双极电阻率对致密岩心形成的影响
The effect of ambipolar resistivity on the formation of dense cores
摘要：
我们的目标是了解形成致密的核心磁声波的热磁压力比小的地区。由于分子云中的电离率很低，中性粒子和带电粒子之间的耦合很弱。双极扩散则在形成过程中起着重要作用。静止的均匀等离子体受到快模波的扰动。使用二维数值模拟，我们遵循的快模波的演变。模拟是用一个多流体，自适应网格细化MHD代码。初始扰动的波长是2个数量级大于耗散长度的强烈影响的离子中性漂移。只有在对应于高度湍流气体的磁场变化很大的情况下，快模波才能产生致密的核。这意味着，在大多数核心，没有子结构可以产生。然而，TMC-1的核心D是这种情况的例外。由于其高的电离率，波长比耗散长度大3个数量级的波可以存在。这样的波仅受双极扩散的微弱影响，并且可以产生致密的子结构而没有大的波幅。我们的研究结果也解释了观察到的过渡从Alfvenic湍流运动在大尺度上的亚音速运动的水平上的密集的核心。
英文摘要：
We aim to understand the formation of dense cores by magnetosonic waves in regions where the thermal to magnetic pressure ratio is small. Because of the low-ionisation fraction in molecular clouds, neutral and charged particles are weakly coupled. Ambipolar diffusion then plays an important role in the formation process. A quiescent, uniform plasma is perturbed by a fast-mode wave. Using 2D numerical simulations, we follow the evolution of the fast-mode wave. The simulations are done with a multifluid, adaptive mesh refinement MHD code. Initial perturbations with wavelengths that are 2 orders of magnitude larger than the dissipation length are strongly affected by the ion-neutral drift. Only in situations where there are large variations in the magnetic field corresponding to a highly turbulent gas can fast-mode waves generate dense cores. This means that, in most cores, no substructure can be produced. However, Core D of TMC-1 is an exception to this case. Due to its atypically high ionisation fraction, waves with wavelengths up to 3 orders of magnitude greater than the dissipation length can be present. Such waves are only weakly affected by ambipolar diffusion and can produce dense substructure without large wave-amplitudes. Our results also explain the observed transition from Alfvenic turbulent motion at large scales to subsonic motions at the level of dense cores.
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