The Dynamics of Charged Dust in Magnetized Molecular Clouds
  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 (2016) directly simulated
the dynamics of neutral dust grains in super-sonic MHD 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 behavior 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 micron), Lorentz forces suppress dust-to-gas ratio
fluctuations, while for large grains (sizes >1 micron), 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.
