Understanding Protostellar Jet Feedback on Disc and Cloud Scales
Creators: name: Nolan, Christopher
Publisher: name: The Australian National University
Year: 2020

description: Protostellar jets are a vital part of the star formation process. They are responsible for the removal of excess angular momentum critical to the growth of protostars, while feeding that angular momentum back into molecular clouds to regulate the formation of stellar cores and gravitational collapse. To better understand the origin and impact of protostellar jets, this thesis investigates the launching of protostellar disc winds and the driving of non-isothermal turbulence in the interstellar medium. In the first part of this thesis, we explore how the structure of protostellar discs relates to the properties of the wind-launching region, which directly effects the large-scale properties of the jet. In order to study the launching of disc winds, we first design a 1+1.5D magnetohydrodynamic (MHD) model of the launching region in the [r,z] plane. We take into account the three diffusion mechanisms of non-ideal MHD (Ohm, Hall, and ambipolar) by calculating their contributions at the disc midplane and using a simplified, vertically-scaled approach for higher z. We observe that most of the mass launched by the wind is concentrated within a radially localized region a fraction of an astronomical unit (au) in width, in agreement with current observations. We find that the footprint radius and the wind efficiency, measured by the ratio of the wind mass-loss rate to the rate of material accreted onto the star, are a strong function of the model parameters, namely the mass accretion rate, magnetic field strength, and surface density profile of the disc. Understanding the structure of the wind-launching region has important We subsequently improve the 1+1.5D models by removing the vertical scaling approximation to the non-ideal MHD terms and calculate the magnetic diffusivities self-consistently at all heights above the disc midplane. This results in increased field-matter coupling surrounding the midplane, increasing the poloidal magnetic field bending and compressing the disc via enhanced magnetic pressure gradients. It also shifts the wind-launching region to smaller radii, decreases the overall wind mass-loss rate by an order of magnitude, and generates a radially symmetric wind mass-loss profile. In the second part of this thesis, we investigate the properties of driven, turbulent, adiabatic gas. The density variance--Mach number relation of the turbulent interstellar medium is a key ingredient for analytical models of star formation. We examine the robustness of the standard, isothermal form of this relation in the non-isothermal regime, specifically testing ideal gases with diatomic molecular and monatomic adiabatic indices. Stirring the gas with purely solenoidal forcing at low wavenumbers, we find that as the gas heats in adiabatic compressions, it evolves along a curve in the density variance-Mach number plane, but deviates significantly from the standard isothermal relation. We provide new empirical and theoretical relations that take the adiabatic index into account and provide good fits for a range of Mach numbers.