How do velocity structure functions trace gas dynamics in simulated molecular clouds?
How do velocity structure functions trace gas dynamics in simulated molecular clouds?
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速度结构函数如何追踪模拟分子云中的气体动力学？
DOI：
10.1051/0004-6361/201833970
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发表时间：
2019
影响因子：
6.5
通讯作者：
Henning, Th.
中科院分区：
文献类型：
--
作者：
Chira, R.-A.;Ibáñez-Mejía, J. C.;Mac Low, M.-M.;Henning, Th.
背景超音速无序流伴随着分子云（MC）的形成和演化。有人认为，这是可以支持重力塌陷并形成分层子结构的湍流。目的我们通过检查模拟 MC 的时间演化来研究：什么物理过程主导了湍流的驱动？如何描述这些流动的特征？它们与均匀湍流或引力塌缩一致吗？模拟的流动与观测结果一致吗？方法我们分析了在千秒差距尺度的星际介质（ISM）数值模拟中自洽形成的三个MC。模拟的ISM在物理过程的影响下演化，包括自重力、分层、磁场、超新星驱动的湍流以及辐射加热和冷却。我们使用具有或不具有密度加权或密度截断的速度结构函数（VSF）来表征流动，并在一维或三维中进行计算。然而，我们不包括可以隐藏最稠密气体中的运动的光学深度效应，从而限制了我们的结果与观测结果的比较。结果在具有足够分辨率的区域中，密度加权VSF最初似乎遵循均匀湍流的预期，其一阶幂律指数与拉尔森的尺寸-速度关系一致。超新星爆炸波对MC的影响会在大尺度上产生短暂的相干运动，从而增加穿越时间的标度指数。引力收缩驱动小尺度运动，产生的尺度系数随着小尺度占主导地位而下降甚至变成负值。去除密度加权可以消除这种效应，因为它强调了弥散的 ISM。结论我们得出结论，两种不同的效应同时再现了拉森的尺寸速度关系。最初，均匀湍流占主导地位，因此能量级联产生与拉尔森关系一致的 VSF。后来，收缩占主导地位，密度加权的 VSF 变得更浅甚至倒置，但 MC 的全局平均速度色散与其半径的关系遵循拉森关系，反映了维里平衡或自由落体塌陷。通过冲击注入的能量在 VSF 中是可见的，但会在交叉时间内衰减。
ContextSupersonic disordered flows accompany the formation and evolution of molecular clouds (MCs). It has been argued that this is turbulence that can support against gravitational collapse and form hierarchical sub-structures.AimsWe examine the time evolution of simulated MCs to investigate: What physical process dominates the driving of turbulent flows? How can these flows be characterised? Are they consistent with uniform turbulence or gravitational collapse? Do the simulated flows agree with observations?MethodsWe analysed three MCs that have formed self-consistently within kiloparsec-scale numerical simulations of the interstellar medium (ISM). The simulated ISM evolves under the influence of physical processes including self-gravity, stratification, magnetic fields, supernova-driven turbulence, and radiative heating and cooling. We characterise the flows using velocity structure functions (VSFs) with and without density weighting or a density cutoff, and computed in one or three dimensions. However, we do not include optical depth effects that can hide motions in the densest gas, limiting comparison of our results with observations.ResultsIn regions with sufficient resolution, the density-weighted VSFs initially appear to follow the expectations for uniform turbulence, with a first-order power-law exponent consistent with Larson’s size-velocity relationship. Supernova blast wave impacts on MCs produce short-lived coherent motions at large scales, increasing the scaling exponents for a crossing time. Gravitational contraction drives small-scale motions, producing scaling coefficients that drop or even turn negative as small scales become dominant. Removing the density weighting eliminates this effect as it emphasises the diffuse ISM.ConclusionsWe conclude that two different effects coincidentally reproduce Larson’s size velocity relationship. Initially, uniform turbulence dominates, so the energy cascade produces VSFs that are consistent with Larson’s relationship. Later, contraction dominates and the density-weighted VSFs become much shallower or even inverted, but the relationship of the global average velocity dispersion of the MCs to their radius follows Larson’s relationship, reflecting virial equilibrium or free-fall collapse. The injection of energy by shocks is visible in the VSFs, but decays within a crossing time.
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DOI：
10.1093/mnras/stu2498
发表时间：
2014
期刊：
arXiv: Astrophysics of Galaxies
影响因子：
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作者：
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DOI：
10.1103/physreve.97.023107
发表时间：
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期刊：
Physical review. E
影响因子：
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DOI：
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发表时间：
2011
期刊：
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