Stability of Interfaces with Self‐Gravity, Relative Flow, andBField

Abstract

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 interfaces between different media in molecular clouds. Instability and nonlinear growth of crenelations of interfaces can lead to mass concentrations that in turn lead to star formation. In an earlier study of the stability of interfaces, we took into account the important physical effects—the different densities and temperatures of the media, the relative motion (Kelvin-Helmholtz instability), the gravitational acceleration perpendicular to the interface (Rayleigh-Taylor instability), and self-gravity. A new self-gravitational 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 length is approached from below. The e-folding time for the instability is comparable with the free-fall collapse time for the denser fluid. In the present work, we generalize our earlier theory in two ways: by including ordered magnetic fields parallel to the interface, and by examining the stability of long cylindrical interfaces. We show that dynamically important magnetic fields in the media can quench instabilities if the fields are oriented in different directions (that is, crossed); however, for astronomically plausible geometries in which the fields are closer to being parallel, but of different strengths in the two media, instabilities are free to grow in directions normal to the fields. A cylindrical interface between an interior medium of density ρ1 and an exterior medium of density ρ2 provides a model for the long filaments of dense gas observed in some molecular clouds. We show that such an interface with ρ1 > ρ2 is stable to kink modes but unstable to sausage modes owing to self-gravity for long axial wavelengths, λz > 3.8d, where d is the diameter of the cylinder. This instability will tend to form prolate ellipsoidal density concentrations aligned with the cylinder axis.