"Twinkle, Twinkle Little Stars"— The Importance of Finding RR Lyrae Stars in Ultra-Faint Dwarf Galaxies

In the last decade, wide-area multi-band optical imaging surveys such as SDSS, DES, SMASH, MagLiteS,  DELVE, ATLAS, Pans-STARRS1, and Gaia have contributed to the discovery of more than 50 ultra-faint dwarf galaxies (UFDs, M_V > —8 mag), satellites of the Milky Way, which were undetectable in the past. The low mass, the scarcity of stars and the large contamination by field stars make the determination of morphological parameters and distances for these galaxies a challenging task. An attractive method to improve the distance determination to these UFDs —and thus to clarify their nature— is to find RR Lyrae (RRL) star members. As pulsating variable stars with periods ranging from 0.2 to 1.2 days, RRLs are readily identifiable with suitable time-series photometry. Since RRLs obey well-calibrated period-luminosity relations, they are an excellent distance indicator, and are observable throughout the Local Group. Moreover, since RRLs are older than 10 Gyr, they can also provide insight on the properties of the old stellar population in any systems in which they are found. Our team has concentrated on increasing the census of RRLs in UFDs. We detected RRL members for the first time in several UFDs from time-domain studies made from ground-based telescopes (Martínez-Vázquez et al. 2019, 2021b,c) and using the Gaia DR2 RRL catalog (Vivas, Martínez-Vázquez & Walker, 2021). We also identify possible candidate extra-tidal RRLs in some of the UFDs. Furthermore, we made comprehensive and updated analysis of the number of RRLs in dwarf galaxies. This allows us to predict that the method of finding new UFDs by using two or more clumped RRLs will work only for systems brighter than MV ~ —5 mag. Finally, our works not only offer independent and accurate distances to the host but also provide clues about the contribution of UFDs in the formation of the halo of the Milky Way to ascertain how much of the long-period tail of field halo RRL stars can be attributed to disrupted UFDs.