Abstract
In a realistic collapsing cloud, deviations from perfect axial symmetry will produce gravitational torques that alter the distribution of angular momentum. If differential rotation leads to the development of trailing spiral features of large amplitude, angular momentum will be transferred outward on an orbital time scale. Ar'y disk formed by the collapse of a rotating cloud will be a fragile structure, and almost certainly highly unstable to the growth of trailing spiral density perturbations of large amplitude. Thus it seems unavoidable that rapid outward transfer of angular momentum by gravitational torques will eventually occur in any realistic collapsing cloud. The only stable outcome of the collapse is a system in which most of the mass is in a central star or binary system, and less than half of the mass remains in a disk around the star. If many newly formed stars are surrounded by residual disks whose mass is not much smaller than the maximum stable mass, accretion of material from such disks, possibly driven by gravitational torques induced by protoplanetary condensations, could have significant consequences for the observable properties of young stars. In particular, FU Orionis-type outbursts could be caused by sporadic runaway accretion of the inner part of a residual disk, occurring when frictional heating of the outer layers of the star causes it to expand rapidly. Mass ejection could also be powered by disk accretion if part of the kinetic energy released is converted into magnetic energy and causes strong flaring activity.
