A study of U Aquarii and the nucleosynthesis of neutrons and S-process elements in evolved stars of low mass
Abstract
The origin of the extreme hydrogen deficiency observed in the R
Coronae Borealis (RCrB) stars remains poorly understood. A likely
mechanism for producing this deficiency is one whereby the original
hydrogen envelope of the star is engulfed and destroyed in its
interior. One of the few RCrB stars so far analysed, U Aquarii, in fact
shows emphatic evidence of such an event having indeed occurred. This
evidence takes the form of the large s-process enhancements observed in
U Aquarii. Such enhancements are caused by the production of
significant neutron fluxes which are directly produced by envelope
engulfment
giving rise to the ¹²C(p, γ) ¹³N(β⁺γ) ¹³C(α,n) ¹⁶O sequence of
nuclear reactions. In order to obtain vital information regarding the
nature of the envelope mixing event from observed s-process
enhancements, detailed nucleosynthesis calculations investigating
neutron production and s-process synthesis are carried out using a
series of nuclear reaction networks and covering a wide range of
parameter space. These calculations are mainly based on low-mass AGB
and post-AGB stellar models since it is widely believed that the RCrB's
are in some way related to this group of stars. It is shown how the
ingestion rate of envelope material, the initial abundances and the type
of mixing model used have a large influence on the neutron and s-process
production.
New spectral observations of U Aquarii are presented, and it is
shown how these new observations allow a new interpretation of the
mixing event which occurred in this star and how further improved
abundance data of the star will lead to an unambiguous determination of
the nature of the mixing event. The RCrB stars are also thought to be
related to the extreme helium stars. Discovery of variability in two of
these latter stars is presented.
Type
Thesis, PhD Doctor of Philosophy
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http://creativecommons.org/licenses/by-nc-nd/4.0/
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