PSR B1557 $-$ 50发现的微小尺度结构
Tiny-scale Structure Discovered toward PSR B1557$-$50
摘要：
银河系中性星际介质（ISM）中的光学深度变化的空间尺度从几百到几千天文单位已经被观测到，而空间尺度为几十个天文单位的极小结构还没有被探测到。这种微小尺度原子结构（TSAS）的性质和形成需要更好地理解。我们报告了一个TSAS的试探性检测的信噪比为3.2对PSR B 1557 $-$ 50在第二个时代的两个Parkes会议只有0.36年，这是最近的间隔光谱观测到这个脉冲星。已确定一个吸收部分显示出微小的变化。根据脉冲星自行为14 mas $\rm yr^{-1}$，分量运动学距离为3.3 kpc，对应的特征空间尺度为17 au，这是已知TSAS的最小尺度之一。假设一个类似的视线（LOS）的深度，这里检测到的试探TSAS云相对于冷中性介质（CNM）是过密和超压的，并且可以辐射冷却得足够快，与周围环境处于热平衡。我们发现湍流不足以限制超压TSAS。我们探讨了在典型的CNM压力下，试验性TSAS所需的LOS伸长，发现它是$\sim5000$ --比ISM中观察到的细丝大得多。我们看到了一些证据的线宽和温度变化的CNM组件在两个时代观察到的，TSAS-like云的形成碰撞温暖的中性介质流的模型预测。
英文摘要：
Optical depth variations in the Galactic neutral interstellar medium (ISM) with spatial scales from hundreds to thousands of astronomical units have been observed through HI absorption against pulsars and continuum sources, while extremely small structures with spatial scales of tens of astronomical units remain largely unexplored. The nature and formation of such tiny-scale atomic structures (TSAS) need to be better understood. We report a tentative detection of TSAS with a signal-to-noise ratio of 3.2 toward PSR B1557$-$50 in the second epoch of two Parkes sessions just 0.36 yr apart, which are the closest-spaced spectral observations toward this pulsar. One absorption component showing marginal variations has been identified. Based on the pulsar's proper motion of 14 mas $\rm yr^{-1}$ and the component's kinematic distance of 3.3 kpc, the corresponding characteristic spatial scale is 17 au, which is among the smallest sizes of known TSAS. Assuming a similar line-of-sight (LOS) depth, the tentative TSAS cloud detected here is overdense and overpressured relative to the cold neutral medium (CNM), and can radiatively cool fast enough to be in thermal equilibrium with the ambient environment. We find that turbulence is not sufficient to confine the overpressured TSAS. We explore the LOS elongation that would be required for the tentative TSAS to be at the canonical CNM pressure, and find that it is $\sim5000$ -- much larger than filaments observed in the ISM. We see some evidence of line width and temperature variations in the CNM components observed at the two epochs, as predicted by models of TSAS-like cloud formation colliding warm neutral medium flows.
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