Three-dimensional GRMHD Simulations of Neutrino-cooled Accretion Disks from Neutron Star Mergers
Three-dimensional GRMHD Simulations of Neutrino-cooled Accretion Disks from Neutron Star Mergers
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DOI：
10.3847/1538-4357/aabaec
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发表时间：
2017-11
期刊：
影响因子：
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通讯作者：
D. Siegel;B. Metzger
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文献类型：
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作者：
D. Siegel;B. Metzger
由两颗中子星（NS）或一颗中子星和一个恒星质量黑洞组成的合并双星通常会在残余黑洞或长寿中子星周围形成一个巨大的吸积环。从这些中微子冷却的吸积盘流出代表了r-过程核合成和kilonovae产生的重要场所。我们提出了第一个三维的，广义相对论磁流体动力学（GRMHD）模拟，包括弱相互作用和现实的状态方程，这种吸积盘在粘性时间尺度（380毫秒）。我们见证了稳态MHD湍流的出现，一个周期为1.20毫秒的磁发电机，以及一个“热”盘日冕的产生，该日冕在自由核子重组成α粒子时释放的能量的帮助下发射强大的热外流。我们确定了一个自我调节机制，保持中平面电子分数低（叶0.1）在粘性的时间尺度。这个富含中子的储层，反过来，饲料流出，保持足够低的值Ye 0.2，以稳健地合成第三峰r-过程元素。的准球形流出预计unbind 40%的初始磁盘质量与典型的渐近逃逸速度为0.1c，因此可能代表的主导质量喷射机制在NS-NS合并。包括中微子吸收，我们的研究结果与以前的流体动力学α盘模拟一致，即从第一到第三个r过程峰的整个r过程核范围都可以在流出物中合成，与观测到的太阳系丰度非常一致。渐近逃逸速度和喷出物的数量，外推到适度较高的磁盘质量时，是一致的，需要解释的红色kilonova排放后，NS合并GW 170817。
Merging binaries consisting of two neutron stars (NSs) or an NS and a stellar-mass black hole typically form a massive accretion torus around the remnant black hole or long-lived NS. Outflows from these neutrino-cooled accretion disks represent an important site for r-process nucleosynthesis and the generation of kilonovae. We present the first three-dimensional, general-relativistic magnetohydrodynamic (GRMHD) simulations including weak interactions and a realistic equation of state of such accretion disks over viscous timescales (380 ms). We witness the emergence of steady-state MHD turbulence, a magnetic dynamo with an ∼20 ms cycle, and the generation of a “hot” disk corona that launches powerful thermal outflows aided by the energy released as free nucleons recombine into α-particles. We identify a self-regulation mechanism that keeps the midplane electron fraction low (Ye ∼ 0.1) over viscous timescales. This neutron-rich reservoir, in turn, feeds outflows that retain a sufficiently low value of Ye ≈ 0.2 to robustly synthesize third-peak r-process elements. The quasi-spherical outflows are projected to unbind 40% of the initial disk mass with typical asymptotic escape velocities of 0.1c and may thus represent the dominant mass ejection mechanism in NS–NS mergers. Including neutrino absorption, our findings agree with previous hydrodynamical α-disk simulations that the entire range of r-process nuclei from the first to the third r-process peak can be synthesized in the outflows, in good agreement with observed solar system abundances. The asymptotic escape velocities and quantity of ejecta, when extrapolated to moderately higher disk masses, are consistent with those needed to explain the red kilonova emission following the NS merger GW170817.