Stability of interfaces with self-gravity, relative flow, and B field

Observations with the Hubble Space Telescope (Hester et al.) of spectacular "fingers" or "elephant trunks" of gas protruding from a large star-forming cloud in the Eagle Nebula stimulate renewed interest in the stability of i nterfaces between different media in molecular clouds. Instability and nonl inear growth of crenelations of interfaces can lead to mass concentrations that in turn lead to star formation. In an earlier study of the stability o f interfaces, we took into account the important physical effects-the diffe rent densities and temperatures of the media, the relative motion (Kelvin-H elmholtz instability), the gravitational acceleration perpendicular to the interface (Rayleigh-Taylor instability), and self-gravity. A new self-gravi tational instability of an interface was found that was independent of the wavelength of the perturbation. At short wavelengths, the perturbations are essentially distortional, but compression becomes important as the Jeans l ength is approached from below. The e-folding time for the instability is c omparable with the free-fall collapse time for the denser fluid. In the pre sent work, we generalize our earlier theory in two ways: by including order ed magnetic fields parallel to the interface, and by examining the stabilit y of long cylindrical interfaces. We show that dynamically important magnet ic fields in the media can quench instabilities if the fields are oriented in different directions (that is, crossed); however, for astronomically pla usible geometries in which the fields are closer to being parallel, but of different strengths in the two media, instabilities are free to grow in dir ections normal to the fields. A cylindrical interface between an interior m edium of density rho(1) and an exterior medium of density rho(2) provides a model for the long filaments of dense gas observed in some molecular cloud s. We show that such an interface with rho(1) > rho(2) is stable to ''kink modes" but unstable to "sausage modes" owing to self-gravity for long axial wavelengths, lambda(z) > 3.8d, where d is the diameter of the cylinder. Th is instability will tend to form prolate ellipsoidal density concentrations aligned with the cylinder axis.