Postmerger Mass Ejection of Low-mass Binary Neutron Stars
Postmerger Mass Ejection of Low-mass Binary Neutron Stars
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DOI：
10.3847/1538-4357/abafc2
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发表时间：
2020-07
期刊：
影响因子：
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通讯作者：
S. Fujibayashi;S. Wanajo;K. Kiuchi;K. Kyutoku;Y. Sekiguchi;M. Shibata
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文献类型：
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作者：
S. Fujibayashi;S. Wanajo;K. Kiuchi;K. Kyutoku;Y. Sekiguchi;M. Shibata
我们研究了系统质量为2.5 M⊙的低质量双中子星（NSs）合并后的质量抛射和随后的核合成，并进行了轴对称的广义相对论、中微子辐射粘流体动力学模拟。我们发现，无论采用何种核状态方程（EOSs），合并残余物都是存活数秒以上的长寿命大质量NSs。我们的基准模型的喷出物质量为~ 0.06-0.1 M⊙（取决于EOS），是初始磁盘质量（~ 0.15-0.3 M⊙）的~ 30%。后处理核合成计算表明，喷射物主要由轻的r过程原子核组成，其中含有少量镧系元素（质量分数~ 0.002-0.004）和较重的镧系元素（由于黏性系数的合理值，平均电子分数适中）（~ 0.32-0.34）。这种丰度分布与弱r过程恒星（如HD 122563）中的丰度分布是相容的，但与所有测量到的r过程增强的贫金属恒星中发现的类似太阳r过程的丰度模式不相容。因此，低质量双子星NS合并应该是罕见的。如果这种低质量的NS合并发生，它们的电磁对应体，千新星，将以早期明亮的蓝色发射为特征，因为喷出物质量大，镧系元素含量小。然而，我们还表明，如果有效湍流粘度非常高，则喷射物的电子分数可能足够低，从而重现太阳r过程的丰度模式，并且镧系元素分数变得如此之高，以至于千新星的特征将是早期的亮蓝色和晚期的亮红色发射。
We study the postmerger mass ejection of low-mass binary neutron stars (NSs) with the system mass of 2.5 M ⊙ and subsequent nucleosynthesis by performing general-relativistic, neutrino-radiation viscous-hydrodynamics simulations in axial symmetry. We find that the merger remnants are long-lived massive NSs surviving more than several seconds, irrespective of the nuclear equations of state (EOSs) adopted. The ejecta masses of our fiducial models are ∼0.06–0.1 M ⊙ (depending on the EOS), being ∼30% of the initial disk masses (∼0.15–0.3 M ⊙). Postprocessing nucleosynthesis calculations indicate that the ejecta is composed mainly of light r-process nuclei with small amounts of lanthanides (mass fraction ∼0.002–0.004) and heavier species due to the modest average electron fraction (∼0.32–0.34) for a reasonable value of the viscous coefficient. Such abundance distributions are compatible with those in weak r-process stars such as HD 122563 but not with the solar r-process-like abundance patterns found in all measured r-process-enhanced metal-poor stars. Therefore, low-mass binary NS mergers should be rare. If such low-mass NS mergers occur, their electromagnetic counterparts, kilonovae, will be characterized by an early bright blue emission because of the large ejecta mass as well as the small lanthanide fraction. We also show, however, that if the effective turbulent viscosity is very high, the electron fraction of the ejecta could be low enough that the solar r-process-like abundance pattern is reproduced and the lanthanide fraction becomes so high that the kilonova would be characterized by early bright blue and late bright red emissions.