RR Lyrae Stars in the Andromeda Halo from Deep Imaging with the Advanced Camera for Surveys

We present a complete census of RR Lyrae stars in a halo field of theAndromeda galaxy. These deep observations, taken as part of a program tomeasure the star formation history in the halo, spanned a period of 41 dayswith sampling on a variety of time scales, enabling the identification of shortand long period variables. Although the long period variables cannot be fullycharacterized within the time span of this program, the enormous advance insensitivity provided by the Advanced Camera for Surveys on the Hubble SpaceTelescope allows accurate characterization of the RR Lyrae population in thisfield. We find 29 RRab stars with a mean period of 0.594 days, 25 RRc starswith a mean period of 0.316 days, and 1 RRd star with a fundamental period of0.473 days and a first overtone period of 0.353 days. These 55 RR Lyrae starsimply a specific frequency S_RR=5.6, which is large given the high meanmetallicity of the halo, but not surprising given that these stars arise fromthe old, metal-poor tail of the distribution. This old population in theAndromeda halo cannot be clearly placed into one of the Oosterhoff types: theratio of RRc/RRabc stars is within the range seen in Oosterhoff II globularclusters, the mean RRab period is in the gap between Oosterhoff types, and themean RRc period is in the range seen in Oosterhoff I globular clusters. Theperiods of these RR Lyraes suggest a mean metallicity of [Fe/H]=-1.6, whiletheir brightness implies a distance modulus to Andromeda of 24.5+/-0.1, in goodagreement with the Cepheid distance.Comment: 15 pages, latex. Accepted for publication in The Astronomical Journa