Chemical Abundance Analysis of Three $\alpha$-Poor, Metal-Poor Stars in the Ultra-Faint Dwarf Galaxy Horologium I

TL;DR

This study analyzes the chemical composition of three stars in the ultra-faint dwarf galaxy Horologium I, revealing unusual abundance patterns that challenge existing models of galaxy enrichment.

Contribution

It provides the first detailed chemical abundance measurements of stars in Horologium I, highlighting its unique nucleosynthetic history compared to other dwarf galaxies.

Findings

Stars have low metallicity (~ -2.6) and are not alpha-enhanced.

Abundance patterns are consistent among the three stars.

Results suggest a different enrichment mechanism for Hor I.

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] $\sim -2.6$ and are not $\alpha$-enhanced ([$\alpha$/Fe] $\sim 0.0$). 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.