[论文解读] Ferromagnetic Phase Transitions in Neutron Stars

J. P. W. Diener|arXiv (Cornell University)|May 31, 2013

Pulsars and Gravitational Waves Research参考文献 18被引用 3

一句话总结

本文利用包含重子磁偶极矩与电荷-磁场耦合的量子强子场论（QHD），研究了中子星物质中的铁磁相变。研究发现，由于朗道能级跃迁，铁磁磁场在特定密度下出现急剧跃迁，显著影响中子星的磁性，但对质量-半径关系影响甚微。

ABSTRACT

The ferromagnetic phase in pure neutron matter as well as charge neutral, beta-equilibrated nuclear matter is considered. We employ Quantum Hadrodynamics, a relativistic field theory description of nuclear matter with meson degrees of freedom, and include couplings between the baryon (proton and neutron) magnetic dipole moment as well as between their charge and the magnetic field in the Lagrangian density describing such a system. We vary the strength of the baryon magnetic dipole moment till a non-zero value of the magnetic field, for which the total energy density of the magnetised system is at a minimum, is found. The system is then assumed to be in the ferromagnetic state. The ferromagnetic equation of state is employed to study matter in the neutron star interior. We find that as the density increases the ferromagnetic field does not increase continuously, but exhibit sudden rapid increases. These sudden increases in the magnetic field correspond to shifts between different configurations of the charged particle's Landau levels and can have significant observational consequences for neutron stars. We also found that although the ferromagnetic phase softens the neutron star equation of state it does not significantly alter the star's mass-radius relationship. The properties of magnetised symmetric nuclear matter were also studied. We confirm that magnetised matter tends to be more proton-rich but become more weakly bound for stronger magnetic fields. We show that the behaviour of the compressibility of nuclear matter is influenced by the Landau quantisation and tends to have an oscillatory character as it increases with the magnetic field. The symmetry energy also exhibits similar behaviour.

研究动机与目标

• 探索在极端强磁场下，致密中子物质中出现铁磁性的条件。
• 建立模型，分析重子磁偶极矩与电荷-磁场耦合对中子星中态方程的影响。
• 确定中子星内部自发磁化出现的条件。
• 评估铁磁相变对磁星及其他中子星现象的观测影响。
• 分析朗道量化的角色如何改变磁化核物质中的可压缩性与对称性能量。

提出的方法

• 采用量子强子场论（QHD），一种基于介子交换的相对论场论，描述中子物质中的核子相互作用。
• 在拉格朗日量中显式引入重子磁偶极矩与磁场之间的耦合，以及重子电荷与电磁场之间的耦合。
• 使用相对论平均场（RMF）近似求解基态，并自洽地确定磁场分布。
• 通过调节重子磁偶极矩的强度，寻找能量最低的非零磁场状态，以表征铁磁序。
• 计算在带电费米子朗道量化的条件下，粒子密度、能量密度、压强与标量密度。
• 将所得的铁磁态方程应用于中子星内部，分析其对质量-半径关系与辐射特性的影响。

实验结果

研究问题

• RQ1 在纯中子物质中，何种条件下会发生自发铁磁相变？
• RQ2 朗道能级跃迁如何影响致密、磁化中子星物质中磁场的行为？
• RQ3 铁磁性对中子星态方程及其质量-半径关系有何影响？
• RQ4 朗道量化如何影响磁化核物质中的可压缩性与对称性能量？
• RQ5 中子星中磁感应强度的急剧跃迁，特别是对磁星而言，具有何种观测后果？

主要发现

• 铁磁磁场在特定密度下因朗道能级构型的跃迁而出现突然、急剧的上升，而非连续增加。
• 这些急剧的磁场跃迁直接与带电粒子在朗道能级中量子数分布的变化相关，表明其具有相变特征。
• 尽管铁磁相使态方程变软，但对中子星的质量-半径关系影响甚微。
• 随着磁场强度增加，磁化对称核物质变得更富含质子，且结合能降低。
• 由于朗道量化的效应，核物质的可压缩性随磁场增强表现出振荡行为。
• 对称性能量也随磁场强度呈现振荡变化，反映出朗道能级量化对核相互作用的影响。