Neutron Stars with Hyperons subject to Strong Magnetic Field

Abstract

Neutron stars are one of the most exotic objects in the universe and a unique laboratory to study the nuclear matter above the nuclear saturation density. In this work, we study the equation of state of the nuclear matter within a relativistic model subjected to a strong magnetic field. We then apply this EoS to study and describe some of the physical characteristics of neutron star, especially the massradius relation and chemical compositions. To study the influence of a the magnetic field and the hyperons in the stellar interior, we consider altogether four solutions: two different values of magnetic field to obtain a weak and a strong influence, and two configurations: a family of neutron stars formed only by protons, electrons and neutrons and a family formed by protons, electrons, neutrons, muons and hyperons. The limit and the validity of the results found are discussed with some care. In all cases the particles that constitute the neutron star are in β equilibrium and zero total net charge. Our work indicates that the effect of a strong magnetic field has to be taken into account in the description of magnetars, mainly if we believe that there are hyperons in their interior, in which case, the influence of the magnetic field can increase the mass by more than 10%. We have also seen that although a magnetar can reach 2.48M ⊙ , a natural explanation of why we do not know pulsars with masses above 2.0M ⊙ arises. We also discuss how the magnetic field affects the strangeness fraction in some standard neutron star masses and, to conclude our paper, we revisit the direct URCA process related to the cooling of the neutron stars and show how it is affected by the hyperons and the magnetic field.

Hyperon threshold and stellar radii

“…We start by reviewing the effects of hyperon threshold in dense nuclear matter and studying the influence of the coupling constant of the Λ hyperon. The consequent softening of the EoS and reduction of the maximum mass is a well-known theme in the literature [25,27,29]. The "turn to the left" due to the hyperon threshold has also been noted in some works.…”

“…The mesonic fields are obtained via mean field approximation (MFA) [24,25,27] and the EoS by thermodynamic relations [25,28].…”

“…To describe the properties of nuclear matter we use a slightly modified version of the well-known GM1 parametrization [29], a widely accepted parametrization [7,25,27,[30][31][32][33][34][35] that is able to reasonably describe both, nuclear matter and stellar structure, consistent with experimental and astrophysical observations [34]. In this work we just reduce the strength of the ρ coupling, reducing the symmetry energy slope L from 94 MeV to 87.9 MeV [7], a value closer to what is inferred in recent observations [9,11,12].…”

“…The φ field is analogous to the ω field and its expected value is also obtained via MFA [24,25,27]. Adding a new mesonic field has two advantages.…”

Magnetic neutron stars in f(R) gravity

“…The simplest model with interacting npeµ gas is investigated in [22]. Models with hyperons and quarks are considered in [23]- [27]. It has been demonstrated that the Landau quantization leads to the softening of the EoS for matter but account for contributions of magnetic field into pressure and density.…”