arXiv subjects
Publications and source records attributed to Luiz L. Lopes.
In this second part, I discuss how to introduce and the role played by non-atomic degrees of freedom in neutron stars' core using the formalism developed in Part I.
By varying the symmetry energy slope ($L$), I investigate how the exotic content within the interiors of neutron stars changes and how it affects both macroscopic and microscopic quantities. Using two different parametrizations (L3$ωρ$ and BigApple), and three different possibilities about the neutron star core (nucleons+hyperons, nucleons+deltas, nucleons+hyperons+deltas), I show that, for the models analyzed in this work, changing the slope barely changes the amount of hyperons, but it can strongly suppress the $Δ$ resonances for large values of $L$. I also show that, in general, the presence of exotic content will be more evident for lower values of $L$ than for large ones. Differences and similarities between the two parametrizations are also analyzed.
In this work, we study the effect of the symmetry slope on the observables of weakly and strongly magnetized neutron stars within the chaotic magnetic field approximation. We investigate the impact of the symmetry energy slope in the equation of state, as well as on the observables of neutron stars, by calculating their masses, radii, redshifts, tidal deformabilities, and fundamental-mode gravitational-wave frequencies. We show that the effect of the magnetic field is strong on low mass stars, producing a softer equation of state and correspondingly lower values of radii. Furthermore, the magnetic field also causes a significant drop in the dimensionless tidal parameter even when the effects on the radii are small. At the end of the paper, we discuss the effects of the magnetic field in neutron stars' universal relations.
In this work, I discuss neutron stars with hyperons and anti-kaon condensate. To fix their coupling constants with the vector mesons of the Quantum Hadrodynamics, I use a unified scheme imposing that the Yukawa coupling is an invariant under SU(3) and SU(6) groups. Combining with the G-Parity, I show that some expected results of the kaon and anti-kaon interaction with the nucleus are re-obtained. In the same sense, the naive quark-isospin counting rule is restored in the SU(6) limit. Furthermore, the G-Parity combined with the SU(3) gives us a clear picture of the role played by each meson in the kaon condensation. Numerical results show that the presence of anti-kaons severely compromises the stiffening of the EOS by breaking the SU(6) symmetry.
In this tutorial, I discuss how to model a neutron star from the Quantum Hadrodynamics microscopic approach. After a brief discussion about hydrostatic equilibrium, I discuss the role of each meson of the model and how to calculate the corresponding equation of state and the expected values. Each meson is introduced individually. Its effects are analyzed from both an analytical and a numerical point of view. To explicitly show the effects of a given meson, the coupling constant is varied in an arbitrary range before being fixed to reproduce well-known constraints. This work is intended for late undergraduate students as well as early graduate students. The equation of states is obtained from the statistical mechanics formalism, which is more familiar to students at this stage of their research career, instead of the traditional quantum field theory formalism.
In this letter, I discuss the macroscopic properties of the ultracompact object XTE J1814-338, whose inferred mass and radius read $M$ = 1.21 $\pm$ 0.05 $M_\odot$ and R = 7.0 $\pm$ 0.4 km. By using the neutralino as WIMP dark matter with a fixed Fermi momentum, I calculated the maximum possible mass of this object, the moment of inertia, the gravitational redshift, the dimensionless tidal parameter, and the total amount of dark matter for a 1.2$M_\odot$ star.
We investigated the radial and non-radial fundamental ($f$) mode oscillations of self-bound (quark) stars obtained after employing the Vector MIT (vMIT) bag model. Within this model, we computed the equation of state for strange quark matter satisfying thermodynamic consistency. This allowed us to obtain the corresponding behavior of the speed of sound, mass-radius relation, and gravitational redshift. In particular, our choice of $G_V$ = 0.30 fm$^2$ produces masses and radii in agreement with recent astronomical data (e.g. from NICER and HESS J1731). In fact, we tested that variations of the remaining vMIT parameters slightly modify this conclusion. Then, we proceeded to compute the radial oscillation frequencies of the $f$-mode, which is tightly connected to the dynamical stability of these compact stars. We found that increments of the $G_V$ parameter have a stabilizing property around the maximal-mass stars for a given stellar family. We also calculated the gravitational-wave frequencies of the non-radial $f$-mode. Our results show that they are restricted to be in the range (1.6 - 1.8) kHz for high-mass stars and to (1.5 - 1.6) kHz for low-mass stars. Finally, we propose a universal relation between these frequencies and the square root of the average density. All these last results are important in distinguishing strange stars from ordinary neutron stars in future gravitational-wave detections coming from compact sources with activated non-radial modes.
The existence of the ultracompact object XTE J1814-338, with an inferred mass and radius of $M$ = 1.21 $\pm$ 0.05 $M_\odot$ and R = 7.0 $\pm$ 0.4 km, presents a great challenge for the theory of neutron stars. Within this context, we revisit the theory of dark-matter-admixed neutron stars and infer the physical properties of this compact object, such as the Fermi momentum of dark matter necessary to compress the star at such low radius, its equation of state, speed of sound, and some macroscopic properties, such as the moment of inertia and the dimensionless tidal parameter. We also compare the physical properties of the XTE J1814-338 with other pulsars, such as the canonical 1.4 M$_\odot$, the PSR J0740 + 6620, and the HESS J1731-347.
Using the Bower-Liang model, we discuss how pressure anisotropies affect the microscopic and macroscopic properties of hybrid stars. We find that anisotropies affect the maximum mass, central density, and radius of the canonical stars. Anisotropies also affect the minimum neutron star mass that presents quarks in their core, as well as the total amount of quarks for the maximally massive stars. We also confront our results with standard constraints, such as the radius and the tidal parameter of the canonical star, as well as the mass and radius of the PSR J0740+6620 pulsar. We observe that moderate values for anisotropies could fulfill these constraints simultaneously. On the other hand, within more extreme degrees of anisotropies, more speculative constraints such as black widow pulsars PSR J0952-0607 and the mass-gap object in the GW190814 event can be explained as hybrid stars. We also investigate the role of anisotropies in the neutron stars' moment of inertia.
Using six different parametrizations of the quantum hadrodynamics (one of which is original), I study how different values of the symmetry energy slope ($L)$ affect some microscopic and macroscopic properties of neutron stars, such as the proton fraction, the maximum mass, the radius of the canonical 1.4$M_\odot$ star and its dimensionless tidal parameter $Λ$. I show that while most quantities present the same qualitative results, the tidal parameter can increase or decrease with the slope, depending on the model. Moreover, special attention is given to the minimum mass that enables the direct URCA process to occur in neutron stars' interiors ($M_{DU})$. Assuming the weak constraint $M_{DU}~>~1.35M_\odot$, we see that the maximum value of $L$ that satisfies it lies between 79 and 86 MeV. A range of only 7 MeV. Therefore, $M_{DU}$ is an easy way to impose upper bounds to the slope.
In this work, we perform a comparative analysis between the density-dependent quark model and the vector MIT bag model using Bayesian analysis. We use the equations of state generated by these two models to describe quark stars. We impose four recent observational astrophysical constraints on both models to determine their model-dependent parameters in an optimized manner assuming that the compact objects observed are composed entirely of self-bound quarks. The restrictions are aimed at producing stars with maximum masses $2 - 2.35$ M$_\odot$ and a mass-radii diagram compatible with the observed pulsars: PSR J0740+6620, PSR J0952-0607, PSR J0030+0451 and the compact object XMMU J173203.3-344518. With this analysis, the parameter dependence of the nuclear equation of state (EoS) of both models is restricted.
We study how the nuclear symmetry energy slope ($L$) can affect the hadron-quark phase transition and neutron star properties. We show that the main physical quantities as the critical chemical potential and pressure are strongly influenced by the symmetry energy slope. In extreme cases, the total amount of deconfined quarks can reach up to 99$\%$ of the hybrid star mass.
In December 2023, the Fermi LAT Catalog announced the discovery of 33 new millisecond pulsars. Motivated by that, in this work, I study how different values of the symmetry energy slope $(L)$ affect the properties of static and slowly rotating neutron stars. For fixed values of angular velocity, I study how the slope influences the increase of the maximum mass, the radii of the canonical 1.4 solar mass, its eccentricity, as well the same quantities for the 2.01 $M_\odot$ stars. I show that different slope values cause different variations not only the absolute quantities but also in relative ones. Indeed, different slope values predict different values for the eccentricity, which does not depend on the absolute value of the neutron stars' radii. Therefore, this quantity can be a powerful tool to constrain the symmetry energy slope.
In this work, we made an extensive study about the possible presence of anisotropies in strange stars. To accomplish this task, we use three different configurations for the strange matter: the unpaired matter, a two-flavor super-conducting (2SC) strange matter, and a fully three-flavor super-conducting strange matter (CFL). For each configuration, we calculate the relevant quantities for the strange stars, such as the mass-radius relation, the dimensionless tidal parameter, the moment of inertia, and the surface curvature for different degrees of anisotropies. Whenever possible, we compare our results with constraints found in the literature, especially focusing on the existence of very massive pulsars (PSR J0952-0607), as well as very light compact objects (HESS J1731-347).
I investigate the use of the SU(3) Clebsch-Gordan coefficients in light of the relations of completeness and closure. I show that in the case of $α_V = F/(F+D)~\neq$ 1, there is an additional interaction: the exchange of a $ρ$ meson between a $Λ$ and a $Σ^0$ hyperon that only affects the symmetric coupling. I then calculate these additional coupling constants and show that this recovers the completeness and closure of the SU(3) Clebsch-Gordan coefficients for all values of $α_V$. Besides, it increases the symmetry of the theory, once now we can group the baryon octet into four doublets. Finally, I add the new coupling constants to study numerical results in the hyperon onset in dense nuclear matter assuming $α_V$ as a free parameter.
We investigate possible traces of the nuclear symmetry energy slope ($L$) in the gravitational wave emission of neutron stars. For fixed stellar mass values, we examine how the slope influences the stellar radius, compactness, the tidal deformability, the frequency of the quadrupole fundamental fluid mode, and the damping time of the mode due to the gravitational wave emission. We demonstrate that all these physical quantities are sensitive to the slope and could potentially impose significant constraints on it.
Motivated by the recent suggestions that very massive pulsar (PSR J0952-0607) and very light compact object (HESS J1731-347) exist, in this article, we revisit the possibility of such objects being strange stars instead of the standard hadronic neutron stars. We study the possible presence of local anisotropy and how it affects the macroscopic properties of strange stars and compare our results with the recent constraints presented in the literature. We found that the presence of anisotropy increases the maximum mass, the radius of the canonical star, and its tidal deformability for positive values of $λ_{\rm BL}$ and the opposite for negative values. We also show that although we cannot rule out the possibility of very compact objects being standard hadronic neutron stars, strange stars easily fulfill most of the observational constraints.
In this paper the QCD phase diagram is obtained by the crossing of two effective models: the MIT bag based models are used to describe quark matter and QHD type models to describe hadronic matter, being the use of the former something new to this kind of approach and the latter used with improved parameterizations as compared with previous calculations. We use the Gibbs' conditions to establish the crossing points of the pressure in function of the chemical potential obtained in both phases. We first analyze two-flavour symmetric matter constrained to both the freeze-out and the liquid-gas phase transition at the hadronic phase. Later we analyse the results for $β$-stable and charge neutral stellar matter and compare two different prescriptions: one that assumes flavour conservation, so that the quark phase is completely determined from the hadronic phase, a prescription never applied to finite temperatures before, and the other based on the Maxwell construction, where the quark phase is also $β$-stable. At the end, we compute the latent heat to find a signature of the critical end point.