FORMATION OF COLLAPSING CORES IN SUBCRITICAL MAGNETIC CLOUDS: THREE-DIMENSIONAL MAGNETOHYDRODYNAMIC SIMULATIONS WITH AMBIPOLAR DIFFUSION
FORMATION OF COLLAPSING CORES IN SUBCRITICAL MAGNETIC CLOUDS: THREE-DIMENSIONAL MAGNETOHYDRODYNAMIC SIMULATIONS WITH AMBIPOLAR DIFFUSION
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亚临界磁云中塌缩核的形成：双极扩散的三维磁流体动力学模拟
DOI：
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发表时间：
2010
期刊：
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通讯作者：
S. Basu
中科院分区：
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作者：
T. Kudoh;S. Basu
我们采用包括双极扩散在内的三维磁流体动力学模拟来研究亚临界分子云的重力驱动破碎，其中只要存在磁通量冻结，重力破碎就会稳定。模拟表明，最初的亚临界云中的核心通常会在双极扩散时间内逐渐发展，这在典型的分子云中约为几×107年。另一方面，超音速非线性流加速了亚临界云中塌缩核心的形成。我们的参数研究表明，随着云中初始流速强度的增加，核心形成发生得更快。我们发现核心形成时间大致与超音速非线性流产生的增强密度的平方根的倒数成正比。尽管核心形成条件是由我们模拟中的非线性流创建的，但密度依赖性与准静态收缩磁支撑云中的密度依赖性相似。我们还发现，如果云是高度亚临界的，则加速形成时间并不强烈依赖于磁场的初始强度。我们的模拟表明，由于存在大规模超音速流（〜3倍声速），我们的模型亚临界云的核心形成时间为数×106年。一旦塌缩的核心形成，核心的密度、速度和磁场结构并不强烈依赖于速度波动的初始强度。核心的落入速度是亚音速的，磁力线呈现弱沙漏形状。
We employ three-dimensional magnetohydrodynamic simulations 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 ×107yr 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 ×106 yr due to the presence of large-scale supersonic flows (∼3 times sound speed). Once a collapsing core forms, the density, velocity, and magnetic field structure of the core do 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.