What makes a star turn violent? : exploring high-mass star formation and stellar magnetic helicity

  • Kristin Lund

Student thesis: Doctoral Thesis (PhD)


High-mass stars play an important role in both the dynamical and chemical evolution of the
galaxy, hence how they form has long been a topic of interest. In this thesis I explore accretion
as a possible means of forming high-mass stars. I use a radiation hydrodynamics code to
simulate accretion onto a star which emits ionising radiation. Usually accretion is assumed
to end once ionisation begins, however I find accretion continues through the ionisation front
whilst the ionised region is gravitationally trapped. This extended period of accretion increases
the amount of material added to the star. As most high-mass stars are observed in close binary
systems, next I develop a semi-analytic model to investigate whether accretion onto wide lowmass
binaries can increase the mass of the stars and simultaneously decrease their separation.
The result suggests magnetic braking of the accretion flow is a feasible way of forming the
most massive close binary systems.

The second part of this thesis focuses on the magnetic helicity density of low-mass stars.
There has been growing interest in magnetic helicity as a potential tracer of stellar eruptions,
but measurements based on observations have been limited to the Sun. I present an analytic
expression for the magnetic helicity density across any stellar surface given only observable
quantities. This expression is used to calculate the magnetic helicity density of 52 stars, which
is then plotted against other stellar parameters. I find mostly different behaviours for the
partially and largely convective stars, except when plotting helicity density against toroidal
magnetic energy. In that case the entire stellar sample follows the same power law. Comparing
the Sun to the other stars I find the variation in solar helicity densities across a solar cycle falls
within the scatter of the stellar values.
Date of Award1 Dec 2020
Original languageEnglish
Awarding Institution
  • University of St Andrews
SupervisorIan Alexander Bonnell (Supervisor), Kenny Wood (Supervisor) & Moira Mary Jardine (Supervisor)

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