Magnetic clumping of charged dust in the dense interstellar medium

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 develop a dusty model that goes beyond the standard non-ideal MHD and use the code {\ttfamily 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: We identify a mechanism similar to the parametric instability that efficiently forms dust clumps even in presence of dissipative processes. 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 which is controlled by: transverse Mach number and plasma parameter. We find that a substantial fraction of dust experiences a density increase of more than a factor of 10 under reasonable conditions, thus promoting dust growth. Conclusion: 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 (sub-mm) in protostellar envelopes a plausible scenario.