arXiv subjects
Publications and source records attributed to Luiz L. Lopes.
In this work, we study dark matter (DM) admixed strange quark stars exploring the different possibilities about the nature of the DM and their effects on the macroscopic properties of strange stars, such as maximum masses, radii, as well the dimensionless tidal parameter. We observe that the DM significantly affects the macroscopic properties that depend on its mass, type, and fraction inside the star.
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.
We calculate the baryon-meson coupling constants for the spin-1/2 baryonic octet and spin-3/2 decuplet in a unified approach relying on symmetry arguments such as the fact that the Yukawa couplings, present in the Lagrangian density of the Walecka-type models, must be an invariant under SU(3) and SU(6) group transformations. The coupling constants of the baryon with the scalar $σ$ meson are fixed to reproduce the known potential depths for the hyperons and $Δ$ resonances, in an approach that can be extended to all particles. We then apply the calculated coupling constants to study neutron star matter with hyperons and deltas admixed to its composition. We conclude that the $Δ^-$ is by far the most important exotic particle that can be present in the neutron star interior. It is always present, independent of the chosen parameterization, and might appear in almost every known neutron star, once its onset happens at very low density. Yet, its presence affects the astrophysical properties of the canonical 1.4 M$_\odot$ star, and, in some cases, it can even contribute to an increase in the maximum mass reached.
In this work, we make a very extensive study on the conditions that allow the mass-gap object in the GW190814 event to be faced as a degenerate star instead of a black hole. We begin revisiting some parametrizations of the Quantum Hadrodynamics (QHD) and then study under which conditions the hyperons are present in such a massive star. Afterward, using a vector MIT based model, we study if self-bound quark stars, satisfying the Bodmer-Witten conjecture fulfill all the observational constraints. Finally, we study hybrid stars within a Maxwell construction and check for what values of the bag, as well as the vector interaction, a quark core star with only nucleons, and with nucleons admixed with hyperons can reach at least 2.50 M$_\odot$. We conclude that, depending on the choice of parameters, none of the possibilities can be completely ruled out, i.e., the mass-gap object can be a hadronic (either nucleonic or hyperonic), a quark, or a hybrid star. However, some cases are more probable than others.
Using a extended version of the Quantum Hadrodynamics (QHD), I propose a new microscopic equation of state (EoS) able to correctly reproduce the main properties of symmetric nuclear matter at the saturation density, as well produce massive neutron stars and satisfactory results for the radius and the tidal parameter $Λ$. I show that even when hyperons are present, this EoS is able to reproduce at least 2.00 solar masses neutron star. The constraints about the radius of a $2.00M_\odot$ and the minimum mass that enable direct Urca effect are also checked.
Using a quantum hadrodynamics (QHD) and MIT based models we construct hybrid stars within the Maxwell criteria of hadron-quark phase transition. We are able to produce a hybrid star with maximum mass of 2.15$M_\odot$. Furthermore, a 2.03$M_\odot$ star with a quark core corresponding to more than $80\%$ of both, its total mass and radius, is also possible.
In this work we study how the small contribution of the inner crust to the total equation of state (EoS) of a neutron star affects its mass-radius relation, focusing on the canonical mass of 1.4$M_\odot$. We build empirical EoS of the kind - $p = Kε^γ + b$ - in the range of 0.003 fm$^{-3}~<~n~<~$0.08 fm$^{-3}$ and also calculate the speed of sound in this region. We see that different behaviours of the speed of sound can affect the radius of the canonical star by more than 1.1 km. This result can help us understand extreme results as GW170817, where some studies indicate that the radius of the canonical star cannot exceed 11.9 km.
In this work we study different variations of the MIT bag model. We start with the so called non-ideal bag model and discuss it in detail. Then we implement a vector interaction in the MIT bag model that simulates a meson exchange interaction and fix the quark-meson coupling constants via symmetry group theory. At the end we propose an original model, inspired by the Boguta-Bodmer models, which allows us to control the repulsion interaction at high densities. For each version of the model we obtain a stability window as predicted by the Bodmer-Witten conjecture and discuss its thermodynamic consistency.
In the present work we use the modified versions of the MIT bag model, on which both a vector field and a self-interacting term are introduced, to obtain hot quark matter and to investigate the QCD phase diagram. We first analyze two-flavored quark matter constrained to both the freeze-out and the liquid-gas phase transition at the hadronic phase. Later, three-flavored quark matter subject to $β$ equilibrium and charge neutrality is used to compute quark star macroscopic properties, which are confronted with recent observational massive and canonical star radius results. Finally, a comparison with QCD phase diagrams obtained from the Nambu-Jona-Lasinio model is performed.
In this work we revisit the quantum hadrodynamics (QHD) formalism to investigate hyperonic and hybrid stars with hyperon-meson couplings fixed via broken SU(6) group, in favor of a more general flavor group SU(3). We also employ an additional channel, the strangeness-hidden $ϕ$ meson, which couples only to the hyperons. In hybrid stars, the quark phase is built with the SU(3) NJL model also with an additional vector channel in the Lagrangian. We found that within the models chosen the hyperon puzzle cannot be avoided by the quark-hadron phase transition, once the hyperon threshold always happens at lower density. Also, the contribution of the quark core in a hybrid star is more relavant to the radius than to the mass, and strongly depends on the quark equation of state. We are able to reproduce 2.21 $M_\odot$ hyperonic star and 2.10 $M_\odot$ hybrid star. Both results are in agreement with the recently detected hyper massive pulsar MSP J0740+6620.
Motivated by the recent gravitational wave detection by the LIGO-VIRGO observatories, we study the Love number and dimensionless tidal polarizability of highly magnetized stars. We also investigate the fundamental quasi-normal mode of neutron stars subject to high magnetic fields. To perform our calculations we use the chaotic field approximation and consider both nucleonic and hyperonic stars. As far as the fundamental mode is concerned, we conclude that the role played by the constitution of the stars is far more relevant than the intensity of the magnetic field and if massive stars are considered, the ones constituted by nucleons only present frequencies somewhat lower than the ones with hyperonic cores, a feature that can be used to point out the real internal structure of neutron stars. Moreover, our studies clearly indicate that strong magnetic fields play a crucial role in the deformability of low mass neutron stars, with possible consequences on the interpretation of the detected gravitational waves signatures.
We study how the magnetic field and non-standard vector channels affect hadronic, quarkionic and hybrid stars. In the hadronic phase, we use the QHD model, within its standard $σωρ$ mesons, and compare the results with the ones obtained with the inclusion of the strangeness hidden $ϕ$ meson. In the quark phase, we use the standard SU(3) NJL and compare the results with the version that takes into account the vector channel $G_v(\barψγ^μψ)$. Magnetic fields are taken into account via chaotic magnetic field approximation.
We study how the $Λ$ hyperon threshold influences the radius of the canonical $1.4 ~M_\odot$ neutron star in the light of the measurements found in the recent literature. We show that the onset of a new degree of freedom not only causes the well known reduction of the maximum mass, but also compacts the neutron stars with high central density. With the help of the strange mesons $ϕ$ and $σ^*$, we show that it is possible to simulate very compact neutron stars keeping realistic hyperon potentials, $U_Λ(n_0)= -28$ MeV and $ U_Λ^Λ(n_0/5)$ in agreement with recents measurements. In the end we generalize these results showing that the onset of a yet not known dark matter particle with mass of 1.04 GeV is able to produce simultaneously a 2 $M_\odot$ neutron star and a canonical one with a radius of only 11.62 km.
We revisit three of the mathematical formalisms used to describe magnetized quark matter in compact objects within the MIT and the Nambu-Jona-Lasinio models and then compare their results. The tree formalisms are based on 1) isotropic equations of state, 2) anisotropic equations of state with different parallel and perpendicular pressures and 3) the assumption of a chaotic field approximation that results in a truly isotropic equation of state. We have seen that the magnetization obtained with both models is very different: while the MIT model produces well-behaved curves that are always positive for large magnetic fields, the NJL model yields a magnetization with lots of spikes and negative values. This fact has strong consequences on the results based on the existence of anisotropic equations of state. We have also seen that, while the isotropic formalism results in maximum stellar masses that increase considerably when the magnetic fields increase, maximum masses obtained with the chaotic field approximation never vary more than 5.5$\%$. The effect of the magnetic field on the radii is opposed in the MIT and NJL models: with both formalisms, isotropic and chaotic field approximation, for a fixed mass, the radii increase with the increase of the magnetic field in the MIT bag model and decrease in the NJL, the radii of quark stars described by the NJL model being smaller than the ones described by the MIT model.
In this work we review the formalism normally used in the literature about the effects of density-dependent magnetic fields on the properties of neutron and quark stars, expose some ambiguities that arise and propose a way to solve the related problem. Our approach explores more deeply the concept of pressure, yielding the so called chaotic magnetic field formalism for the stress tensor. We also use a different way of introducing a variable magnetic field, which depends on the energy density rather than on the baryonic density, which allows us to build a parameter free model.
Using the well known quantum hadrodynamics (QHD), we study the effects of meson-hyperon coupling constants on the onset of hyperons in dense nuclear matter. We use the SU(3) symmetry group to fix all these coupling constants, constrained to experimental nuclear matter results and astrophysical observations. While the discovery of massive pulsars PSR J1614-2230 and PSR J0348+0432 points towards a very stiff equation of state at very large densities, results from heavy ion collisions point in opposite direction for densities below five times the nuclear saturation density. We study some well known parametrizations and see that most of them cannot satisfy both types of constraints. Indeed, although in our model we can simulate a 2.25 $M_\odot$ hyperonic neutron star, the heavy ion collision constraints limits the maximum mass around 2.06 $M_\odot$.
The physics of neutron stars leads historically towards Landau's speculation. Even before the discovery of the neutron, he postulated the possible existence of stars more compact than white dwarfs, containing matter of the order of nuclear density. From a modern point of view neutron stars are compact objects maintained by the equilibrium between gravity and the degeneracy pressure of the fermions together with a strong nuclear repulsion force due to the high density reached in their interior. While the physics in the vicinity of nuclear saturation density is well know from phenomenology, the physics of ultra-dense nuclear matter is still an open puzzle. In this work we study dense nuclear matter within a relativistic model, allowing hyperons to be present through beta equilibrium. The presence of hyperons is justifiable since the constituents of neutron stars are fermions. So, according to the Pauli principle, as the baryon density increases, so do the Fermi momentum and the Fermi energy. On the other hand, this hyperonic matter softens the equation of state (EoS) and a recent measurement of pulsar PSR J1614-2230 implies that the EoS has to be stiff enough to produce a 2.0 $M_{\odot}$ pulsar. We also consider Duncan's magnetar ideas and study the influence of strong magnetic fields on the EoS. We see that a strong magnetic field produces very massive neutron stars, in agreement with the astronomical observations.