Kelvin-Helmholtz and Thermal-dynamic Instabilities with Self-Gravity: A New Gravitational Interface Instability

In the vortex sheet limit, we generalize our previous work on compressible, anisentropic Kelvin-Helmholtz, and related instabilities by including self-gravity in the calculations. In addition to significantly modifying the Kelvin-Helmholtz modes, if the background media are of unequal density, self-gravity gives rise to a new instability that persists in the static limit. If the media have significant density contrast ({rho}{sub 1}/{rho}{sub 2}=1/2, say), the growth rate of this new gravitational interface instability is of the order of the free-fall time in the denser medium, and, unlike a Jeans instability, it depends only weakly on the perturbation wavelength. Such instabilities may initiate star formation near the boundaries of molecular clouds in the ISM on timescales of {approximately}10{sup 6}{minus}10{sup 7}yr. {copyright} {ital 1997} {ital The American Astronomical Society}