arXiv 1202.44982012-02-20astro-ph.GAastro-ph.SRnlin.CDphysics.flu-dyn

星际Turbulence的物理过程

Physical Processes of Interstellar Turbulence

Enrique Vazquez-Semadeni

备注：: Invited review for the "Enzo Levi International Seminar", organized by the Fluid Dynamics Division of the Mexican Physical Society, on May 5-6, Morelia, Mexico. To appear in the proceedings of the XVII Congress of the Fluid Dynamics Division, Springer

摘要：

我讨论了自引力和辐射加热和冷却在塑造我们银河系的星际介质（ISM）的湍流的性质的作用。加热和冷却导致它是高度可压缩的，并且在某些密度和温度范围内，变得热不稳定，倾向于自发地分离成暖/扩散和冷/致密相。另一方面，湍流是一种固有的混合过程，倾向于补充单独在热过程下被禁止的密度和温度范围。电离ISM中的紊流似乎是跨音速的（即马赫数为$\Ms \sim 1$），因此基本上是不可压缩的。然而，在中性介质中，热不稳定性导致气体的声速波动高达$\sim 30$的因子，因此相对于稠密/冷气体，流动可以是高度超音速的，尽管数值模拟表明，这种行为更多地对应于冷团块的集合，而不是团块的内部速度分散。最后，温暖中性介质中的相干大尺度压缩（例如，由螺旋臂或超新星冲击波引起的）可以产生大而致密的分子云，这些分子云受到自身重力的影响，并开始在重力作用下收缩。因为它们是由非线性密度涨落组成的，其局部自由落体时间明显小于整个云的自由落体时间，这些涨落更早地终止了它们的坍缩，从而产生了一种分级的引力碎裂机制，小尺度的坍缩发生在大尺度的坍缩中。因此，分子云中的“湍流”可能在所有尺度上都由引力收缩分量主导。

英文摘要：

I discuss the role of self-gravity and radiative heating and cooling in shaping the nature of the turbulence in the interstellar medium (ISM) of our galaxy. The heating and cooling cause it to be highly compressible, and, in some regimes of density and temperature, to become thermally unstable, tending to spontaneously segregate into warm/diffuse and cold/dense phases. On the other hand, turbulence is an inherently mixing process, tending to replenish the density and temperature ranges that would be forbidden under thermal processes alone. The turbulence in the ionized ISM appears to be transonic (i.e, with Mach numbers $\Ms \sim 1$), and thus to behave essentially incompressibly. However, in the neutral medium, thermal instability causes the sound speed of the gas to fluctuate by up to factors of $\sim 30$, and thus the flow can be highly supersonic with respect to the dense/cold gas, although numerical simulations suggest that this behavior corresponds more to the ensemble of cold clumps than to the clumps' internal velocity dispersion. Finally, coherent large-scale compressions in the warm neutral medium (induced by, say, the passage of spiral arms or by supernova shock waves) can produce large, dense molecular clouds that are subject to their own self-gravity, and begin to contract gravitationally. Because they are populated by nonlinear density fluctuations, whose local free-fall times are significantly smaller than that of the whole cloud, the fluctuations terminate their collapse earlier, giving rise to a regime of hierarchical gravitational fragmentation, with small-scale collapses occurring within larger-scale ones. Thus, the "turbulence" in molecular clouds may be dominated by a gravitationally contracting component at all scales.

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