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arXiv 1012.5707astro-ph.SR
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亚临界磁云中坍缩核的形成：三维双极扩散MHD模拟

Formation of collapsing cores in subcritical magnetic clouds: three-dimensional MHD simulations with ambipolar diffusion

Takahiro Kudoh, Shantanu Basu

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43 pages, 21 figures, accepted for publication in ApJ
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摘要：

采用三维磁流体动力学模拟方法研究了亚临界分子云的引力驱动碎裂过程，发现只要施加磁通量冻结，引力驱动碎裂过程是稳定的.模拟结果表明，在一个初始亚临界云的核心一般是逐渐发展的双极扩散时间，这是大约几个$\times 10^7$年在一个典型的分子云。另一方面，超音速非线性流加速了亚临界云中坍缩核的形成。我们的参数研究表明，核心的形成发生更快的云的初始流速的强度增加。我们发现，核心的形成时间大致成正比的平方根的倒数的增强密度所产生的超音速非线性流。的密度依赖性是类似的，在准静态收缩磁支持的云，虽然核心形成的条件是由我们的模拟中的非线性流动。我们还发现，如果云是高度亚临界的，加速形成时间并不强烈地依赖于磁场的初始强度。我们的模拟表明，在我们的模型亚临界云的核心形成时间是几个$\times 10^6$年，由于存在大规模的超音速流（$\sim 3$倍音速）。一旦坍缩核心形成，核心的密度、速度和磁场结构并不强烈地依赖于速度波动的初始强度。芯的下落速度为亚音速，磁场线呈弱沙漏形。

英文摘要：

We employ the three-dimensional magnetohydrodynamic simulation including ambipolar diffusion to study the gravitationally-driven fragmentation of subcritical molecular clouds, in which the gravitational fragmentation is stabilized as long as magnetic flux-freezing applies. The simulations show that the cores in an initially subcritical cloud generally develop gradually over an ambipolar diffusion time, which is about a few $\times 10^7$ years in a typical molecular cloud. On the other hand, the formation of collapsing cores in subcritical clouds is accelerated by supersonic nonlinear flows. Our parameter study demonstrates that core formation occurs faster as the strength of the initial flow speed in the cloud increases. We found that the core formation time is roughly proportional to the inverse of the square root of the enhanced density created by the supersonic nonlinear flows. The density dependence is similar to that derived in quasistatically contracting magnetically supported clouds, although the core formation conditions are created by the nonlinear flows in our simulations. We have also found that the accelerated formation time is not strongly dependent on the initial strength of the magnetic field if the cloud is highly subcritical. Our simulation shows that the core formation time in our model subcritical clouds is several $\times 10^6$ years, due to the presence of large-scale supersonic flows ($\sim 3$ times sound speed). Once a collapsing core forms, the density, velocity, and magnetic field structure of the core does not strongly depend on the initial strength of the velocity fluctuation. The infall velocities of the cores are subsonic and the magnetic field lines show weak hourglass shapes.

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