2012MNRAS.419.1362Y

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 <P_ab>= 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 (P_ab ∼ 0.5d), yet their period-V-band amplitude [P- Amp (V)] relations track the relation defined by the M31 field halo RRL populations at ∼ 11 kpc from the centre of M31. The metallicities of the RRab stars are calculated via a relationship between [Fe/H], log P_ab 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 luminosity-metallicity relation of Local Group dwarf galaxies.

Based on observations taken with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute.