Magnetic clumping of charged dust in the dense interstellar medium

TL;DR

The paper investigates how charged dust concentrates in dense, magnetized turbulent ISM using a 1D multifluid MHD framework (gas plus a single charged dust species) with both dusty ideal and non-ideal regimes. A parametric-like instability driven by compressive magnetic effects in shocks forms strong dust clumps, with sustained clumping under turbulent driving; the effect is amplified by higher $B_\perp/B_\parallel$ controlled by $\beta$ and the transverse Mach number, and by larger dust sizes that decouple from gas. Non-ideal MHD introduces magnetic drag from ions, damping but not eliminating clumping, underscoring the need for realistic ionization chemistry and a broader dust-size distribution. The results imply that significant dust density enhancements can occur on small scales, potentially accelerating in-situ growth of large grains in protostellar envelopes and contributing to early planetesimal formation under certain conditions. Overall, the work highlights a viable mechanism—magnetically driven clumping via a parametric-like instability—that goes beyond hydrodynamic dust concentration in turbulent dense environments.

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.

Figures (19)

• Figure 1: Maximum dust density fluctuations and maximum transverse magnetic field as a function of time upon propagation of a circularly polarized Alfvén wave, for different values of wavelength (as a fraction of the box length $L$) and wave amplitudes $\delta_B$ within the dusty ideal MHD regime and dusty non-ideal MHD regime (see Sect. \ref{['dusty ideal MHD']} and Sect. \ref{['Dusty non-ideal MHD']}). The early sharp rise in dust density is due to a mechanism similar to the parametric instability DelZana2001. Note that growth rates measured here compare well with analytical predictions of the standard parametric instability (see Sect. \ref{['Appendix: growth rate parametric']}). Afterwards, the dust density decreases as a consequence of gas dust friction. The gas density is not displayed because subject to negligible variations. Parameters: $n_\mathrm{H} = 10^6 \ \mathrm{cm^{-3}}$, $s_\mathrm{d} = 10 \ \mathrm{\mu m} \ (\mathrm{St} = 0.01)$ and $\beta = 0.1$.
• Figure 2: Different fields at times $t=0.23$, $t=0.44$ and $t=0.82$ in a simulation with driven turbulence. First panel: dust (solid lines) and gas (dotted lines) density fluctuations. Second panel: x-wise dust (solid) and gas (dotted) Mach number. Third panel: magnetic pressure gradient. Fourth panel: total (x,y,z-wise) dust Alfvénic Mach number. Parameters: $n_\mathrm{H} = 10^6 \ \mathrm{cm^{-3}}$, $s_\mathrm{d} = 10 \ \mathrm{\mu m} \ (\mathrm{St} = 0.01)$ and $\beta = 1$.
• Figure 3: Schematic illustration of the parameters controlling the transverse-to-longitudinal magnetic field ratio $B_\perp / B_\parallel$ and thus the intensity of dust clumping via the parametric instability in the simulations with turbulence.
• Figure 4: Maximum dust density (solid lines) and gas density (dotted lines) fluctuations as a function of time for different values of the dust grain size $s_\mathrm{d}$, longitudinal Mach number (perpendicular one being varied too) $\mathcal{M}_\parallel$, gas initial density $n_\mathrm{H}$ and plasma parameter $\beta$. The dust density mean value for pure hydrodynamics simulations is displayed for reference as dashed lines. When not varied, $\beta=0.7$.
• Figure 5: Same as Fig. \ref{['rhod vs time 0.7']} but depicting space averages (mass weighted) instead of maximum values. On the first row is displayed the dust-to-gas ratio (dust density weighted), on the second row the dust density (dust density weighted) and on the third row the gas density average (gas density weighted).