在自引力盘中形成Pop III双星：如何保持轨道angular momentum
Forming Pop III binaries in self-gravitating disks: how to keep the orbital angular momentum
摘要：
星盘碎裂是导致星族III双星系统形成的一个可能过程。然而，数值模拟显示了碎片的不同命运;一些演变成稳定的双星，另一些则与中心恒星合并。为了澄清这种多样性背后的物理学，我们以可控的方式进行了一系列三维流体动力学模拟。我们在自引力盘中插入一个模仿碎片的点粒子，初始质量和位置为自由参数，跟踪了几十个轨道的轨道演化。即使是这样简单的实验，结果也显示出很大的差异。一些粒子向内迁移后很快就融合了，但另一些粒子则随着圆盘中的间隙打开而迁移停止。我们发现，我们的结果是很好的解释假设提取的轨道角动量（i）从磁盘螺旋结构的引力扭矩，和（ii）潮汐破坏的引力约束的信封周围的粒子。我们的分析评估表明，过程（i）和（ii）是有效的，在外部和内部的磁盘分别。如果包层质量足够大，则在中间存在间隙开口的窗口。这些都与我们的数值结果相一致。我们进一步表明，双星，出现的“生存”的情况下，逐渐扩大，而吸积盘气体。我们的理论框架可以自由扩展，适用于当今的恒星和行星形成。
英文摘要：
The disk fragmentation is a possible process leading to the formation of Population III stellar binary systems. However, numerical simulations show diverse fates of the fragments; some evolve into stable binaries and others merge away with a central star. To clarify the physics behind such diversity, we perform a series of three dimensional hydrodynamics simulations in a controlled manner. We insert a point particle mimicking a fragment in a self-gravitating disk, where the initial mass and position are free parameters, and follow the orbital evolution for several tens of orbits. The results show great diversity even with such simple experiments. Some particles shortly merge away after migrating inward, but others survive as the migration stalls with the gap-opening in the disk. We find that our results are well interpreted postulating that the orbital angular momentum is extracted by (i) the gravitational torque from the disk spiral structure, and (ii) tidal disruption of a gravitationally-bound envelope around the particle. Our analytic evaluations show the processes (i) and (ii) are effective in an outer and inner part of the disk respectively. There is a window of the gap-opening in the middle, if the envelope mass is sufficiently large. These all agree with our numerical results. We further show that the binaries, which appear for the "survival" cases, gradually expand while accreting the disk gas. Our theoretical framework is freely scalable to be applied for the present-day star and planet formation.
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