The Dynamics of Charged Dust in Magnetized Molecular Clouds

Hyunseok Lee, Philip F. Hopkins, Jonathan Squire

Published 2016-12-15Version 1

We study the dynamics of charged dust grains in turbulent molecular clouds (GMCs). Massive grains behave as aerodynamic particles in primarily neutral 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 supersonic MHD turbulence and showed that dust-to-gas fluctuations can exceed factor ~1000 on small scales, with important implications for star formation, stellar abundances, and dust 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 Lorentz forces on charged grains (in addition to drag). For small charged grains (<<0.1 micron), Lorentz forces suppress dust-to-gas ratio fluctuations, while for large grains (~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. For physically expected scalings of dust charge with grain size, we find the most important effects depend on grain size 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.