Galactic Archaeology with Metal-Poor Stars
2020
Metal-poor stars are time capsules into the
earliest epochs of the Universe. Their stellar populations,
chemical abundances, and proper motions allow for insight into the
nucleosynthesis paths and proto-Galactic systems which became the
building blocks of the Milky Way. Through a wide variety of
methods, the aim of this thesis is to advance the understanding of
the chemodynamical evolution of the Galaxy. In
“Metal-poor Stars from the South African Large Telescope”, the
first release of a large-scale study of relatively bright (V
< 13.5) metal-poor stars is presented. The elemental
abundances of C, Sr, Ba, and Eu are reported, as well as several
alpha-elements (Mg, Ca, Sc, Ti, V) and iron-peak elements (Mn, Co,
Ni, Zn). We find a diverse array of abundance patterns, including
several consistent with the signatures of carbon-enhanced
metal-poor CEMP-i and CEMP-r stars. We find that 15 of 50 (30%) are
carbon-enhanced (with [C/Fe] > +0.70), and that a large
fraction (26 of 50, 52%) are enhanced in r-process elements. Among
the r-process-enhanced stars, five are strongly enhanced r-II stars
(two of which are newly discovered) and 21 are moderately enhanced
r-I stars. There are eight stars in our sample that, on the basis
of their abundances and kinematics, are possible members of the
metal-weak thick-disk population. We also compare our measured
abundances to progenitor-enrichment models,and find that the
abundance patterns for the majority of our stars can be attributed
to a single (rather than multiple) enrichment event.
Next, an abundance measurement of uranium in
the metal-poor r-process-enhanced star BD+17 3248 is reported. The
heavy-element abundance pattern of 20 elements produced by the
r-process from this optical spectrum and an archival ultraviolet
spectrum taken with the Hubble Space Telescope is re-analyzed. The
U abundance is log_epsilon(U) = -1.45 +- 0.15, the highest yet
detected in a metal-poor star, and so far the only detection made
in an r-I star. Our analysis finds the actinide content much higher
than the values previously reported, leading to the classification
of BD+17 3248 as an actinide-boost star. Additionally, it has an
unusually high log_epsilon(U/Th) value compared to the other five
known stars with detected uranium. We consider the implications of
this for r-process nucleosynthesis models and suggest that further
studies be done to determine whether this ratio is evidence of
nuance within our extant understanding of r-process channels or
whether it suggests a new site altogether. The
assembly of the Milky Way’s halo is an open question in the field
of Galactic chemodynamical evolution. Kinematic analyses propose
that the halo is composed of the remnants of small stellar systems
as well as at least one major accretion event; however, such
studies are not well equipped to tell us about the chemical nature
of such systems. Instead, chemical tagging of halo stars is needed
to gain insight into their nucleosynthetic histories. With the
advent of the R-Process…
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