HST/WFPC2 imaging of the dwarf satellites And XI and And XIII: horizontal branch morphology and RR Lyraes

We present a study of the stellar populations in two faint M31 dwarf satellites, Andromeda XI and Andromeda XIII. Using archival images from the Wide Field Planetary Camera 2 onboard the Hubble Space Telescope, we characterize the horizontal branch (HB) morphologies and the RR Lyrae (RRL) populations of these two faint dwarf satellites. Our new template light-curve fitting routine [Robust RR Lyrae light curve FITing (RRFIT)] has been used to detect and characterize RRL populations in 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 0.648 +/- 0.026 (error1) +/- 0.022 (error2), respectively, where error1 represents the standard error of the mean, while error2 is based on our synthetic light-curve simulations. The RRL populations in these galaxies show a lack of RRab stars with high amplitudes [Amp (V) > 1.0 mag] and relatively short periods (Pab similar to 0.5 d), yet their period-V-band amplitude [P-Amp (V)] relations track the relation defined by the M31 field halo RRL populations at similar to 11 kpc from the centre of M31. The metallicities of the RRab stars are calculated via a relationship between [Fe/H], log Pab and Amp(V). The resultant abundances ([Fe/H]And XI=-1.75; [Fe/H]And XIII=-1.74) are consistent with the values calculated from the red giant branch slope, indicating that our measurements are not significantly affected by RRL evolutionary away from the zero-age HB. The distance to each galaxy, based on the absolute V magnitudes of the RRab stars, is (m-M)0, V= 24.33 +/- 0.05 for And XI and (m-M)0, V= 24.62 +/- 0.05 for And XIII. We discuss the origins of And XI and And XIII based on a comparative analysis of the luminositymetallicity relation of Local Group dwarf galaxies.