The Dwarf Spheroidal Companions to M31: Variable Stars in Andromeda II

Abstract

We present the results of a variable star search in Andromeda II, a dwarf spheroidal galaxy companion to M31, using Hubble Space Telescope Wide Field Planetary Camera 2 observations. Seventy-three variables were found, one of which is an anomalous Cepheid while the others are RR Lyrae stars. The anomalous Cepheid has properties consistent with those found in other dwarf spheroidal galaxies. For the RR Lyrae stars, the mean periods are 0.571 and 0.363 days for the fundamental mode and first-overtone mode stars, respectively. With this fundamental mode mean period and the mean metallicity determined from the red giant branch (〈[Fe/H]〉 = -1.49), Andromeda II follows the period-metallicity relation defined by the Galactic globular clusters and other dwarf spheroidal galaxies. We also find that the properties of the RR Lyrae stars themselves indicate a mean abundance that is consistent with that determined from the red giants. There is, however, a significant spread among the RR Lyrae stars in the period-amplitude diagram, which is possibly related to the metallicity spread in Andromeda II indicated by the width of the red giant branch, which was determined by Da Costa et al. In addition, the abundance distribution of the RR Lyrae stars is notably wider than the distribution expected from the abundance determination errors alone. The mean magnitude of the RR Lyrae stars, 〈VRR〉 = 24.87 ± 0.03, implies a distance d = 665 ± 20 kpc to Andromeda II. This matches the distance derived from the mean magnitude of the horizontal branch stars by Da Costa et al., d = 680 ± 20 kpc. We also demonstrate that the specific frequency of anomalous Cepheids in dwarf spheroidal galaxies correlates with the mean metallicity of their parent galaxy and that the Andromeda II and Andromeda II anomalous Cepheids appear to follow the same relation as those in the Galactic dwarf spheroidal galaxies.