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S. S. Avancini

Publications and source records attributed to S. S. Avancini.

At least 19 recordsLinked to original sources

Pairing effects in nuclear pasta phase within the relativistic Thomas-Fermi formalism

Pairing effects in non-uniform nuclear matter, surrounded by electrons, are studied in the protoneutron star early stage and in other conditions. The so-called nuclear pasta phases at subsaturation densities are solved in a Wigner-Seitz cell, within the Thomas-Fermi approximation. The solution of this problem is important for the understanding of the physics of a newly born neutron star after a supernova explosion. It is shown that the pasta phase is more stable than uniform nuclear matter on some conditions and the pairing force relevance is studied in the determination of these stable phases.

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The Magnetic Field Distribution in Strongly Magnetized Neutron Stars

In this work, we expand on a previously reported realistic calculation of the magnetic field profile for the equation of state inside strongly magnetized neutron stars. In addition to showing that magnetic fields increase quadratically with increasing baryon chemical potential of magnetized matter (instead of exponentially, as previously assumed), we show here that the magnetic field increase with baryon number density is more complex and harder to model. We do so by the analysis of several different realistic models for the microscopic description of matter in the star (including hadronic, hybrid and quark models) combined with general relativistic solutions by solving Einstein-Maxwell's field equations in a self-consistent way for stars endowed with a poloidal magnetic field.

astro-ph.HE

What is the magnetic field distribution for the equation of state of magnetized neutron stars?

In this Letter, we report a realistic calculation of the magnetic field profile for the equation of state inside strongly magnetized neutron stars. Unlike previous estimates, which are widely used in the literature, we find that magnetic fields increase relatively slowly with increasing baryon chemical potential (or baryon density) of magnetized matter. More precisely, the increase is polynomial instead of exponential, as previously assumed. Through the analysis of several different realistic models for the microscopic description of stellar matter (including hadronic, hybrid and quark models) combined with general relativistic solutions endowed with a poloidal magnetic field obtained by solving Einstein-Maxwell's field equations in a self-consistent way, we generate a phenomenological fit for the magnetic field distribution in the stellar polar direction to be used as input in microscopic calculations.

astro-ph.HE

Thermo-magnetic effects in quark matter: Nambu--Jona-Lasinio model constrained by lattice QCD

The phenomenon of inverse magnetic catalysis of chiral symmetry in QCD predicted by lattice simulations can be reproduced within the Nambu--Jona-Lasinio model if the coupling~$G$ of the model decreases with the strength $B$ of the magnetic field and temperature~$T$. The thermo-magnetic dependence of $G(B,T)$ is obtained by fitting recent lattice QCD predictions for the chiral transition order parameter. Different thermodynamic quantities of magnetized quark matter evaluated with $G(B, T)$ are compared with the ones obtained at constant coupling, $G$. The model with $G(B,T)$ predicts a more dramatic chiral transition as the field intensity increases. In addition, the pressure and magnetization always increase with $B$ for a given temperature. Being parametrized by four magnetic field dependent coefficients and having a rather simple exponential thermal dependence our accurate ansatz for the coupling constant can be easily implemented to improve typical model applications to magnetized quark matter.

hep-ph

(No) neutron star maximum mass constraint from hypernuclei

(Abridged) The recent measurement of the mass of two $2\, M_\odot$ pulsars has raised the question whether such large masses allow for the existence of exotic degrees of freedom, such as hyperons, inside neutron stars. In the present work we will investigate how the existing hypernuclei properties may constrain the neutron star equation of state and confront the neutron star maximum masses obtained with equations of state calibrated to hypernuclei properties with the astrophysical $2\,M_\odot$ constraint. The study is performed using a relativistic mean field approach to describe both the hypernuclei and the neutron star equations of state. A set of five models consistent with $2\,M_\odot$ for a purely nucleonic composition are employed. The $Λ$-meson couplings are determined for all the models considered. Hyperonic stars with the complete baryonic octet are studied, restricting the coupling of the $Σ$ and $Ξ$ hyperons to the $ω-$, $ρ-$ and $σ-$mesons due to the lack of experimental data, and maximum star masses calculated for unified equations of state. We conclude that the currently available hypernuclei experimental data and the lack of constraints on the asymmetric equation of state of nuclear matter at high densities do not allow to further constrain the neutron star matter equation of state using the recent $2\, M_\odot$ observations. It is also shown that the $Λ$ potential in symmetric nuclear matter takes a value $\sim 30-32$ MeV at saturation for the $g_{ωΛ}$ coupling given by the SU(6) symmetry, close to values generally used in the literature. However, the $Λ$ potential in $Λ$ matter varies between -16 and -8 MeV taking for vector mesons couplings the SU(6) values, at variance with generally employed values between $-1$ and $-5$ MeV.

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Neutrino diffusion in the pasta phase matter within the Thomas-Fermi approach

The behavior and properties of neutrinos in non-uniform nuclear matter, surrounded by electrons and other neutrinos are studied. The nuclear matter itself is modeled by the non-linear Walecka model, where the so-called nuclear pasta phase is described using the Thomas-Fermi approximation, solved in a Wigner-Seitz cell. We obtain the total cross-section and mean-free path for the neutrinos, taking into account scattering and neutrino absorption, and compare the final results for two known kind of model parametrizations: one in which non-linear effects in the strong sector are explicitly written in the model Lagrangian and another one in which the coupling constants are density dependent. The solution for this problem is important for the understanding of neutrino diffusion in a newly born neutron star after a supernova explosion.

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Relativistic Mean-Field Hadronic Models under Nuclear Matter Constraints

Relativistic mean-field (RMF) models have been widely used in the study of many hadronic frameworks because of several important aspects not always present in nonrelativistic models, such as intrinsic Lorentz covariance, automatic inclusion of spin, appropriate saturation mechanism for nuclear matter, causality and, therefore, no problems related to superluminal speed of sound. With the aim of identifying the models which best satisfy well known properties of nuclear matter, we have analyzed $263$ parameterizations of seven different types of RMF models under three different sets of constraints related to symmetric nuclear matter, pure neutron matter, symmetry energy, and its derivatives. One of these (SET1) is formed of the same constraints used in a recent work [M. Dutra et al., Phys. Rev. C 85, 035201 (2012)] in which we analyzed $240$ Skyrme parameterizations. The results pointed to $2$ models consistent with all constraints. By using another set of constraints, namely, SET2a, formed by the updated versions of the previous one, we found $4$ models approved simultaneously. Finally, in the third set, named SET2b, in which the values of the constraints are more restrictive, we found $3$ consistent models. Another interesting feature of our analysis is that the results change dramatically if we do not consider the constraint regarding the volume part of the isospin incompressibility ($K_{τ,\rm v}$). In this case, we have $35$ approved models in SET2a and $30$ in SET2b.

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Relativistic Mean-Field Models and Nuclear Matter Constraints

This work presents a preliminary study of 147 relativistic mean-field (RMF) hadronic models used in the literature, regarding their behavior in the nuclear matter regime. We analyze here different kinds of such models, namely: (i) linear models, (ii) nonlinear σ^3+σ^4 models, (iii) σ^3+σ^4+ω^4 models, (iv) models containing mixing terms in the fields σand ω, (v) density dependent models, and (vi) point-coupling ones. In the finite range models, the attractive (repulsive) interaction is described in the Lagrangian density by the σ(ω) field. The isospin dependence of the interaction is modeled by the ρmeson field. We submit these sets of RMF models to eleven macroscopic (experimental and empirical) constraints, used in a recent study in which 240 Skyrme parametrizations were analyzed. Such constraints cover a wide range of properties related to symmetric nuclear matter (SNM), pure neutron matter (PNM), and both SNM and PNM.

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Warm pasta phase in the Thomas-Fermi approximation

In the present paper, the pasta phase is studied at finite temperatures within a Thomas-Fermi (TF) approach. Relativistic mean field models, both with constant and density-dependent couplings, are used to describe this frustrated system. We compare the present results with previous ones obtained within a phase-coexistence description and conclude that the TF approximation gives rise to a richer inner pasta phase structure and the homogeneous matter appears at higher densities. Finally, the transition density calculated within TF is compared with the results for this quantity obtained with other methods.

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Light clusters and the pasta phase

The effects of including light clusters in nuclear matter at low densities are investigated within four different parametrizations of relativistic models at finite temperature. Both homogeneous and inhomogeneous matter (pasta phase) are described for neutral nuclear matter with fixed proton fractions. We discuss the effect of the density dependence of the symmetry energy, the temperature and the proton fraction on the non-homogeneous matter forming the inner crust of proto-neutron stars. It is shown that the number of nucleons in the clusters, the cluster proton fraction and the sizes of the Wigner Seitz cell and of the cluster are very sensitive to the density dependence of the symmetry energy.

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$α$ particles and the pasta phase

The effects of the $α$ particles in nuclear matter at low densities are investigated within three different parametrizations of relativistic models at finite temperature. Both homogeneous and inhomogeneous matter (pasta phase) are described for neutral nuclear matter with fixed proton fractions and stellar matter subject to $β$-equilibrium and trapped neutrinos. In homogeneous matter, $α$ particles are only present at densities below 0.02 fm$^{-3}$ and their presence decreases with the increase of the temperature and, for a fixed temperature, the $α$ particle fraction decreases for smaller proton fractions. A repulsive interaction is important to mimic the dissolution of the clusters in homogeneous matter. The effects of the $α$ particles on the pasta structure is very small except close to the critical temperatures and / or proton fractions when it may still predict a pasta phase while no pasta phase would occur in the absence of light clusters. It is shown that for densities above 0.01 fm$^{-3}$ the $α$ particle fraction in the pasta phase is much larger than the $α$ particle fraction in homogeneous matter.

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Quark matter under strong magnetic fields in the su(3) Nambu-Jona-Lasinio Model

In the present work we use the mean field approximation to investigate quark matter described by the su(3) Nambu-Jona-Lasinio model subject to a strong magnetic field. We consider two cases: pure quark matter and quark matter in beta-equilibrium possibly present in magnetars. The results are compared with the ones obtained with the su(2) version of the model. The energy per baryon of magnetized quark matter becomes more bound than nuclear matter made of iron nuclei, for B around 2x10^(19)G. When the su(3) NJL model is applied to stellar matter, the maximum mass configurations are always above 1.45 solar masses and may be as high as 1.86 solar masses for a central magnetic field of 5x10^(18)G. These numbers are within the masses of observed neutron stars.

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The pasta phase within density dependent hadronic models

In the present paper we investigate the onset of the pasta phase with different parametrisations of the density dependent hadronic model and compare the results with one of the usual parametrisation of the non-linear Walecka model. The influence of the scalar-isovector virtual delta meson is shown. At zero temperature two different methods are used, one based on coexistent phases and the other on the Thomas-Fermi approximation. At finite temperature only the coexistence phases method is used. npe matter with fixed proton fractions and in beta-equilibrium are studied. We compare our results with restrictions imposed on the the values of the density and pressure at the inner edge of the crust, obtained from observations of the Vela pulsar and recent isospin diffusion data from heavy-ion reactions, and with predictions from spinodal calculations.

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Quark matter under strong magnetic fields in the Nambu--Jona-Lasinio Model

In the present work we use the large-$N_c$ approximation to investigate quark matter described by the SU(2) Nambu--Jona-Lasinio model subject to a strong magnetic field. The Landau levels are filled in such a way that usual kinks appear in the effective mass and other related quantities. $β$-equilibrium is also considered and the macroscopic properties of a magnetar described by this quark matter is obtained. Our study shows that the magnetar masses and radii are larger if the magnetic field increases but only very large fields ($\ge 10^{18}$ G) affect the EoS in a non negligible way.

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Reexamining the neutron skin thickness within a density dependent hadronic model

We calculate the skin thickness, binding energy, surface energy and density profiles within the Dirac solution of a density dependent hadronic model and compare with the NL3 parametrization results of the non-linear Wallecka model. The asymmetry for polarized electron scattering is also obtained using a partial wave expansion calculation. Results for 208Pb, 40Ca, 48Ca, 66Ni and 90Zr are shown and discussed.

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Center-of-Mass Correction in a Relativistic Hartree Approximation Including the Meson Degrees of Freedom

The Peierls-Yoccoz projection method is used to study the motion of a relativistic system of nucleons interacting with sigma and omega mesons. The nuclear system is described in a mean-field Hartree approach, including explicitly the meson contribution and the formalism is applied to some selected N=Z spherical nuclei. The center-of-mass motion correction makes the system too much bounded, but we show that an appropriate refitting of the model brings the radii and binding energies close to the experimental values.

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Density dependent hadronic models and the relation between neutron stars and neutron skin thickness

In the present work we investigate the main differences in the lead neutron skin thickness, binding energy, surface energy and density profiles obtained with two different density dependent relativistic hadronic models, within the Thomas-Fermi approximation. We show that the asymmetry parameter for low momentum transfer polarized electron scattering is not sensitive to the model parametrization differences.

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Collective modes in relativistic npe matter at finite temperature

Isospin and density waves in neutral neutron-proton-electron (npe) matter are studied within a relativistic mean-field hadron model at finite temperature with the inclusion of the electromagnetic field. The dispersion relation is calculated and the collective modes are obtained. The unstable modes are discussed and the spinodals, which separate the stable from the unstable regions, are shown for different values of the momentum transfer at various temperatures. The critical temperatures are compared with the ones obtained in a system without electrons. The largest critical temperature, 12.39 MeV, occurs for a proton fraction y_p=0.47. For y_p=0.3 we get $T_{cr}$ =5 MeV and for y_p>0.495 $T_cr\lesssim 8$ MeV. It is shown that at finite temperature the distillation effect in asymmetric matter is not so efficient and that electron effects are particularly important for small momentum transfers.

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