Interfacial instabilities driven by self-gravity: first numerical results

The evolution of the interstellar medium (ISM) is driven by a variety of ph enomena, including turbulence, shearing flows, magnetic fields and the ther mal properties of the gas. Among the most important forces at work is self- gravity, which ultimately drives protostellar collapse. As part of an ongoi ng study of instabilities in the ISM, Hunter, Whitaker & Lovelace have disc overed another process driven by self-gravity: the instability of an interf ace 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 ar e found to grow on a time-scale of the order of the free-fall time, even wh en the perturbation wavelength is much less than the Jeans length. Here we present the first numerical simulations of this instability. The theoretica l growth rate is confirmed and the non-linear morphology displayed. The sel f-gravity interfacial instability is shown to be fundamentally different fr om the Rayleigh-Taylor instability, although both exhibit similar morpholog ies under the condition of a high density contrast, such as is commonly fou nd in the ISM. Such instabilities are a possible mechanism by which observe d features, such as the pillars of gas seen near the boundaries of interste llar clouds, are formed.