The acoustic resonant drag instability with a spectrum of grain sizes

Squire, Jonathan; Moroianu, Stefania; Hopkins, Philip F.

doi:10.1093/mnras/stab3377

Cited by 7 publications

(1 citation statement)

References 67 publications

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“…In particular, observations of a proto-stellar system presented in Kwon et al (2019) strongly suggest that more than one grain alignment mechanism seems to be simultaneously at work. The required gas-dust drift may arise due to by magnetohydrodynamic turbulence (Yan & Lazarian 2003) or resonant drag instabilities (Hopkins & Squire 2018;Squire et al 2021), respectively. Up to this date an analytical description of mechanic alignment comparable to the AMO of RAT alignment is still a matter of ongoing research.…”

Section: Introductionmentioning

confidence: 99%

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

Reißl¹,

Meehan²,

Klessen³

2022

Preprint

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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...

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Section: Introductionmentioning

confidence: 99%

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

Reißl¹,

Meehan²,

Klessen³

2022

Preprint

View full text Add to dashboard Cite

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The mechanical alignment of dust (MAD)

S.¹,

Meehan²,

Klessen³

2023

A&A

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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 scanned 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 were 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 was taken into account to estimate the upper limit of possible grain rotation. The spin-up efficiencies were analyzed within a mathematical framework of grain alignment dynamics in order to identify long-term stable grain alignment points in the parameter space. Here, we distinguish between the cases of grain alignment in the direction of the gas-dust drift and the alignment along the magnetic field lines. Finally, the net dust polarization was 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 drastically reduces. In the presence of a magnetic field, the drift velocity required for the most elongated 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 and report a possible dust polarization efficiency of 0.6–0.9, indicating that a gas-dust drift alone can provide the conditions required to observationally probe the magnetic field structure. We predict that magnetic field alignment is highly inefficient when the direction of the gas-dust drift and 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 canonical radiative torque alignment theory fails to account for the full spectrum of available dust polarization observations.

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Resonant instabilities mediated by drag and electrostatic interactions in laboratory and astrophysical dusty plasmas

2023

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Dusty plasmas are known to support a diverse range of instabilities, including both generalizations of standard plasma instabilities and ones caused by effects specific to dusty systems. It has been recently demonstrated that a novel broad class of streaming instabilities, termed resonant drag instabilities (RDIs), can be attributed to a particular resonance phenomenon, manifested by defective eigenvalues of the linearized dust/fluid system. In this work, it is demonstrated that this resonance phenomenon is not unique to RDIs and can be used as a framework to understand a wider range of instabilities, termed resonant instabilities. Particular attention is given to the filamentary ionization instability seen in laboratory dusty plasmas and to the two-stream instability. It is shown that, due to the commonalities in underlying physics between the dust-ion-acoustic two-stream instability and the acoustic RDI, these instabilities should be relevant in strongly overlapping regimes in astrophysical dusty plasmas. It is proposed that a similar overlap in the experimental accessibility of these modes (and of the filamentary instability) allows for the possibility of experimental investigation in the laboratory of complex and astrophysically relevant instability dynamics.

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The acoustic resonant drag instability with a spectrum of grain sizes

Cited by 7 publications

References 67 publications

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

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

The mechanical alignment of dust (MAD)

Resonant instabilities mediated by drag and electrostatic interactions in laboratory and astrophysical dusty plasmas

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