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Title: The effects of wind energetics on the angular momentum outflow from late-type stars
Author: Pantolmos, G.
ISNI:       0000 0004 7969 5270
Awarding Body: University of Exeter
Current Institution: University of Exeter
Date of Award: 2019
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Sun-like and low-mass stars (i.e. late-type stars) possess high temperature coronae and lose mass in the form of stellar (or coronal) winds, driven by thermal pressure and complex magnetohydrodynamic processes. The effect of coronal winds on the stellar mass during a star's main-sequence life is relatively small, but they have an enormous impact on stellar rotation by exerting a spin-down torque on the stellar surface, a mechanism known as magnetic braking. The critical parameter, which determines the stellar-angular-momentum loss, is the Alfvén radius, the radius at which the flow reaches the local Alfvénic speed. This critical radius represents the effective lever-arm of the outflow and determines the efficiency of the braking torque. From a theoretical perspective, the objective is to provide analytic stellar-torque prescriptions based on fundamental stellar parameters (e.g. stellar mass, radius, rotation rate, magnetic field properties, and coronal conditions). Studies, employing multidimensional stellar wind simulations, demonstrated that the effective lever arm (or Alfvén radius) scales as a power law with a quantity called the wind magnetization, which depends on stellar parameters (i.e stellar mass, radius, mass-loss rate, and surface magnetic field strength). Using this method, we investigate how the wind energetics, which affect the flow velocity and acceleration profile, can influence the magnetic braking of late-type stars. In this work, with the use of 2.5D stellar-wind numerical simulations, we show that a faster wind has a smaller magnetic lever arm, and therefore the braking torque exerted on the star decreases. We derive new predictive torque formulae that quantify this effect over a wide range of flow acceleration profiles. We further show how numerical-diffusion effects (due to different approaches in the simulation setup) can influence the accuracy of the torque prescriptions presented here.
Supervisor: Matt, S. ; Browning, M. K. M. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available