However, a linear combination of the slope and curvature parameters at $ρ=0.1$ fm$^{-3}$ is well correlated with the transition pressure. In the second part we analyze the effect of the symmetry energy on the pasta phase. It is shown that the size of the pasta clusters, number of nucleons and the cluster proton fraction depend on the density dependence of the symmetry energy: a small $L$ gives rise to larger clusters. The influence of the equation of state at subsaturation densities on the extension of the inner crust of the neutron star is also discussed. Finally, the effect of the effect of the density dependence of the symmetry energy on the strangeness content of neutron stars is studied in the last part of the work. It is found that charged (neutral) hyperons appear at smaller (larger) densities for smaller values of the slope parameter $L$. A linear correlation between the radius and the strangeness content of a star with a fixed mass is also found.

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Interplay between the symmetry energy and the strangeness content of neutron stars

The effect of the density dependence of the nucleonic equation of state and the hyperon meson couplings on the star properties, including strangeness content, mass and radius, are studied within a relativistic mean field formalism. It is shown that there is still lacking information on the nucleonic equation of state at supra-saturation densities and on the hyperon interactions in nuclear matter that will allow a clear answer to the question whether the mass of the pulsar J1614-2230 could rule out exotic degrees of freedom from the interior of compact stars. We show that some star properties are affected in a similar way by the density dependence of the symmetry energy and the hyperon content of the star. To disentangle these two effects it is essential to have a good knowledge of the equation of state at supra-saturation densities. A linear correlation between the radius and the strangeness content of a star with a fixed mass is obtained.

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Description of light clusters in relativistic nuclear models

Light clusters are included in the equation of state of nuclear matter within the relativistic mean field theory. The effect of the cluster-meson coupling constants on the dissolution density is discussed. Theoretical and experimental constraints are used to fix the cluster-meson couplings. The relative light cluster fractions are calculated for asymmetric matter in chemical equilibrium at finite temperature. It is found that above T = 5 MeV deuterons and tritons are the clusters in larger abundances. The results do not depend strongly on the relativistic mean field interaction chosen.

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The Inner Crust and its Structure

In this chapter we discuss some possible physical pictures that describe the constitution of the inner crust of compact objects. Different relativistic models both with constant couplings and density dependent ones are used. We calculate the liquid-gas phase transition in asymmetric nuclear matter from the thermodynamic and dynamic instabilities. The equations of state used to describe the crust are related to the crust-core transition properties. Cold and warm pasta phases with and without alpha particles are constructed. The influence of the pasta phase and its internal structure on the diffusion coefficients associated with Boltzman transport equations used to simulate the evolution of protoneutron stars are shown. Finally, the possible existence of bare quark stars and the effects of strong magnetic fields on quark matter are considered. Open questions are pointed out.

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Quark matter nucleation with a microscopic hadronic equation of state

The nucleation process of quark matter in cold (T = 0) stellar matter is investigated using the microscopic Brueckner-Hartree-Fock approach to describe the hadronic phase, and the MIT bag model, the Nambu-Jona-Lasinio, and the Chromo Dielectric models to describe the deconfined phase of quark matter. The consequences of the nucleation process for neutron star physics are outlined. Hyperonic stars are metastable only for some of the quark matter equations of state considered. The effect of an hyperonic three body force on the metastability of compact stars is estimated, and it is shown that, except for the Nambu-Jona-Lasinio model and the MIT bag model with a large bag pressure, the other models predict the formation of hybrid stars with a maximum mass not larger than \sim 1.62 M\odot .

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Compact stars within an asy-soft quark-meson-coupling model

We investigate compact star properties within the quark meson coupling model (QMC) with a soft symmetry energy density dependence at large densities. In particular, the hyperon content and the mass/radius curves for the families of stars obtained within the model are discussed. The hyperon-meson couplings are chosen according to experimental values of the hyperon nuclear matter potentials, and possible uncertainties are considered. It is shown that a softer symmetry energy gives rise to stars with less hyperons, smaller radii and larger masses. Hyperon-meson couplings may also have a strong effect on the mass of the star.

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The neutron star inner crust and symmetry energy

The cell structure of clusters in the inner crust of a cold β-equilibrium neutron star is studied within a Thomas Fermi approach and compared with other approaches which include shell effects. Relativistic nuclear models are considered. We conclude that the symmetry energy slope L may have quite dramatic effects on the cell structure if it is very large or small. Rod-like and slab-like pasta clusters have been obtained in all models except one with a large slope L.

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Chiral model approach to quark matter nucleation in neutron stars

The nucleation process of quark matter in both cold and hot dense hadronic matter is investigated using a chiral approach to describe the quark phase. We use the Nambu-Jona-Lasinio and the Chromo Dielectric models to describe the deconfined phase and the non-linear Walecka model for the hadronic one. The effect of hyperons on the transition phase between hadronic and quark matter is studied. The consequences of the nucleation process for neutron star physics are outlined.

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The QCD Critical End Point Under Strong Magnetic Fields

We use the three-flavor Nambu--Jona-Lasinio model, which includes strangeness and quark physical masses in the mean field approximation, to investigate the influence of strong magnetic fields on the QCD phase diagram covering the whole $T-μ$ plane. It is found that the size of the first order transition line increases as the field strength increases so that a larger coexistence region for hadronic and quark matter should be expected for strong magnetic fields. The location of the critical end point is also affected by the presence of magnetic fields which invariably increase the temperature value at which the first order line terminates. On the other hand, the critical end point chemical potential value displays a subtle oscillation around the B=0 value for magnetic fields within the $10^{17}-10^{20} \, {\rm G}$ range. These findings may have non trivial consequences for the physics of magnetars and heavy ion collisions.

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BCS theory of hadronic matter at high densities

The equilibrium between the so-called 2SC and CFL phases of strange quark matter at high densities is investigated in the framework of a simple schematic model of the NJL type. Equal densities are assumed for quarks $u,d$ and $s$. The 2SC phase is here described by a color-flavor symmetric state, in which the quark numbers are independent of the color-flavor combination. In the CFL phase the quark numbers depend on the color-flavor combination, that is, the number of quarks associated with the color-flavor combinations $ur,dg,sb$ is different from the number of quarks associated with the color flavor combinations $ug,ub,dr,db,sr,sg$. We find that the 2SC phase is stable for a chemical potential $μ$ below $μ_c=0.505$ GeV, while the CFL phase is stable above, the equilibrium pressure being $P_c=0.003$ GeV$^4$. We have used a 3-momentum regularizing cutoff $Λ=0.8$ GeV, which is somewhat larger than is usual in NJL type models. This should be adequate if the relevant chemical potential does not exceed 0.6 GeV.

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Neutron star properties and the symmetry energy

The effect of the symmetry energy on the properties of compact stars is discussed. It is shown that, for stars with masses above 1 $M_\odot$, the radius of the star varies linearly with the symmetry energy slope $L$. The dependence of the hyperon content and onset density of the direct Urca process on the symmetry energy and meson coupling parametrization are also analyzed.

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Core-crust transition in neutron stars: predictivity of density developments

The possibility to draw links between the isospin properties of nuclei and the structure of compact stars is a stimulating perspective. In order to pursue this objective on a sound basis, the correlations from which such links can be deduced have to be carefully checked against model dependence. Using a variety of nuclear effective models and a microscopic approach, we study the relation between the predictions of a given model and those of a Taylor density development of the corresponding equation of state: this establishes to what extent a limited set of phenomenological constraints can determine the core-crust transition properties. From a correlation analysis we show that a) the transition density $ρ_t$ is mainly correlated with the symmetry energy slope $L$, b) the proton fraction $Y_{p,t}$ with the symmetry energy and symmetry energy slope $(J,L)$ defined at saturation density, or, even better, with the same quantities defined at $ρ=0.1$ fm$^{-3}$, and c) the transition pressure $P_t$ with the symmetry energy slope and curvature $(J,K_{\rm sym})$ defined at $ρ=0.1$ fm$^{-3}$.

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Quark matter under strong magnetic field in chiral models

The chiral model is used to describe quark matter under strong magnetic fields and compared to other models, the MIT bag model and the two flavors Nambu-Jona-Lasinio model. The effect of vacuum corrections due to the magnetic field is discussed. It is shown that if the magnetic field vacuum corrections are not taken into account explicitly the parameters of the models should be fitted to low density meson properties in the presence of the magnetic field.

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Hadron-quark phase transition in asymmetric matter with boson condensation

In the present work we study the hadron-quark phase transition with boson condensation in asymmetric matter by investigating the binodal surface and extending it to finite temperature in order to mimic the QCD phase diagram. We consider a system with two conserved charges (isospin and baryon densities) using the Gibbs' criteria for phase equilibrium. In order to obtain these conditions we use two different models for the two possible phases, namely the non-linear Walecka model (NLWM) for the hadron matter (also including hyperons) and the MIT bag model for the quark phase. It is shown that the phase transition is very sensitive to the density dependence of the equation of state and the symmetry energy. For isospin asymmetry of 0.2 and a mixed phase with a fraction of 20% of quarks, a transition density in the interval 2 rho_0 to 4 rho_0 was obtained for temperatures 30 < T < 65 MeV.

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Isospin Constraints on the Parametric Coupling Model for Nuclear Matter

We make use of isospin constraints to study the parametric coupling model and the properties of asymmetric nuclear matter. Besides the usual constraints for nuclear matter - effective nucleon mass and the incompressibility at saturation density - and the neutron star constraints - maximum mass and radius - we have studied the properties related with the symmetry energy. These properties have constrained to a small range the parameters of the model. We have applied our results to study the thermodynamic instabilities in the liquid-gas phase transition as well as the neutron star configurations.

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Color symmetric superconductivity in a phenomenological QCD model

In this paper, we construct a theory of the NJL-type where superconductivity is present, and yet the super-conducting state remains, in the average, color symmetric. This shows that the present approach to color superconductivity is consistent with color singlet-ness. Indeed, quarks are free in the deconfined phase, but the deconfined phase itself is believed to be a color singlet. The usual description of the color superconducting state violates color singlet-ness. On the other hand, the color superconducting state here proposed, is color symmetric in the sense that an arbitrary color rotation leads to an equivalent state, with precisely the same physical properties.

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