| Protostellar disks play a central role in star and planet formation, but the details of their formation are not known. Recent studies have shown that rotationally supported disk formation is not guaranteed, as magnetic braking may remove sufficient angular momentum. However, circumstellar disks are ubiquitously observed in young stellar objects (YSOs). This thesis investigates the formation of protostellar disks in the ideal and ambipolar diffusion limits using magnetohydrodynamics (MHD) simulations.;Magnetic fields are usually considered dynamically important in star formation when the dimensionless mass-to-flux ratio is close to, or less than, unity. We show that, in disk formation, the requirement is far less stringent. This conclusion is drawn from a set of 2D simulations of the collapse of rotating, singular isothermal cores magnetized to different degrees. We find that a weak field can begin to disrupt the rotationally supported disk through magnetic braking, by creating regions of rapid, supersonic collapse in the disk. We further find that rotationally supported disks are not formed in the main accretion phase of star formation due to efficient magnetic braking in the ideal MHD limit. As circumstellar disks are observed in YSOs, we suggest that the efficiency of magnetic braking must be reduced via non-ideal MHD effects or due to outflow-envelope outflows.;We concentrate on one non-ideal MHD effect, ambipolar diffusion, which enables the field lines to slip relative to the bulk neutral matter. We find that the slippage does not sufficiently weaken the braking to allow rotationally supported disks to form for realistic levels of cloud magnetization and cosmic ray ionization rate; in some cases, the magnetic braking is even enhanced. Only in dense cores with both exceptionally weak fields and unreasonably low ionization rate do such disks start to form in our simulations. We conclude that additional processes, such as Ohmic dissipation or Hall effect, are needed to enable disk formation. Alternatively, the disk may form at late times when the massive envelope that anchors the magnetic brake is dissipated, perhaps by a protostellar wind. |
