Mechanism of Magnetic Flux Loss in Molecular Clouds
Mechanism of Magnetic Flux Loss in Molecular Clouds
复制标题
分子云中磁通量损失的机制
DOI：
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发表时间：
2002
期刊：
影响因子：
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通讯作者：
T. Umebayashi
中科院分区：
文献类型：
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作者：
T. Nakano;R. Nishi;T. Umebayashi
我们研究了分子云中磁场漂移的详细过程。对于磁力传递给中性分子的摩擦力，离子仅在云密度nH = 104 cm-3时贡献了一半以上，而带电颗粒在nH = 106 cm-3时贡献了约90%以上。因此，晶粒在磁通损失过程中起着决定性的作用。用幂律tB <$B-γ来近似通量损失时间tB，其中B是云中的平均场强，我们发现只有在nH <$107 cm-3处才有γ <$2，这是双极扩散的特征，在这个位置上，离子和最小的颗粒被磁场冻结得很好。当nH > 107 cm-3时，γ随nH急剧下降，最后在nH下降到约几× 1011 cm-3时，磁场与气体有效地解耦，达到γ 1011，使人联想到欧姆耗散，尽管通量损失比纯欧姆耗散快10倍左右。因为即使离子在nH > 107 cm-3时也不能很好地冻结，所以离子和颗粒的漂移速度比磁场慢。这种不充分的冻结使得随着nH的增加，tB对B越来越不敏感。欧姆耗散仅在nH = 1 × 1012 cm-3处占主导地位。虽然离子和电子在所有密度下都沿着磁力的方向漂移，但具有相反电荷的颗粒在高密度下沿着相反的方向漂移，在高密度下，颗粒是摩擦力的主要贡献者。虽然磁通量损失发生明显快于欧姆耗散，即使在非常高的密度，如nH nndec，在高密度进行的过程是完全不同的双极扩散，其中相反电荷的粒子应该作为一个单位漂移。
We investigate the detailed processes at work in the drift of magnetic fields in molecular clouds. To the frictional force, whereby the magnetic force is transmitted to neutral molecules, ions contribute more than half only at cloud densities nH ≲ 104 cm-3, and charged grains contribute more than about 90% at nH ≳ 106 cm-3. Thus, grains play a decisive role in the process of magnetic flux loss. Approximating the flux loss time tB by a power law tB ∝ B-γ, where B is the mean field strength in the cloud, we find γ ≈ 2, characteristic of ambipolar diffusion, only at nH ≲ 107 cm-3, at which ions and the smallest grains are pretty well frozen to the magnetic fields. At nH > 107 cm-3, γ decreases steeply with nH, and finally at nH ≈ ndec ≈ a few × 1011 cm-3, at which the magnetic fields effectively decouple from the gas, γ ≪ 1 is attained, reminiscent of Ohmic dissipation, although flux loss occurs about 10 times faster than by pure Ohmic dissipation. Because even ions are not very well frozen at nH > 107 cm-3, ions and grains drift slower than the magnetic fields. This insufficient freezing makes tB more and more insensitive to B as nH increases. Ohmic dissipation is dominant only at nH ≳ 1 × 1012 cm-3. While ions and electrons drift in the direction of the magnetic force at all densities, grains of opposite charges drift in opposite directions at high densities, at which grains are major contributors to the frictional force. Although magnetic flux loss occurs significantly faster than by Ohmic dissipation even at very high densities, such as nH ≈ ndec, the process going on at high densities is quite different from ambipolar diffusion, in which particles of opposite charges are supposed to drift as one unit.