Interstellar Phase Transitions Stimulated by Time-dependent Heating

We use three-dimensional hydrodynamic numerical simulations to study phase transformations occurring in a clumpy interstellar gas exposed to time-dependent volumetric heating. To mimic conditions in the Galactic interstellar medium, we take a numerical model of a turbulent multiphase medium from Kritsuk & Norman (2002) computed in a periodic box with mean density n_0=0.25 cm^-3 and mean pressure P_0/k~10^3.4 K cm^-3. A second model with n_0=1 cm^-3 is also considered. Variations of the heating rate on a timescale of 1-10 Myr applied thereafter cause pressure variations in the gas and shifting of the thermal equilibrium curve in the phase plane. This stimulates mass transfer between the gas phases via thermal instability, converting 5-10% of the thermal energy into kinetic energy of gas motions. The experiments demonstrate that recurrent substantial heating episodes can maintain turbulence at this level. Possible applications to the interstellar gas heated by variable far-ultraviolet background radiation produced by short-living massive stars are discussed.