The Chemical Evolution of the Draco Dwarf Spheroidal Galaxy

TL;DR

This study analyzes the chemical abundances of stars in the Draco dwarf galaxy, revealing unique chemical evolution patterns that differ from the Milky Way and other dwarf galaxies, shedding light on galaxy formation processes.

Contribution

It provides detailed high-resolution spectroscopic data for Draco stars and compares their chemical evolution to other galaxies, highlighting differences in enrichment timescales and processes.

Findings

Draco stars show lower alpha-element ratios at higher metallicities.

Heavy element s-process begins at lower metallicity in Draco.

Evidence supports galaxy formation through accretion of dwarf satellites.

Abstract

We present an abundance analysis based on high resolution spectra of 8 stars selected to span the full range in metallicity in the Draco dwarf spheroidal galaxy. We find [Fe/H] for the sample stars ranges from -1.5 to -3.0 dex. Combining our sample with previously published work for a total of 14 luminous Draco giants, we show that the abundance ratios [Na/Fe], [Mg/Fe] and [Si/Fe] for the Draco giants overlap those of Galactic halo giants at the lowest [Fe/H] probed, but are significantly lower for the higher Fe-metallicity Draco stars. For the explosive alpha-elements Ca and Ti, the abundance ratios for Draco giants with [Fe/H] > -2.4 dex are approximately constant and slightly sub-solar, well below values characteristic of Galactic halo stars. The s-process contribution to the production of heavy elements begins at significantly lower Fe-metallicity than in the Galactic halo. Using a toy model we compare the behavior of the abundance ratios within the sample of Draco giants with those from the literature of Galactic globular clusters, and the Carina and Sgr dSph galaxies. The differences appear to be related to the timescale for buildup of the heavy elements, with Draco having the slowest rate. We note the presence of a Draco giant with [Fe/H] < -3.0 dex in our sample, and reaffirm that the inner Galactic halo could have been formed by early accretion of Galactic satellite galaxies and dissolution of young globular clusters, while the outer halo could have formed from those satellite galaxies accreted later.