The dynamics of charged dust in magnetized molecular clouds
The dynamics of charged dust in magnetized molecular clouds
复制标题
磁化分子云中带电尘埃的动力学
DOI：
10.1093/mnras/stx1097
复制
发表时间：
2016
影响因子：
4.8
通讯作者：
J. Squire
中科院分区：
文献类型：
--
作者：
Hyunseok Lee;P. Hopkins;J. Squire
我们研究了湍流巨型分子云(GMCs)中大颗粒带电尘埃的动力学。大量的尘埃颗粒在主要是中性的致密气体中表现为空气动力学颗粒，因此能够在尘气比中产生戏剧性的小范围波动。霍普金斯和李直接模拟了超音速磁流体湍流中中性尘埃颗粒的动力学，表明尘埃到气体的起伏在小尺度上可以超过因子∼1000，这对恒星形成、恒星丰度和尘埃行为和增长具有重要意义。然而，即使在GMC中主要是中性气体中，尘埃颗粒也是带负电荷的，洛伦兹力是不可忽略的。因此，我们扩展了我们之前的研究，包括洛伦兹力对带电颗粒的影响(除了阻力)。对于小电荷颗粒(尺寸≪0.1 μm)，洛伦兹力抑制尘气比波动，而对于大颗粒(尺寸≳1 μm)，洛伦兹力基本上没有影响，这一趋势可以用尘埃磁化的简单理论很好地解释。在一些特殊的中间情况下，洛伦兹力可以加强尘气分离。无论如何，对于尘埃电荷随颗粒大小的物理预期标度，我们发现最重要的影响取决于颗粒大小(通过阻力方程)，洛伦兹力/电荷作为二阶修正。我们发现，在弱背景磁场的极限下，我们所考虑的动力学由三个无量纲数决定：湍流马赫数、尘埃阻力参数(与颗粒大小成正比)和尘埃洛伦兹参数(与颗粒电荷成正比)；这些允许我们将我们的模拟推广到更广泛的条件。
We study the dynamics of large, charged dust grains in turbulent giant molecular clouds (GMCs). Massive dust grains behave as aerodynamic particles in primarily neutral dense gas, and thus are able to produce dramatic small-scale fluctuations in the dust-to-gas ratio. Hopkins & Lee directly simulated the dynamics of neutral dust grains in supersonic magnetohydrodynamic turbulence, typical of GMCs, and showed that the dust-to-gas fluctuations can exceed factor ∼1000 on small scales, with important implications for star formation, stellar abundances and dust behaviour and growth. However, even in primarily neutral gas in GMCs, dust grains are negatively charged and Lorentz forces are non-negligible. Therefore, we extend our previous study by including the effects of Lorentz forces on charged grains (in addition to drag). For small-charged grains (sizes ≪ 0.1 μm), Lorentz forces suppress dust-to-gas ratio fluctuations, while for large grains (sizes ≳ 1 μm), Lorentz forces have essentially no effect, trends that are well explained with a simple theory of dust magnetization. In some special intermediate cases, Lorentz forces can enhance dust–gas segregation. Regardless, for the physically expected scaling of dust charge with grain size, we find the most important effects depend on grain size (via the drag equation) with Lorentz forces/charge as a second-order correction. We show that the dynamics we consider are determined by three dimensionless numbers in the limit of weak background magnetic fields: the turbulent Mach number, a dust drag parameter (proportional to grain size) and a dust Lorentz parameter (proportional to grain charge); these allow us to generalize our simulations to a wide range of conditions.