Evidence and Implications of Pressure Fluctuations in the ISM |
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Authors: | Edward B. Jenkins |
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Affiliation: | 1. Princeton University Observatory, Princeton, NJ, 08544-1001, USA
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Abstract: | A recent survey of the fine-structure excitation of neutral carbonreveals that the interstellar medium in the Galactic plane exhibits athermal pressure, nT/k, that ranges from about 103to 104cm-3K from one location to the next, with occasional excursions in excess of about 105 cm-3K. The large excitations for small amounts of gas indicate that some regions are either subjected to shocks or must be pressurized within time scales much shorter than the time needed to reach thermal equilibrium. These rapidfluctuations probably arise from the cascade of macroscopic mechanical energyto small scales through a turbulent cascade. One consequence of thiseffect is that changes in gas temperature can arise from near adiabaticcompressions and expansions, and this may explain why investigations of21-cm emission and absorption reveal the presence of hydrogen attemperatures well below the expected values derived from the balance ofvarious known heating and cooling processes. |
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