Hyperon Cores Research Articles

Effect of the σ -cut potential on the properties of neutron stars with or without a hyperonic core

Motivated by the recent observation of high-mass pulsars ($M \simeq 2 M_{\odot}$), we employ the $\sigma$-cut potential on the equation of state (EOS) of high-density matter and the properties of neutron stars within the relativistic mean-field (RMF) model using TM1$^{*}$ parameter set. The $\sigma$-cut potential is known to reduce the contributions of the $\sigma$ field, resulting in a stiffer EOS at high densities and hence leading to larger neutron star masses without affecting the properties of nuclear matter at normal saturation density. We also analyzed the effect of the same on pure neutron matter and also on the neutron star matter with and without hyperonic core and compared it with the available theoretical, experimental, and observational data. The corresponding tidal deformability ($\Lambda_{1.4}$) is also calculated. With the choice of meson-hyperon coupling fixed to hypernuclear potentials, we obtain $\approx 10~\%$ increase in mass by employing the $\sigma$-cut potential for $f_{s} = 0.6$. Our results are in good agreement with various experimental constraints and observational data, particularly with the GW170817 data.

Hyperon bulk viscosity and r-modes of neutron stars

ABSTRACT We propose and apply a new parametrization of the modified chiral effective model to study rotating neutron stars with hyperon cores in the framework of the relativistic mean-field theory. The inclusion of mesonic cross couplings in the model has improved the density content of the symmetry energy slope parameters, which are in agreement with the findings from recent terrestrial experiments. The bulk viscosity of the hyperonic medium is analyzed to investigate its role in the suppression of gravitationally driven r-modes. The hyperonic bulk viscosity coefficient caused by non-leptonic weak interactions and the corresponding damping time-scales are calculated and the r-mode instability windows are obtained. The present model predicts a significant reduction of the unstable region due to a more effective damping of oscillations. We find that from ∼108 K to ∼109 K, hyperonic bulk viscosity completely suppresses the r-modes leading to a stable region between the instability windows. Our analysis indicates that the instability can reduce the angular velocity of the star up to ∼0.3 ΩK, where ΩK is the Kepler frequency of the star.

Thermal luminosity degeneracy of magnetized neutron stars with and without hyperon cores

ABSTRACT The dissipation of intense crustal electric currents produces high Joule heating rates in cooling neutron stars. Here, it is shown that Joule heating can counterbalance fast cooling, making it difficult to infer the presence of hyperons (which accelerate cooling) from measurements of the observed thermal luminosity Lγ. Models with and without hyperon cores match Lγ of young magnetars (with poloidal–dipolar field Bdip ≳ 1014 G at the polar surface and Lγ ≳ 1034 erg s−1 at t ≲ 105 yr) as well as mature, moderately magnetized stars (with Bdip ≲ 1014 G and 1031 erg s−1 ≲ Lγ ≲ 1032 erg s−1 at t ≳ 105 yr). In magnetars, the crustal temperature is almost independent of hyperon direct Urca cooling in the core, regardless of whether the latter is suppressed or not by hyperon superfluidity. The thermal luminosities of light magnetars without hyperons and heavy magnetars with hyperons have Lγ in the same range and are almost indistinguishable. Likewise, Lγ data of neutron stars with Bdip ≲ 1014 G but with strong internal fields are not suitable to extract information about the equation of state as long as hyperons are superfluid, with maximum amplitude of the energy gaps of the order ≈1 MeV.

Hyperonic equation of state at finite temperature for neutron stars

We review the composition and the equation of state of the hyperonic core of neutron stars at finite temperature within a relativistic mean-field approach. We make use of the new FSU2H∗ model, which is built upon the FSU2H scheme by improving on the Ξ potential according to the recent analysis on the Ξ atoms, and we extend it to include finite temperature corrections. The calculations are done for a wide range of densities, temperatures and charge fractions, thus exploring the different conditions that can be found in protoneutron stars, binary mergers remnants and supernovae explosions. The inclusion of hyperons has a strong effect on the composition and the equation of state at finite temperature, which consequently would lead to significant changes in the properties and evolution of hot neutron stars.

Complexity and neutron stars with crust and hyperon core

Continuing the work of K. Ch. Chatzisavvas and others [1], a measure of complexity proposed by R. López-Ruiz, H.L Mancini, and X. Calbet [2] is used to study a model of neutron star (NS) with crust and hyperon core. We employed the relativistic-mean-field approximation theory to build the Lagrangian density model of the neutron star’s core and from it, obtained the equation of state (EoS), ∈(ρ). This ∈(ρ) was then put into the complexity equation and the complexity-values were obtained using numerical methods. Plotted against the corresponding mass values, the results show that neutron star’s complexity-mass curves behave very similarly to an isolated-system phase complexity diagram, which was not apparent in the previous work by Chatzisavvas et al.. The reason for this is that the crust EoS put an upper limit to the complexity value and it follows that the crust itself is an ordered system of low complexity. We also show that NS have the properties of a perfect crystal although they are modeled as liquid.

Bulk viscosity in neutron stars with hyperon cores

It is well-known that r-mode oscillations of rotating neutron stars may be unstable with respect to the gravitational wave emission. It is highly unlikely to observe a neutron star with the parameters within the instability window, a domain where this instability is not suppressed. But if one adopts the `minimal' (nucleonic) composition of the stellar interior, a lot of observed stars appear to be within the r-mode instability window. One of the possible solutions to this problem is to account for hyperons in the neutron star core. The presence of hyperons allows for a set of powerful (lepton-free) non-equilibrium weak processes, which increase the bulk viscosity, and thus suppress the r-mode instability. Existing calculations of the instability windows for hyperon NSs generally use reaction rates calculated for the $\Sigma^-\Lambda$ hyperonic composition via the contact $W$ boson exchange interaction. In contrast, here we employ hyperonic equations of state where the $\Lambda$ and $\Xi^-$ are the first hyperons to appear (the $\Sigma^-$'s, if they are present, appear at much larger densities), and consider the meson exchange channel, which is more effective for the lepton-free weak processes. We calculate the bulk viscosity for the non-paired $npe\mu\Lambda\Xi^-$ matter using the meson exchange weak interaction. A number of viscosity-generating non-equilibrium processes is considered (some of them for the first time in the neutron-star context). The calculated reaction rates and bulk viscosity are approximated by simple analytic formulas, easy-to-use in applications. Applying our results to calculation of the instability window, we argue that accounting for hyperons may be a viable solution to the r-mode problem.

R-mode stabilization in neutron stars with hyperon cores

R-mode oscillations of neutron stars are known to be generally unstable with respect to the emission of gravitational waves. This instability can be suppressed by dissipative processes in the neutron star core. The standard minimal nucleonic model of the core cannot stabilize r-modes for many observed neutron stars, at the same time the probability to observe them unstable should be very low. One of the solutions to this problem is to account for possible presence of hyperons in the stellar core. Hyperons strongly enhance bulk viscosity in the core, which effectively suppresses the r-mode instability. We revisit the bulk viscosity calculations in the non-superfluid hyperon matter and briefly discuss the effect of baryon superfluidity. Our results suggest that the hyperon solution to the r-mode problem is more promising than it was thought before.

Properties of Neutron stars with hyperon cores in parametrized hydrostatic conditions

Models of neutron stars (NSs) with hyperon cores are constructed with an effective chiral model in mean-field approximation. The hyperon couplings are fixed by reproducing their experimentally determined binding energies. The impact of these couplings on population of different particles and the equation of state (EoS) are studied in this work. The global properties of NSs like gravitational mass, radius, baryonic mass and central density are calculated using parametrized Tolman–Oppenheimer–Volkoff equations (PTOVs) with special emphasis on two effects of pressure — one contributing to total mass density and the other to self-gravity of the star. We find that with PTOV solutions in static conditions, a softer EoS (including hyperons) can also lead to massive stellar configurations of NSs, which are in good agreement with the observed maximum mass bound of [Formula: see text] (PSR J0348-0432). Estimates of [Formula: see text] and [Formula: see text], obtained with the PTOV equations, are consistent with the recent findings of the same from the data analysis of gravitational waves (GW170817) observation.

Dynamical tides in coalescing superfluid neutron star binaries with hyperon cores and their detectability with third-generation gravitational-wave detectors

The dynamical tide in a coalescing neutron star binary induces phase shifts in the gravitational waveform as the orbit sweeps through resonances with individual g-modes. Unlike the phase shift due to the equilibrium tide, the phase shifts due to the dynamical tide are sensitive to the stratification, composition, and superfluid state of the core. We extend our previous study of the dynamical tide in superfluid neutron stars by allowing for hyperons in the core. Hyperon gradients give rise to a new type of composition g-mode. Compared to g-modes due to muon-to-electron gradients, those due to hyperon gradients are concentrated much deeper in the core and therefore probe higher density regions. We find that the phase shifts due to resonantly excited hyperonic modes are ~ 10^{-3} rad, an order of magnitude smaller than those due to muonic modes. We show that by stacking events, third generation gravitational-wave detectors should be able to detect the phase shifts due to muonic modes. Those due to hyperonic modes will, however, be difficult to detect due to their smaller magnitude.

The Equation of State for the Nucleonic and Hyperonic Core of Neutron Stars

AbstractWe re-examine the equation of state for the nucleonic and hyperonic inner core of neutron stars that satisfies the 2M⊙ observations as well as the recent determinations of stellar radii below 13 km, while fulfilling the saturation properties of nuclear matter and finite nuclei together with the constraints on the high-density nuclear pressure coming from heavy-ion collisions. The recent nucleonic FSU2R and hyperonic FSU2H models are updated in order to improve the behaviour of pure neutron matter at subsaturation densities. The corresponding nuclear matter properties at saturation, the symmetry energy, and its slope turn out to be compatible with recent experimental and theoretical determinations. We obtain the mass, radius, and composition of neutron stars for the two updated models and study the impact on these properties of the uncertainties in the hyperon–nucleon couplings estimated from hypernuclear data. We find that the onset of appearance of each hyperon strongly depends on the hyperon–nuclear uncertainties, whereas the maximum masses for neutron stars differ by at most 0.1M⊙, although a larger deviation should be expected tied to the lack of knowledge of the hyperon potentials at the high densities present in the centre of 2M⊙ stars. For easier use, we provide tables with the results from the FSU2R and FSU2H models for the equation of state and the neutron star mass–radius relation.

Theory of neutrino emission from nucleon–hyperon matter in neutron stars: angular integrals

Investigations of thermal evolution of neutron stars with hyperon cores require neutrino emissivities for many neutrino reactions involving strongly degenerate particles (nucleons, hyperons, electrons, muons). We calculate the angular integrals $I_n$ (over orientations of momenta of $n$ degenerate particles) for major neutrino reactions with $n$ =3, 4, 5 at all possible combinations of particle Fermi momenta. The integrals $I_n$ are necessary ingredients for constructing a uniform database of neutrino emissivities in dense nucleon-hyperon matter. The results can also be used in many problems of physical kinetics of strongly degenerate systems.

Neutron stars with hyperon cores: stellar radii and equation of state near nuclear density

The existence of 2 Msun pulsars puts very strong constraints on the equation of state (EOS) of neutron stars (NSs) with hyperon cores, which can be satisfied only by special models of hadronic matter. The radius-mass relation for these models is sufficiently specific that it could be subjected to an observational test with future X-ray observatories. We want to study the impact of the presence of hyperon cores on the radius-mass relation for NS. We aim to find out how, and for which particular stellar mass range, a specific relation R(M), where M is the gravitational mass, and R is the circumferential radius, is associated with the presence of a hyperon core. We consider a set of 14 theoretical EOS of dense matter, based on the relativistic mean-field (RMF) approximation, allowing for the presence of hyperons in NSs. We seek correlations between R(M) and the stiffness of the EOS below the hyperon threshold needed to pass the 2 Msun test. For NS masses 1.0<M/Msun<1.6, we get R>13km, because of a very stiff pre-hyperon segment of the EOS. At nuclear density, the pressure is significantly higher than a robust upper bound obtained recently using chiral effective field theory. If massive NSs do have a sizable hyperon core, then according to current models the radii for M=1.0-1.6 Msun are necessarily >13km. If, on the contrary, a NS with a radius R<12 km is observed in this mass domain, then sizable hyperon cores in NSs, as we model them now, are ruled out. Future X-ray missions with <5% precision for a simultaneous M and R measurement will have the potential to solve the problem with observations of NSs. Irrespective of this observational test, present EOS allowing for hyperons that fulfill condition M_max>2 Msun yield a pressure at nuclear density that is too high relative to up-to-date microscopic calculations of this quantity.

Anisotropic pressure and hyperons in neutron stars

We study the effects of anisotropic pressure (AI-P) on properties of the neutron stars (NSs) with hyperons inside its core within the framework of extended relativistic mean field. It is found that the main effects of AI-P on NS matter is to increase the stiffness of the equation of state EOS, which compensates for the softening of the EOS due to the hyperons. The maximum mass and redshift predictions of anisotropic neutron star with hyperonic core are quite compatible with the result of recent observational constraints if we use the parameter of AI-P model h ≤ 0.8 [L. Herrera and W. Barreto, Phys. Rev. D 88 (2013) 084022.] and Λ ≤ -1.15 [D. D. Doneva and S. S. Yazadjiev, Phys. Rev. D 85 (2012) 124023.]. The radius of the corresponding NS at M = 1.4 M⊙ is more than 13 km, while the effect of AI-P on the minimum mass of NS is insignificant. Furthermore, due to the AI-P in the NS, the maximum mass limit of higher than 2.1 M⊙ cannot rule out the presence of hyperons in the NS core.

Impact of the Strangeness on the Structure of a Neutron Star

The comparative analysis of the impact of strangeness on the structure and evolution of a neutron star was performed on the basis of two theoretical models. These models are indistinguishable for the density range relevant for the outer core of a neutron star. However, they gave qualitatively dierent results for the hyperon core of a neutron star.

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.

Maximum mass of neutron stars and strange neutron-star cores

Recent measurement of mass of PSR J1614-2230 rules out most of existing models of equation of state (EOS) of dense matter with high-density softening due to hyperonization, based on the recent hyperon-nucleon and hyperon-hyperon interactions, leading to a "hyperon puzzle". We study a specific solution of "hyperon puzzle", consisting in replacing a too soft hyperon core by a sufficiently stiff quark core. We construct an analytic approximation fitting very well modern EOSs of 2SC and CFL color superconducting phases of quark matter. This allows us for simulating continua of sequences of first-order phase transitions from hadronic matter to the 2SC, and then to the CFL state of color superconducting quark matter. We obtain constraints in the parameter space of the EOS of superconducting quark cores, resulting from M_max> 2 M_sol. We also derive constraints that would result from significantly higher measured masses. For 2.4 M_sol required stiffness of the CFL quark core should have been close to the causality limit, the density jump at the phase transition being very small. Condition M_max > 2 M_sol puts strong constraints on the EOSs of the 2SC and CFL phases of quark matter. Density jumps at the phase transitions have to be sufficiently small and sound speeds in quark matter - sufficiently large. A strict condition of thermodynamic stability of quark phase results in the maximum mass of hybrid stars similar to that of purely baryon stars. Therefore, to get M_max>2 M_sol for stable hybrid stars, both sufficiently strong additional hyperon repulsion at high density baryon matter and a sufficiently stiff EOS of quark matter would be needed. However, it is likely that the high density instability of quark matter (reconfinement) indicates actually the inadequacy of the point-particle model of baryons in dense matter at very high densities.

Dynamics of dissipative multifluid neutron star cores

We present a Newtonian multifluid formalism for superfluid neutron star cores, focusing on the additional dissipative terms which arise when one takes into account the individual dynamical degrees of freedom associated with the coupled ``fluids.'' The problem is of direct astrophysical interest as the nature of the dissipative terms can have significant impact on the damping of the various oscillation modes of the star and the associated gravitational-wave signatures. A particularly interesting application concerns the gravitational-wave driven instability of $f$- and $r$-modes. We apply the developed formalism to two specific three-fluid systems: (i) a hyperon core in which both $\ensuremath{\Lambda}$ and ${\ensuremath{\Sigma}}^{\ensuremath{-}}$ hyperons are present and (ii) a core of deconfined quarks in the color-flavor-locked phase in which a population of neutral ${K}^{0}$ kaons is present. The formalism is, however, general and can be applied to other problems in neutron-star dynamics (such as the effect of thermal excitations close to the superfluid transition temperature) as well as laboratory multifluid systems.

Hyperons in neutron-star cores and a 2 M⊙pulsar

Recent measurement of mass of PSR J1614-2230 rules out most of existing models of equation of state (EOS) of dense matter with high-density softening due to hyperonization or a phase transition to quark matter or a boson condensate. We look for a solution of an apparent contradiction between the consequences stemming from up-to-date hypernuclear data, indicating appearance of hyperons at 3 nuclear densities and existence of a two-solar-mass neutron star. We consider a non-linear relativistic mean field (RMF) model involving baryon octet coupled to meson fields. An effective lagrangian includes quartic terms in the meson fields. The values of the parameters of the model are obtained by fitting semi-empirical parameters of nuclear matter at the saturation point, as well as potential wells for hyperons in nuclear matter and the strength of the Lambda-Lambda attraction in double-Lambda hypernuclei. We propose a non-linear RMF model which is consistent with up-to-date semiempirical nuclear and hypernuclear data and allows for neutron stars with hyperon cores and M larger than 2 solar masses. The model involves hidden-strangenes scalar and vector mesons, coupled to hyperons only, and quartic terms involving vector meson fields. Our EOS involving hyperons is stiffer than the corresponding nucleonic EOS (with hyperons artificially suppressed) above five nuclear densities. Required stiffening is generated by the quartic terms involving hidden-strangeness vector meson.

Oscillations in the angular velocity of pulsars

Observational data on long-period oscillations in the angular velocity of pulsars are examined. The characteristic radii of the superfluid regions of pulsars are determined assuming that these oscillations are collective elastic oscillations of a superfluid vortex lattice (Tkachenko oscillations). These radii are compared with values obtained in various theoretical models of neutron stars which assume the existence of a superfluid quark “CFL”-core or a superfluid hyperon core. This method can be used to estimate the radii of pulsars.

SPECTROSCOPY OF MEDIUM-MASS HYPERNUCLEAR PRODUCTION

Theoretical hypernuclear photoproduction spectra are demonstrated by choosing typical medium-heavy nuclear targets such as 28 Si , 40 Ca , and 52 Cr . The predictions provide us with interesting opportunities of spectroscopic study beyond p-shell on the dynamical coupling between a hyperon and nuclear core excitations.