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arXiv · 1904.01599

RR Lyrae Stars in the Field of Sagittarius II

Seok-Joo Joo·Jaemann Kyeong·Soung-Chul Yang·Sang-Il Han·Eon-Chang Sung·Soo-Chang Rey·Helmut Jerjen·Hak-Sub Kim·Dongwon Kim·Hyunjin Jeong·Chang H. Ree·Sang-Mok Cha·Yongseok Lee
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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 (~4.8') of the galaxy indicating their association with Sgr II, while the RRab star V4 is located ~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 = 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 Oosterhoff-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 = -2.1 +- 0.3) and heliocentric distance (~64 +- 3 kpc) of Sgr II from the RR Lyrae stars, and estimate its age (~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.

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Seok-Joo Joo, Jaemann Kyeong, Soung-Chul Yang, Sang-Il Han, Eon-Chang Sung, Soo-Chang Rey, Helmut Jerjen, Hak-Sub Kim, Dongwon Kim, Hyunjin Jeong, Chang H. Ree, Sang-Mok Cha, Yongseok Lee. 2019-04-02. RR Lyrae Stars in the Field of Sagittarius II. https://doi.org/10.3847/1538-4357%2Fab11ca

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