Turbulent Angular Momentum Transport in Weakly-ionized Accretion Disks

Accretion disks are ubiquitous in the universe. Although difficult to observe directly, their presence is often inferred from the unique signature they imprint on the spectra of the systems in which they are observed. In addition, many properties of accretion-disk systems that would be otherwise mysterious are easily accounted for by the presence of matter accreting (accumulating) onto a central object. Since the angular momentum of the infalling material is conserved, a disk naturally forms as a repository of angular momentum. Dissipation removes energy and angular momentum from the system and allows the disk to accrete. It is the energy lost in this process and ultimately converted to radiation that we observe. Understanding the mechanism that drives accretion has been the primary challenge in accretion disk theory. Turbulence provides a natural means of dissipation and the removal of angular momentum, but firmly establishing its presence in disks proved for many years to be difficult. The realization in the 1990s that a weak magnetic field will destabilize a disk and result in a vigorous turbulent transport of angular momentum has revolutionized the field. Much of accretion disk research now focuses on understanding the implications of this mechanism for astrophysical observations. At the same time, the success of this mechanism depends upon a sufficient ionization level in the disk for the flow to be well-coupled to the magnetic field. Many disks, such as disks around young stars and disks in binary systems that are in quiescence, are too cold to be sufficiently ionized, and so efforts to establish the presence of turbulence in these disks continues. This dissertation focuses on several possible mechanisms for the turbulent transport of angular momentum in weakly-ionized accretion disks: gravitational instability, radial convection and vortices driving compressive motions. It appears that none of these mechanisms are very robust in driving accretion. A discussion is given, based on these results, as to the most promising directions to take in the search for a turbulent transport mechanism that does not require magnetic fields. Also discussed are the implications of assuming that no turbulent transport mechanism exists for weakly-ionized disks.