The Highly Unusual Chemical Composition of the Hercules Dwarf Spheroidal Galaxy

TL;DR

This study reveals that stars in the Hercules dwarf galaxy have unusual chemical signatures, notably high alpha-element levels and neutron-capture deficiencies, indicating enrichment by massive supernovae and stochastic star formation.

Contribution

First detailed chemical abundance analysis of Hercules dSph stars showing unique neutron-capture deficiencies and high-mass supernova enrichment signatures.

Findings

Stars show strong alpha-element enhancement (~+0.8 dex)

Neutron-capture elements are severely deficient (<-2 dex)

Enrichment likely dominated by high-mass (~35 M_sun) Type II supernovae

Abstract

We report on the abundance analysis of two red giants in the faint Hercules dwarf spheroidal (dSph) galaxy. These stars show a remarkable deficiency in the neutron-capture elements, while the hydrostatic alpha-elements (O, Mg) are strongly enhanced. Our data indicate [Ba/Fe] and [Mg/Fe] abundance ratios of <-2 dex and ~+0.8 dex, respectively, with essentially no detection of other n-capture elements. In contrast to the only other dSph star with similar abundance patterns, Dra 119, which has a very low metallicity at [Fe/H]=-2.95 dex, our objects, at [Fe/H]~-2.0 dex, are only moderately metal poor. The measured ratio of hydrostatic/explosive alpha-elements indicates that high-mass (~35 M_sun) Type II supernovae progenitors are the main, if not only, contributors to the enrichment of this galaxy. This suggests that star formation and chemical enrichment in the ultrafaint dSphs proceeds stochastically and inhomogeneously on small scales, or that the IMF was strongly skewed to high mass stars. The neutron capture deficiencies and the [Co/Fe] and [Cr/Fe] abundance ratios in our stars are similar to those in the extremely low metallicity Galactic halo. This suggests that either our stars are composed mainly of the ejecta from the first, massive, population III stars (but at moderately high [Fe/H]), or that SN ejecta in the Hercules galaxy were diluted with ~30 times less hydrogen than typical for extreme metal-poor stars.