Dynamical condensation in a thermally bistable flow Application to interstellar cirrus

Astrophysical multi-phase media display rich spatial structures, as for example the cold cirrus clouds found in the neutral atomic interstellar medium. Condensation of cold structures in locally compressed regions of turbulent thermally bistable flows is a plausible, not exclusive, origin for the structuration of the medium. Simulations in plane-parallel geometry show that trans-sonic convergent flows of warm gas can indeed produce long-lived cold condensations, whose properties are consistent with cirrus observations. Self-similar, non-isobaric, analytical solutions capable of describing the beginning of the condensation process in the context of a convergent cooling flow are derived. The quantitative conditions for condensation are discussed (size and velocity thresholds) as functions of the ambient pressure. The process efficiency is also addressed as well as the long-term evolution. Although the simplified framework proposed leaves aside important features of realistic 3-d flows, we argue that compression-induced condensation within trans-sonic flows, at scales close to the size threshold, is a central mechanism in the formation of interstellar structures.