Ubiquitous instabilities of dust moving in magnetized gas

Abstract

Squire & Hopkins showed that coupled dust-gas mixtures are generically subject to 'resonant drag instabilities' (RDIs), which drive violently growing fluctuations in both. But the role of magnetic fields and charged dust has not yet been studied. We therefore explore the RDI in gas that obeys ideal MHD and is coupled to dust via both Lorentz forces and drag, with an external acceleration (e.g. gravity, radiation) driving dust drift through gas. We show this is always unstable, at all wavelengths and non-zero values of dust-togas ratio, drift velocity, dust charge, 'stopping time' or drag coefficient (for any drag law), or field strength; moreover, growth rates depend only weakly (sub-linearly) on these parameters. Dust charge and magnetic fields do not suppress instabilities, but give rise to a large number of new instability 'families,' each with distinct behavior. The 'MHD-wave' (magnetosonic or Alfvén) RDIs exhibit maximal growth along 'resonant' angles where the modes have a phase velocity matching the corresponding MHD wave, and growth rates increase without limit with wavenumber. The 'gyro' RDIs are driven by resonances between drift and Larmor frequencies, giving growth rates sharply peaked at specific wavelengths. Other instabilities include 'acoustic' and 'pressurefree' modes (previously studied), and a family akin to cosmic ray instabilities that appear when Lorentz forces are strong and dust streams super-Alfvénically along field lines. We discuss astrophysical applications in the warm ISM, circum-galactic medium/inter-galactic medium (CGM/IGM), H II regions, SNe ejecta/remnants, Solar corona, cool-star winds, GMCs, and AGN.

Spatial power spectra of dust across the Local Group: No constraint on disc scale height

“…Furthermore, dust may be a passive tracer in ISM turbulence, meaning that it may not actively contribute to the turbulence and may have different properties from the gas (e.g., Goldman 2000). Dust may further be subject to additional drag instabilities (Hopkins 2014;Hopkins & Squire 2018). The comparisons between the dust, H I and CO we show here may results from any of these sources.…”

Formation of Dust Filaments in the Diffuse Envelopes of Molecular Clouds

“…The main trend has been to consider the aerodynamic drag of dust in the framework of protoplanetary disks (Bai & Stone 2010;Price et al 2018;Mignone et al 2019). Up to the knowledge of the authors, the only ones that have included the physics of charged grains in their MHD simulations are Lee et al (2017); they have devoted a series of papers to the exhaustive study of resonant instabilities that arise between gas and charged dust over different phases of the ISM when dust motions are driven by an external force (Hopkins & Squire 2018;Seligman et al 2019;Hopkins et al 2020).…”

Dust cleansing of star-forming gas

“…However, such fields would also reduce the drift of the photoelectrically charged dust grains and thus diminish radiative cleansing considerably; see Draine (2011a). As recently shown by Hopkins & Squire (2018b), the wealth of the newly discovered RDIs in magnetized dusty gases may, however, lead to modifications of this conclusion due to more or less vastly growing magneto-sonic RDIs. At any rate, the inhomogeneities and local instabilities discussed in this work would probably not contribute a systematic global dust cleansing of as much mass as needed for a cluster such as M 67.…”