arXiv subjects

Yeunhwan Lim

Publications and source records attributed to Yeunhwan Lim.

At least 19 recordsLinked to original sources

arXiv2026

Constraining hyperonic relativistic mean-field models with rapidly rotating neutron stars

Motivated by the recent mass measurement of the black-widow pulsar PSR~J0952$-$0607 with $M=2.35\pm0.11\,M_\odot$, we investigate how the masses of heavy, rapidly rotating millisecond pulsars can be used to constrain relativistic mean-field (RMF) models containing hyperonic degrees of freedom. In our approach, hyperons are incorporated following the spin-flavor SU(6) symmetry scheme for the vector-meson couplings. We find that increasing the nonlinear $ω$-meson vector self-coupling parameter $ζ$ suppresses the hyperon fraction and can alter the onset ordering of the $Σ^-$ and $Ξ^-$ hyperons. By computing rotating neutron-star configurations at the observed spin frequency $707\,\mathrm{Hz}$ of PSR~J0952$-$0607, we identify RMF models compatible with this pulsar's observed lower-mass bound. Using an empirical relation for the maximum neutron star mass, the PSR~J0952$-$0607 observational contraint is mapped onto the allowed RMF parameter space in $n_0$, $m^\ast$, and $ζ$.

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arXiv2026

Sensitivity of neutron drip lines and neutron star properties to the symmetry energy

We investigate the influence of the nuclear symmetry energy and its density slope parameter on the neutron dripline and neutron star properties using a semi-classical liquid drop model (LDM) and energy density functionals constrained by chiral effective field theory. To analyze finite nuclei and mass tables, the nuclear symmetry energy at saturation density is fixed, and the surface tension is determined to minimize the root-mean-square deviation of the total binding energy for 2208 nuclei. Correlations between symmetry energy parameters and neutron driplines, crust-core transition densities, and the radii of $1.4\,\msun$ neutron stars are explored using the LDM framework. Additionally, we examine the relationship between macroscopic properties, such as neutron star radii ($R_{1.4}$), and microscopic properties, including the number of isotopes and the last bound nucleus for $Z=28$, within the LDM context.

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arXiv2025

Universal Relation for the Neutron Star Maximum Mass within Relativistic Mean-Field Theories

We obtain a universal relation for the neutron star maximum mass arising from a particular combination of the saturation density ($n_0$), the effective mass ($m^*$), and (when present) the vector meson self-coupling constant ($ζ$) within the relativistic mean-field model framework. Observations of massive neutron stars heavier than $\sim 2M_{\odot}$ have eliminated the softest equation of state from consideration and impose strong constraints on nuclear interactions used to model dense nuclear matter. To date there have been numerous attempts to refine relativistic mean-field models by including the presence of additional mesons, such as the delta meson, and couplings. We show that current RMF models, including our own constructions, exhibit a maximum neutron star mass that is primarily determined by the combination of the saturation density, the effective mass at saturation, and the vector meson self-coupling constant. When constraining the pure neutron matter equation of state using chiral effective field theory (ChEFT) at low densities, 250 parameter sets were generated to derive an empirical formula for the maximum mass of neutron stars and apply the formula with the present relativistic mean field models.

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arXiv2024

Symmetry energy and neutron star properties constrained by chiral effective field theory calculations

We investigate the nuclear symmetry energy and neutron star properties using a Bayesian analysis based on constraints from different chiral effective field theory calculations using new energy density functionals that allow for large variations at high densities. Constraints at high densities are included from observations of GW170817 and from NICER. In particular, we show that both NICER analyses lead to very similar posterior results for the symmetry energy and neutron star properties when folded into our equation-of-state framework. Using the posteriors, we provide results for the symmetry energy and the slope parameter, as well as for the proton fraction, the speed of sound, and the central density in neutron stars. Moreover, we explore correlations of neutron star radii with the pressure and the speed of sound in neutron stars. Our 95\% credibility ranges for the symmetry energy $S_v$, the slope parameter $L$, and the radius of a 1.4$\msun$ neutron star, $R_{1.4}$, are $S_v=(30.6\text{--}33.9)$\,MeV, $L=(43.7\text{--}70.0)$\,MeV, and $R_{1.4}=(11.6\text{--}13.2)$\,km. Our analysis for the proton fraction shows that larger and/or heavier neutron stars are more likely to cool rapidly via the direct Urca process. Within our equation-of-state framework a maximum mass of neutron stars $M_{\rm max}>2.1\msun$ indicates that the speed of sound needs to exceed the conformal limit.

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arXiv2023

Shell-Model Description of the Isospin-Symmetry-Breaking Correction to Gamow-Teller $β$-Decay Rates and Their Mirror Asymmetries

The isospin-symmetry breaking correction, denoted as $δ_C$, is introduced for the first time within the shell-model framework to the nuclear matrix element of Gamow-Teller transitions. $δ_C$ is separated into two components: the isospin mixing term, $δ_{C1}$, induced by the Coulomb and nuclear charge-dependent forces in the effective Hamiltonian, and the radial mismatch term, $δ_{C2}$, arising from differences between proton and neutron realistic wave functions. Consequently, the refinement strategy developed for superallowed $0^+\rightarrow 0^+$ Fermi transitions is applied to Gamow-Teller transitions as well. It is demonstrated that, to a given precision level, the shell model calculation of $δ_C$ converges much faster than the calculation of the transition matrix element. Furthermore, higher-order correction terms are investigated and considered for consistent study of our works. Various interesting properties of the leading correction terms are discovered within the two-level model and parentage expansion of the one-body transition densities in angular momentum and isospin spaces. One such property is the dependence of $δ_{C1}$ on the isospin admixture amplitude, $α$, starting from the first order, while the same model yields $δ_{C1} = α^2$ for Fermi transitions. The calculated $δ_C$ values are then utilized to evaluate the mirror asymmetry of Gamow-Teller transition strengths, which are compared with available experimental data and other theoretical calculations. Due to the refined fitting procedure of the Woods-Saxon potential parameters and the improved convergence as a function of intermediate state number, our results show better agreement on average compared to those of Smirnova and Volpe [Nucl. Phys. {\bf A 714}, 441 (2003)].

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arXiv2023

Improved determination of the oscillator parameters in nuclei

The oscillator parameter in nuclei is refitted to reproduce the available charge radius data. As an important improvement, we include the Coulomb term evaluated within the assumption of a uniformly charged sphere, and take into account the symmetry effect induced by the difference between N and Z numbers in a straightforward manner using the conventional parameterization. The Coulomb interaction has repulsive effect, causing the wave functions to extend further toward the nucleus exterior, resulting in an effectively larger oscillator length parameter. The symmetry effect is attractive for protons in neutron-rich nuclei and for neutrons in proton-rich nuclei, and repulsive for the other cases. Therefore, three distinct oscillator parameters are determined: one for protons, one for neutrons, and one isospin-invariant version, which is obtained by subtracting the Coulomb and symmetry contributions. Additionally, we explore the direct fit of the harmonic oscillator wave functions to the eigenfunctions of the Hartree-Fock mean field using the Skyrme interaction. Generally, this method agrees well with the others for light nuclei, typically up to $^{40}$Ca. Beyond this nucleus, however, the results begin to diverge over the orbits chosen for the fit. Only the parameters values obtained for the last occupied states agree remarkably well with the conventional ones throughout the mass range under consideration.

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arXiv2022

Neutron star radii, deformabilities, and moments of inertia from experimental and ab initio theory constraints on the 208Pb neutron skin thickness

Recent experimental and ab initio theory investigations of the 208Pb neutron skin thickness are sufficiently precise to inform the neutron star equation of state. In particular, the strong correlation between the 208Pb neutron skin thickness and the pressure of neutron matter at normal nuclear densities leads to modified predictions for the radii, tidal deformabilities, and moments of inertia of typical 1.4 solar-mass neutron stars. In the present work, we study the relative impact of these recent analyses of the 208Pb neutron skin thickness on bulk properties of neutron stars within a Bayesian statistical analysis. Two models for the equation of state prior are employed in order to study the role of the highly uncertain high-density equation of state. From our combined Bayesian analysis of nuclear theory, nuclear experiment, and observational constraints on the dense matter equation of state, we find at the 90% credibility level $R_{1.4}=12.36^{+0.38}_{-0.73}$ km for the radius of a 1.4 solar-mass neutron star, $R_{2.0}=11.96^{+0.94}_{-0.71}$ km for the radius of a 2.0 solar-mass neutron star, $Λ_{1.4}=440^{+103}_{-144}$ for the tidal deformability of a 1.4 solar-mass neutron star, and $I_{1.338}=1.425^{+0.074}_{-0.146}\, \times 10^{45}\,\rm{g\,cm}^{2}$ for the moment of inertia of PSR J0737-3039A whose mass is 1.338 solar masses.

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arXiv2021

Radius and equation of state constraints from massive neutron stars and GW190814

Motivated by the unknown nature of the $2.50-2.67\,M_\odot$ compact object in the binary merger event GW190814, we study the maximum neutron star mass based on constraints from low-energy nuclear physics, neutron star tidal deformabilities from GW170817, and simultaneous mass-radius measurements of PSR J0030+045 from NICER. Our prior distribution is based on a combination of nuclear modeling valid in the vicinity of normal nuclear densities together with the assumption of a maximally stiff equation of state at high densities, a choice that enables us to probe the connection between observed heavy neutron stars and the transition density at which conventional nuclear physics models must break down. We demonstrate that a modification of the highly uncertain supra-saturation density equation of state beyond 2.64 times normal nuclear density is required in order for chiral effective field theory models to be consistent with current neutron star observations and the existence of $2.6\,M_\odot$ neutron stars. We also show that the existence of very massive neutron stars strongly impacts the radii of $\sim 2.0\,M_\odot$ neutron stars (but not necessarily the radii of $1.4\,M_\odot$ neutron stars), which further motivates future NICER radius measurements of PSR J1614-2230 and PSR J0740+6620.

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arXiv2021

Proton pairing in neutron stars from chiral effective field theory

We study the ${}^{1}S_0$ proton pairing gap in beta-equilibrated neutron star matter within the framework of chiral effective field theory. We focus on the role of three-body forces, which strongly modify the effective proton-proton spin-singlet interaction in dense matter. We find that three-body forces generically reduce both the size of the pairing gap and the maximum density at which proton pairing may occur. The pairing gap is computed within BCS theory, and model uncertainties are estimated by varying the nuclear potential and the choice of single-particle spectrum in the gap equation. We find that a second-order perturbative treatment of the single-particle spectrum suppresses the proton ${}^{1}S_0$ pairing gap relative to the use of a free spectrum. We estimate the critical temperature for the onset of proton superconductivity to be $T_c = (3.7 - 6.0)\times 10^{9} $ K, which is consistent with previous theoretical results in the literature and marginally within the range deduced from a recent Bayesian analysis of neutron star cooling observations.

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arXiv2019

Enhanced adiabatic index for hot neutron-rich matter from microscopic nuclear forces

We investigate the adiabatic index $Γ_{\mathrm{th}}$ of hot and dense nuclear matter from chiral effective field theory and find that the results are systematically larger than from typical mean field models. We start by constructing the finite-temperature equation of state from chiral two- and three-nucleon forces, which we then use to fit a class of extended Skyrme energy density functionals. This allows for modeling the thermal index across the full range of densities and temperatures that may be probed in simulations of core-collapse supernovae and neutron star mergers, including the low-density inhomogeneous mixed phase. For uniform matter we compare the results to analytical expressions for $Γ_{\mathrm{th}}$ based on Fermi liquid theory. The correlation between the thermal index and the effective masses at nuclear saturation density is studied systematically through Bayesian modeling of the nuclear equation of state. We then study the behavior of $Γ_{\mathrm{th}}$ in both relativistic and non-relativistic mean field models used in the astrophysical simulation community to complement those based on chiral effective field theory constraints from our own study. We derive compact parameterization formulas for $Γ_{\mathrm{th}}$ across the range of densities and temperatures encountered in core collapse supernovae and binary neutron star mergers, which we suggest may be useful for the numerical simulation community.

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arXiv2019

Predicting the moment of inertia of pulsar J0737-3039A from Bayesian modeling of the nuclear equation of state

We investigate neutron star moments of inertia from Bayesian posterior probability distributions of the nuclear equation of state that incorporate information from microscopic many-body theory and empirical data of finite nuclei. We focus on PSR J0737-3039A and predict that for this 1.338 M_sun neutron star the moment of inertia lies in the range $1.04 \times 10^{45}$ g cm$^{2} < I < 1.51 \times 10^{45}$ g cm$^{2}$ at the 95% credibility level, while the most probable value for the moment of inertia is $\tilde I = 1.36 \times 10^{45}$ g cm$^{2}$. Assuming a measurement of the PSR J0737-3039A moment of inertia to 10% precision, we study the implications for neutron star radii and tidal deformabilities. We also determine the crustal component of the moment of inertia and find that for typical neutron star masses of 1.3 M_sun < M < 1.5 M_sun the crust contributes 1% - 6% of the total moment of inertia, below what is needed to explain large pulsar glitches in the scenario of strong neutron entrainment.

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arXiv2019

Dense matter equation of state and neutron star properties from nuclear theory and experiment

The equation of state of dense matter determines the structure of neutron stars, their typical radii, and maximum masses. Recent improvements in theoretical modeling of nuclear forces from the low-energy effective field theory of QCD has led to tighter constraints on the equation of state of neutron-rich matter at and somewhat above the densities of atomic nuclei, while the equation of state and composition of matter at high densities remains largely uncertain and open to a multitude of theoretical speculations. In the present work we review the latest advances in microscopic modeling of the nuclear equation of state and demonstrate how to consistently include also empirical nuclear data into a Bayesian posterior probability distribution for the model parameters. Derived bulk neutron star properties such as radii, moments of inertia, and tidal deformabilities are computed, and we discuss as well the limitations of our modeling.

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arXiv2018

Neutron star tidal deformabilities constrained by nuclear theory and experiment

We confront observational data from gravitational wave event GW170817 with microscopic modeling of the cold neutron star equation of state. We develop and employ a Bayesian statistical framework that enables us to implement constraints on the equation of state from laboratory measurements of nuclei and state-of-the-art chiral effective field theory methods. The energy density functionals constructed from the posterior probability distributions are then used to compute consistently the neutron star equation of state from the outer crust to the inner core, assuming a composition consisting of protons, neutrons, electrons, and muons. In contrast to previous studies, we find that the 95% credibility range of predicted neutron star tidal deformabilities ($136 < Λ< 519$) for a 1.4 solar-mass neutron star is already consistent with the upper bound deduced from observations of the GW170817 event. However, we find that lower bounds on the neutron star tidal deformability will very strongly constrain microscopic models of the dense matter equation of state. We also demonstrate a strong correlation between the neutron star tidal deformability and the pressure of beta-equilibrated matter at twice saturation density.

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arXiv2018

Universal correlations in the nuclear symmetry energy, slope parameter, and curvature

From general Fermi liquid theory arguments, we derive correlations among the symmetry energy (J), its slope parameter (L), and curvature (K_sym) at nuclear matter saturation density. We argue that certain properties of these correlations do not depend on details of the nuclear forces used in the calculation. We derive as well a global parametrization of the density dependence of the symmetry energy that we show is more reliable, especially at low densities, than the usual Taylor series expansion around saturation density. We then benchmark these predictions against explicit results from chiral effective field theory.

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arXiv2018

Tidal Deformability of Neutron Stars with Realistic Nuclear Energy Density Functionals

We investigate the constraints on the mass and radius of neutron stars by considering the tidal deformability in the merge of neutron star binaries. In order to extract the most reliable range of uncertainty from theory, we employ models based upon the Skyrme force and density functional theory and select models that are consistent with empirical data of finite nuclei, measured properties of nuclear matter around the saturation density, and observation of the maximum mass of neutron stars. From the selected models, we calculate the Love number $k_2$, dimensionless tidal deformability $Λ$, and mass-weighted deformability $\tildeΛ$ in the binary system. We find that all the models considered in this work give $\tildeΛ$ less than 800 which is the upper limit obtained from the measurement of GW170817. However, the model dependence of tidal deformability is manifest such that our results on the tidal deformability exhibit critical sensitivity to the size of neutron stars.

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arXiv2017

Effective interactions of hyperons and mass-radius relation of neutron stars

We examine the role of hyperons in a neutron star based on the relativistic mean field approach. For nuclear matter below 1.5 times the normal nuclear density we constrain the model parameters by using the symmetric nuclear matter properties and theoretical investigations for neutron matter in the literature. We then extend the model to higher densities by including hyperons and isoscalar vector mesons that contain strangeness degree of freedom. We confirm that the $ϕ$ meson induces a $Λ$ repulsive force and hardens the equation of state. The hardening arising from the $ϕ$ meson compensates the softening from the existence of hyperons. The flavor SU(3) and spin-flavor SU(6) relations are examined as well. We found that the coupling constants fitted by neutron matter properties could yield high enough maximum mass of a neutron star and the obtained results satisfy both the mass and radius constraints. The onset of the hyperon direct Urca process in neutron stars is also investigated using our parametrization.

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arXiv2017

Nuclear dipole polarizability from mean-field modeling constrained by chiral effective field theory

We construct a new Skyrme interaction Sk$χ$m$^*$ by fitting the equation of state and nucleon effective masses in asymmetric nuclear matter from chiral two- and three-body forces as well as the binding energies of finite nuclei. Employing this interaction to study the electric dipole polarizabilities of $^{48}$Ca, $^{68}$Ni, $^{120}$Sn, and $^{208}$Pb in the random-phase approximation, we find that the theoretical predictions are in good agreement with experimentally measured values without additional fine tuning of the Skyrme interaction, thus confirming the usefulness of the new Skyrme interaction in studying the properties of nuclei. We further use this interaction to study the neutron skin thicknesses of $^{48}$Ca and $^{208}$Pb, and they are found to be consistent with the experimental data.

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arXiv2017

Density dependence of the nuclear energy-density functional

The explicit density (rho) dependence in the coupling coefficients of the non-relativistic nuclear energy-density functional (EDF) encodes effects of three-nucleon forces and dynamical correlations. The necessity for a coupling coefficient in the form of a small fractional power of rho is empirical and the power often chosen arbitrarily. Consequently, precision-oriented parameterisations risk overfitting and loss of predictive power. Observing that the Fermi momentum kF~rho^1/3 is a key variable in Fermi systems, we examine if a power hierarchy in kF can be inferred from the properties of homogeneous matter in a domain of densities which is relevant for nuclear structure and neutron stars. For later applications we want to determine an EDF that is of good quality but not overtrained. We fit polynomial and other functions of rho^1/3 to existing microscopic calculations of the energy of symmetric and pure neutron matter and analyze the fits. We select a form and parameter set which we found robust and examine the parameters' naturalness and the resulting extrapolations. A statistical analysis confirms that low-order terms like rho^1/3 and rho^2/3 are the most relevant ones. It also hints at a different power hierarchy for symmetric vs. pure neutron matter, supporting the need for more than one rho^a terms in non-relativistic EDFs. The EDF we propose accommodates adopted properties of nuclear matter near saturation. Importantly, its extrapolation to dilute or asymmetric matter reproduces a range of existing microscopic results, to which it has not been fitted. It also predicts neutron-star properties consistent with observations. The coefficients display naturalness. Once determined for homogeneous matter, EDFs of the present form can be mapped onto Skyrme-type ones for use in nuclei. The statistical analysis can be extended to higher orders and for different ab initio calculations.

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