A Search for RR Lyrae Stars in Segue 2 and Segue 3

TL;DR

This study searches for RR Lyrae stars in the ultra-faint Milky Way companions Segue 2 and Segue 3, discovering a new RR Lyrae in Segue 2 and deriving its distance, while also identifying a candidate binary in Segue 3.

Contribution

First detection of an RR Lyrae star in Segue 2 and robust distance measurement; also reports a candidate binary in Segue 3.

Findings

Discovered a new RRab star in Segue 2 with P_ab = 0.748 days.

Derived the first robust distance to Segue 2 (~37 kpc).

Found a candidate eclipsing binary in Segue 3.

Abstract

We present an extensive search for RR Lyrae stars in and around the ultra-faint Milky Way companions Segue 2 and Segue 3. The former (M_V = -2.5, Belokurov et al. 2009) appears to be an extremely faint dwarf galaxy companion of the Milky Way. The latter (M_V = 0.0, Fadely et al. 2011) is among the faintest star clusters known. We use B and V band time-series imaging obtained at the WIYN 0.9 meter telescope at Kitt Peak National Observatory to search for RR Lyrae in these objects. In our Segue 2 observations, we present a previously unknown fundamental mode (RRab) RR Lyrae 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 RR Lyrae. 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 d_RRL = 36.6 +2.5/-2.4 and 37.7 +/- 2.7 kpc; assuming [Fe/H] = -2.257 dex, we find d_BHB = 34.4 +/- 2.6 kpc. Although no RR Lyrae were present in the Segue 3 field, we found a candidate eclipsing binary star system.