驻波吸积激波不稳定性的螺旋模态
The Spiral Modes of the Standing Accretion Shock Instability
摘要：
一个停滞的球形吸积激波，例如在核心坍缩超新星中产生的，对非球形扰动是不稳定的。在三维空间中，这种静止吸积激波不稳定性（SASI）可以发展出螺旋模式，使质子中子星旋转起来。在这里，我们研究这些非轴对称模式相结合的线性稳定性分析和三维，随时间变化的流体动力学模拟与Zeus-MP，专注于表征其空间结构和角动量的内容。我们不对背景吸积流施加任何旋转，并使用简化的微观物理，没有中微子加热或核离解。螺旋模式进行检查，通过选择流动参数，只有基本模式是不稳定的，对于一个给定的极性指数，导致良好的协议与线性理论隔离。我们发现，任何非零相对相位的晃动模式的叠加生存在非线性制度，并导致角动量重新分配。因此，获得自旋向上所需的扰动范围比获得π/2相移的极限情况所需的扰动范围更宽。角动量重分布的大部分发生在指数增长的阶段，并从内部扭矩，是第二阶的扰动幅度。这种重新分布至少产生了两个反向旋转的区域，给定符号的最大角动量接近质量吸积率乘以激波半径平方的一个重要分数（Mdot * r_shock^2 ~ 1 E +47 g/cm^2/s，自旋周期~60 ms）。饱和时的非线性模式耦合导致角动量在所有方向上以小得多的幅度波动。
英文摘要：
A stalled spherical accretion shock, such as that arising in core-collapse supernovae, is unstable to non-spherical perturbations. In three dimensions, this Standing Accretion Shock Instability (SASI) can develop spiral modes that spin-up the protoneutron star. Here we study these non-axisymmetric modes by combining linear stability analysis and three-dimensional, time-dependent hydrodynamic simulations with Zeus-MP, focusing on characterizing their spatial structure and angular momentum content. We do not impose any rotation on the background accretion flow, and use simplified microphysics with no neutrino heating or nuclear dissociation. Spiral modes are examined in isolation by choosing flow parameters such that only the fundamental mode is unstable for a given polar index, leading to good agreement with linear theory. We find that any superposition of sloshing modes with non-zero relative phases survives in the nonlinear regime and leads to angular momentum redistribution. It follows that the range of perturbations required to obtain spin-up is broader than that needed to obtain the limiting case of a phase shift of pi/2. The bulk of the angular momentum redistribution occurs during a phase of exponential growth, and arises from internal torques that are second order in the perturbation amplitude. This redistribution gives rise to at least two counter rotating regions, with the maximum angular momentum of a given sign approaching a significant fraction of the mass accretion rate times the shock radius squared (Mdot * r_shock^2 ~ 1E+47 g/cm^2/s, spin period ~60 ms). Nonlinear mode coupling at saturation causes the angular momentum to fluctuate in all directions with much smaller amplitudes.
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