Variable stars in the field of the Hydra II ultra-faint dwarf galaxy

TL;DR

This paper reports the discovery of an RR Lyrae star in Hydra II, refining its distance, size, and luminosity estimates, and compares its pulsational properties with other ultra-faint dwarf galaxy stars.

Contribution

The study provides the first RR Lyrae detection in Hydra II, offering new insights into its distance, size, and stellar population characteristics.

Findings

Hydra II is approximately 151 kpc away.

The RR Lyrae star has a period of 0.645 days.

Hydra II's properties suggest it is a dwarf galaxy within the Oosterhoff group.

Abstract

We report the discovery of one RR Lyrae star in the ultra--faint satellite galaxy Hydra II based on time series photometry in the g, r and i bands obtained with the Dark Energy Camera at Cerro Tololo Interamerican Observatory, Chile. The RR Lyrae star has a mean magnitude of $i = 21.30\pm 0.04$ which translates to a heliocentric distance of $151\pm 8$ kpc for Hydra II; this value is $\sim 13\%$ larger than the estimate from the discovery paper based on the average magnitude of several blue horizontal branch star candidates. The new distance implies a slightly larger half-light radius of $76^{+12}_{-10}$ pc and a brighter absolute magnitude of $M_V = -5.1 \pm 0.3$, which keeps this object within the realm of the dwarf galaxies. The pulsational properties of the RR Lyrae star ($P=0.645$ d, $\Delta g = 0.68$ mag) suggest Hydra II may be a member of the intermediate Oosterhoff or Oosterhoff II group. A comparison with other RR Lyrae stars in ultra--faint systems indicates similar pulsational properties among them, which are different to those found among halo field stars and those in the largest of the Milky Way satellites. We also report the discovery of 31 additional short period variables in the field of view (RR Lyrae, SX Phe, eclipsing binaries, and a likely anomalous cepheid). However, given their magnitudes and large angular separation from Hydra II, they must be field stars not related to Hydra II.