Electronic Thesis and Dissertation Repository


Doctor of Philosophy




Shantanu Basu


We investigate magnetic diffusion on scales from molecular clouds over prestellar and
protostellar cores down to young stellar objects (YSOs) and their surrounding protoplanetary disk.
In Chapter 2, we present thin-sheet simulations that exhibit long-lived magnetic-tension-driven oscillations, founded in the interaction of the cloud's magnetic field with that anchored in an external medium. In contrast with "local" simulations in a periodic box, where turbulence decays away in approximately a sound crossing time, and needs to be continually replenished by driving, our simulation has "global" aspects, and retains some kinetic energy indefinitely. We provide an analytical explanation for these modes, that occur in the flux-freezing limit, as may be applicable to photoionized molecular cloud envelopes. The motions decay rapidly if ambipolar diffusion is introduced.
Chapter 3 introduces a new analytical three-parameter column density profile to fit prestellar cores. It is a replacement for the Bonnor-Ebert sphere model which has severe drawbacks, not the least of which is that fitting it often produces unrealistic temperatures. Our model instead fits the size of the flat region of both collapsing cores and those in equilibrium. It uses temperature as an input parameter. It can also be used to fit flattened cores, as well as cores with rotation and magnetic fields. Finally, our model provides a quantitative measure to judge whether a core is collapsing or in equilibrium. We apply it to B68 and L1689B.
In Chapters 4 and 5, we present numerical simulations that show that catastrophic magnetic braking can be avoided in the collapse of a prestellar core. Non-ideal MHD effects (ambipolar diffusion and Ohmic dissipation) weaken the magnetic field in the first core, inactivate magnetic braking there, and allow a disk to form close to the protostar. The formation of a small disk is consistent with observations that do not show evidence of a large centrifugal disk around Class 0 and I protostars. We propose a scenario where over time, a small but initially massive disk can expand to sizes of approximately 100 AU, as commonly observed around Class II protostars.