The Galactic Shock Pump: A Source of Supersonic Internal Motions in the Cool Interstellar Medium
The Galactic Shock Pump: A Source of Supersonic Internal Motions in the Cool Interstellar Medium
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银河激波泵：冷星际介质中超音速内部运动的来源
DOI：
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发表时间：
1997
期刊：
影响因子：
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通讯作者：
J. Scalo
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文献类型：
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作者：
P. Kornreich;J. Scalo
我们认为，银河系冲击通过星际密度波动传播提供了一种机制，间歇性的补充，或“泵”的超音速运动和内部密度增强普遍观察到的冷原子和分子星际结构，而不一定需要存在的自引力，磁场，或年轻的恒星。这些冲击被认为是由于一系列尺度上的各种星系源。本文导出了激波通过径向分层的二维等压模型云时所产生的运动学涡量的解析结果，假定马赫数不至于大到使云破裂的程度，并忽略激波曲率和云畸变。在适当马赫数下的二维格子气流体动力学模拟被用来验证分析结果。诱导的内部速度最初是激波速度除以密度对比的平方根的一个重要部分，占所观察到的线宽振幅和明显的云到云的线宽-密度标度。谱线宽度与尺寸的关系可以用众所周知的冲击系统的功率谱来解释。诱导的涡能应迅速转换为可压缩和MHD模式，因此很难直接观察到，即使它仍然是其他模式的动力源。因此，冲击泵产生密度结构，而不需要任何形式的不稳定性。我们认为，冲击泵应导致嵌套的冲击诱导结构，提供了一个级联机制的超音速“湍流”和冷星际介质的分形结构的物理解释。在我们的银河系中的一个理想化的云的冲击曝光之间的平均时间进行了估计，发现是足够小的冲击泵是能够维持超音速运动对重新调整和耗散，除了最小的结构。这就解释了在小的“致密核”中，谱线的宽度在空间上是大致均匀的，而且接近音速。“我们推测，避免冲击泵可能是局部区域形成恒星所必需的，并且避免概率对区域大小的反向依赖可能是确定恒星初始质量函数的重要因素。
We propose that galactic shocks propagating through interstellar density fluctuations provide a mechanism for the intermittent replenishment, or "pumping," of the supersonic motions and internal density enhancements observed pervasively within cool atomic and molecular interstellar structures, without necessarily requiring the presence of self-gravity, magnetic fields, or young stars. The shocks are assumed to be due to a variety of galactic sources on a range of scales. An analytic result for the kinematic vorticity generated by a shock passing through a radially stratified two-dimensional isobaric model cloud is derived, assuming that the Mach number is not so large that the cloud is disrupted, and neglecting the shock curvature and cloud distortion. Two-dimensional lattice gas hydrodynamic simulations at modest Mach numbers were used to verify the analytic result. The induced internal velocities are initially a significant fraction of the shock speed divided by the square root of the density contrast, accounting for both the observed line width amplitudes and the apparent cloud-to-cloud line width-density scaling. The line width-size relation could then be interpreted in terms of the well-known power spectrum of a system of shocks. The induced vortical energy should quickly be converted to compressible and MHD modes and so would be difficult to observe directly, even though it would still be the power source for the other modes. The shock pump thus produces density structure without the necessity of any sort of instability. We argue that the shock pump should lead to nested shock-induced structures, providing a cascade mechanism for supersonic "turbulence" and a physical explanation for the fractal-like structure of the cool interstellar medium. The average time between shock exposures for an idealized cloud in our Galaxy is estimated and found to be small enough that the shock pump is capable of sustaining the supersonic motions against readjustment and dissipation, except for the smallest structures. This suggests an explanation of the roughly spatially uniform and nearly sonic line widths in small "dense cores." We speculate that the avoidance of shock pumping may be necessary for a localized region to form stars and that the inverse dependence of probability of avoidance on region size may be an important factor in determining the stellar initial mass function.