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