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arXiv 2204.03005astro-ph.HEastro-ph.SRgr-qcnucl-th
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中子星并合黑洞吸积盘的长期三维MHD模拟

Long-term 3D-MHD Simulations of Black Hole Accretion Disks formed in Neutron Star Mergers

Steven Fahlman, Rodrigo Fernández

摘要:

我们研究的长期演化的吸积环面周围的黑洞(BH)的残余物的紧凑的对象合并涉及至少一个中子星,以更好地了解他们的贡献kilonovae和r-过程元素的合成。为此,我们修改了FLASH4.5中的未分裂磁流体动力学(MHD)求解器,使其在非均匀三维球坐标下工作,从而能够更有效地覆盖长度尺度上的大动态范围,同时利用系统中的对称性。这个修改后的代码是用来执行BH吸积盘模拟,改变初始磁场的几何形状和磁盘的紧凑性,利用物理状态方程,中微子泄漏计划的发射和吸收,并模拟BH的重力与伪牛顿势。模拟运行足够长的时间,以在磁盘中实现辐射效率低下的状态。我们发现强大的质量喷射与极向和环向初始场的几何形状,并在高磁盘紧凑性抑制外流。与所包括的物理,我们得到双峰速度分布,追溯到质量喷射的磁应力在早期,和热过程中的辐射效率低的状态在后期。盘流出的电子分数分布是广泛的,在所有模型中,喷出物的几何形状如下特征沙漏形状。我们测试的效果,消除中微子吸收或核复合与轴对称模型,发现$\sim 50\%$更少的质量喷射和更多的中子丰富的组成没有中微子吸收,和一个次主导的贡献核复合。测试的MHD和中微子泄漏的实现包括在内。

英文摘要:

We examine the long-term evolution of accretion tori around black hole (BH) remnants of compact object mergers involving at least one neutron star, to better understand their contribution to kilonovae and the synthesis of r-process elements. To this end, we modify the unsplit magnetohydrodynamic (MHD) solver in FLASH4.5 to work in non-uniform three-dimensional spherical coordinates, enabling more efficient coverage of a large dynamic range in length scales while exploiting symmetries in the system. This modified code is used to perform BH accretion disk simulations that vary the initial magnetic field geometry and disk compactness, utilizing a physical equation of state, a neutrino leakage scheme for emission and absorption, and modeling the BH's gravity with a pseudo-Newtonian potential. Simulations run for long enough to achieve a radiatively-inefficient state in the disk. We find robust mass ejection with both poloidal and toroidal initial field geometries, and suppressed outflow at high disk compactness. With the included physics, we obtain bimodal velocity distributions that trace back to mass ejection by magnetic stresses at early times, and to thermal processes in the radiatively-inefficient state at late times. The electron fraction distribution of the disk outflow is broad in all models, and the ejecta geometry follows a characteristic hourglass shape. We test the effect of removing neutrino absorption or nuclear recombination with axisymmetric models, finding $\sim 50\%$ less mass ejection and more neutron-rich composition without neutrino absorption, and a subdominant contribution from nuclear recombination. Tests of the MHD and neutrino leakage implementations are included.

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