Abstract
We present chemical abundance measurements of three stars in the ultra-faint
dwarf galaxy Horologium I, a Milky Way satellite discovered by the Dark Energy
Survey. Using high resolution spectroscopic observations we measure the
metallicity of the three stars as well as abundance ratios of several
$\alpha$-elements, iron-peak elements, and neutron-capture elements. The
abundance pattern is relatively consistent among all three stars, which have a
low average metallicity of Fe/H $-2.6$ and are not $\alpha$-enhanced
($\alpha$/Fe $0.0$). This result is unexpected when compared to other
low-metallicity stars in the Galactic halo and other ultra-faint dwarfs and
hints at an entirely different mechanism for the enrichment of Hor I compared
to other satellites. We discuss possible scenarios that could lead to this
observed nucleosynthetic signature including extended star formation, a
Population III supernova, and a possible association with the Large Magellanic
Cloud.
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