Linear Gravitational Instability of Filamentary and Sheetlike Molecular Clouds with Magnetic Fields

Abstract

We study the linear evolution of small perturbations in self-gravitating fluid systems with magnetic fields. We consider wave-like perturbations to nonuniform filamentary and sheet-like hydrostatic equilibria in the presence of a uniform parallel magnetic field. Motivated by observations of molecular clouds that suggest substantial nonthermal (turbulent) pressure, we adopt equations of state that are softer than isothermal. We numerically determine the dispersion relation and the form of the perturbations in the regime of instability. The form of the dispersion relation is the same for all equations of state considered, for all magnetic field strengths, and for both geometries examined. We demonstrate the existence of a fastest growing mode for the system and study how its characteristics depend on the amount of turbulence and the strength of the magnetic field. Generally, turbulence tends to increase the rate and the length scale of fragmentation. While tending to slow the fragmentation, the magnetic field has little effect on the fragmentation length scale until reaching some threshold, above which the length scale decreases significantly. Finally, we discuss the implications of these results for star formation in molecular clouds.