Density Dependent Relativistic Mean Field Research Articles

Probing the impact of delta-baryons on nuclear matter and non-radial oscillations in neutron stars

Non-radial oscillations of Neutron Stars (NSs) provide a means to learn important details regarding their interior composition and equation of state. We consider the effects of Δ-baryons on non-radial f-mode oscillations and other NS properties within the Density-Dependent Relativistic Mean Field formalism. Calculations are performed for Δ-admixed NS matter with and without hyperons. Our study of the non-radial f-mode oscillations revealed a distinct increase in frequency due to the addition of the Δ-baryons with upto 20% increase in frequency being seen for canonical NSs. Other bulk properties of NSs, including mass, radii, and dimensionless tidal deformability (Λ) were also affected by these additional baryons. Comparing our results with available observational data from pulsars (NICER) and gravitational waves (LIGO-VIRGO collaboration), we found strong agreement, particularly concerning Λ.

Density-dependent relativistic mean field approach and its application to single-Λ hypernuclei in oxygen hyperisotopes* *Supported by the Fundamental Research Funds for the Central Universities, Lanzhou University (lzujbky-2022-sp02, lzujbky-2023-stlt01), the National Natural Science Foundation of China (11875152, 12275111), and the Strategic Priority Research Program of Chinese Academy of Sciences (XDB34000000)

The in-medium feature of nuclear force, which includes both nucleon-nucleon () and hyperon-nucleon () interactions, impacts the description of single-Λ hypernuclei. With the alternated mass number or isospin of hypernuclei, such effects may be unveiled by analyzing the systematic evolution of the bulk and single-particle properties. From a density-dependent meson-nucleon/hyperon coupling perspective, a new effective interaction in the covariant density functional (CDF) theory, namely, DD-LZ1-, is obtained by fitting the experimental data of Λ separation energies for several single-Λ hypernuclei. It is then used to study the structure and transition properties of single-Λ hypernuclei in oxygen hyperisotopes, in comparison with those determined using several selected CDF Lagrangians. A discrepancy is explicitly observed in the isospin evolution of spin-orbit splitting with various effective interactions, which is attributed to the divergence of the meson-hyperon coupling strengths with increasing density. In particular, the density-dependent CDFs introduce an extra contribution to reduce the value but enhance the isospin dependence of the splitting, which originates from the rearrangement terms of Λ self-energies. In addition, the characteristics of hypernuclear radii are studied along the isotopic chain. Owing to the impurity effect of the Λ hyperon, a size shrinkage is observed in the matter radii of hypernuclei compared with the cores of normal nuclei, and its magnitude is further elucidated to correlate with the incompressibility of nuclear matter. Moreover, there is a sizable model-dependent trend in which the Λ hyperon radii evolve with neutron number, which is decided partly by the in-medium interactions and core polarization effects.

Magnetic-field Induced Deformation in Hybrid Stars

The effects of strong magnetic fields on the deconfinement phase transition expected to take place in the interior of massive neutron stars are studied in detail for the first time. For hadronic matter, the very general density-dependent relativistic mean field model is employed, while the simple, but effective vector-enhanced bag model is used to study quark matter. Magnetic-field effects are incorporated into the matter equation of state and in the general-relativity solutions, which also satisfy Maxwell’s equations. We find that for large values of magnetic dipole moment, the maximum mass, canonical mass radius, and dimensionless tidal deformability obtained for stars using spherically symmetric Tolman–Oppenheimer–Volkoff (TOV) equations and axisymmetric solutions attained through the LORENE library differ considerably. The deviations depend on the stiffness of the equation of state and on the star mass being analyzed. This points to the fact that, unlike what was assumed previously in the literature, magnetic field thresholds for the approximation of isotropic stars and the acceptable use of TOV equations depend on the matter composition and interactions.

Bayesian inference of signatures of hyperons inside neutron stars

The possible signatures of the presence of hyperons inside neutron stars are discussed within a Bayesian inference framework applied to a set of models based on a density-dependent relativistic mean-field description of hadronic matter. Nuclear matter properties, hypernuclei properties, and observational information are used to constrain the models. General properties of neutron stars such as the maximum mass, radius, tidal deformability, proton fraction, hyperon fraction, and speed of sound are discussed. It is shown that the two solar mass constraint imposes that neutron stars described by equations of state that include hyperons have on average a larger radius, $\gtrsim 0.5$km, and a larger tidal deformability, $\gtrsim 150$, than the stars determined from a nucleonic equation of state, while the speed of sound at the center of the star is more than 25\% smaller. If a 1.4 M$_\odot$ star with a radius $\lesssim 12.5$ km is measured it is quite improbable that a massive star described by the same model contains hyperons. A similar conclusion is drawn if a two solar mass star with a radius $\lesssim 11.5$ km or a neutron star with a mass above 2.2 M$_\odot$ is observed: the possible hyperon content of these stars is ruled out or very reduced. The hyperon presence inside neutron stars is compatible with the present NICER mass-radius observations of the pulsars PSR J0030+0451 and PSR J0740+6620 and the gravitational wave detection GW170817. {It is shown that if the polytropic index $\gamma=\partial\ln p/\partial\ln\epsilon$ takes values of the order of 1.75 at not too large densities, it may indicate the onset of some kind of exotic matter, but not necessarily of deconfined quark matter.

Effects of the ϕ Meson on the Properties of Hyperon Stars in the Density-dependent Relativistic Mean Field Model

Abstract The effects of the ϕ meson on the properties of hyperon stars are studied systematically in the framework of the density-dependent relativistic mean field (DDRMF) model. The ϕ meson shifts the hyperon threshold to a higher density and reduces the hyperon fractions in neutron star cores. It also strongly stiffens the equation of state calculated with various DDRMF effective interactions and increases the maximum mass of hyperon stars, but only a few effective interactions survive under the constraints from recent astrophysical observations. In the DDRMF model, the conformal limit of the sound velocity is still in strong tension with the fact that the maximum mass of neutron stars obtained in theoretical calculations reaches about 2 M ⊙. Based on different interior composition assumptions, we discuss the possibility of the secondary object of GW190814 as a neutron star. When the ϕ meson is considered, DD-ME2 and DD-MEX support the possibility that the secondary object of GW190814 is a hyperon star rapidly rotating with Kepler frequency.

New effective interactions for hypernuclei in a density-dependent relativistic mean field model

New effective $\Lambda N$ interactions are proposed for the density dependent relativistic mean field model. The multidimensionally constrained relativistic mean field model is used to calculate ground state properties of eleven known $\Lambda$ hypernuclei with $A\ge 12$ and the corresponding core nuclei. Based on effective $NN$ interactions DD-ME2 and PKDD, the ratios $R_\sigma$ and $R_\omega$ of scalar and vector coupling constants between $\Lambda N$ and $NN$ interactions are determined by fitting calculated $\Lambda$ separation energies to experimental values. We propose six new effective interactions for $\Lambda$ hypernuclei: DD-ME2-Y1, DD-ME2-Y2, DD-ME2-Y3, PKDD-Y1, PKDD-Y2 and PKDD-Y3 with three ways of grouping and including these eleven hypernuclei in the fitting. It is found that the two ratios $R_\sigma$ and $R_\omega$ are correlated well and there holds a good linear relation between them. The statistical errors of the ratio parameters in these effective interactions are analyzed. These new effective interactions are used to study the equation of state of hypernuclear matter and neutron star properties with hyperons.

Tidal deformability of neutron stars with exotic particles within a density dependent relativistic mean field model in R-squared gravity

There is a growing interest in investigating modified theories of gravity, primarily, with the aim of explaining the universe's accelerated expansion, which has been confirmed by several independent observations. Compact objects, like neutron stars, exhibit strong gravity effects and therefore are used to study modified gravity theories. We use the f(R)=R+aR 2 model, where R is the Ricci scalar and a is a free parameter. This model has been studied both perturbatively and non-perturbatively. However, it was found that perturbative methods results in nonphysical solutions for the neutron stars. In this paper, we examine neutron star properties, such as mass, radius, tidal deformability in non-perturbative f(R) gravity model with density dependent relativistic equation of state with different particle compositions. The strange particles in the core of NS in the form of Λ hyperons, K - condensate, and quarks are considered. We have observed that while the mass-radius relation allows for a wide range of parameter a, when tidal deformability is considered, the parameter a is constrained down by one order.

Heavy Magnetic Neutron Stars

Abstract We systematically study the properties of pure nucleonic and hyperonic magnetic stars using a density-dependent relativistic mean-field (DD-RMF) equations of state. We explore several parameter sets and hyperon coupling schemes within the DD-RMF formalism. We focus on sets that are in better agreement with nuclear and other astrophysical data while generating heavy neutron stars. Magnetic field effects are included in the matter equation of state and in general relativity solutions, which in addition fulfill Maxwell’s equations. We find that pure nucleonic matter, even without magnetic field effects, generates neutron stars that satisfy the potential GW 190814 mass constraint; however, this is not the case for hyperonic matter, which instead only satisfies the more conservative 2.1 M ⊙ constraint. In the presence of strong but still somehow realistic internal magnetic fields ≈1017 G, the stellar charged particle population re-leptonizes and de-hyperonizes. As a consequence, magnetic fields stiffen hyperonic equations of state and generate more massive neutron stars, which can satisfy the possible GW 190814 mass constraint but present a large deformation with respect to spherical symmetry.

Role of antikaon condensation on the universality relations of hot and rapidly rotating neutron stars

We study cold as well as hot neutron star (NS) at finite entropy using density dependent relativistic mean field model in the presence of nucleons and antikaon condensates. The parameters like gravitational mass(M), radius(R), moment of inertia(I) and quadrupole moment(Q) are calculated as a function of rotation frequency for a NS with fixed baryonic mass. Next, we investigate the relation of normalized I with compactness (M/R). Finally, we extend our study to calculate the tidal deformability parameter and tidal love number and show their variation with compactness.

The Possibility of the Secondary Object in GW190814 as a Neutron Star

Abstract A compact object was observed with a mass of by LIGO Scientific and Virgo collaborations (LVC) in GW190814, which provides a great challenge to investigations of supranuclear matter. To study this object, the properties of the neutron star are systematically calculated within the latest density-dependent relativistic mean-field (DDRMF) parameterizations, which are determined by the ground-state properties of spherical nuclei. The maximum masses of the neutron star calculated by DD-MEX and DD-LZ1 sets can be around with quite stiff equations of state generated by their strong repulsive contributions from vector potentials at high densities. Their maximum speeds of sound c s /c are smaller than at the center of the neutron star, and the dimensionless tidal deformabilities at are less than 800. Furthermore, the radii of also satisfy the constraint from the observation of simultaneous mass–radius measurements (Neutron star Interior Composition Explorer). Therefore, we conclude that one cannot exclude the possibility of the secondary object in GW190814 as a neutron star composed of hadron matter from DDRMF models.

Properties of massive rotating protoneutron stars with hyperons: structure and universality

In this work, we study the properties and structure of a massive and rapidly rotating protoneutron star (PNS) with hyperon content. We follow several stages of quasi-stationary evolution in an approximate way at four discrete steps. We use a density-dependent relativistic mean field theory model and calculate different quantities such as mass, equatorial radius, moment of inertia, and quadrupole moment to get different rotating configurations up to the mass-shedding limit. We study the effect of the appearance of Λ, the lightest of all hyperons, on each of the evolutionary stages of the PNS. We also check its sensitivity to the inclusion of ϕ vector meson as a mediator of Λ–Λ interaction in detail. Finally, we investigate the universal relations between moment of inertia and compactness in the context of a hot and young compact object.

Moments of inertia of neutron stars in relativistic mean field theory: The role of the isovector scalar channel

With the inclusion of the isovector scalar channel in the meson-nucleon couplings, taking DD-ME$\delta$ as an effective interaction, the moments of inertia of neutron stars possessing various stellar masses are studied within the density dependent relativistic mean field (RMF) theory. The isovector scalar channel contributes to the softening of the neutron-star matter equation of state (EOS) and therefore the reduction of the maximum mass and radius of neutron stars. Smaller values of the total moment of inertia $I$ and the crustal moment of inertia $\Delta{I}$ are then obtained in DD-ME$\delta$ via numerical procedure in comparison with those in other selected RMF functionals. In addition, the involvement of the isovector scalar channel lowers the thickness of the neutron star crust and its mass fraction as well. The sensitivity to both the crustal mass and stellar radius causes the crustal moment of inertia to be more obviously reduced than the total one, eventually leading to a suppression on the fraction of crustal moment of inertia $\Delta{I}/I$ in DD-ME$\delta$. The results indicate the crustal moment of inertia as a more sensitive probe of the neutron-star matter EOS than the total one, and demonstrate that the isovector scalar meson-nucleon couplings in the RMF theory could exert influence over the physics of pulsar glitches.

Nuclear fourth-order symmetry energy and its effects on neutron star properties in the relativistic Hartree-Fock theory

Adopting the density dependent relativistic mean-field (RMF) and relativistic Hartree-Fock (RHF) approaches, the properties of the nuclear fourth-order symmetry energy $S_4$ are studied within the covariant density functional (CDF) theory. It is found that the fourth-order symmetry energies are suppressed in RHF at both saturation and supranuclear densities, where the extra contribution from the Fock terms is demonstrated, specifically via the isoscalar meson-nucleon coupling channels. The reservation of $S_4$ and higher-order symmetry energies in the nuclear equation of state then affects essentially the prediction of neutron star properties, which is illustrated in the quantities such as the proton fraction, the core-crust transition density as well as the fraction of crustal moment of inertia. Since the Fock terms enhance the density dependence of the thermodynamical potential, the RHF calculations predict systematically smaller values of density, proton fraction and pressure at the core-crust transition boundary of neutron stars than density dependent RMF ones. In addition, a linear anti-correlation between the core-crust transition density $\rho_t$ and the density slope of symmetry energy $L$ is found which is then utilized to constrain the core-crust transition density as $\rho_t\thicksim[0.069, 0.098]~\rm{fm}^{-3}$ with the recent empirical information on $L$. The study clarifies the important role of the fourth-order symmetry energy in determining the properties of nuclear matter at extreme isospin or density conditions.

Neutron star properties in density-dependent relativistic mean field theory with consideration of an isovector scalar meson

Based on the density-dependent relativistic mean field theory, the properties for nuclear matter and neutron stars with the effective interaction $\mathrm{DD}\text{\ensuremath{-}}\mathrm{ME}\ensuremath{\delta}$ including the isovector scalar channel which disentangle the effects of isovector scalar and isovector vector channels by fitting microscopic calculations. The influences of the isovector scalar $\ensuremath{\delta}$ meson on properties of asymmetric nuclear matter at high densities are discussed in detail. The results support that the isovector scalar channel can soften the equation of state through the effects on the nucleon effective mass and the scalar $\ensuremath{\sigma}$ field and impact the behavior of the nuclear matter symmetry energy. Because of the influence on the symmetry energy by the $\ensuremath{\delta}$ meson, a larger proton fraction in neutron stars is predicted by the $\mathrm{DD}\text{\ensuremath{-}}\mathrm{ME}\ensuremath{\delta}$ calculation, which strongly affects the cooling process of the star. The maximum masses of neutron stars given by the $\mathrm{DD}\text{\ensuremath{-}}\mathrm{ME}\ensuremath{\delta}$ calculation is $1.97{M}_{⨀}$ which is in reasonable agreement with PSR J1614 \ensuremath{-} 2230 ($1.97 \ifmmode\pm\else\textpm\fi{} 0.04 {M}_{⨀}$) and PSR J0348 + 0432 $(2.01 \ifmmode\pm\else\textpm\fi{} 0.04 {M}_{⨀})$. Among all the selected interactions, $\mathrm{DD}\text{\ensuremath{-}}\mathrm{ME}\ensuremath{\delta}$ gives the smallest radius range. The radius for the 1.4 solar mass neutron star calculated by $\mathrm{DD}\text{\ensuremath{-}}\mathrm{ME}\ensuremath{\delta}$ is in good agreement with the prediction [Astrophys. J. Lett. 765, L5 (2013)] according to the resent observations.

Effects of the symmetry energy on strongly-magnetized neutron stars in the density-dependent RMF model

We investigate the structure of neutron stars with strong magnetic fields by using a density-dependent relativistic mean field model. The coupling constant between nucleons and the rho meson in the model is obtained by using the polytropic formula for the symmetry energy in the neutron star. With the density-dependent coupling constant, the effects of the symmetry energy on strongly-magnetized neutron stars are considered in detail for particle fractions and the equation of state. The consequent changes in the masses and the radii of the neutron star are discussed by using the roles of the symmetry energy in the neutron star.

Liquid-gas phase transition in hot asymmetric nuclear matter

We use a density-dependent relativistic mean field model to study the properties of nuclear systems at finite temperature. The liquid-gas phase transition of symmetric and asymmetric nuclear matter is discussed. A limiting pressure plim for hot asymmetric nuclear matter has been found because of the density dependence of the nucleon-nucleon-rho meson coupling. It is found that the liquid-gas phase transition cannot take place if p > plim. The binodal surface for this model is addressed. In addition, we calculated the asymmetry parameter dependence of the critical temperature.

Equation of state of dense matter from a density dependent relativistic mean field model

We calculate the equation of state (EoS) of dense matter, using a relativistic mean field (RMF) model with a density dependent coupling that is a slightly modified form of the original NL3 interaction. For nonuniform nuclear matter we approximate the unit lattice as a spherical Wigner-Seitz cell, wherein the meson mean fields and nucleon Dirac wave functions are solved fully self-consistently. We also calculate uniform nuclear matter for a wide range of temperatures, densities, and proton fractions, and match them to non-uniform matter as the density decreases. The calculations took over 6,000 CPU days in Indiana University's supercomputer clusters. We tabulate the resulting EoS at over 107,000 grid points in the proton fraction range $Y_P$ = 0 to 0.56. For the temperature range $T$ = 0.16 to 15.8 MeV we cover the density range $n_B$ = 10$^{-4}$ to 1.6 fm$^{-3}$; and for the higher temperature range $T$ = 15.8 to 80 MeV we cover the larger density range $n_B$ = 10$^{-8}$ to 1.6 fm$^{-3}$. In the future we plan to study low density, low temperature (T$<$15.8 MeV), nuclear matter using a Virial expansion, and we will match the low density and high density results to generate a complete EoS table for use in astrophysical simulations of supernova and neutron star mergers.

Dirac-Brueckner-Hartree-Fock calculations for isospin asymmetric nuclear matter based on improved approximation schemes

We present Dirac-Brueckner-Hartree-Fock calculations for isospin asymmetric nuclear matter which are based on improved approximations schemes. The potential matrix elements have been adapted for isospin asymmetric nuclear matter in order to account for the proton-neutron mass splitting in a more consistent way. The proton properties are particularly sensitive to this adaption and its consequences, whereas the neutron properties remains almost unaffected in neutron rich matter. Although at present full Brueckner calculations are still too complex to apply to finite nuclei, these relativistic Brueckner results can be used as a guidance to construct a density dependent relativistic mean field theory, which can be applied to finite nuclei. It is found that an accurate reproduction of the Dirac-Brueckner-Hartree-Fock equation of state requires a renormalization of these coupling functions.

Antikaon condensation and in-medium (anti)kaon production inβequilibrium nuclear matter

The density dependent relativistic mean field (DDRMF) model in the new parametrization used recently is extended to investigate antikaon condensation and in-medium (anti)kaon production in $\ensuremath{\beta}$ equilibrium nuclear matter, and the obtained results are compared for different models. Our results show that the mean critical density for antikaon condensation is $\ensuremath{\approx}2.8{\ensuremath{\rho}}_{0}$ in the DDRMF model with the two model parameter sets and in the nonlinear $\ensuremath{\sigma}\ensuremath{-}\ensuremath{\omega}$ model with the parameter set Tm1, while a little larger critical density $3.3{\ensuremath{\rho}}_{0}$ is obtained in chiral perturbation theory. The subthreshold energies for (anti)kaon production are calculated, and indicate that the in-medium effects predicted by the DDRMF model are essential for explaining the antikaon enhanced production, while kaon production rates are possibly enhanced only in the high density region. As an application for $\ensuremath{\beta}$ equilibrium nuclear matter, the masses against the central densities of neutron stars are studied in the DDRMF model too.

Phase transition in warm nuclear matter

A density-dependent relativistic mean field (DRMF) theory is extended to that at finite temperature and density, and the liquid-gas phase transition of warm nuclear matter is investigated. Our results are compared with those obtained from the Walecka's mean field theory. The critical values of temperature, density, and pressure are calculated by the DRMF. The obtained critical temperature is ${T}_{c}=12.66 \mathrm{MeV}$ by using the parameter set A, which is in accord with recent experimental observation.