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The Fundamentally Different Dynamics of Dust and Gas in Molecular Clouds

Hopkins, Philip F. and Lee, Hyunseok (2016) The Fundamentally Different Dynamics of Dust and Gas in Molecular Clouds. Monthly Notices of the Royal Astronomical Society, 456 (4). pp. 4174-4190. ISSN 0035-8711.

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We study the behaviour of large dust grains in turbulent molecular clouds (MCs). In primarily neutral regions, dust grains move as aerodynamic particles, not necessarily with the gas. We therefore directly simulate, for the first time, the behaviour of aerodynamic grains in highly supersonic, magnetohydrodynamic turbulence typical of MCs. We show that, under these conditions, grains with sizes a ≳ 0.01 micron exhibit dramatic (exceeding factor ∼1000) fluctuations in the local dust-to-gas ratio (implying large small-scale variations in abundances, dust cooling rates, and dynamics). The dust can form highly filamentary structures (which would be observed in both dust emission and extinction), which can be much thinner than the characteristic width of gas filaments. Sometimes, the dust and gas filaments are not even in the same location. The ‘clumping factor’ ⟨n^2_(dust)⟩/⟨n_(dust)⟩^2 of the dust (critical for dust growth/coagulation/shattering) can reach ∼100, for grains in the ideal size range. The dust clustering is maximized around scales ∼ 0.2 pc (a/μm) (n_(gas)/100 cm^(−3))^(−1), and is ‘averaged out’ on larger scales. However, because the density varies widely in supersonic turbulence, the dynamic range of scales (and interesting grain sizes) for these fluctuations is much broader than in the subsonic case. Our results are applicable to MCs of essentially all sizes and densities, but we note how Lorentz forces and other physics (neglected here) may change them in some regimes. We discuss the potentially dramatic consequences for star formation, dust growth and destruction, and dust-based observations of MCs.

Item Type:Article
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URLURL TypeDescription Paper
Hopkins, Philip F.0000-0003-3729-1684
Additional Information:© 2016 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. Accepted 2015 November 18. Received 2015 November 10; in original form 2015 October 8. First published online January 15, 2016. We thank Jim Stone, Paolo Padoan, Jessie Christiansen, Charlie Conroy, Evan Kirby, and Paul Torrey for helpful discussions and contributions motivating this work. Support for PFH was provided by the Gordon and Betty Moore Foundation through Grant #776 to the Caltech Moore Center for Theoretical Cosmology and Physics, an Alfred P. Sloan Research Fellowship, NASA ATP Grant NNX14AH35G, and NSF Collaborative Research Grant #1411920. Numerical calculations were run on the Caltech compute cluster “Zwicky” (NSF MRI award #PHY-0960291) and allocation TG-AST130039 granted by the Extreme Science and Engineering Discovery Environment (XSEDE) supported by the NSF.
Group:TAPIR, Moore Center for Theoretical Cosmology and Physics
Funding AgencyGrant Number
Gordon and Betty Moore Foundation776
Alfred P. Sloan FoundationUNSPECIFIED
Subject Keywords:accretion, accretion discs instabilities turbulence planets and satellites: formation galaxies: formation cosmology: theory
Issue or Number:4
Record Number:CaltechAUTHORS:20151022-134506351
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Official Citation: Philip F. Hopkins and Hyunseok Lee The fundamentally different dynamics of dust and gas in molecular clouds MNRAS (March 11, 2016) Vol. 456 4174-4190 doi:10.1093/mnras/stv2745 First published online January 15, 2016
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:61430
Deposited By: Joy Painter
Deposited On:22 Oct 2015 21:19
Last Modified:03 Oct 2019 09:08

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