| abstract: |
Star formation occurs in Giant Molecular Clouds in cold condensed cores triggered toward collapse by external mechanisms, such as shock waves or stellar winds. For initially static cores, numerical simulations have shown that collapse is spherically symmetric and solar-type stars form on timescales of tens of millions of years. Observations have shown, however, that typical cloud cores have large specific angular momenta ~ 1021 m2/s, the Sun has ~ 1015 m2/s. The specific angular momenta of protostellar clouds are far too high to allow them to collapse directly into a star, only a few percent of the matter expected to fall into the central object for a typical low-mass core, the rest forming a circumstellar disk. The process of star formation is then a question of how the disk material accretes onto the central object and thus requires knowledge of the viscosity of the disk material. Ordinary molecular viscosity cannot supply the dissipation and nonaxisymmetric hydrodynamic and/or magnetohydrodynamic instabilities are usually invoked to supply the transport directly or to generate turbulence to enhance the viscosity. We investigated the hydrodynamic stability properties of massive, self-gravitating disks to study this issue. We modeled disks in the linear, quasi-linear, and nonlinear regimes with the aim of shedding light on the general nature of global nonaxisymmetric instabilities in massive disks by mapping the regimes of instability in the relevant parameter space, and then developing a quasi-linear theory to model the evolution of disks linearly unstable to global nonaxisymmetric instabilities. We consider implications of our results for the star formation process.
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