FRAGMENTATION INSTABILITY OF MOLECULAR CLOUDS

Turbulent motions observed in molecular clouds are thought to reflect initial conditions associated with cloud formation and may be sustaine d over the cloud lifetime by mechanical energy sources associated with star formation. This paper demonstrates that free energy stored in th e magnetic fields of clouds represents another source of turbulent ene rgy, 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 general ly susceptible, and a strong form, which appears if the cloud is withi n about 30% of critical. In the strong form, the instability grows at a rate intermediate between the slow rate of ambipolar drift and the m ore rapid rates associated with dynamical processes. In the weak form of the instability the growth rate is close to the ambipolar drift rat e. The instability drives turbulent motions, both compressive and vort ical, and may accelerate the fragmentation of a molecular cloud into s ubstructures.