偏离、摆动和散射：非球面势场中轨道和流的angular momentum演化
Stray, swing and scatter: angular momentum evolution of orbits and streams in aspherical potentials
摘要：
在非球面势中，轨道平面不断演化。引力矩促使角动量矢量进动，即围绕对称轴缓慢偏离，并章动，即在垂直方向上周期性地上下摆动。这种熟悉的轨道极运动--如果被探测到并测量到--可以揭示潜在的引力势的形状，到目前为止，银河系中只有粗略的测量。在这里，我们证明了恒星潮汐流的碎片极表现出非常相似的漂移和摆动行为，并给出解析表达式连接的振幅和频率的极演化的平坦化的暗物质分布。虽然这些结果是来自近圆轨道，我们表明，他们也是有效的偏心轨道。最重要的是，我们解释了如何微分轨道平面进动导致扩大的流，并表明流极轨道上应该更快地分散。我们提供的流的宽度演变的函数的轴对称潜在的扁平化和从对称平面的角度的表达式，并证明我们的模型是在良好的协议与流N体模拟。有趣的是，同样的直觉也适用于其祖先位于三轴势中的短轴或长轴环上的流。最后，我们提出了银河系冷流数据的汇编，并讨论了如何简单的图片开发这里，随着流建模，可以用来约束对称轴和扁平的银河系。
英文摘要：
In aspherical potentials orbital planes continuously evolve. The gravitational torques impel the angular momentum vector to precess, that is to slowly stray around the symmetry axis, and nutate, i.e. swing up and down periodically in the perpendicular direction. This familiar orbital pole motion - if detected and measured - can reveal the shape of the underlying gravitational potential, the quantity only crudely gauged in the Galaxy so far. Here we demonstrate that the debris poles of stellar tidal streams show a very similar straying and swinging behavior, and give analytic expressions to link the amplitude and the frequency of the pole evolution to the flattening of the dark matter distribution. While these results are derived for near-circular orbits, we show they are also valid for eccentric orbits. Most importantly, we explain how the differential orbital plane precession leads to the broadening of the stream and show that streams on polar orbits ought to scatter faster. We provide expressions for the stream width evolution as a function of the axisymmetric potential flattening and the angle from the symmetry plane and prove that our models are in good agreement with streams produced in N-body simulations. Interestingly, the same intuition applies to streams whose progenitors are on short or long-axis loops in a triaxial potential. Finally, we present a compilation of the Galactic cold stream data, and discuss how the simple picture developed here, along with stream modelling, can be used to constrain the symmetry axes and flattening of the Milky Way.
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