RR Lyrae Stars in the Field of Sagittarius II

Abstract

We present the detection of RR Lyrae variable stars in the field of the Sagittarius. II (Sgr II) ultra-faint dwarf (UFD) galaxy. Using B, V time-series photometry obtained with the Korea Microlensing Telescope Network (KMTNet) 1.6 m telescope at CTIO and G-band data from Gaia Data Release 2 (DR2), we identified and characterized two ab-type and four c-type RR Lyrae variables. Five out of the six stars are clustered within three half-light radii (similar to 4'.8) of the galaxy indicating their association with Sgr. II, while the RRab star V4 is located similar to 22' from the galaxy center. By excluding V4, the high c-type fraction (0.8) and the period of the only RRab star V3 (P-V3 similar or equal to 0.666 days) suggest an Oosterhoff. II (Oo II) classification for Sgr. II. Located close to the locus of Oo. II clusters in the period-amplitude diagram, V3 is similar to RRab stars in other UFDs having Oosterh off-intermediate and Oo. II properties. Sgr. II is, however, more compact than usual UFDs, placed in between star clusters and dwarf galaxies in the size-luminosity plane, and therefore spectroscopic studies are eventually required to ascertain the true nature of this stellar system. We derive the metallicity ([Fe/H] RRab similar or equal to -2.1 +/- 0.3) and heliocentric distance (similar to 64 +/- 3 kpc) of Sgr. II from the RR Lyrae stars, and estimate its age (similar to 12 Gyr) based on our stellar population models. The Oosterhoff properties of UFDs can be explained with the evolution effect of RR Lyrae stars in the instability strip.