Thermal Instability Induced Interstellar Turbulence

We study the dynamics of phase transitions in the interstellar medium (ISM) by means of threedimensional hydrodynamic numerical simulations. We use a realistic cooling function and generic nonequilibrium initial conditions to follow the formation history of a multi-phase medium in detail in the absence of gravity. We outline a number of qualitatively distinct stages of this process, including a linear isobaric evolution, transition to an isochoric regime, formation of filaments and voids (also known as ‘thermal pancakes’), the development and decay of supersonic turbulence, approach to pressure equilibrium, and final relaxation of the multi-phase medium. We refer to previous analytical studies for a number of nonlinear effects of thermal instability in order to identify them in our numerical models. We discuss the role of initial conditions, radiative cooling and heating, and build up a comprehensive scenario for the development of phase transitions in the ISM. While not all initial conditions yield a stable two-phase medium, we examine such a case in detail. We find that the two phases are well mixed with the cold clouds possessing a fine-grained structure near our numerical resolution limit. We find that the amount of gas in the intermediate unstable phase roughly tracks the rms turbulent Mach number, peaking at 25% when Mrms ∼ 8, decreasing to 11% when Mrms ∼ 0.4. Subject headings: hydrodynamics — instabilities — ISM: structure — turbulence