Nebular Emission from Lanthanide-rich Ejecta of Neutron Star Merger
Nebular Emission from Lanthanide-rich Ejecta of Neutron Star Merger
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DOI：
10.1093/mnras/stab1975
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发表时间：
2021-02
影响因子：
4.8
通讯作者：
K. Hotokezaka;Masaomi Tanaka;D. Kato;G. Gaigalas
中科院分区：
文献类型：
--
作者：
K. Hotokezaka;Masaomi Tanaka;D. Kato;G. Gaigalas
用单区模型研究了中子星合并(NSM)富稀土抛射物的星云相，其中原子性质由单一物种--钕(ND)来表示。在假定r过程核的β衰变是热源和电离源的前提下，我们解决了非局域热力学平衡条件下喷射物的电离和热平衡问题。利用原子结构程序GRASP2K和HULLAC获得了包括能级、辐射跃迁速率、碰撞强度和复合速率系数在内的原子数据。我们发现，在星云温度下，允许线和禁止线对NdII和NdIII的冷却速率的贡献大致相等。结果表明，在星云相早期，动力学温度和电离度随时间增加，而在升温速率热化破裂后，动力学温度和电离度与时间基本无关，这是因为与电离和热化平衡有关的过程归因于后期电子和离子之间的两体碰撞。因此，尽管发光度迅速下降，但出射谱的形状在破缺后并不随时间发生明显变化。结果表明，纯钕喷出物的发射线星云光谱由0.5μm到20μm的宽结构组成，在1μm和10μm附近有两个不同的峰。
The nebular phase of lanthanide-rich ejecta of a neutron star merger (NSM) is studied by using a one-zone model, in which the atomic properties are represented by a single species, neodymium (Nd). Under the assumption that β-decay of r-process nuclei is the heat and ionization source, we solve the ionization and thermal balance of the ejecta under non-local thermodynamic equilibrium. The atomic data including energy levels, radiative transition rates, collision strengths, and recombination rate coefficients, are obtained by using atomic structure codes, GRASP2K and HULLAC. We find that both permitted and forbidden lines roughly equally contribute to the cooling rate of Nd II and Nd III at the nebular temperatures. We show that the kinetic temperature and ionization degree increase with time in the early stage of the nebular phase while these quantities become approximately independent of time after the thermalization break of the heating rate because the processes relevant to the ionization and thermalization balance are attributed to two-body collision between electrons and ions at later times. As a result, in spite of the rapid decline of the luminosity, the shape of the emergent spectrum does not change significantly with time after the break. We show that the emission-line nebular spectrum of the pure Nd ejecta consists of a broad structure from 0.5 μm to 20 μm with two distinct peaks around 1 μm and 10 μm.