Magnetic clumping of charged dust in the dense interstellar medium

¹ Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, 91191 Gif-sur-Yvette, France
² Université Paris Cité, Université Paris-Saclay, CEA, CNRS, AIM, 91191 Gif-sur-Yvette, France

^★ Corresponding author: valentin.vallucci@gmail.com

Received: 12 March 2025
Accepted: 16 October 2025

Abstract

Context. Dust grains undergo significant growth in star-forming environments, especially in dense regions prone to gravitational collapse. Although dust is generally assumed to represent 1% of the gas mass, dust density variations are expected on small scales due to differential dynamics with the gas, leading to enhanced coagulation rates in regions of dust enrichment.

Aims. We aim to investigate the clumping of charged dust in the turbulent magnetized dense regions of the interstellar medium.

Methods. We developed a dusty model that goes beyond the standard non-ideal magnetohydrodynamics (MHD) and used the code shark to perform multifluid 1D simulations of a single size charged dust species and neutral gas with large-scale-driven turbulence and including ion-neutral friction.

Results. Propagating nonlinear circularly polarized Alfvén waves, we identify a mechanism similar to the parametric instability that efficiently forms dust clumps even in the presence of dissipative processes such as Ohmic dissipation, Hall effect, and magnetic drag. Such strong clumping survives and is sustained when driving turbulence, and thus high levels of dust concentration are produced due to compressive magnetic effects in regions of shocks. Dust density enhancements are favored by a high transverse-to-longitudinal magnetic ratio B_⊥/B_∥ which is controlled by the two following simulation parameters: transverse Mach number ℳ_⊥ and plasma parameter β. We found that a substantial fraction of dust experiences a density increase of more than a factor of 10 under reasonable conditions (subsonic turbulence, β=0.7 and dust size s_d ≥ 1 μm), thus promoting dust growth.

Conclusions. Our novel dusty non-ideal MHD model shows that dust grains (main charge carriers) are subject to small-scale compressive magnetic effects driven by a parametric instability-like mechanism in regions of shocks, and consequently experience high density enhancements in turbulent environments that go beyond those permitted by pure hydrodynamical processes, making in situ formation of large grains (s_d ∼ 100 μm) in protostellar envelopes a plausible scenario.