RR Lyrae variables in the Andromeda group galaxies

We present the results of an extensive survey of RR Lyrae stars in the companion galaxies (M33, NGC 147, And XI and And XIII) around the Andromeda galaxy (M31). From images taken with the Advanced Camera for Surveys (ACS) Wide Field Channel (WFC) on-board the Hubble Space Telescope (HST) through two passbands (F606W and F814W), we have identified and characterized a total of 119 RR Lyrae variables (96 RRab (RR0) and 23 RRc(RR1)) in M33. Using the properties of 83 RR Lyrae stars (65 RRab and 18 RRc) in the innermost ACS field (hereafter DISK2), we find mean periods of <Pab > = 0.553 +/- 0.008 (error1) +/- 0.05 (error2) and < Pc> = 0.325 +/- 0.008 (error1) +/- 0.05 (error2), where the 'error1' value represents the standard error of the mean and the 'error2' value is based on the error of an individual RRL period calculated from our synthetic light curve simulations. The distribution of RRab periods and the frequency of RRc stars ( Nc=nc /nabc =0.22) strongly suggest that these RR Lyraes follow the general characteristics of those in Oosterhoff type I Galactic globular clusters. The metallicities of 65 individual RRab stars are calculated from the period-amplitude-metallicity relationship, yielding a mean metallicity of <[Fe/H] > = -1.48 +/- 0.05 dex, where the uncertainty is the standard error of the mean. The VI minimum-light colors of the RRab stars are used to calculate a mean line-of-sight reddening toward the DISK2 field of <E(V-I ) >=0.175. By adopting this line-of-sight reddening and using a relation between RR Lyrae luminosity and metallicity (Mv=0.23 Fe/H+0.93), we estimate a mean distance modulus of <( m -M)0 >=24.52 +/- 0.11 for M33, where the error is the quadratic sum of the uncertainties in the absolute and dereddened V magnitudes of the RRLs.;We used both Thuan-Gunn g-band ground-based photometry from the literature and HST Wide Field Planetary Camera 2 (WFPC2) archival data in the F555W and F814W passbands to investigate the pulsation properties of RR Lyrae variable candidates in NGC 147. These datasets represent the two extreme cases often found in RR Lyrae studies with respect to the phase coverage of the observations and the quality of the photometric measurements. Extensive artificial variable star tests for both cases were performed. We conclude that neither dataset is sufficient to confidently determine the pulsation properties of the NGC 147 RR Lyraes. Thus, while we can assert that NGC 147 contains RR Lyrae variables, and therefore a population older than ;We present a comprehensive study of the stellar populations in two faint M31 dwarf satellites, And XI and And XIII. Using deep archival images from the WFPC2 onboard HST, we characterize the horizontal branch (HB) morphologies and the RR Lyrae populations of these two faint dwarf satellites. Our new template light curve fitting routine (RRFIT) detected RR Lyrae populations from both galaxies. The mean periods of RRab (RR0) stars in And XI and And XIII are < Pab >=0.621 +/- 0.026 (error1) +/- 0.022 (error2), and <Pab>=0.648 +/- 0.026 (error1) +/- 0.022 (error2) respectively. The "error1" represents the standard error of the mean, while "error2" is the error subject to the individual RRab periods estimated from our synthetic light curve simulation. Even though the RRL populations show a lack of RRab stars with high amplitudes (Amp(V) > 1.0 mag) and relatively short periods (Pab ∼0.5 days), their period -- V band amplitude (P-Amp(V)) relations track the lower part of the general P-Amp(V) trend in the M31 outer halo RRL populations. The metallicities of RRab stars were calculated via the [Fe/H] - logPab-Amp(V) relationship of Alcock et al. The metallicities thus obtained ([Fe/H]AndXI =-1.75; [Fe/H]AndXIII = -1.74 ) are consistent with the values calculated from the RGB slope indicating that our measurements are not significantly affected by the evolutionary effects of RRL stars. The distance to each galaxy was calculated using the absolute V magnitudes of the RRab stars. We obtained (m-M)0,v =24.54 for And XI and this value becomes (m-M)0,v =24.71 for And XIII. We discuss the origins of And XI and And XIII based on a comparative analysis of the luminosity-metallicity (L-M) relation of Local Group dwarf galaxies.