Constraining the Angular Momentum-loss Rates of the Sun and Other Sun-like Stars

Abstract

Stellar rotation, convection, and magnetism are intricately linked in low-mass stars like the Sun. In their outer convective envelopes, the interplay of rotation and convection form a magnetic dynamo capable of sustaining both large and small scale magnetic fields. The strength of these magnetic fields are observed to grow with increasing rotation rate. The coronae of low-mass stars are heated by these magnetic fields (the exact mechanism of which remains under debate), such that the thermal pressure drives a quasi-steady outflow of plasma, referred to as a stellar wind. Due to the interaction of the large-scale magnetic field with the outflowing plasma, stellar winds are able to efficiently remove angular momentum from these stars. Therefore, the evolution of rotation for low-mass stars (on the the main sequence) is governed by their stellar winds, and by interrelation, the evolution of their magnetic activity and stellar wind output. In this thesis I attempt to better constrain the angular momentum-loss rates of the Sun and other Sun-like stars through the use of magnetohydrodynamic simulations combined with a broad range of observations. Though I do not find a concrete value for the solar case, I reduce the uncertainty in its value to within a factor of a few by locating key factors/quantities which limit our predictions, and further highlight the importance of understanding the solar angular momentum-loss rate in an astrophysical context. For the other Sun-like stars, I find the simulation results largely under-predict the angular momentum-loss rates implied by current rotation-evolution models. The reason(s) for this are uncertain, but likely involve uncertainties in both the observed magnetic field strengths and mass-loss rates of these stars, along with the under-prediction of how much of the surface magnetic field is ``opened'' by the stellar wind.