An abundance study of red-giant-branch stars in the Hercules dwarf spheroidal galaxy

TL;DR

This study uses high-resolution spectroscopy to analyze the metallicity and chemical enrichment history of red giant stars in the Hercules dwarf spheroidal galaxy, revealing insights into its stellar populations and chemical evolution.

Contribution

It provides new spectroscopic metallicity measurements, a simple calibration for Stromgren photometry, and insights into the galaxy's chemical enrichment and age distribution.

Findings

Hercules red giants are more metal-poor than previously estimated.

[Ca/Fe] increases with decreasing [Fe/H], indicating early supernova enrichment.

Red giants are older than 10 Gyr, suggesting an ancient stellar population.

Abstract

Using high-resolution spectroscopy, we provide a determination of [Fe/H] and [Ca/H] for confirmed red-giant branch member stars of the Hercules dwarf spheroidal galaxy. Based on this we explore the ages of the prevailing stellar populations in Hercules, and the enrichment history from supernovae. Additionally, we provide a new simple metallicity calibration for Stromgren photometry for metal-poor, red giant branch stars. We find that the red-giant branch stars of the Hercules dSph galaxy are more metal-poor than estimated in our previous study that was based on photometry alone. Additionally, we find an abundance trend such that [Ca/Fe] is higher for more metal-poor stars, and lower for more metal-rich stars, with a spread of about 0.8 dex. The [Ca/Fe] trend suggests an early rapid chemical enrichment through supernovae of type II, followed by a phase of slow star formation dominated by enrichment through supernovae of type Ia. A comparison with isochrones indicates that the red giants in Hercules are older than 10 Gyr.