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Xia, C.-J.*; 丸山 敏毅; 安武 伸俊*; 巽 敏隆*
Physical Review D, 110(11), p.114024_1 - 114024_14, 2024/12
被引用回数：3 パーセンタイル：37.77(Astronomy & Astrophysics)Based on an extended Nambu-Jona-Lasinio model that treats baryons as clusters of quarks, we investigate the properties and microscopic structures of mixed phases for various types of first-order phase transitions in a unified manner, where the model parameters are fixed by reproducing nuclear matter properties and the binding energies of finite nuclei. In particular, based on the Thomas-Fermi approximation, we investigate the mixed phases that arise from the liquid-gas phase transition of nuclear matter, chiral phase transition, and deconfinement phase transition in dense stellar matter adopting spherical and cylindrical approximations for the Wigner-Seitz cells. It is found that the geometrical structures do not emerge for the chiral phase transition, while the droplet, rod, slab, tube, and bubble phases emerge sequentially as density increases for the liquid-gas and deconfinement phase transitions. Additional attractive interactions between strange quark matter and hyperons are observed as the deconfinement phase transition is entangled with the chiral phase transition of 
quarks. The results obtained here should be useful to understand the properties and structures of dense stellar matter throughout compact stars and, in particular, the matter state in the core regions. Meanwhile, more extensive investigations in a three-dimensional geometry with large box sizes are necessary for our future study.
安武 伸俊*; 丸山 敏毅
Nuclear Physics Review, 41(3), p.801 - 805, 2024/09
We present the current status of molecular dynamics, in which hadron and quark matter are described only within quark models. In our previous calculations, we solved the time evolution of internal degrees of freedom for color. In this article, we show the results simultaneously solved the internal degrees of freedom for spin as well. Unlike conventional methods, our approach also considers the flavor conversion between quarks and electrons during the calculations, which naturally achieves the beta equilibrium under the charge neutrality condition. Based on this latest formulation, the equation of state for dense matter is studied, replicating the saturation properties: symmetric energy, 
-parameter, incompressibility around nuclear density. The resultant mass-radius relations are also consistent with the observations such as the gravitational wave observations, and NICER. Our numerical results suggest that deconfined quark matter appears in the core of neutron stars via crossover. The key difference from previous molecular dynamics simulations is the spontaneous emergence of clustering of quarks or baryons on a local scale. These properties would never be observed in equations of state based on the other approaches. While our formulation does not yet fully include strangeness, we expect that our method has a possibility to show the direct evidence of the quark clustering models such as "the strangeon model" or "the sexaquark model".
Xia, C.-J.*; 丸山 敏毅; 安武 伸俊*; 巽 敏隆*; Zhang, Y.-X.*
Nuclear Physics Review, 41(3), p.818 - 827, 2024/09
Based on the relativistic mean field (RMF) model with Thomas-Fermi approximation, we investigate the elastic properties of neutron star matter. The elastic constants are estimated by introducing deformations on the nuclear pasta structures in 
-equilibrium, where various crystalline configurations are considered in a fully three-dimensional geometry without the Wigner-Seitz approximation. Two scenarios with different symmetry energy slope (=41.34 and 89.39 MeV) are examined, where the elastic constants can vary by ten times. By fitting to the numerical results, we improve the analytic formulae for the elastic properties of nuclear pasta by introducing damping factors.
丸山 敏毅; 安武 伸俊*
Nuclear Physics Review, 41(3), p.806 - 810, 2024/09
We present a microscopic simulation of particles with time-dependent spins, where different interaction and density are considered. Ferromagnetism has been observed for the attractive spin-spin interaction, while antiferromagnetism and paramagnetism have been observed in different density region for the repulsive spin-spin interaction. The method developed here should be extended to include spin-color interaction into "color molecular dynamics".
安武 伸俊*; 丸山 敏毅
Physical Review D, 109(4), p.043056_1 - 043056_11, 2024/02
被引用回数：2 パーセンタイル：26.79(Astronomy & Astrophysics)The equation of state for dense matter is studied with colour molecular dynamics, in which hadron matter and quark matter are automatically distinguished only from quark colour state. The quark-quark interactions are optimized to be consistent with saturation properties: symmetric energy, -parameter, and incompressibility around nuclear density. In the calculations, the degrees of freedom of colours are solved at each numerical step, although the flavours are fixed as up or down quarks. The resultant mass-radius relations of neutron stars also satisfy the observational constraints. In this model deconfined quark matter appears in the core of neutron stars via crossover. Although the current constraints from the observations are not enough to conclude whether quark matter appears at high-density region, our method would help to understand high-density material properties inside neutron stars.
小形 美沙*; 大川 博督*; 藤澤 幸太郎*; 安武 伸俊; 山本 裕*; 山田 章一*
Monthly Notices of the Royal Astronomical Society, 521(2), p.2561 - 2576, 2023/05
被引用回数：1 パーセンタイル：6.51(Astronomy & Astrophysics)We present a new formulation for numerically obtaining axisymmetric equilibrium structures of rotating stars in two spatial dimensions. With a view to apply it to the secular evolution of rotating stars, we base it on the Lagrangian description, i.e. we solve the force-balance equations to find the spatial positions of fluid elements endowed individually with a mass, specific entropy and angular momentum. The system of non-linear equations obtained by finite-differencing the basic equations is solved with the W4 method, which is a new multidimensional root-finding scheme of our own devising. We augment it with a remapping scheme to avoid distortions of the Lagrangian coordinates. In this first one of a series of papers, we will give a detailed description of these methods initially. We then present the results of some test calculations, which include the construction of both rapidly rotating barotropic and baroclinic equilibrium states. We gauge their accuracies quantitatively with some diagnostic quantities as well as via comparisons with the counterparts obtained with an Eulerian code. For a demonstrative purpose, we apply the code to a toy-model cooling calculation of a rotating white dwarf.
Xia, C.-J.*; 丸山 敏毅; 安武 伸俊*; 巽 敏隆*; Zhang, Y.-X.*
Physics Letters B, 839, p.137769_1 - 137769_5, 2023/04
被引用回数：5 パーセンタイル：58.11(Astronomy & Astrophysics)Realistic estimations on the elastic properties of neutron star matter are carried out with a large strain (
0.5) in the framework of relativistic-mean-field model with Thomas-Fermi approximation, where various crystalline configurations are considered in a fully three-dimensional geometry with reflection symmetry. Our calculation confirms the validity of assuming Coulomb crystals for the droplet phase above neutron drip density, which nonetheless does not work at large densities since the elastic constants are found to be decreasing after reaching their peaks. Similarly, the analytic formulae derived in the incompressible liquid-drop model give excellent description for the rod phase at small densities, which overestimates the elastic constants at larger densities. For slabs, due to the negligence on the variations of their thicknesses, the analytic formulae from liquid-drop model agree qualitatively but not quantitatively with our numerical estimations. By fitting to the numerical results, these analytic formulae are improved by introducing dampening factors. The impacts of nuclear symmetry energy are examined adopting two parameter sets, corresponding to the slope of symmetry energy L = 41.34 and 89.39 MeV. Even with the uncertainties caused by the anisotropy in polycrystallines, the elastic properties of neutron star matter obtained with L = 41.34 and 89.39 MeV are distinctively different, results in detectable differences in various neutron star activities.
大川 博督*; 藤澤 幸太郎*; 安武 伸俊; 小形 美沙*; 山本 裕*; 山田 章一*
Monthly Notices of the Royal Astronomical Society, 520(1), p.24 - 43, 2023/03
被引用回数：1 パーセンタイル：6.51(Astronomy & Astrophysics)We present a new formulation to construct numerically equilibrium configurations of rotating stars in general relativity. Having in mind the application to their quasi-static evolutions on a secular time-scale, we adopt a Lagrangian formulation of our own devising, in which we solve force-balance equations to seek for the positions of fluid elements corresponding to the grid points, instead of the ordinary Eulerian formulation. Unlike previous works in the literature, we do not employ the first integral of the Euler equation, which is not obtained analytically in general. We assign a mass, specific angular momentum and entropy to each fluid element in contrast to the previous Eulerian methods, in which the spatial distribution of the angular velocity or angular momentum is specified. These distributions are determined after the positions of all fluid elements (or grid points) are derived in our formulation. We solve the large system of algebraic non-linear equations that are obtained by discretizing the time-independent Euler and Einstein equations in the finite-element method by using our new multidimensional root-finding scheme, named the W4 method. To demonstrate the capability of our new formulation, we construct some rotational configurations, both barotropic and baroclinic. As toy models, we also solve three evolutionary sequences that mimic the cooling, mass-loss, and mass-accretion.
Xia, C.-J.*; 丸山 敏毅; 安武 伸俊*; 巽 敏隆*
Physical Review D, 106(6), p.063020_1 - 063020_14, 2022/09
被引用回数：10 パーセンタイル：55.35(Astronomy & Astrophysics)We study nuclear pasta structures at high temperatures within a relativistic mean field model and Thomas-Fermi approximation. Typical structures (droplet, rod, slab, tube, and bubble) are obtained, which form various crystalline configurations. The properties of those nuclear pastas are examined in a three-dimensional geometry with reflection symmetry, where the optimum lattice constants are fixed minimizing the free energy under spherical or cylindrical approximations for Wigner-Seitz cells. For fixed densities and temperatures, the differences of the free energies per baryon for various shapes and lattice structures are typically on the order of tens of keV, suggesting the coexistence of those structures. If we further increase temperature, eventually the non-uniform structures of nuclear pasta become unstable and are converted into uniform nuclear matter. The phase diagrams of nuclear matter are estimated, which is useful for understanding the various phenomena in neutron stars.
Xia, C.-J.*; 丸山 敏毅; 安武 伸俊*; 巽 敏隆*; Zhang, Y.-X.*
Physical Review C, 103(5), p.055812_1 - 055812_13, 2021/05
被引用回数：21 パーセンタイル：86.64(Physics, Nuclear)In the framework of the relativistic mean field model with Thomas-Fermi approximation, we study the structures of low density nuclear matter in a three-dimensional geometry with reflection symmetry. The effect of finite cell size is treated carefully by searching for the optimum cell size. Typical pasta structures (droplet, rod, slab, tube, and bubble) arranged in various crystalline configurations are obtained for both fixed proton fractions and -equilibration. It is found that the properties of droplets/bubbles are similar in body-centered cubic (BCC) and face-centered cubic (FCC) lattices, where the FCC lattice generally becomes more stable than BCC lattice as density increases. For the rod/tube phases, the honeycomb lattice is always more stable than the simple one. By introducing an 
-
cross coupling term, we further examine the pasta structures with a smaller slope of symmetry energy 
MeV, which predicts larger onset densities for core-crust transition and non spherical nuclei. Such a variation due to the reduction of is expected to have impacts on various properties in neutron stars, supernova dynamics, and binary neutron star mergers.
Xia, C.-J.*; 丸山 敏毅; 安武 伸俊*; 巽 敏隆*; Shen, H.*; 富樫 甫*
Physical Review D, 102(2), p.023031_1 - 023031_18, 2020/07
被引用回数：34 パーセンタイル：80.64(Astronomy & Astrophysics)高密度星を構成する物質のクォーク-ハドロン混合相について調べる。クォーク相とハドロン相はそれぞれいくつかのモデルにより計算し、混合相の形状と状態方程式については、Ravenhallらによる連続的な幾何学的次元による計算法を踏襲する。その際表面張力は固定した値のものの他、多重反射展開法によるものと、equivparticle法によるものとを使う。これによって天体の全体的構造や質量,半径などが得られる。天体の半径と潮汐変形性の相関はハイブリッド星でも中性子星のときと同様であった。重い天体になると中心付近にクォーク物質が現れ、クォーク-ハドロンの境界の表面張力が全体の構造に対して重要になる。0.8fm
以下の密度では観測量により状態方程式に対する制限が良く得られるが、高密度では不定性が大きかった。
Maslov, K.*; 安武 伸俊*; Blaschke, D.*; Ayriyan, A.*; Grigorian, H.*; 丸山 敏毅; 巽 敏隆*; Voskresensky, D. N.*
Physical Review C, 100(2), p.025802_1 - 025802_11, 2019/08
被引用回数：64 パーセンタイル：97.12(Physics, Nuclear)ハドロン-クォーク相転移に対するパスタ相の及ぼす影響を、相対論的平均場による状態方程式に関してしらべた。パスタ相の存在をフルに考慮した数値計算による状態方程式と従来の2相の内挿による混合相の状態方程式を比較した。ハドロン相・クォーク相双方に対して混合相の状態方程式がMaxwell構成法とほぼ一致する表面張力の限界値が得られた。そしてその値は理論による推定値とよく一致した。また、ベータ平衡な中性子星物質まで理論を拡張することで、ハイブリッド高密度星の構造と第3系列のロバストネスに対するパスタ相の影響を議論した。
Xia, C.-J.*; 丸山 敏毅; 安武 伸俊*; 巽 敏隆*
Physical Review D, 99(10), p.103017_1 - 103017_14, 2019/05
被引用回数：36 パーセンタイル：78.76(Astronomy & Astrophysics)We study the interface effects of a quark-hadron mixed phase in compact stars. The properties of nuclear matter are obtained based on the relativistic-mean-field model. For the quark phase, we adopt a perturbation model with running quark masses and coupling constant. At certain choices of parameter sets, it is found that varying the quark-hadron interface tension will have sizable effects on the radii (
m) and tidal deformabilities (
%) of 1.36 solar mass hybrid stars. These provide possibilities for us to constrain the quark-hadron interface tension with future gravitational wave observations as well as the ongoing NICER mission.
Lee, T.-G.*; 安武 伸俊*; 丸山 敏毅; 巽 敏隆*
Proceedings of Science (Internet), 281, p.326_1 - 326_8, 2017/05
We discuss the possibility of a multidimensional structure of inhomogeneous chiral condensates, which might be realized in cold and dense quark matter or the core of compact stars. For one-dimensional structures, the system becomes unstable at finite temperature due to the Landau-Peierls instability attributed to the excitation of the Nambu-Goldstone modes, with the result that there is no long-range order, while quasi-long-range order is realized instead. On the other hand, chirally inhomogeneous phases with two- or three-dimensional modulations may be realized as a true long-range ordered phase at any temperature, as inferred from the Landau-Peierls theorem. We here consider possible strategies to explore a multidimensional structure of chiral crystals.
Lee, T.-G.*; 西山 和也*; 安武 伸俊*; 丸山 敏毅; 巽 敏隆*
JPS Conference Proceedings (Internet), 14, p.020808_1 - 020808_3, 2017/02
被引用回数：0 パーセンタイル：0.00(Astronomy & Astrophysics)We explore the multi-dimensional structure of inhomogeneous chiral condensates in quark matter. For a one-dimensional structure, the system becomes unstable at finite temperature due to the Nambu-Goldstone excitations. However, inhomogeneous chiral condensates with multi-dimensional modulations may be realized as a true long-range order at any temperature, as inferred from the Landau Peierls theorem. We here present some possible strategies for searching the multi-dimensional structure of chiral crystals.
野田 常雄*; 橋本 正章*; 松尾 康秀*; 安武 伸俊*; 丸山 敏毅; 巽 敏隆*
JPS Conference Proceedings (Internet), 14, p.020805_1 - 020805_3, 2017/02
被引用回数：0 パーセンタイル：0.00(Astronomy & Astrophysics)A neutron star is a highly dense object which lasts after a supernova explosion. The density of a neutron star overcomes the nuclear density, and the temperature is high at the beginning of its history. An isolated neutron star does not have any heat sources, and it cools down emitting thermal energy by neutrinos. The neutrino emission process depends on the state of interior matter of the neutron star. To compare theoretical simulations and observations of neutron stars, it can constrain the nuclear theory of high density region. We create a model of neutron stars with colour superconducting quark matter and nucleon super fluidity / superconductivity, to satisfy recent observations, including two 2
neutron stars. We parameterize these super-states and demonstrate the cooling curves, which show heavy stars do not always cool faster than lighter stars.
安武 伸俊*; Chen, H.*; 丸山 敏毅; 巽 敏隆*
Journal of Physics; Conference Series, 665(1), p.012068_1 - 012068_4, 2016/01
被引用回数：8 パーセンタイル：90.32(Astronomy & Astrophysics)We study the hadron-quark phase transition, taking into account the finite-size effects for neutron star matter. For the hadron phase, we adopt a realistic equation of state within the framework of the Brueckner-Hartree-Fock theory. For the quark phase, we apply the Dyson-Schwinger method. The properties of the mixed phase are clarified by considering the finite-size effects. We find that, if the surface tension is strong enough, the equation of state becomes to be close the one with the Maxwell condition, though we properly adopt the Gibbs conditions. This result is qualitatively the same with the one by the use of the simple bag model. We also find that the mass-radius relation by the EoS is consistent with the observations of massive neutron stars.
野田 常雄*; 安武 伸俊*; 橋本 正章*; 丸山 敏毅; 巽 敏隆*; 藤本 正行*
Acta Astronomica Sinica, 56(Suppl.), p.52 - 54, 2015/11
コンパクト星の冷却シナリオを示す。カシオペアA(Cas A)の観測により中央のソースは高い有効温度のコンパクト星で、エキゾチックな相を持たない場合の冷却と矛盾しない。Cas Aの観測データから、Cas Aの質量が太陽質量の1.5倍以上となることが分かる。それは重いコンパクト星ほど早く冷えるという現在のコンパクト星の冷却シナリオと矛盾する可能性がある。我々はコンパクト星の冷却にカラー超伝導のあるクォーク物質の効果を入れ、ギャップエネルギー
が10MeVより大きいと仮定した。冷却曲線を計算すると、重い星はゆっくり冷えて軽い星はその反対の傾向を持つことを示した。
安武 伸俊*; Benic, S.*; Blaschke, D.*; 丸山 敏毅; 巽 敏隆*
Acta Astronomica Sinica, 56(Suppl.), p.85 - 87, 2015/11
中性子星におけるクォークーハドロン相転移について調べた。複数成分の系では、相転移の際に構造を持った混合相の有限サイズ効果を考慮する必要性がある。相転移はクォーク物質やハドロン物質のモデルに強く依存し、2相間の表面張力やニュートリノ含有率、温度にも依存する。しかし、モデルによらず表面張力が強いとMaxwell構成法の状態方程式に似たものになり、弱いとバルクなギブス条件のものになることが分かった。また、我々の提唱する状態方程式が与える天体の質量-半径曲線は、観測と矛盾しないことが分かった。
安武 伸俊*; Lastowiecki, R.*; Benic, S.*; Blaschke, D.*; 丸山 敏毅; 巽 敏隆*
Physical Review C, 89(6), p.065803_1 - 065803_10, 2014/06
被引用回数：102 パーセンタイル：97.71(Physics, Nuclear)We study the role of finite-size effects at the hadron-quark phase transition in a new hybrid equation of state constructed from an ab initio Br
ckner-Hartree-Fock theory, with the realistic Bonn-B potential for the hadronic phase and a covariant nonlocal Nambu-Jona-Lasinio model for the quark phase. We construct static hybrid star sequences and find that our model can support stable hybrid stars with an onset of quark matter below 
and a maximum mass above 
in agreement with recent observations. If the finite-size effects are taken into account the core is composed of pure quark matter. Provided that the quark vector channel interaction is small and the finite size effects are taken into account, quark matter appears at densities 2-3 times the nuclear saturation density. In that case the proton fraction in the hadronic phase remains below the value required by the onset of the direct URCA process, so that the early onset of quark matter shall affect on the rapid cooling of the star.
