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) ultrafaint 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 V 3 ≃ 0.666 days) suggest an Oosterhoff II (Oo II) classification for Sgr II. Located close to the locus of Oo II clusters in the periodamplitude 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.

Gaia RR Lyrae Stars in Nearby Ultra-faint Dwarf Satellite Galaxies

“…From deep photometry, Longeard et al (2020) find a distance of 73.1 +1.1 −0.7 kpc for Sgr II. The Gaia RRLs distance suggest a closer distance, 62 ± 6 kpc, which is in agreement with the distance given by Joo et al (2019) of 64 ± 3 kpc, also based in RRLs.…”

“…It is believed that Sgr II is a case of a satellite of a satellite since it has an orbit similar to the classical Sgr dSph (Longeard et al 2020), which we know is disrupting. Sgr II was searched for RRLs by Joo et al (2019). They found six RRLs in the field, but associated only five of them to Sgr II because one of them, V4, was too far away from the center of the galaxy.…”

Pegasus IV: Discovery and Spectroscopic Confirmation of an Ultra-faint Dwarf Galaxy in the Constellation Pegasus

“…RRL-type variable stars are excellent tracers of old, metalpoor stellar populations in the Milky Way halo, and have been identified in nearly every ultra-faint dwarf galaxy (e.g., Greco et al 2008;Boettcher et al 2013;Medina et al 2017;Joo et al 2018Joo et al , 2019Martínez-Vázquez et al 2019;Vivas et al 2020;Martínez-Vázquez et al 2021). According to the empirical relation derived by Martínez-Vázquez et al (2019), ultra-faint dwarf galaxies with the same absolute magnitude as Pegasus IV (M V = −4.25) are expected to have between two and four RRLs.…”

Pegasus IV: Discovery and Spectroscopic Confirmation of an Ultra-Faint Dwarf Galaxy in the Constellation Pegasus

“…RR-Lyrae-type variable stars (RRLs) are excellent tracers of old, metal-poor stellar populations in the Milky Way halo, and have been identified in nearly every ultra-faint dwarf galaxy (e.g., Greco et al 2008;Boettcher et al 2013;Medina et al 2017;Joo et al 2018;Martínez-Vázquez et al 2019;Joo et al 2019;Vivas et al 2020;Martínez-Vázquez et al 2021). According to the empirical relation derived by Martínez-Vázquez et al (2019), ultra-faint dwarf galaxies with the same absolute magnitude as Pegasus IV (M V = −4.25) are expected to have between 2 and 4 RRLs.…”