Short-Period Variable Stars in the M31 Halo

We report the findings of a new search for RR Lyrae stars in an M31 halo field located 40' from the nucleus of the galaxy along the minor axis. We detected 37 variable stars, of which 24 are classified as RR Lyrae and the others are ambiguous. Estimating a completeness fraction of ~24%, we calculate that there are approximately 100 RR Lyrae stars in the field, which is consistent with what is expected from deep Hubble Space Telescope (HST) color-magnitude diagrams. We calculate a mean magnitude of g = 25.15 ± 0.03, which we interpret to mean that the mean metallicity of RR Lyrae stars is significantly lower than that of the M31 halo as a whole. The presence of ancient, metal-poor stars opens the possibility that initially the M31 halo appeared much like the Milky Way halo.