The Chemical Evolution of the Ursa Minor Dwarf Spheroidal Galaxy

TL;DR

This study analyzes the chemical composition of stars in the Ursa Minor dwarf galaxy, revealing its star formation history and chemical evolution patterns, which differ from those of the Milky Way and other dwarf galaxies.

Contribution

It provides detailed abundance measurements for stars across a wide metallicity range in Ursa Minor, highlighting similarities to the Galactic halo at low metallicities and differences at higher metallicities.

Findings

Stars at low metallicity resemble Galactic halo stars in alpha-element ratios.

The galaxy's neutron-capture elements are dominated by the r-process across metallicities.

Star formation in Ursa Minor was shorter and more intense than in other dwarf galaxies.

Abstract

We present an abundance analysis based on high resolution spectra of 10 stars selected to span the full range in metallicity in the Ursa Minor dwarf spheroidal galaxy. We find [Fe/H] for the sample stars ranges from -1.35 to -3.10 dex and establish the trends of the abundance ratios [X/Fe]. In key cases, particularly for the alpha-elements, these resemble those for stars in the outer part of the Galactic halo, especially at the lowest metallicities probed. The n-capture elements show a r-process distribution over the full range of Fe-metallicity. This suggests that the duration of star formation in the UMi dSph was shorter than in other dSph galaxies. The derived ages for a larger sample of UMi stars with more uncertain metallicities also suggest a population dominated by uniformly old (~13 Gyr) stars, with a hint of an age-metallicity relationship. In comparing our results for UMi, our earlier work in Draco, and published studies of more metal-rich dSph Galactic satellites, there appears to be a pattern of moving from a chemical inventory for dSph giants with [Fe/H] < -2 dex which is very similar to that of stars in the outer part of the Galactic halo (enhanced alpha/Fe relative to the Sun, coupled with subsolar [X/Fe] for the heavy neutron capture elements and r-process domination), switching to subsolar alpha-elements and super-solar s-process dominated neutron capture elements for the highest [Fe/H] dSph stars. The combination of low star formation rates over a varying and sometimes extended duration that produced the stellar populations in the local dSph galaxies with [Fe/H] > -1.5 dex leads to a chemical inventory wildly discrepant from that of any component of the Milky Way.