arXiv subjects

César H. Lenzi

Publications and source records attributed to César H. Lenzi.

At least 19 recordsLinked to original sources

arXiv2026

Bayesian approach study of hybrid neutron stars

In this work, we explore how astronomical observations (specifically measurements of masses and radii) can constrain the presence of quark matter inside neutron stars, namely the phase transition from nuclear matter to deconfined quark matter. Our approach employs Bayesian analysis to study this constraint. In this sense to model Hadronic matter we used the relativistic mean-field approximation, for which we have selected two parameter sets: $NL3^{*}ωρ$, representing hadronic matter with nucleons only, and $EL3ωρ$ with nucleons only and $EL3ωρY$, which includes hyperons. On the other hand deconfined quark matter is modeled using the vector-MIT bag model. Also, for our purpose, the first order phase transition is implemented using the Maxwell construction. Bayesian inference is performed by optimizing three parameters: the bag constant (i.e. $B^{1/4}$), the vector coupling constant $\left(G_{v}\right)$, and the Dirac sea contribution ($b_{4}$). We found that a phase transition could exist at densities below $2.0\,n_{0}$ for both the $EL3ωρ$ and $NL3^{*}ωρ$ parametrizations. As a consequence, the inferred parameters of the vector MIT bag model, based on our likelihood function, imply hybrid stars with large quark cores.

nucl-th↗

arXiv2026

Examining the influence of anisotropy on the fundamental mode of nonradial oscillation in neutron stars on a complete general relativistic scheme

The anisotropic influence on the $f$-mode frequency of oscillations and dimensionless tidal deformability of neutron stars is analyzed by employing the nonradial oscillation equations for the complete general relativity frame and tidal deformability equations, which are derived and modified from their standard form to introduce the anisotropic factor. The fluid inside the compact star obeys an equation of state constructed by matching microscopic nuclear and perturbative QCD calculations through a piecewise polytropic interpolating scheme. For the anisotropic profile, we use a local anisotropy which is regular along the whole star and vanishes both at the center and on the star's surface. We show that the $f$-mode oscillation frequency and dimensionless tidal deformability are noticeably affected by anisotropy. Finally, we investigate the correlation between the dimensionless tidal deformability inferred from the GW$170817$ event and the anisotropy parameter.

gr-qc↗

arXiv2026

Reconciling GW170817 and GW190814 with a Nonmonotonic Sound-Speed Equation of State

We show that the GW170817-GW190814 tension can be reconciled within General Relativity by a structured causal EoS basin found with a Constrained Evolutionary TOV Discovery pipeline (CETD). The basin contains ~1.4x10^4 unique EoS with M_max = 2.3-2.8 M_sun, R_1.4 = 11.97-12.29 km, Lambda_1.4 <= 580, and double-peaked sound speeds reaching c_s^2/c^2 = 0.86-0.99.

astro-ph.HE↗

arXiv2026

Certified Uncertainty for Surrogate Models of Neutron Star Equations of State via Mondrian Conformal Prediction

We present a multitask surrogate for neutron-star equations of state (EoSs) that delivers \emph{distribution-free}, certified uncertainty via split conformal prediction (CP) and its Mondrian variant. The surrogate ingests a six-parameter piecewise-polytropic representation $(\log_{10}p_1,Γ_1,Γ_2,Γ_3,ρ_1,ρ_2)$ -- with fixed transition densities $ρ_1$ and $ρ_2$ -- and jointly performs (i) validity classification under physical/observational constraints and (ii) regression of $M_{\max}$, $R(M_{\max})$, $R_{1.4}$, and $Λ_{1.4}$. Trained on a balanced set of $40{,}000$ EoSs, the model attains near-perfect discrimination (AUC $\approx 0.997$) and sub-percent relative errors for masses and radii, with few-percent error for tidal deformability. Across $α\in[0.05,0.25]$, empirical coverages closely track $1-α$ for both Standard and Mondrian CP; in conservative regimes, Mondrian yields narrower average physical widths at comparable coverage. To our knowledge, this is the first application of class-conditioned (Mondrian) conformal calibration to neutron-star EoS surrogates, enabling efficient, reproducible, and uncertainty-aware inference; the framework is readily extensible to functional targets (e.g., full $R(M)$ curves).

astro-ph.HE↗

arXiv2025

Exploring Temperature Influences on Gravitational Wave Production in Binary White Dwarfs

This study investigates the conditions under which gravitational waves (GWs) are emitted during the merger of hot white dwarfs (WDs) in a binary system. Traditionally, these systems consist of two low-mass stars or a more massive WD paired with a less massive companion. In addition, recent work has investigated the possibility that double white dwarf (DWD) mergers are possibly the leading formation channel of massive, rapidly rotating, high-field magnetic WDs , particularly SDSS J221141.80 + 113604.4 (hereafter J2211+1136) and ZTF J190132.9 + 145808.7 (hereafter J1901 + 14588). Motivated by these findings and the Laser Interferometer Space Antenna (LISA) prospects, this study aims to calculate the tidal Love number, the dimensionless tidal deformability, as well as the frequency and amplitude of GWs of hot WDs. The results indicate that the tidal deformability is more pronounced in stars with higher central temperatures and lower masses, which would lead to reduced emission of GWs. In contrast, more massive stars exhibit less deformability, making them prime candidates for generating stronger GWs. Additionally, the analysis of frequency and amplitude reveals that the frequencies of high-mass binaries are smaller and evolve more rapidly, reaching a limit that aligns with the operational detection capabilities of LISA during its initial phase.

astro-ph.SR↗

arXiv2024

Gravitational wave asteroseismology of neutron stars with unified EOS: on the role of high-order nuclear empirical parameters

We analyze the sensitivity of non-radial fluid oscillation modes and tidal deformations in neutron stars to high-order nuclear empirical parameters (NEP). In particular, we study the impact of the curvature and skewness of the symmetry energy $K_{\rm sym}$, $Q_{\rm sym}$, and the skewness of the binding energy in symmetric nuclear matter $Q_{\rm sat}$. As we are interested in the possibility of gravitational wave detection by future interferometers, we consider that the tidal interaction is the driving force for the quadrupolar non-radial fluid oscillations. We have also studied the correlations between those quantities, which will be useful to understand the strong physics of gravitational wave phenomena. Our main results show that $K_{\rm sym}$ impacts the frequencies of the fundamental mode mainly for low-mass neutron stars. The NEP $Q_{\rm sym}$ and $Q_{\rm sat}$ affect the fundamental modes of intermediate and heavy neutron stars, respectively. In the case of the first pressure mode, $K_{\rm sym}$ shows a small effect, while $Q_{\rm sat}$ shows a considerable decrease in this oscillation mode independent of the neutron star mass. Similarly, for tidal deformability, the NEP $Q_{\rm sat}$ and $Q_{\rm sym}$ show a bigger impact than $K_{\rm sym}$. Given the impact of the NEP on gravitational wave phenomena and the currently large uncertainties of these parameters, the prospect of higher sensitivity in future gravitational wave detectors promise a possible new tool to constrain high-order NEP.

nucl-th↗

arXiv2024

Gravitational wave seismology of charged strange stars in the Cowling approximation: the fluid pulsation modes

In this work we study, within the framework of Cowling approximation, the effect of the electric charge on the gravitational wave frequency of fluid oscillation modes of strange quark stars. For this purpose, the dense matter of the stellar fluid is described by the MIT bag model equation of state (EoS), while for the electric charge profile, we consider that the electric charge density is proportional to the energy density. We find that the gravitational wave frequencies change with the increment of electric charge; these effects are more noticeable at higher total mass values. We obtain that the $f$-mode is very sensitive to the change in the electric charge of the star. Furthermore, in the case of the $p_1$ mode, the effect of the electric charge is not very significant. Our results reveal that the study of the fundamental pulsation mode of an electrically charged compact star is very important in distinguishing whether compact stars could contain electric charge. We also employ another electric charge distribution profile that follows a power law, and it is found that the $f$-mode change is more noticeable than the $p_1$-mode when the electric charge is incremented.

astro-ph.SR↗

arXiv2023

Quark confinement in an equiparticle quark model: application to stellar matter

We perform an improvement in a thermodynamical consistent model with density dependent quark masses ($m'_{u,d,s}$) by introducing effects of quark confinement/deconfinement phase transition, at high density regime and zero temperature, by means of the traced Polyakov loop ($Φ$). We use realistic values for the current quark masses, provided by the Particle Data Group, and replace the constants of the interacting part of $m'_{u,d,s}$ by functions of $Φ$, leading to a first order phase transition structure, for symmetric and stellar quark matter, with $Φ$ being the order parameter. We show that the improved model points out the direction of the chiral symmetry restoration due to the emergence of a deconfined phase. In another application, we construct quark stars mass-radius profiles, obtained from this new model, and show to be possible to satisfy recent astrophysical observational data coming from the LIGO and Virgo Collaboration, and the NICER mission concerning the millisecond pulsars PSR J0030+0451, and PSR J0740+6620.

astro-ph.HE↗

arXiv2023

Fluid pulsation modes and tidal deformability of anisotropic strange stars in light of the GW$170817$ event

The effects of the anisotropy on the fluid pulsation modes adopting the so-called Cowling approximation and tidal deformability of strange quark stars are investigated by using the numerical integration of the hydrostatic equilibrium, nonradial oscillations, and tidal deformability equations, being these equations modified from their standard form to include the anisotropic effects. The fluid matter inside the compact stars is described by the MIT bag model equation of state. For the anisotropy profile, we consider a local anisotropy that is both regular at the center and null at the star's surface. We find that the effect of the anisotropy is reflected in the fluid pulsation modes and tidal deformability. Finally, we analyze the correlation between the tidal deformability of the GW$170817$ event with the anisotropy.

astro-ph.HE↗

arXiv2023

Phase transition and stiffer core fluid in neutron stars: Effects on stellar configurations, dynamical stability, and tidal deformability

In this work, we investigate the influence of the phase transition and a stiffer fluid in neutron stars' cores on the static equilibrium configuration, dynamical stability, and tidal deformability. For this aim, it is taken into account that the fluid in the core and the envelope follow the relativistic polytropic equation of state. We find that the phase transition and a stiffer fluid in the core will reflect in the total mass, radius, speed of sound, core radius, radial stability with a slow and rapid conversion at the interface, and tidal deformability. We also investigate the dimensionless tidal deformability $Λ_1$ and $Λ_2$ for a binary neutron stars system with chirp mass equal to GW$170817$. Finally, we contrast our results with observational data to show the role that phase transition and a stiffer core fluid could play in the study of neutron stars.

astro-ph.HE↗

arXiv2023

Dark matter effects on hybrid star properties

In the present work we investigate the effects of dark matter (DM) on hybrid star properties. We assume that dark matter is mixed with both hadronic and quark matter and interact with them through the exchange of a Higgs boson. The hybrid star properties are obtained from equations of state calculated with a Maxwell prescription. For the hadronic matter we use the NL3* parameter set and for the quark matter, the MIT bag model with a vector interaction. We see that dark matter does not influence the phase transition points (pressure and chemical potential) but shifts the discontinuity on the energy density, which ultimately reduces the minimum mass star that contains a quark core. Moreover, it changes considerably the star family mass-radius diagrams and moves the merger polarizability curves inside the confidence lines. Another interesting feature is the influence of DM in the quark core of the hybrid stars constructed. Our results show an increase of the core radius for higher values of the dark particle Fermi momentum.

hep-ph↗

arXiv2022

Non-radial oscillations and global stellar properties of anisotropic compact stars using realistic equations of state

In this work, we have made a systematic study of how the gravitational wave frequency of the fundamental mode from compact stars is affected by anisotropic effects using realistic equations of state. Our study is an extension of the seminal research performed by Doneva [Phys. Rev. D 85 (2012) 124023], where a polytropic equation of state was used. To achieve our objective, we considered compact stars which were built by using equations of state in the framework of a relativistic mean field theory for the case of hadronic stars and in the framework of the MIT model for the case of quark stars. In order to obtain some pertinent information that could give us the possibility to detect the anisotropy in compact stars, we also studied and analized the behaviour of various global stellar quantities, e.g., gravitational redshift, stellar mass, radius, among others. We concluded that the anisotropic effects can have important consequences, which are strongly related to the anisotropic parameter and the equation of state of high density matter. Additionally, a comparison with observational data has been made and we have shown that the anisotropic parameter $λ$ can be used as a tuning parameter to reproduce mass and radius observational data of neutron stars.

gr-qc↗

arXiv2021

Tidal deformability of strange stars and the GW170817 event

In this work we consider strange stars formed by quark matter in the color-flavor-locked (CFL) phase of color superconductivity. The CFL phase is described by a Nambu-Jona-Lasinio model with four-fermion vector and diquark interaction channels. The effect of the color superconducting medium on the gluons are incorporated into the model by including the gluon self-energy in the thermodynamic potential. We construct parametrizations of the model by varying the vector coupling $G_V$ and comparing the results to the data on tidal deformability from the GW170817 event, the observational data on maximum masses from massive pulsars such as the MSP J0740+6620, and the mass/radius fits to NICER data for PSR J003+0451. Our results points out to windows for the $G_V$ parameter space of the model, with and without gluon effects included, that are compatible with all these astrophysical constraints, namely, $0.21<G_V/G_S<0.4$, and $0.02<G_V/G_S<0.1$, respectively. We also observe a strong correlation between the tidal deformabilites of the GW170817 event and $G_V$. Our results indicate that strange stars cannot be ruled out in collisions of compact binaries from the structural point of view.

astro-ph.HE↗

arXiv2021

Short-range correlations effects on the deformability of neutron stars

In the present work, we investigate the effects of short-range correlations (SRC) on the dimensionless deformability of the binary neutron system related to the GW170817 event. We implemented phenomenological SRC in a relativistic mean-field model in which the bulk parameters, namely, incompressibility ($K_0$), effective nucleon mass ratio ($m^*$), symmetry energy ($J$) and its slope ($J_0$), are independently controlled. Our results point out that the SRC favor the model to pass through the constraints, established by the LIGO/Virgo Collaboration, on the values of $Λ_{1.4}$ and on the $Λ_1\timesΛ_2$ region. We also found a clear linear correlation between $Λ_{1.4}$ with $K_0$ and $L_0$ (increasing dependence), and with $m^*$ and $J$ (decreasing dependence). Finally, we also obtained compatible numbers for $R_{1.4}$ (model with and without SRC) in comparison with recent data from the neutron star interior composition explorer mission.

nucl-th↗

arXiv2020

Fluid pulsation modes from strange stars in a higher-dimensional space-time

In this work, we make the first step to derive non-radial pulsation equations in extra dimensions and investigate how the $f$- and $p_1$-mode frequencies of strange quark stars, within the Cowling approximation, change with the number of dimensions. In this regard, the study is performed by solving numerically the non-radial pulsation equations, adjusted for a $d$-dimensional space-time $(d\geq4)$. We connect the interior to a Schwarzschild-Tangherlini exterior metric and analyze the $f$- and $p_1$- mode frequencies. We found that the frequencies could become higher than those found in four-dimensional space-time. The $f$-mode frequency is essentially constant and only for large gravitational radius values grows monotonically and fast with the gravitational radius. In a gravitational radius range, where $f$-mode frequencies are constant, they increase for space-time dimensions $4\leq d\leq6$ and decrease for $d\geq7$. Regarding $p_1$-mode frequencies they are always larger for higher dimensions and decay monotonically with the increase of the gravitational radius. In extra dimensions, as it happens for four-dimensional space-time, we found $p_1$-mode frequencies are always larger than the $f$-modes ones. In the Newtonian gravity, for a homogeneous star in $d$ dimensions, we observe that the $f$-mode eigenfrequencies are constant and given by the relation $ω^2=l\, M\, G_d/R^{d-1}$; where $l$ represents the spherical harmonic index, $M\,G_d$ being the total star mass and $R$ the stellar radius.

gr-qc↗

arXiv2019

Neutron star cooling and GW170817 constraint within quark-meson coupling models

In the present work we used five different versions of the quark-meson coupling (QMC) model to compute astrophysical quantities related to the GW170817 event and to neutron star cooling process. Two of the models are based on the original bag potential structure and three versions consider a harmonic oscillator potential to confine the quarks. The bag-like models also incorporate the pasta phase used to describe the inner crust of neutron stars. We show that the pasta phase always play a minor or negligible role in all studies. Moreover, while no clear correlation between the models that satisfy the GW170817 constraints and the slope of the symmetry energy is found, a clear correlation is observed between the slope and the fact that the cooling is fast or slow, i.e., fast (slow) cooling is related to higher (lower) values of the slope. We did not find one unique model that can describe, at the same time, GW170817 constraints and give a perfect description of the possible cooling processes.

astro-ph.HE↗

arXiv2019

Consistent relativistic mean field models constrained by GW170817

We have obtained the Love number and corresponding tidal deformabilites ($Λ$) associated with the relativistic mean-field parametrizations shown to be consistent (CRMF) with the nuclear matter, pure neutron matter, symmetry energy and its derivatives [Dutra, et al., Phys. Rev. C 90, 055203 (2014)]. Our results show that CRMF models present very good agreement with the recent data from binary neutron star merger event GW170817. They also confirm the strong correlation between $Λ_{1.4}$ and the radius of canonical stars ($R_{1.4}$). When a recently GW170817 constraint on $Λ_{1.4}$ and the corresponding radius $R_{1.4}$ is used, the majority of the models tested are shown to satisfy it.

nucl-th↗

arXiv2013

Implications of PSR J1614-2230 for NJL hybrid star

The recent determination of the mass of the pulsar PSR J1614-2230 with $1.97 \pm 0.04 M_\odot$ by \cite{Demorest}, renewed the discussions about the possibility of exotic matter being present at the core of neutron stars. Since the description of matter at densities beyond nuclear saturation is model dependent, several works have explored different aspects of the fact that the maximum neutron star mass implied by any equation of state (EoS) must exceed the mass of PSR J1614-2230. In this article we present an extensive study of hybrid star masses using several parametrizations of a relativistic mean-field hadronic EoS together with a typical three-flavor NJL model with scalar, vector and 't Hooft interactions as realized.

astro-ph.HE↗