相空间蜗牛壳的剖析
Dissecting the Phase Space Snail Shell
摘要：
盖亚DR 2数据已经发现了正在进行的垂直相混合，表现为$Z-V_{Z}$相空间中的蜗牛壳。为了更好地理解相位混合过程的起源和性质，我们研究了太阳圈附近$V_{R}-V_ϕ$相空间中拱（包括经典的“移动群”）中的垂直相位混合特征。有趣的是，相空间蜗牛壳仅存在于具有$|V_ϕ - V_{\rm LSR}| \lesssim 30$ km/s的拱中，即，恒星在动态的“冷”轨道上。蜗牛壳变得更弱，并最终消失为越来越大的径向作用（$J_{R}$），量化的“热”的轨道。因此，在未来的相混合研究中，应更加关注较冷的轨道。我们还确认，武仙座流有两个分支（在快速和慢速$V_ϕ$），这可能不是由一个单一的机制来解释，因为只有快速分支显示突出的蜗牛壳功能。较热的轨道可能已经由于径向变化的更大的动力学范围而相位包裹掉，以促进更快的相位混合。为了解释为什么在较热的轨道上没有一个明确的蜗牛壳，这个圆盘应该至少在$500$ Myr之前就被扰动了。我们的结果提供了更多的支持，最近的卫星盘遇到的情况下，比内部杆屈曲扰动的情况下，作为相空间混合的起源。在$V_ϕ$颜色编码相空间中更突出的蜗牛壳的起源也进行了讨论。
英文摘要：
The on-going vertical phase mixing, manifesting itself as a snail shell in the $Z-V_{Z}$ phase space, has been discovered with the Gaia DR2 data. To better understand the origin and properties of the phase mixing process, we study the vertical phase-mixing signatures in arches (including the classical ``moving groups'') of the $V_{R}-V_ϕ$ phase space near the Solar circle. Interestingly, the phase space snail shell exists only in the arches with $|V_ϕ - V_{\rm LSR}| \lesssim 30$ km/s, i.e., stars on dynamically ``colder'' orbits. The snail shell becomes much weaker and eventually disappears for increasingly larger radial action ($J_{R}$), quantifying the ``hotness'' of orbits. Thus one should pay closer attention to the colder orbits in future phase mixing studies. We also confirm that the Hercules stream has two branches (at fast and slow $V_ϕ$), which may not be explained by a single mechanism, since only the fast branch shows the prominent snail shell feature. The hotter orbits may have phase-wrapped away already due to the much larger dynamical range in radial variation to facilitate faster phase mixing. To explain the lack of a well-defined snail shell in the hotter orbits, the disk should have been perturbed at least $500$ Myr ago. Our results offer more support to the recent satellite-disk encounter scenario than the internal bar buckling perturbation scenario as the origin of the phase space mixing. Origin of the more prominent snail shell in the $V_ϕ$ color-coded phase space is also discussed.
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