用双折射技术研究Turbulence和探测磁场：应用于HI-to-H2过渡区
Study Turbulence and Probe Magnetic Field Using Gradients Technique: Application to HI-to-H2 Transition Regions
摘要：
在过去的几十年里，人们通过解析和数值模拟研究了光解区的原子-分子（HI-to-H $_2$）跃迁。然而，经典的PDR模型通常假设均匀密度的气体，忽略了星际介质的湍流性质。最近，Bialy等人（2017 b，2019）提出了一个理论框架，用于研究具有非均匀密度结构的现实湍流介质中的HI到H $_2$。在这里，我们扩展了这些磁致化学模型，探索可能性的跟踪磁场方向的湍流PDR使用磁致技术。我们利用亚音速和超音速磁流体动力学数值模拟化学HI/H $_2$平衡计算。我们证实了湍流引起的密度涨落可以分散H $_2$和HI分数的分布。我们发现，随着音速马赫数MS的增加，力矩图的能谱变浅。我们探讨的能力，在磁场跟踪梯度的高阶速度质心，并比较其性能与传统的速度质心梯度（VCG）和强度梯度（IG）。我们发现，二阶质心（VC $_2$ GS）的速度梯度比VCG和IG在探测磁场方向更准确。
英文摘要：
The atomic-to-molecular (HI-to-H$_2$) transition in photodissociation regions (PDRs) has been investigated over the last several decades through analytic and numerical modeling. However, classical PDR models typically assume uniform density gas, ignoring the turbulent nature of the interstellar medium. Recently, Bialy et al. (2017b, 2019) have presented a theoretical framework for studying the HI-to-H$_2$ in a realistic turbulent medium with a non-homogeneous density structure. Here we extend these turbulent-chemical models to explore the possibility of tracing the magnetic field direction in turbulent PDRs using the Gradients Technique. We utilize both subsonic and supersonic magnetohydrodynamic numerical simulations for chemical HI/H$_2$ balance calculations. We confirm that the density fluctuations induced by turbulence can disperse the distribution of H$_2$ and HI fraction. We find that the energy spectrum of moment maps gets shallower when the sonic Mach number MS increases. We explore the ability in magnetic field tracing of gradients of higher-order velocity centroids and compare their performance with that of traditional velocity centroid gradients (VCGs) and with intensity gradients (IGs). We find that the velocity gradients of the second-order centroids (VC$_2$Gs) are more accurate than VCGs and IGs in probing the magnetic field orientation.
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