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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：0 パーセンタイル：0(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：1 パーセンタイル：68.16(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：50.15(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：4 パーセンタイル：49.9(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：13 パーセンタイル：90.39(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：16 パーセンタイル：74.34(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：44 パーセンタイル：97.28(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。
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.
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：0.03(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.
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：0.03(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.
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：6 パーセンタイル：88.68(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：80 パーセンタイル：97.6(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.
丸山 敏毅; 安武 伸俊*; 巽 敏隆*
Proceedings of Science (Internet), 146, p.125_1 - 125_6, 2013/04
ニュートリノがトラップされた原子核物質の非一様構造と物質の性質を調べた。理論的枠組みとしては相対論的平均場とトーマス-フェルミ近似を用いた。ニュートリノを含まないわれわれの以前の研究では、電子含有率を固定した場合、いわゆるパスタ構造が現れたが、ベータ平衡物質では、球形核のみが見られた。今回の研究では、レプトン含有率を固定した計算を行ったところ、ニュートリノの圧力を下げるように陽子密度が上昇し、陽子-中性子の強い引力がもたらすクラスター生成により非一様パスタ構造の出現が促進されることがわかった。また、ニュートリノの存在が状態方程式に与える影響についても報告する。
野田 常雄*; 橋本 正章*; 安武 伸俊*; 丸山 敏毅; 巽 敏隆*; 藤本 正行*
Astrophysical Journal, 765(1), p.1_1 - 1_5, 2013/03
被引用回数：32 パーセンタイル：66.33(Astronomy & Astrophysics)The Cas A observation shows that the central source is a compact star that has high effective temperature, and it is consistent with the cooling without exotic phases. The observation also gives the mass range of 
, which may conflict with the current plausible cooling scenario of compact stars. There are some cooled compact stars such as Vela or 3C58, which can not be explained by the minimal cooling scenario with the neutrino emission by nucleon superfluidity (PBF). Then we invoke the exotic cooling processes where heavier stars cool faster than lighter ones. However, the result is inconsistent with the observation of Cas A. Therefore, we present a new cooling scenario by constructing models that include a quark color superconducting (CSC) phase with a large energy gap; this phase appears at ultrahigh density regions and reduces neutrino emissivity. In our model, a compact star has a CSC quark core with a low neutrino emissivity surrounded by high emissivity region made by normal quarks. Our result without considering nucleon superfluidity, shows that while heavier stars cool slowly and lighter ones quickly. Furthermore, we show that our scenario is consistent with the recent observations of the effective temperature of Cas A during the last 10 years, including nucleon superfluidity.
安武 伸俊*; 野田 常雄*; 祖谷 元*; 丸山 敏毅; 巽 敏隆*
Recent Advances in Quarks Research, p.63 - 111, 2013/01
Using a baryon-baryon effective interaction for the hadron phase with hyperons and the MIT bag model for the quark phase, we derive the equation of state (EOS) of hadron-quark (HQ) matter, based on the Gibbs conditions for phase equilibrium. One of the interesting consequences may be the appearance of the inhomogeneous structures called "pasta", which are brought about by the balance between the surface and the Coulomb interactions. We explore the properties of cold catalyzed matter, hot matter and neutrino-trapped matter. The pasta structures become unstable by the charge screening of the Coulomb interaction, high temperatures or the neutrino trapping effect. HQ matter with pasta structures may affect astrophysical phenomena through its elasticity or thermal properties. We also discuss some implications on supernova explosion, the gravitational waves and the cooling of compact stars.
安武 伸俊*; 丸山 敏毅; 巽 敏隆*
Physical Review D, 86(10), p.101302_1 - 101302_4, 2012/11
被引用回数：10 パーセンタイル：38.3(Astronomy & Astrophysics)全バリオン数,全電荷,全レプトン数を保存するターナリーな系として原始中性子星におけるハドロン-クォーク混合相を研究した。その結果、レプトン数が大きい場合に"パスタ"の結晶構造が現れないことがわかった。すなわち、荷電遮蔽によってパスタ構造のサイズが発散し、系が力学的に不安定になる。したがって、アモルファスのように全系がバルクな2つの領域に分離し、表面の効果が無視できるようになる。そこでは局所荷電中性が近似的に成り立ち、系の状態方程式は実質的にはターナリーではなくバイナリーな系のものに帰着する。しかし、共存相におけるレプトンの割合やバリオン密度が異なるため純粋系にはならず、その特徴である密度の不連続は原始中性子星物質では起こらないと結論できる。
巽 敏隆*; 安武 伸俊*; 丸山 敏毅
Neutron Stars; The Aspect of High Density Matter, Equations of State and Observables, p.199 - 231, 2012/10
高密度天体の内部構造を知るためには相転移の熱力学が必要である。非対称原子核物質の液相気相相転移を例として、Gibbs条件を精密に考慮に入れた相平衡を考える。相対論的平均場とThomas-Fermi近似による粒子密度分布を無撞着に解くことにより、混合相の特徴を明らかにした。混合相は共存する平衡な2相からなる非一様な物質で、「パスタ」と呼ばれる特徴的な形状をした1つの相がもう1つの相の中に埋め込まれた結晶構造をとるが、われわれの計算ではこの非一様構造による表面張力とクーロン斥力とを勘案することができる。それにより非一様構造の力学的不安定が起こる場合を明らかにした。また、液相気相相転移における有限温度の効果についても調べた。同様に、ハドロン-クォークの非閉じ込め相転移をハイペロン入りの物質について調べた。そこではニュートリノがトラップされた場合の効果及び有限温度の効果について議論し、混合相の特性と状態方程式を導き出した。
安武 伸俊*; 丸山 敏毅; 巽 敏隆*
Journal of Physics; Conference Series, 312, p.042027_1 - 042027_6, 2011/09
被引用回数：9 パーセンタイル：93.4(Astronomy & Astrophysics)原始中性子星で実現している有限温度での高密度物質の構造と性質を調べた。ハドロン-クォークの混合相では、パスタと呼ばれる非一様構造が現れることが予想されていて、有限サイズ効果(表面張力やクーロン斥力)を考えないバルクな計算では、Gibbs条件を満たす混合相が広い密度領域で現れるが、非一様なパスタ構造に対する有限サイズ効果を考慮すると、力学的不安定な領域が増え、混合相が狭い領域に制限される。今回、特に有限温度の効果を調べ、混合相が温度の上昇によって不安定になるという結果が得られた。これは、表面張力の変化がないのに対し、正荷電のハドロン相に負電荷のハイペロン
が混じるためにクーロン斥力が弱まり、非一様構造が不安定になるためである。また、ニュートリノがトラップされた状況も調べ、ニュートリノを含まないこれまでの結果との違いを議論する。
