Small Ionized and Neutral Structures: A Theoretical Review

Abstract

The workshop on Small Ionized and Neutral Structures in the Interstellar Medium featured many contributions on the theory of the objects which are responsible for ``Tiny Scale Atomic Structures'' (TSAS) and ``Extreme Scattering Events'' (ESE). The main demand on theory is accounting for objects that have the high densities and small sizes apparently required by the observations, but also persist over a sufficiently long time to be observable. One extensively-discussed mechanism is compressions by transonic turbulence in the warm interstellar medium, followed by thermal instabilities leading to an even more compressed state. In addressing the requirements for overpressured but persistent objects, workshop participants also discussed fundamental topics in the physics of the interstellar medium, such as the timescale for evaporation of cool dense clouds, the relevance of thermodynamically-defined phases of the ISM, the effect of magnetic fields, statistical effects, and the length and time scales introduced by interstellar processes.