Fragmentation Instability of Molecular Clouds

Abstract

Turbulent motions observed in molecular clouds are thought to reflect initial conditions associated with cloud formation and may be sustained over the cloud lifetime by mechanical energy sources associated with star formation. This paper demonstrates that free energy stored in the magnetic fields of clouds represents another source of turbulent energy, which can be released through an instability driven by ambipolar drift. The instability operates even in cases in which the cloud would be dynamically stable if the magnetic field were completely frozen to the gas. The instability has a weak form, to which clouds are generally susceptible, and a strong form, which appears if the cloud is within about 30% of critical. In the strong form, the instability grows at a rate intermediate between the slow rate of ambipolar drift and the more rapid rates associated with dynamical processes. In the weak form of the instability the growth rate is close to the ambipolar drift rate. The instability drives turbulent motions, both compressive and vortical, and may accelerate the fragmentation of a molecular cloud into substructures.