辐射冷却冲击波磁场强度的新诊断
A New Diagnostic of Magnetic Field Strengths in Radiatively-Cooled Shocks
摘要：
我们表明，它是可能的测量阿尔文马赫数，定义为在流中的激波速度除以阿尔文速度，低速（V $_{shock}$ $\lesssim$ 100 km $\,$ s $^{-1}$）辐射激波。该方法结合了明亮的禁线的观测与冷却区的大小的测量，后者通常从巴耳末发射线和禁线之间的空间分离获得。由于磁场在激波后区域的气体冷却时会被压缩，因此即使是相对较弱的震前磁场也可以用这种方法探测到。我们推导出解析公式，解释了空间分离如何与发射线比相关，并计算了一个大网格的辐射冷却激波模型，以开发诊断图，可用于推导流动中的Alfvénic马赫数。将该方法应用于HH 111喷流中亮结的现有数据，我们获得了相对较低的阿尔夫文马赫数$\sim$ 2，这表明磁化喷流内部具有超磁声速度扰动。
英文摘要：
We show that it is possible to measure Alfvénic Mach numbers, defined as the shock velocity in the flow divided by the Alfvén velocity, for low-velocity (V$_{shock}$ $\lesssim$ 100 km$\,$s$^{-1}$) radiative shocks. The method combines observations of bright forbidden lines with a measure of the size of the cooling zone, the latter typically obtained from spatial separation between the Balmer emission lines and the forbidden lines. Because magnetic fields become compressed as gas in the postshock region cools, even relatively weak preshock magnetic fields can be detected with this method. We derive analytical formulae that explain how the spatial separations relate to emission-line ratios, and compute a large grid of radiatively-cooled shock models to develop diagnostic diagrams that can be used to derive Alfvénic Mach numbers in flows. Applying the method to existing data for a bright knot in the HH 111 jet, we obtain a relatively low Alfvénic Mach number of $\sim$ 2, indicative of a magnetized jet that has super-magnetosonic velocity perturbations within it.
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