Electronic Thesis and Dissertation Repository

Thesis Format

Monograph

Degree

Doctor of Philosophy

Program

Applied Mathematics

Supervisor

Shantanu, Basu

Abstract

Recent observations of protostellar cores reveal complex magnetic field configurations that are distorted in the innermost disk region. Unlike the prestellar phase, where the magnetic field geometry is simpler with an hourglass configuration, magnetic fields in the protostellar phase are sculpted by the formation of outflows and rapid rotation. This gives rise to a significant azimuthal (or toroidal) component that has not yet been analytically modelled in the literature. Moreover, the onset of outflows, which act as angular momentum transport mechanisms, have received considerable attention in the past few decades. Two mechanisms: 1) the driving by the gradient of a twisted magnetic field (magnetic pressure gradient force or MPGF); and 2) the driving by magneto-centrifugal winds (MCW), are invoked in the literature and sometimes applied to different launch regions. The former arises when the toroidal component is dominant whereas the latter arises when the poloidal component is dominant near the surface of the accretion disk. By employing three-dimensional resistive non-ideal magnetohydronamics (MHD), the magnetic field of the disk-outflow system has been modelled and analyzed. A mathematical model for the azimuthal component is constructed via a pseudo-Fourier approach, while the poloidal component is an extension of the previous work done by Ewertowski \& Basu (2013). After fitting to simulation data, our results show that the full model is within reasonable agreement with the MHD data with a combined root mean squared error of ~10^{-4}. In addition, by generating a series of azimuthally averaged heat-maps, the driving mechanisms of the wide-angle outflow region is investigated. The heat-maps reveal not one single driver, but a hybrid driving mechanism arising from the MPGF or MCW mechanism in different regions, and extending well beyond the confines of the protostellar disk itself. A flow that is launched along field lines that are very inclined from the vertical is initially MCW-driven until it surpasses the Alfven surface, where it becomes MPGF-driven.

Summary for Lay Audience

Protostars are young stars, born out of the self-gravity of a collapsing molecular cloud (or prestellar core), which are still accreting from the parent envelope of material. Throughout their life, protostars are characterized by two distinctive features; the rapidly rotating disk and the onset of outflows emanating from its near environment. The rapidly spinning disk twists the magnetic field lines, thereby distorting them in the innermost region. Also, because of the rapid rotation of the protostellar disk, accretion would not be possible if not for some mechanism channelling the excess angular momentum into interstellar space. As a result, the outflows are formed to solve this angular momentum problem. Despite being extensively studied, the exact driving mechanisms of the outflow system remains unclear. Two prominent drivers are often invoked in the literature, where one is analogous to a low-tension wire and the other, to a high-tension wire; In the former case, twisting the wire will give rise to a spring-like effect whereas, in the latter, twisting it will yield co-rotation. In addition to proposing a magnetic field model for the protostar, this thesis investigates the role of the magnetic field in driving the outflows, via a simulation approach known as magnetohydrodynamics. It suggests that the driver of the outflows is a combination of the two aforementioned mechanisms, and ultimately depends on the degree of twisting of the magnetic field lines.

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