Stellar Archaeology in the Galactic halo with Ultra-Faint Dwarfs: VII. Hercules

TL;DR

This study conducts the first detailed time-series analysis of Hercules, an ultra-faint dwarf galaxy, identifying variable stars to determine its properties and stellar population characteristics.

Contribution

It provides the first characterization of variable stars in Hercules, deriving its metallicity, distance, and age, and confirming its classification within the Oosterhoff II group.

Findings

Hercules hosts 10 pulsating variable stars, including RR Lyrae and an Anomalous Cepheid.

The galaxy's properties are [Fe/H] = -2.30, E(B-V) = 0.09, and distance modulus 20.6 mag.

The stellar population is old and metal-poor, similar to globular cluster M68.

Abstract

We present the first time-series study of the ultra-faint dwarf galaxy Hercules. Using a variety of telescope/instrument facilities we secured about 50 V and 80 B epochs. These data allowed us to detect and characterize 10 pulsating variable stars in Hercules. Our final sample includes 6 fundamental-mode (ab-type) and 3 first overtone (c-type) RR Lyrae stars, and one Anomalous Cepheid. The average period of the ab-type RR Lyrae stars, < Pab >= 0.68 d (sigma = 0.03 d), places Hercules in the Oosterhoff II group, as found for almost the totality of the ultra-faint dwarf galaxies investigated so far for variability. The RR Lyrae stars were used to obtain independent estimates of the metallicity, reddening and distance to Hercules, for which we find: [Fe/H] = -2.30+-0.15 dex, E(B -V) = 0.09+-0.02 mag, and (m-M)o = 20.6+-0.1 mag, in good agreement with the literature values. We have obtained a V, B - V color-magnitude diagram (CMD) of Hercules that reaches V ~ 25 mag and extends beyond the galaxy's half-light radius over a total area of 40' {\times} 36'. The CMD and the RR Lyrae stars indicate the presence of a population as old and metal-poor as (at least) the Galactic globular clusters M68.