A computational study of interfacial instabilities in the interstellar medium

One of the most important problems in astronomy today is the development of a comprehensive theory for the origins of stars and planetary systems. In order to understand star formation, we must understand the interstellar medium (ISM) in which the process occurs. The evolution of the ISM is driven by a variety of phenomena, including gravity, turbulence, shearing flows, magnetic fields, and the thermal properties of the gas. As a result, the ISM is susceptible to a wide range of hydrodynamic instabilities. In this dissertation, two very different types of instabilities are studied using computational hydrodynamics simulations. The first is a recently discovered instability which acts upon an interface of discontinuous density. Theory predicts that this self-gravity driven interfacial instability persists in the static limit and in the absence of a constant background acceleration. Disturbances to a density interface are found to grow on a time-scale of the order of the free-fall time, even when the perturbation wavelength is much less than the Jeans length. Here, I present the first numerical simulations of this instability. The theoretical growth rate is confirmed and the nonlinear morphology displayed. The self-gravity interfacial instability is shown to be fundamentally different from the Rayleigh-Taylor instability, although both exhibit similar morphologies under the condition of a high density contrast such as is commonly found in the ISM. Such instabilities are a possible mechanism by which observed features, such as the pillars of gas seen near the boundaries of interstellar clouds, are formed.; The second instability considered in this study is a dynamic instability which occurs when supersonic gas streams collide. It is driven by a combination of ram pressure and strong cooling. I examine the stability of a cold, dense slab formed by the head-on collision of supersonic gas streams. Special emphasis is placed on the effects of altering the cooling rate. A series of models are performed in cylindrical geometry to determine whether the instability of the slab acts to increase or decrease the likelihood of forming collapsing regions capable of undergoing star formation.