Short-Period Variable Stars in the M31 Halo

Abstract

We report the findings of a new search for RR Lyraes in an M31 halo field located 40 arcminutes from the nucleus of the galaxy along the minor axis. We detected 37 variable stars, of which 24 are classified as RR Lyraes and the others are ambiguous. Estimating a completeness fraction of ~24%, we calculate that there are approximately 100 RR Lyraes in the field, which is consistent with what is expected from deep 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 Lyraes 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.