The Magnetic Field of Protostar-Disk-Outflow Systems
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.