TWO-FLUID MAGNETOHYDRODYNAMICS SIMULATIONS OF CONVERGING H i FLOWS IN THE INTERSTELLAR MEDIUM. II. ARE MOLECULAR CLOUDS GENERATED DIRECTLY FROM A WARM NEUTRAL MEDIUM?
TWO-FLUID MAGNETOHYDRODYNAMICS SIMULATIONS OF CONVERGING H i FLOWS IN THE INTERSTELLAR MEDIUM. II. ARE MOLECULAR CLOUDS GENERATED DIRECTLY FROM A WARM NEUTRAL MEDIUM?
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DOI：
10.1088/0004-637x/704/1/161
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发表时间：
2009-08
期刊：
影响因子：
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通讯作者：
T. Inoue;S. Inutsuka
中科院分区：
文献类型：
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作者：
T. Inoue;S. Inutsuka
利用二维双流体磁流体动力学模拟研究了热不稳定性导致的星际云的形成。我们考虑的情况下，收敛，超音速流动的温暖的中性介质中的星际介质，产生一个冲击板的热不稳定的气体云的形式。我们发现，正如论文I所推测的，在冲击板中，磁压占主导地位，热不稳定性在等容冷却的热不稳定板中增长，导致H i云的形成，其数密度通常为n × 100 cm-3，即使磁场和会聚流之间的角度很小。我们还发现，即使有一个很大的分散的磁场，冲击板的演变基本上是由平均磁场和会聚流之间的角度。因此，由热中性介质堆积直接形成分子云似乎不是典型的分子云形成过程，除非超声速会聚流的方向通过某种机制偏向于平均磁场的方向。然而，当该角度较小时，由于会聚流而产生的H i壳层是巨大的，并且可能演化成分子云，前提是在巨大的H i壳层中的气体沿着磁场线再次堆积。我们预计另一个后续的冲击波可以再次堆积大质量壳层的气体并产生更大的云。因此，我们强调的重要性，多集的收敛流，作为一个典型的形成过程的分子云。
Formation of interstellar clouds as a consequence of thermal instability is studied using two-dimensional two-fluid magnetohydrodynamic simulations. We consider the situation of converging, supersonic flows of warm neutral medium in the interstellar medium that generate a shocked slab of thermally unstable gas in which clouds form. We find, as speculated in Paper I, that in the shocked slab magnetic pressure dominates thermal pressure and the thermal instability grows in the isochorically cooling, thermally unstable slab that leads to the formation of H i clouds whose number density is typically n ≲ 100 cm−3, even if the angle between magnetic field and converging flows is small. We also find that even if there is a large dispersion of magnetic field, evolution of the shocked slab is essentially determined by the angle between the mean magnetic field and converging flows. Thus, the direct formation of molecular clouds by piling up warm neutral medium does not seem to be a typical molecular cloud formation process, unless the direction of supersonic converging flows is biased to the orientation of mean magnetic field by some mechanism. However, when the angle is small, the H i shell generated as a result of converging flows is massive and possibly evolves into molecular clouds, provided gas in the massive H i shell is piled up again along the magnetic field line. We expect that another subsequent shock wave can again pile up the gas of the massive shell and produce a larger cloud. We thus emphasize the importance of multiple episodes of converging flows, as a typical formation process of molecular clouds.