Hyperonic Matter Research Articles

Equation of state for neutron stars with hyperons using a variational method

We investigate the effects of the odd-state part of bare $\Lambda \Lambda$ interactions on the structure of neutron stars (NSs) by constructing equations of state (EOSs) for uniform nuclear matter containing $\Lambda$ and $\Sigma^-$ hyperons with use of the cluster variational method. The isoscalar part of the Argonne v18 two-nucleon potential and the Urbana IX three-nucleon potential are employed as the interactions between nucleons, whereas, as the bare $\Lambda N$ and even-state $\Lambda \Lambda$ interactions, two-body central potentials that are determined so as to reproduce the experimental data on single- and double-$\Lambda$ hypernuclei are adopted. In addition, the $\Sigma^- N$ interaction is constructed so as to reproduce the empirical single-particle potential of $\Sigma^-$ in symmetric nuclear matter. Since the odd-state part of the $\Lambda \Lambda$ interaction is not known owing to lack of experimental data, we construct four EOSs of hyperonic nuclear matter, each with a different odd-state part of the $\Lambda \Lambda$ interaction. The EOS obtained for NS matter becomes stiffer as the odd-state $\Lambda \Lambda$ interaction becomes more repulsive, and correspondingly the maximum mass of NSs increases. It is interesting that the onset density of $\Sigma^-$ depends strongly on the repulsion of the odd-state $\Lambda \Lambda$ interaction. Furthermore, we take into account the three-baryon repulsive force to obtain results that are consistent with observational data on heavy NSs.

Axial quasinormal modes of Einstein-Gauss-Bonnet-dilaton neutron stars

We investigate axial quasi-normal modes of realistic neutron stars in Einstein-Gauss-Bonnet-dilaton gravity. We consider 8 realistic equations of state containing nuclear, hyperonic, and hybrid matter. We focus on the fundamental curvature mode and compare the results with those of pure Einstein theory. We observe that the frequency of the modes is increased by the presence of the Gauss-Bonnet-dilaton, while the impact on the damping time is typically smaller. Interestingly, we obtain that universal relations valid in pure Einstein theory still hold for Einstein-Gauss-Bonnet-dilaton gravity, and we propose a method to use these phenomenological relations to constrain the value of the Gauss-Bonnet coupling.

An effective field theory for neutron stars with many‐body forces, strong ∑– repulsion, and K– and $\bar K^0$ condensation

AbstractThe role of many‐body correlations (many‐body forces) and K–‐ condensation in β‐equilibrated hyperonic matter is investigated in order to shed some light in the hyperonization puzzle, ie that neutron star mass of 2M® cannot be obtained in the presence of exotic degree of freedoms. In this investigation, we use an effective relativistic QHD‐model with parameterized couplings which represents an extended compilation of other effective models found in the literature. Our theoretical approach exhausts the whole fundamental baryon octet (n, p, ∑–, ∑0, ∑+, Λ, Ξ–, Ξ0) and simulates n‐order corrections to the minimal Yukawa couplings by considering many‐body nonlinear self‐couplings and meson‐meson interaction terms involving scalar‐isoscalar (σ, σ*), vector‐isoscalar (ω, φ), vector‐isovector (ϱ), and scalar‐isovector (δ) sectors. Following recent experimental results, we consider in our calculations the extreme case where the ∑– experiences such a strong repulsion that its influence in the nuclear structure of a neutron star is excluded at all. We study the effects of this exclusion on the phase transition of conventional exotic hadronic matter to hadronic matter containing a condensate of kaons and anti‐kaons. As a novelty in the treatment of kaon and anti‐kaon condensation in high density nuclear matter, we consider a Lagrangian formulation which describes, in addition to the interaction involving baryons and mesons and the contribution of kaons and anti‐kaons in free propagation, the presence of many‐body forces involving kaon, anti‐kaon and meson fields. To implement the corresponding phase transition we considered the Gibbs conditions combined with the mean‐field approximation, giving rise to a mixed phase of coexistence between baryon matter and the condensed of kaons and anti‐kaons. Our investigation show that even with kaon condensation, the nuclear equation of state satisfies both the maximum mass and the allowed ranges of mass and radius of neutron stars. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Simulations of stellar collapses to black holes: Influence of hyperons

Stellar core collapse to a black hole is an event where extremely high temperatures and densities are reached. It is then expected that additional particles to nucleons and electrons appear, as hyperons. We present numerical simulations of the collapse of massive stars to black holes, with an equation of state based on the Lattimer-Swesty one, but allowing for the presence of Λ hyperons. A feature of these equations of state is that its cold and beta-equilibrium version allows for neutrons stars with almost two solar masses, marginally compatible with recent observations. Spherically symmetric simulations, under some conditions, may show the effect of a phase transition to hyperonic matter before the appearance of the black hole. We show axisymmetric simulations, too, where this phase transition would be smoothed, but where gravitational waves show clear imprint of the presence of hyperons.

MANY-BODY FORCES IN THE EQUATION OF STATE OF HYPERONIC MATTER

In this work we introduce an extended version of the formalism proposed originally by Taurines et al. that considers the effects of many-body forces simulated by non-linear self-couplings and meson-meson interaction contributions. In this extended version of the model, we assume that matter is at zero temperature, charge neutral and in beta-equilibrium, considering that the baryon octet interacts by the exchange of scalar-isoscalar ($\sigma$,$\,\sigma^*$), vector-isoscalar ($\omega$,$\,\phi$), vector-isovector ($\varrho$) and scalar-isovector ($\delta$) meson fields. Using nuclear matter properties, we constrain the parameters of the model that describe the intensity of the indirectly density dependent baryon-meson couplings to a small range of possible values. We then investigate asymmetric hyperonic matter properties. We report that the formalism developed in this work is in agreement with experimental data and also allows for the existence of massive hyperon stars (with more than $2M_{\odot}$) with small radii, compatible with astrophysical observations.

Hyperons in neutron stars

Using the Dirac–Brueckner–Hartree–Fock approach, the properties of neutron-star matter including hyperons are investigated. In the calculation, we consider both time and space components of the vector self-energies of baryons as well as the scalar ones. Furthermore, the effect of negative-energy states of baryons is partly taken into account. We obtain the maximum neutron-star mass of 2.08M⊙, which is consistent with the recently observed, massive neutron stars. We discuss a universal, repulsive three-body force for hyperons in matter.

Hardon-Quark Hybrid Stars Constructed by the Nonlinear σ-ω-ρ Mean-Field Model and MIT-Bag Model

Density-dependent relations among saturation properties of symmetric nuclear matter and hyperonic matter, properties of hadron-(strange) quark hybrid stars are discussed by applying the conserving nonlinear s-w-r hadronic mean-field theory. Nonlinear inter...

Density-Dependent Properties of Hadronic Matter in an Extended Chiral (σ, π, ω) Mean-Field Model

Density-dependent relations among saturation properties of symmetric nuclear matter and hyperonic matter, the coupling ratios (strengths) of hyperon matter, and properties of hadronic stars are discussed by applying the conserving chiral nonlinear (s, p, w...

Bottom-strange mesons in hyperonic matter

The in-medium behavior of bottom-strange pseudoscalar mesons in hot, isospin asymmetric and dense hadronic environment is studied using a chiral effective model. The same was recently generalized to the heavy quark sector and employed to study the behavior of open-charm and open-bottom mesons. The heavy quark (anti-quark) is treated as frozen and all medium modifications of these bottom-strange mesons are due to their strange anti-quark (quark) content. We observe a pronounced dependence of their medium mass on baryonic density and strangeness content of the medium. Certain aspects of these in-medium interactions are similar to those observed for the strange-charmed mesons in a preceding investigation, such as the lifting of mass-degeneracy of [Formula: see text] and [Formula: see text] mesons in hyperonic matter, while the same is respected in vacuum as well as in nuclear matter. In general, however, there is a remarkable distinction between the two species, even though the formalism predicts a completely analogous in-medium interaction Lagrangian density. We discuss in detail the reason for different in-medium behavior of these bottom-strange mesons as compared to charmed-strange mesons, despite the dynamics of the heavy quark being treated as frozen in both cases.

Hybrid neutron stars with the field correlator method

We study the quark deconfinement phase transition in cold (T = 0) neutron star matter and we calculate various structural properties of hybrid stars. For the quark phase, we use an equation of state (EOS) based on the Field Correlator Method (FCM) extended to the case of nonzero baryon density. For the confined hadronic phase we use a relativistic mean field model considering both pure nucleonic and hyperonic matter. We constrain the values of the gluon condensate G2, which is one of the EOS parameter within the FCM, making use of the measured mass, M = 1.97 ± 0.04 M⊙, of the neutron star in PSR J1614-2230. Our results show that the values of G2 extracted from the mass measurement of PSR J1614-2230 are consistent with the values of the same quantity derived, within the FCM, from recent lattice QCD calculations of the deconfinement transition temperature at zero baryon chemical potential.

Massive neutron stars with a hyperonic core: A case study with the IUFSU relativistic effective interaction

The recent discoveries of massive neutron stars, such as PSR J$0348+0432$ and PSR J$1614--2230$, have raised questions about the existence of exotic matter such as hyperons in the neutron star core. The validity of many established equations of states (EoSs) like GM1 and FSUGold are also questioned. We investigate the existence of hyperonic matter in the central regions of massive neutron stars by using relativistic mean field (RMF) theory with the recently proposed Indiana University Florida State University (IUFSU) model. The IUFSU model is extended by including hyperons to study the neutron star in $\ensuremath{\beta}$ equilibrium. The effect of different hyperonic potentials, namely $\ensuremath{\Sigma}$ and $\ensuremath{\Xi}$ potentials, on the EoS and hence the maximum mass of neutron stars has been studied. We have also considered the effect of stellar rotation since the observed massive stars are pulsars. It has been found that a maximum mass of $1.93{M}_{\ensuremath{\bigodot}}$, which is within the $3\ensuremath{\sigma}$ limit of the observed mass of PSR J$0348+0432$, can be obtained for rotating stars, with certain choices of the hyperonic potentials. The said star contains a fair amount of hyperons near the core.

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⊙.

Quark deconfinement transition in neutron stars with the field correlator method

A phase of strong interacting matter with deconfined quarks is expected in the core of massive neutron stars. In this article, we perform a study of the hadron-quark phase transition in cold ($T=0$) neutron star matter and we calculate various structural properties of hybrid stars. For the quark phase, we make use of an equation of state (EOS) derived with the field correlator method (FCM) recently extended to the case of nonzero baryon density. For the hadronic phase, we consider both pure nucleonic and hyperonic matter, and we derive the corresponding EOS within a relativistic mean field approach. We make use of measured neutron star masses, and particularly the mass $M=1.97\ifmmode\pm\else\textpm\fi{}0.04{M}_{\ensuremath{\bigodot}}$ of PSR $\mathrm{J}1614\ensuremath{-}2230$ to constrain the values of the gluon condensate ${G}_{2}$, which is one of the EOS parameters within the FCM. We find that the values of ${G}_{2}$ extracted from the mass measurement of PSR $\mathrm{J}1614\ensuremath{-}2230$ are consistent with the values of the same quantity derived within the FCM from recent lattice QCD calculations of the deconfinement transition temperature at zero baryon chemical potential. The FCM thus provides a powerful tool to link numerical calculations of QCD on a space-time lattice with measured neutron star masses.

Updates of the nuclear equation of state for core-collapse supernovae and neutron stars: effects of 3-body forces, QCD, and magnetic fields

We summarize several new developments in the nuclear equation of state for supernova simulations and neutron stars. We discuss an updated and improved Notre-Dame-Livermore Equation of State (NDL EoS) for use in supernovae simulations. This Eos contains many updates. Among them are the effects of 3- body nuclear forces at high densities and the possible transition to a QCD chiral and/or super-conducting color phase at densities. We also consider the neutron star equation of state and neutrino transport in the presence of strong magnetic fields. We study a new quantum hadrodynamic (QHD) equation of state for neutron stars (with and without hyperons) in the presence of strong magnetic fields. The parameters are constrained by deduced masses and radii. The calculated adiabatic index for these magnetized neutron stars exhibit rapid changes with density. This may provide a mechanism for star-quakes and flares in magnetars. We also investigate the strong magnetic field effects on the moments of inertia and spin down of neutron stars. The change of the moment of inertia associated with emitted magnetic flares is shown to match well with observed glitches in some magnetars. We also discuss a perturbative calculation of neutrino scattering and absorption in hot and dense hyperonic neutron-star matter in the presence of a strong magnetic field. The absorption cross-sections show a remarkable angular dependence in that the neutrino absorption strength is reduced in a direction parallel to the magnetic field and enhanced in the opposite direction. The pulsar kick velocities associated with this asymmetry comparable to observed pulsar velocities and may affect the early spin down rate of proto-neutron star magnetars with a toroidal field configuration.

Power-law quantum distributions in protoneutron stars

We investigate the bulk properties of protoneutron stars in the framework of a relativistic mean field theory based on nonextensive statistical mechanics, originally proposed by C. Tsallis and characterized by power-law quantum distributions. We study the relevance of nonextensive statistical effects on the β-stable equation of state at fixed entropy per baryon, for nucleonic and hyperonic matter. We concentrate our analysis in the maximum heating and entropy per baryon s = 2 stage and T ≈ 40 ÷ 80 MeV. This is the phase, at high temperature and high baryon density, in which the presence of nonextensive effects may alter more sensibly the thermodynamical and mechanical properties of the protoneutron star. We show that nonextensive power-law effects could play a crucial role in the structure and in the evolution of the protoneutron stars also for small deviations from the standard Boltzmann-Gibbs statistics.

A Way to Quark Matter: Possible Intermediate Stages

Possibl~ intermediate stages from hadronic matter to the uniform quark matter are discussed. After a brief description on the energy properties of the uniform quark matter, two examples of intermediate phases are presented and discussed by taking reference to the uniform quark matter. The naive deconfinement phase transition to the uniform matter may occur at very high density, once we want to give a consistent description with low-lying hadron spectra. Then the concept of partial deconfinement turns out to play an important role, which suggests appearance of intermediate stages prior to the transition to the uniform quark matter. One possibility is a quark-layered structure with a specific spin-flavor order under the well-developed ;r 0 condensation which still persists between the quark matter slabs. The other is the matter consisting of H-particles in the flavor singlet and spinless six-quark state, which may stem from hyperonic matter and come into existence as a boson superfluid. In these stages some cooperative effects of hadronic and subhadronic degrees of freedom are appreciated.

Neutron star cooling in various sets of nucleon coupling constants

The influences of nucleon coupling constants on the neutrino scatting and cooling properties of neutron stars are investigated. The results in the GM1, GPS250 and NL-SH parameter sets show that the magnitude of the neutrino emissivity and density ranges where the dUrca process of nucleons is allowed differ obviously between the three parameter sets in nucleon-only and hyperonic matter. Furthermore, the neutron stars in the GPS250 set cool very quickly, whereas those in the NL-SH set cool slowly. The cooling rate of the former can be almost three times more that of the latter. It can be concluded that the stiffer the equation of state, the slower the corresponding neutron stars cool. The hyperon Λ makes neutrino emissivity due to the direct Urca process of nucleons lower compared with nucleon-only matter, and postpones the dUrca process with muons. However, these Λ effects are relatively weaker in the GPS250 set than in the GM1 set.

Influence of pions and hyperons on stellar black hole formation

We present numerical simulations of stellar core-collapse with spherically symmetric, general relativistic hydrodynamics up to black hole formation. Using the CoCoNuT code, with a newly developed grey leakage scheme for the neutrino treatment, we investigate the effects of including pions and \Lambda-hyperons into the equation of state at high densities and temperatures on the black hole formation process. Results show non-negligible differences between the models with reference equation of state without any additional particles and models with the extended ones. For the latter, the maximum masses supported by the proto-neutron star are smaller and the collapse to a black hole occurs earlier. A phase transition to hyperonic matter is observed when the progenitor allows for a high enough accretion rate onto the proto-neutron star. Rough estimates of neutrino luminosity from these collapses are given, too.

Two-meson exchange hyperonic three-body forces and consequences for neutron stars

Abstract We construct two-meson exchange three-baryon potentials between two nucleons and one hyperon ( NNY ) consistent with the two body nucleon–hyperon ( NY ) J04 potential of the Julich group. In particular, we focus on the NNΛ and N N Σ − forces since Λ and Σ − are the first hyperons expected to appear in microscopic calculations of neutron star matter. Brueckner–Hartree–Fock calculations of the hyperonic matter are then performed including the effect of these three-body forces. Our results show that at high densities the total contribution of the NNY force is repulsive making the resulting equation of state stiffer, as required from neutron star mass observations.

Relativistic mean-field treatment of pulsar kicks from neutrino propagation in magnetized proto-neutron stars

We make a perturbative calculation of neutrino scattering and absorption in hot and dense hyperonic neutron-star matter in the presence of a strong magnetic field. We calculate that the absorption cross-sections in a fully relativistic mean-field theory. We find that there is a remarkable angular dependence, i.e., the neutrino absorption strength is reduced in a direction parallel to the magnetic field and enhanced in the opposite direction. This asymmetry in the neutrino absorption is estimated to be as much as 2.2% of the entire neutrino momentum for an interior magnetic field of $\ensuremath{\sim}2\ifmmode\times\else\texttimes\fi{}{10}^{17}\text{ }\text{ }\mathrm{G}$. The pulsar kick velocities associated with this asymmetry are shown to be comparable to observed velocities.