The first stars and the convective-reactive regime
Date
2021-01-11
Authors
Clarkson, Ondrea
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Abstract
Due to their initially metal-free composition, the fi rst stars in the Universe, which
are termed Population III (Pop III) stars, were fundamentally different than later
generations of stars. As of now, we have yet to observe a truly metal-free star although
much effort has been placed on this task and that of nding the second
generation of stars. Given they were the first stars, Pop III stars are expected to
have made the fi rst contributions to elements heavier than those produced during
the Big Bang. For decades signi cant mixing between H and He burning layers has
been reported in simulations of massive Pop III stars. In this thesis I investigate
this poorly understood phenomenon and I posit that interactions between hydrogen
and helium-burning layers in Pop III stars may have had a profound impact on their
nucleosynthetic contribution to the early universe, and second generation of stars.
First, I examined a single massive Pop III star. This was done using a combination
of stellar evolution and single-zone nucleosynthesis calculations. For this project I
investigated whether the abundances in the most iron-poor stars observed at the
time of publication, were reproducible by an interaction between H and He-burning
layers. Here it was found that the i process may operate under such conditions. The
neutrons are able to ll in odd elements such as Na, creating what is sometimes called
the `light-element abundance signature' in observed CEMP stars. I also present the
finding that it is possible to produce elements heavier than iron as a result of the
i process operating in massive Pop III stars.
A parameter study I conducted on H-He interactions in a grid of 22/26 MESA
stellar evolution simulations is then described. I grouped these interactions into
four categories based on the core-contraction phase they occur in and the convective
stability of the helium-burning layer involved. I also examine in detail the hydrogen burning
conditions within massive Pop III stars and the behaviour of the CN cycle
during H-He interactions. The latter is compared to observed CN ratios in CEMP
stars.
Finally, I describe the first ever 4pi 3D hydrodynamic simulations of H-He shells
in Pop III stars. I also examine the challenges in modelling such con gurations and
demonstrate the contributions I have made in modelling Pop III H and He shell
systems in the PPMStar hydrodynamics code. My contributions apply to other stellar
modelling applications as well.
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Keywords
Stars, Population III, Stellar Evolution, Nucleosynthesis