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Paper: Small-Scale Dissipative Structures of Diffuse ISM Turbulence: II Chemical Diagnostics
Volume: 365, SINS — Small Ionized and Neutral Structures in the Diffuse Interstellar Medium
Page: 190
Authors: Falgarone, E.; Hily-Blant, P.; Pety, J.; Pineau des Forets, G.
Abstract: A new kind of small scale velocity structures has been found on statistical grounds in diffuse molecular gas. They are the locus of the largest velocity shears in the field and form a network of elongated structures. Three independent properties make them the plausible sites of intermittent dissipation of turbulence: (1) gas there is warmer and more diluted than average, (2) it bears the signatures (i.e. large HCO+ abundances) of a non-equilibrium chemistry triggered by bursts of supra-thermal heating, and (3) the power that these warm structures radiate in their gas cooling lines (mostly H2) is so large that it balances the total energy injection rate of the turbulent cascade, for a volume filling factor of only a few percent. This low value is consistent with other observations in the Solar neighborhood. A model of the thermal and chemical evolution of the gas, comprising an impulsive heating due to a dissipation burst followed by a slow relaxation, is described. The whole evolution is out-of-equilibrium and results in the formation of tiny seeds of cold dense gas embedded in the diffuse molecular gas.
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