A search for RR Lyrae stars in segue 2 and segue 3

Abstract

We present an extensive search for RR Lyrae (RRL) stars in and around the ultra-faint Milky Way companions Segue 2 and Segue 3. The former (MV = -2.5) appears to be an extremely faint dwarf galaxy companion of the Milky Way. The latter (MV = 0.0) is among the faintest star clusters known. We use B and V band time-series imaging obtained at the WIYN 0.9 m telescope at Kitt Peak National Observatory to search for RRL in these objects. In our Segue 2 observations, we present a previously unknown fundamental mode (RRab) RRL star with a period of P ab = 0.748 days. With this measurement, we revisit the inverse correlation between 〈P ab〉 and 〈[Fe/H]〉 established in the literature for Milky Way dwarf galaxies and their RRL. In this context, the long period of Segue 2's RRab star as well as the known significant spread in metallicity in this dwarf galaxy are consistent with the observed trend in 〈P ab〉 and 〈[Fe/H]〉. We derive the first robust distance to Segue 2, using both its RRab star and spectroscopically confirmed blue horizontal branch stars. Using [Fe/H] = -2.16 and -2.44 dex, we find and kpc; assuming [Fe/H] = -2.257 dex, we find dBHB = 34.4 ± 2.6 kpc. Although no RRL were present in the Segue 3 field, we found a candidate eclipsing binary star system. © 2013. The American Astronomical Society. All rights reserved.