Hyperonic Neutron Stars Research Articles

Correction to: Equation of state for hot hyperonic neutron star matter

<b>Correction to:</b> Equation of state for hot hyperonic neutron star matter

Equation of state for hot hyperonic neutron star matter

Abstract The FSU2H equation-of-state model, originally developed to describe cold neutron star matter with hyperonic cores, is extended to finite temperature. Results are presented for a wide range of temperatures and lepton fractions, which cover the conditions met in protoneutron star matter, neutron star mergers, and supernova explosions. It is found that the temperature effects on the thermodynamical observables and the composition of the neutron star core are stronger when the hyperonic degrees of freedom are considered. An evaluation of the temperature and density dependence of the thermal index leads to the observation that the so-called Γ law, widely used in neutron star merger simulations, is not appropriate to reproduce the true thermal effects, specially when hyperons start to be abundant in the neutron star core. To make finite temperature equations of state easily accessible, simple parametrizations of the thermal index for nucleonic and hyperonic β-stable neutrino-free matter are provided.

Multi-Physics Constraints at Different Densities to Probe Nuclear Symmetry Energy in Hyperonic Neutron Stars

The appearance of strangeness in the form of hyperons within the inner core of neutron stars is expected to affect its detectable properties, such as its global structure or gravitational wave emission. This work explores the parameter space of hyperonic stars within the framework of the Relativistic Mean Field model allowed by the present uncertainties in the state-of-the-art nuclear and hypernuclear experimental data. We impose multi-physics constraints at different density regimes to restrict the parameter space: Chiral effective field theory, heavy-ion collision data, and multi-messenger astrophysical observations of neutron stars. We investigate possible correlations between empirical nuclear and hypernuclear parameters, particularly the symmetry energy and its slope, with observable properties of neutron stars. We do not find a correlation for the hyperon parameters and the astrophysical data. However, the inclusion of hyperons generates a tension between the astrophysical and heavy-ion data constraining considerably the available parameter space.

Hyperonic neutron stars: reconciliation between nuclear properties and NICER and LIGO/VIRGO results

Using an extended version of quantum hadrodynamics, I propose a new microscopic equation of state (EoS) that is able to correctly reproduce the main properties of symmetric nuclear matter at the saturation density, as well as produce massive neutron stars and satisfactory results for the radius and the tidal parameter. I show that this EoS can reproduce at least a 2.00 solar mass neutron star, even when hyperons are present. The constraints about the radius of a 2.00 M ⊙ and the minimum mass that enables a direct Urca effect are also checked.

Hyperonic neutron star matter in light of GW170817

Since 2013, the mass of pulsar PSR J0348 + 0432 (M = 2.01M⊙) has provided a tight constraint on neutron star (NS) equation of state. However, a number of different analyses of the recently detected binary neutron star (BNS) merger (GW170817) point to a maximum NS mass around 2.16M⊙. In addition, a recent study determined the mass of the millisecond pulsar PSR J2215 + 5135 to be . In this work, we investigate the presence of hyperons in NS matter in light of these new mass measurements using equations of state calculated in the relativistic mean‐field approximation. Particular attention is paid to the use of the available empirical data from the study of hypernuclei and that of the SU(3) symmetry relations in fixing the meson‐hyperon coupling constants. We find that hyperonic equations of state with reasonable choices for the meson‐hyperon coupling constants can satisfy these new mass constraints, with hyperons potentially accounting for more than 10% of the baryons in the core of a NS.

Neutrino emissivity of the nucleon direct URCA process for rotational traditional and hyperonic neutron stars**Supported by National Natural Science Foundation of China (11545011, 11405096), the Shandong Natural Science Foundation (ZR2014AQ012), the Young Scholars Program of Shandong University, Weihai (2015WHWLJH01) and the Fundamental Research Funds for the Central Universities (lzujbky-2016-30)

Based on covariant density functional theory, we study the effects of rotation on the nucleon direct URCA (N-DURCA) process for traditional and hyperonic neutron stars. The calculated results indicate that, for a fixed mass sequence of rotational traditional neutron stars, the neutrino emissivity of the star is nearly invariant with increasing frequency, while it always increases for rotational hyperonic neutron stars. Thus, rotation has different effects on the N-DURCA process for these two kinds of neutron stars.

A key factor to the spin parameter of uniformly rotating compact stars: crust structure

We study the dimensionless spin parameter j ≡ cJ/(GM2) of different kinds of uniformly rotating compact stars, including traditional neutron stars, hyperonic neutron stars and hybrid stars, based on relativistic mean field theory and the MIT bag model. It is found that jmax ∼ 0.7, which had been suggested in traditional neutron stars, is sustained for hyperonic neutron stars and hybrid stars with M > 0.5 M⊙. Not the interior but rather the crust structure of the stars is a key factor to determine jmax for three kinds of selected compact stars. Furthermore, a universal formula j = 0.63(f/fK) − 0.42(f/fK)2 + 0.48(f/fK)3 is suggested to determine the spin parameter at any rotational frequency f smaller than the Keplerian frequency fK.

Singlet pairing gaps of neutrons and protons in hyperonic neutron stars**Supported by the National Natural Science Foundation of China.

The 1S0 nucleonic superfluids are investigated within the relativistic mean-field model and Bardeen-Cooper-Schrieffer theory in hyperonic neutron stars. The 1S0 pairing gaps of neutrons and protons are calculated based on the Reid soft-core interaction as the nucleon-nucleon interaction. In particular, we have studied the influence of degrees of freedom for hyperons on the 1S0 nucleonic pairing gap in neutron star matter. It is found that the appearance of hyperons has little impact on the baryonic density range and the size of the 1S0 neutronic pairing gap; the 1S0 protonic pairing gap also decreases slightly in this region where ρB = 0.0–0.393 fm−3. However, if baryonic density becomes greater than 0.393 fm−3, the 1S0 protonic pairing gap obviously increases. In addition, the possible range for a protonic superfluid is obviously enlarged due to the presence of hyperons. In our results, the hyperons change the 1S0 protonic pairing gap, which must change the cooling properties of neutron stars.

Hypernuclear matter in a complete SU(3) symmetry group

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 $M_\odot$.

Vacuum fluctuation effects on hyperonic neutron star matter

The vacuum fluctuation (VF) effects on the properties of the hyperonic neutron star matter are investigated in the framework of the relativistic mean field (RMF) theory. The VF corrections result in the density dependence of in-medium baryon and meson masses. We compare our results obtained by adopting three kinds of meson-hyperon couplings. The introduction of both hyperons and VF corrections softens the equation of state (EoS) for the hyperonic neutron star matter and hence reduces hyperonic neutron star masses. The presence of the δ field enlarges the masses and radii of hyperonic neutron stars. Taking into account the uncertainty of meson-hyperon couplings, the obtained maximum masses of hyperonic neutron stars are in the range of 1.33M⊙–1.55M⊙.

Influences of Both Δ− and Δ0 Particles on the Neutron Star Cooling

The influences of Δ quartet, especially Δ− and Δ0 particles, on the neutrino scatting and cooling properties of neutron stars are discussed. The results show that Δ quartet can change the density range of direct Urca process of nucleons for muons, as well as the neutrino emissivity and luminosity. The role of Δ on the cooling rates is complicated. It is found that Δ quartet slows the neutron star cooling obviously in hyperonic neutron star matter. However in nuclear matter, Δ can not influence the cooling too much, unless the neutron star is quite massive. The Δ-meson coupling constants can also impact on the results, and the relatively smaller value, the stronger effects of Δ quartet.

Spin change of a proto-neutron star by the emission of neutrinos

We investigate the structure of proto-neutron stars (PNSs) with trapped neutrinos by using a quark-meson coupling model. We adopt a phenomenological lepton density which is diffuse near the surface. We calculate the populations of baryons and leptons, the equations of state, and the mass-radius relation for isentropic PNS models. In addition, the moment of inertia is calculated for both PNS and cold-neutron-star (CNS) models as a means to study the change of the spin period due to the neutrino emission from a PNS. Neutrino emission from a hyperonic neutron star is shown to increase the spin by about 10$%$ of the initial spin, while the spin of a nucleonic neutron star with a central density above ${\ensuremath{\rho}}_{C}\ensuremath{\approx}5{\ensuremath{\rho}}_{0}$ is decreased by a few $%$ by the emission of neutrinos. Therefore, the spin change owing to the leakage of neutrinos from a PNS is a small ($&lt;$10$%$) correction compared to other processes related to the spin change.

Cooling of Hyperonic Neutron Stars with Antikaons

We discuss the impact of hyperons on the neutrino emitting and the gross cooling features of neutron stars with K− and K̄0 condensations. The results show that hyperons change the density ranges of the direct Urca process with nucleons and the Urca processes of K− and K̄0 condensations, as well as the values of neutrino emissivity. Moreover, interactions between hyperons and antikaons make the neutrino luminosity complicated. It is found that various hyperons play different roles in neutron stars. For massive stars, Σ hyperons make the cooling slower. However, Λ can hardly change the cooling history but it reduces the mass of neutron stars.

Influence of the isovector-scalar channel interaction on neutron star matter with hyperons and antikaon condensation

The relativistic mean field approach including isovector-scalar channel (i.e., exchanging $\ensuremath{\delta}$ mesons) interaction is taken to study the properties of neutron star matter including hyperons and antikaon condensation. For hyperonic neutron stars, it shows that the $\ensuremath{\delta}$-meson channel interaction stiffens the equation of state at lower densities but it softens the equation of state after hyperons appear. This leads to the neutron star having a lower central density and a larger radius than the one with the same mass but without the $\ensuremath{\delta}$-meson channel interaction. For neutron star matter including both hyperons and antikaon condensation, the $\ensuremath{\delta}$-meson channel interaction increases the onset density of the antikaon condensation. At the same time, the stability of the kaonic neutron star and its dependence on the kaon optical potential are discussed. For stable kaonic neutron stars with larger radii, those with the inclusion of the $\ensuremath{\delta}$-meson channel interaction have larger masses than those without the $\ensuremath{\delta}$-meson interaction, but the result is reversed for those with smaller radii. Calculated results are also compared with neutron star observations. Constraints on the model parameters are then provided.

The Density‐Dependent Correlations among Observables in Nuclear Matter and Hyperon‐Rich Neutron Stars

The conserving σ‐ω‐ρ mean‐field approximation with nonlinear interactions of hadrons has been applied to examine properties of nuclear matter and hyperonic neutron stars. The nonlinear interactions that will produce density‐dependent effective masses and coupling constants of hadrons are included in order to examine density correlations among properties of nuclear matter and neutron stars such as binding energy, incompressibility, K, symmetry energy, a4, hyperon‐onset density, and maximum masses of neutron stars. The conditions of conserving approximations in order to maintain thermodynamic consistency to an approximation are essential for the analysis of density‐dependent correlations.

THE EFFECT OF THE SCALAR-ISOVECTOR MESON FIELD ON HYPERON-RICH NEUTRON STAR MATTER

We investigate the effect of the scalar-isovector δ-meson field on the equation of state (EOS) and composition of hyperonic neutron star matter, and the properties of hyperonic neutron stars within the framework of the relativistic mean field theory. The influence of the δ-field turns out to be quite different and generally weaker for hyperonic neutron star matter as compared to that for npeμ neutron star matter. We find that inclusion of the δ-field enhances the strangeness content slightly and consequently moderately softens the EOS of neutron star matter in its hyperonic phase. As for the composition of hyperonic star matter, the effect of the δ-field is shown to shift the onset of the negatively-charged (positively-charged) hyperons to slightly lower (higher) densities and to enhance (reduce) their abundances. The influence of the δ-field on the maximum mass of hyperonic neutron stars is found to be fairly weak, whereas inclusion of the δ-field turns out to enhance sizably both the radii and the moments of inertia of neutron stars with given masses. It is also shown that the effects of the δ-field on the properties of hyperonic neutron stars remain similar in the case of switching off the Σ hyperons.

Evolutionary Sequences of Rotating Protoneutron Stars with Hyperons

Escape of the trapped neutrinos from a protoneutron star might cause a phase transition from a core nucleonic neutron star to a core hyperonic neutron star, which leads to the softening of the equation of state. We study the effects of the phase transition on the rotational evolution of a protoneutron star. It is found that the star keeps shrinking and spinning up until all the trapped neutrinos escape. If the hyperonic star is metastable, its rotational frequency accelerates distinguishedly before it collapses to a black hole.