NONAXISYMMETRIC EVOLUTION IN PROTOSTELLAR DISKS

We present a two-dimensional, multigridded hydrodynamical simulation o f the collapse of an axisymmetric, rotating 1 M. protostellar cloud, w hich forms a resolved, hydrostatic disk. The code includes the effects of physical viscosity, radiative transfer and radiative acceleration but not magnetic fields. We examine how the disk is affected by the in clusion of turbulent viscosity by comparing a viscous simulation with an inviscid model evolved from the same initial conditions, and we der ive a disk evolutionary timescale on the order of 300,000 years if alp ha = 0.01. Effects arising from non-axisymmetric gravitational instabi lities in the protostellar disk are followed with a three-dimensional SPH code, starting from the two-dimensional structure. We find that th e disk is prone to a series of spiral instabilities with primacy azimu thal mode number m = 1 and m = 2. The torques induced by these nonaxis ymmetric structures elicit material transport of angular momentum and mass through the disk, readjusting the surface density profile toward more stable configurations. We present a series of analyses which char acterize both the development and the likely source of the instabiliti es. We speculate that an evolving disk which maintains a minimum Toomr e Q-value of approximately 1.4 will have a total evolutionary span of several times 10(5) years, comparable to, but somewhat shorter than th e evolutionary timescale resulting from viscous turbulence alone. We c ompare the evolution resulting from nonaxisymmetric instabilities with solutions of a one-dimensional viscous diffusion equation applied to the initial surface density and temperature profile, We find that an e ffective alpha-value of 0.03 is a good fit to the results of the simul ation. However, the effective alpha will depend on the minimum Q in th e disk at the time the instability is activated. We argue that the maj or fraction of the transport characterized by the value of alpha is du e to the action of gravitational torques, and does not arise from inhe rent viscosity within the smoothed particle hydrodynamics method.