The Early Stage of Molecular Cloud Formation by Compression of Two-phase Atomic Gases
The Early Stage of Molecular Cloud Formation by Compression of Two-phase Atomic Gases
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DOI：
10.3847/1538-4357/ab02ff
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发表时间：
2018-06
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影响因子：
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通讯作者：
K. Iwasaki;K. Tomida;T. Inoue;S. Inutsuka
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文献类型：
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作者：
K. Iwasaki;K. Tomida;T. Inoue;S. Inutsuka
我们用三维磁流体力学模拟研究了原子气体分子云的形成，包括非平衡化学反应和加热/冷却过程。我们考虑原子气体的超AlfvéNic迎头碰撞流，它具有由HI云和周围热扩散气体组成的两相结构。我们研究了分子云的形成如何依赖于上游流动和平均磁场之间的角度θ。我们发现，存在一个临界角θcr，在该临界角以上，激波放大磁场控制激波后气体动力学。如果原子气体几乎沿平均磁场(θ≪θcr)压缩，则上游高度不均匀的原子气体的吸积将维持超Alfvénic各向异性湍流。结果，产生了一个以湍流为主的激波层。在θ∼θcr附近，激波放大的磁场减弱了激波后的湍流，导致了致密的激波后层。对于θ≫θcr，强磁压抑制了冷致密云的形成。如果θ小于θcr的几倍，则有望形成有效的分子云。通过建立解析模型并进行参数测量，我们得到了临界角随上游原子气体平均密度、碰撞速度和场强的函数的解析式。当场强大于1∼G时，临界夹角小于μ15°，说明当激波来自不同方向时，发生θ<；θcr压缩的概率是有限的。
We investigate the formation of molecular clouds from atomic gas by using three-dimensional magnetohydrodynamic simulations, including non-equilibrium chemical reactions and heating/cooling processes. We consider super-Alfvénic head-on colliding flows of atomic gas possessing the two-phase structure that consists of H i clouds and surrounding warm diffuse gas. We examine how the formation of molecular clouds depends on the angle θ between the upstream flow and the mean magnetic field. We find that there is a critical angle θcr above which the shock-amplified magnetic field controls the post-shock gas dynamics. If the atomic gas is compressed almost along the mean magnetic field (θ ≪ θcr), super-Alfvénic anisotropic turbulence is maintained by the accretion of the highly inhomogeneous upstream atomic gas. As a result, a greatly extended turbulence-dominated post-shock layer is generated. Around θ ∼ θcr, the shock-amplified magnetic field weakens the post-shock turbulence, leading to a dense post-shock layer. For θ ≫ θcr, the strong magnetic pressure suppresses the formation of cold dense clouds. Efficient molecular cloud formation is expected if θ is less than a few times θcr. Developing an analytic model and performing a parameter survey, we obtain an analytic formula for the critical angle as a function of the mean density, collision speed, and field strength of the upstream atomic gas. The critical angle is found to be less than ∼15° as long as the field strength is larger than 1 μG, indicating that the probability of occurrence of compression with θ < θcr is limited if shock waves come from various directions.