强磁场对中子星壳层结构的影响

Influence of strong magnetic fields on inner crust of neutron star

中子星内壳层是由原子核与其周围的自由中子气和电子气共存而构成的非均匀物质.天文观测表明部分中子星内部可能存在高达10^(18)G的强磁场.因此中子星的内壳层结构会受到强磁场的明显影响.本文采用相对论平均场理论描述核子间相互作用,并考虑了质子和中子的反常磁矩.利用Wigner-Seitz近似描述中子星内壳层中的非均匀分布物质,并采用自洽Thomas-Fermi近似方法处理在强磁场环境中WignerSeitz原胞内的核子以及电子分布,从而研究强磁场对中子星内壳层中的非均匀相结构以及壳核相变等性质的影响.计算结果表明,与无磁场情况相比,内壳层中非均匀物质的每核子结合能、原子核pasta相结构和壳核相变密度等性质在磁场B≤10^(17)G的环境中不会发生明显变化.而B≥10^(18)G的强磁场会对中子星内壳层中的pasta相结构产生重要影响,使各种pasta相的每核子结合能降低,非球形原子核出现的阈密度和壳核相变密度减小.随着磁场强度B的增加,球形Wigner-Seitz原胞的半径减小,同时原胞内的核子分布变得更加弥散.

It is generally believed that a neutron star mainly consists of four parts, that is to say, outer crust, inner crust, outer core,and inner core. From neutron drip density to crust-core transition density, the inner crust of neutron star is a nonuniform system where nuclei coexist with nucleon and electron gases surrounding them. As density increases, the nuclear pasta phases with different shapes, such as droplet, rod, slab, tube, and bubble may appear in the inner crust. Astronomical observations indicate that strong magnetic fields, which may be as high as 1018 G, may exist in some neutron stars. So,the crust structure of neutron star may be influenced significantly by the strong magnetic fields. Consequently, a number of astrophysical observations, such as the neutron star oscillations and glitches in the spin rate of pulsars may be affected by the properties of inner crust. In this paper, we adopt the relativistic mean-field theory to describe the nuclear interaction,and anomalous magnetic moments of proton and neutron are considered as well. In order to study the effects of strong magnetic fields on the properties of nonuniform nuclear matter in inner crust and crust-core transition of neutron stars,we adopt the Wigner-Seitz approximation to describe the nonuniform matter in inner crust, where only one nucleus exists in a Wigner-Seitz cell. Meanwhile, the charge neutrality and β equilibrium conditions are satisfied. We adopt the selfconsistent Thomas-Fermi approximation to obtain the distributions of nucleons and electrons in a Wigner-Seitz cell under strong magnetic fields, and then we calculate the binding energy per nucleon and the properties of Wigner-Seitz cell. It turns out that the properties of inner crust structure and the crust-core transition would not change much with strong magnetic fields B 1017 G comparing with B = 0. The nucleon distribution, binding energy per nucleon, radius of Wigner-Seitz cell, onset density of various nonspherical pasta phases, and crust-core density with B = 1017 G are very similar to the corresponding results with B = 0. However, the strong magnetic fields B 1018 G play an important role in determining the pasta phase structure. As a result, the radius of spherical Wigner-Seitz cell, the binding energy per nucleon of various kinds of pasta phases, and the onset densities of nonspherical nuclei and crust-core transition decrease with increasing B,while the area of high density in Wigner-Seitz cell increases. For fixed strength of magnetic fields, as the average baryon density in Wigner-Seitz cell increases, the radius of Wigner-Seitz cell decreases, but the area of high density in the center of the cell increases. The nucleon density at the boundary of Wigner-Seitz cell increases with increasing average baryon density for fixed strength magnetic fields, while the nucleon density at the center area of the cell decreases, which leads to the nucleon distribution in the cell becomes more dispersive.