AI 中文总结
研究吸积到银河系的高速云，通过解析建模与三维流体动力学模拟结合研究终端速度范式动力学极限，推导广义运动方程，发现终端速度是局部平衡，受多种因素影响，合成可观测量再现观测特征并给出阻力系数测量结果。
AI 中文摘要
长期以来，终端速度范式一直用于解释吸积到银河系的高速云（HVCs）的运动并推断其距离，但其在现实银河条件下的有效性仍不确定。我们通过将解析建模与三维流体动力学模拟相结合来研究其动力学极限，该模拟是关于云在分层的银河系晕中移动的情况。我们推导了一个广义运动方程，包括引力、冲压阻力、一个捕获质量损失和增长的唯象质量交换模型以及伯努利驱动的云膨胀。对于性质恒定的云的解析解提供了一个参考框架，而完整的演化则通过使用包含绝热物理、辐射冷却和热传导的模拟来评估。终端速度是局部平衡而非全局吸引子：密集云在其大部分轨迹上保持准弹道运动，仅在到达银盘前不久接近终端运动。流体动力学效应进一步限制了该范式。在绝热流中，不稳定性会迅速破坏云，使终端速度描述不适用。辐射冷却反而促进凝聚和动量加载，在很长一段时间内保持与背景气体的强耦合并恢复类似终端速度的状态，而热传导主要影响小尺度结构。合成可观测量，包括位置 - 速度图、光学消光和软 X 射线发射，再现了观测到的 HVCs 的关键特征，如速度桥和压缩驱动发射。我们还提供了对下落云的有效阻力系数的直接测量，发现其值约为 1 但强烈依赖时间。总体而言，终端速度范式是一个由云结构、质量交换和周围介质决定的条件性描述。
英文摘要
The terminal-velocity paradigm has long been used to interpret the motion and infer the distances of HVCs accreting onto the Milky Way, yet its validity under realistic Galactic conditions remains uncertain. We investigate its dynamical limits by combining analytical modelling with three-dimensional hydrodynamical simulations of clouds moving through a stratified Milky Way halo. We derive a generalized equation of motion including gravity, ram-pressure drag, a phenomenological mass-exchange model capturing mass loss and growth, and Bernoulli-driven cloud expansion. Analytical solutions for constant-property clouds provide a reference framework, while the full evolution is assessed using simulations with adiabatic physics, radiative cooling, and thermal conduction. Terminal velocity is a local equilibrium but not a global attractor: dense clouds remain quasi-ballistic over most of their trajectories and approach terminal motion only shortly before reaching the Galactic disc. Hydrodynamical effects further limit the paradigm. In adiabatic flows, instabilities rapidly disrupt the cloud, rendering the terminal-velocity description inapplicable. Radiative cooling instead promotes condensation and momentum loading, maintaining strong coupling to the background gas and restoring a terminal-velocity-like regime over extended periods, while thermal conduction mainly affects small-scale structure. Synthetic observables, including position--velocity diagrams, optical extinction, and soft X-ray emission, reproduce key features of observed HVCs such as velocity bridges and compression-driven emission. We also provide direct measurements of the effective drag coefficient for infalling clouds, finding values of order unity but strongly time-dependent. Overall, the terminal-velocity paradigm is a conditional description governed by cloud structure, mass exchange, and the ambient medium.
Comments32 pages, 15 figures, 1 table, accepted for publication in MNRAS
