The Mechanical Alignment of Dust (MAD) I: On the spin-up process of fractal grains by a gas-dust drift

Abstract

Context. Observations of polarized light emerging from aligned dust grains are commonly exploited to probe the magnetic field orientation in astrophysical environments. However, the exact physical processes that result in a coherent large-scale grain alignment are still far from being fully constrained. Aims. In this work, we aim to investigate the impact of a gas-dust drift on a microscopic level potentially leading to a mechanical alignment of fractal dust grains and subsequently to dust polarization. Methods. We scan a wide range of parameters of fractal dust aggregates in order to statistically analyze the average grain alignment behavior of distinct grain ensembles. In detail, the spin-up efficiencies for individual aggregates are determined utilizing a Monte-Carlo approach to simulate the collision, scattering, sticking, and evaporation processes of gas on the grain surface. Furthermore, the rotational disruption of dust grains is taken into account to estimate the upper limit of possible grain rotation. The spin-up efficiencies are analyzed within a mathematical framework of grain alignment dynamic in order to identify long-term stable grain alignment points in the parameter space. Here, we distinguish between the cases of grain alignment in direction of the gas-dust drift and the alignment along the magnetic field lines. Finally, the net dust polarization is calculated for each collection of stable alignment points per grain ensemble.Results. We find the purely mechanical spin-up processes within the cold neutral medium to be sufficient enough to drive elongated grains to a stable alignment. The most likely mechanical grain alignment configuration is with a rotation axis parallel to the drift direction. Here, roundish grains require a supersonic drift velocity while rod-like elongated grains can already align for subsonic conditions. We predict a possible dust polarization efficiency in the order of unity resulting from mechanical alignment. Furthermore, a supersonic drift may result in a rapid grain rotation where dust grains may become rotationally disrupted by centrifugal forces. Hence, the net contribution of such a grain ensemble to polarization becomes drastically reduces. In the presence of a magnetic field the drift velocity required for grains to reach a stable alignment is roughly one order of magnitude higher compared to the purely mechanical case. We demonstrate that a considerable fraction of a grain ensemble can stably align with the magnetic field lines. We report a possibly dust polarization efficiency of 0.8 − 0.9 indicating that a gas-dust drift alone can provide the conditions required to observationally probe the magnetic field structure. We predict that the magnetic field alignment is highly inefficient when the direction of the gas-dust drift and the magnetic field lines are perpendicular. Conclusions. Our results strongly suggest that mechanical alignment has to be taken into consideration as an alternative driving mechanism where the standard RAT alignment theory fails to account f...

On Internal and External Alignment of Dust Grains in Protostellar Environments

“…For RATs, the variation of Q spinup with grain shapes is rather small, within an order of magnitude. However, for METs, Q spinup can vary by three to four orders of magnitudes, and are about two orders of magnitude lower than the precession component (Das & Weingartner 2016;Reissl et al 2022). Therefore, the ratio of the MET components is Q prec /Q spinup 1.…”

“…For irregular shapes, numerical calculations in Hoang et al (2018) report the range of Q spinup ∼ 10 −6 − 10 −3 (see the spin-up component in their Figure 5). Recent Monte-Carlo simulations in Reissl et al (2022) show that the MET efficiency Q spinup changes significantly, upto three orders of magnitude, with grain shapes. Therefore, for an arbitrary shape, we treat Q spinup as a parameter, with the conservative value of Q spinup = 10 −3 .…”

Spinning Nanoparticles Impacted by C-shock: Implications for Radio-millimeter Emission from Star-forming Regions

“…This mechanical torque (called MET) was originally proposed for grain alignment (Lazarian & Hoang 2007b) and known to be much more efficient for spinning up nanoparticles than stochastic mechanical torque (Lazarian & Hoang 2021). Our understanding of the impact of MET is primarily based on qualitative guidance from a simple model (Lazarian & Hoang 2007b); however, more complicated models and analyses (e.g., Das & Weingartner 2016;Reissl et al 2022) support the idea that irregular grains exhibit helicity while interacting with gaseous flows, and the corresponding regular torques dominate the stochastic mechanical torques (Lazarian & Hoang 2021). The MET is difficult to calculate and depends strongly on grain shape , and there is no available tool for numerical studies to characterize its performance in astrophysical situations (Lazarian & Hoang 2021).…”

Spinning Nanoparticles Impacted by C-shock: Implications for Radio-millimeter Emission from Star-forming Regions

“…This mechanical torque (called MET) was originally proposed for grain alignment (Lazarian & Hoang 2007b) and known to be much more efficient for spinning up nanoparticles than stochastic mechanical torque (Lazarian & Hoang 2021). Our understanding of the impact of MET is primarily based on qualitative guidance from a simple model (Lazarian & Hoang 2007b); however, more complicated models and analyses (e.g., Das & Weingartner 2016;Reissl et al 2022) support the idea that irregular grains exhibit helicity while interacting with gaseous flows, and the corresponding regular torques dominate the stochastic mechanical torques (Lazarian & Hoang 2021). The MET is difficult to calculate and depends strongly on grain shape , and there is no available tool for numerical studies to char-acterize its performance in astrophysical situations (Lazarian & Hoang 2021).…”