Hydromagnetic Signal Speeds In Weakly Ionized, Dusty Interstellar Clouds Having A Spectrum Of Grain Radii

Details Hydromagnetic Signal Speeds In Weakly Ionized, Dusty Interstellar Clouds Having A Spectrum Of Grain Radii Hydromagnetic Signal Speeds In Weakly Ionized, Dusty Interstellar Clouds Having A Spectrum Of Grain Radii

Jan 2011

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011aas...21725101c&link_type=abstract

American Astronomical Society, AAS Meeting #217, #251.01; Bulletin of the American Astronomical Society, Vol. 43, 2011

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We study the propagation of magnetohydrodynamic waves traveling perpendicular to the direction of the magnetic field in weakly ionized, dusty molecular clouds and cores, for the physically relevant situation in which there is a continuous distribution of grain sizes. The effect of collisions of charged species with the predominantly neutral gas is accounted for in our system of governing equations. Also included is the interaction of charged grains with electron-shielded ion quasiparticles, which aids attachment of grains to magnetic field lines for frequencies less than the grain-quasiparticle hybrid frequency. The dispersion relation for dusty clouds is analyzed and we present the results for models having grain distribution functions that are consistent with interstellar extinction observations (Mathis et al 1977; Weingartner & Draine 2001). Magnetosound waves and plasma diffusion modes dependent on the inertia of the grains are identified and their behavior as a function of frequency is described.
Our results indicate that when there is a spectrum of grain sizes the dust population consists of two distinct dynamical sub-groups: grains that are small enough to be actually coupled to the magnetic field (Hall parameter > 1), referred to as "MHD active" grains, and larger grains that are decoupled from the field by collisions with neutral gas particles (Hall parameter < 1), which can be called "MHD inactive" grains. The limiting MHD signal speed for the lowest-frequency, farthest-traveling waves is set by the net mass of the "MHD active" grains that are loaded onto magnetic field lines, which can be a substantial portion ( 30% - 70%) of the total dust mass. As a result, the limiting MHD signal speed in dusty clouds is significantly smaller than what it would be in dust-free clouds. This can have important consequences for the formation of MHD precursors and shocks in clouds and cores.

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