The dynamics of charged dust in magnetized molecular clouds

Abstract

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 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.

Non-linear evolution of the resonant drag instability in magnetized gas

“…equilibrium t s and w s (Figure 3), the acoustic RDI manifests eventually (after growing much more slowly), but the geometric structure of the dust is quite different, and the dust concentration is vastly weaker. Our fiducial simulation is also qualitatively distinct from cases examined in Lee et al (2017), which included externally-driven MHD turbulence with similar Mach number but without the appropriate back-reaction from the dust on the gas (what actually drives the instabilities here). In all cases in that study, the dust density was either essentially uniform in the box, or strongly correlated with gas density, and at the (very small) gas Mach numbers here, the dust density fluctuations were ∼ 1%-level.…”

Simulating diverse instabilities of dust in magnetized gas

“…(1) below, but each represents an ensemble of dust grains with similar size/mass/charge (denoted grain, mgrain, qgrain, respectively). Numerical methods for the integration are described and tested in Hopkins & Lee (2016); Lee et al (2017) with the back-reaction accounted for as in Moseley et al (2018) where aext,dust is a constant external acceleration (e.g. from radiation pressure), ts is the drag coefficient or "stopping time," tL the gyro or Larmor time, and ws ≡ v d − ug the "drift velocity" (difference between grain velocity v d and gas velocity ug).…”

“…In addition to the generic code validation tests described in § 2, previous papers have extensively tested the numerical methods here (Carballido et al 2008;Johansen et al 2009;Bai & Stone 2010;Pan et al 2011;Hopkins & Raives 2016;Hopkins 2016bHopkins , 2017Su et al 2017;Hopkins & Lee 2016;Lee et al 2017;Moseley et al 2018;Seligman et al 2018). For example Hopkins & Lee (2016) show that the "finite-sampling" effects in super-particle methods, which introduce some shot noise in the particle densities and divergence between particle trajectories and gas (at the integration error level) in the perfectly-coupled limit (see Genel et al 2013), introduce ∼ 0.01 − 0.05 dex scatter in the dust density in a supersonically turbulent medium (δug/cs 1).…”

Density and tracer statistics in compressible turbulence: Phase transition to multifractality

“…If so then we get insight in the dustto-gas ratio of molecular clouds: approximating dust particles by tracers, which neglects their inertia and other effects [1], the ratio is constant. This ratio plays key role in many astrophysical applications including absorption of light in the interstellar medium, evolution of galactic composition and ISM tracking [2]. Its non-constancy can have far-reaching consequences [1].…”