中子星壳的成分冻结
Compositional Freeze-Out of Neutron Star Crusts
摘要：
我们研究了没有回退吸积的中子星的地壳性质。我们计算了三种不同情况下中子星地壳的化学演化（没有地壳热影响的改进型Urca过程、厚地壳和薄地壳的直接Urca过程），以确定核反应冻结时包层和大气的详细组成。利用一个直到锝的核反应网络，我们计算了中子星不同深度的原子核分布。当冷却时间尺度短于反应速率的倒数时，核反应淬灭。计算出的同位素中的微量轻元素可能有足够的时间在层结晶并形成中子星的大气或外壳之前漂浮到表面。中子星包层的组成决定了来自表面的总光子通量，而大气层的组成决定了出射光谱。我们的计算使用的三个冷却模型表明，没有吸积的回落的中子星的气氛是依赖于假设的冷却过程的中子星。每种冷却方法都有不同的大气成分：对于改良的Urca过程，大气成分是$^{28}$ Si，厚地壳的大气成分是$^{50}$ Cr，薄地壳的大气成分是$^{40}$ Ca。在这三种情况下，大气都是由比铁轻的元素组成的。
英文摘要：
We have investigated the crustal properties of neutron stars without fallback accretion. We have calculated the chemical evolution of the neutron star crust in three different cases (a modified Urca process without the thermal influence of a crust, a thick crust, and a direct Urca process with a thin crust) in order to determine the detailed composition of the envelope and atmosphere as the nuclear reactions freeze out. Using a nuclear reaction network up to technetium, we calculate the distribution of nuclei at various depths of the neutron star. The nuclear reactions quench when the cooling timescale is shorter than the inverse of the reaction rate. Trace light elements among the calculated isotopes may have enough time to float to the surface before the layer crystallizes and form the atmosphere or envelope of the neutron star. The composition of the neutron-star envelope determines the total photon flux from the surface, and the composition of the atmosphere determines the emergent spectrum. Our calculations using each of the three cooling models indicate that without accretion of fallback the neutron star atmospheres are dependent on the assumed cooling process of the neutron star. Each of the cooling methods have different elements composing the atmosphere: for the modified Urca process the atmosphere is $^{28}$Si, the thick crust has an atmosphere of $^{50}$Cr, and the thin crust has an atmosphere of $^{40}$Ca. In all three cases the atmospheres are composed of elements which are lighter then iron.
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