HST/WFPC2 Imaging of the Dwarf Satellites And XI and And XIII : HB
  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 (WFPC2) onboard the Hubble Space Telescope (HST),
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 (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 $\pm$ 0.026 (error1) $\pm$ 0.022 (error2),
and $<P_{ab}>$=0.648 $\pm$ 0.026 (error1) $\pm$ 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 ($P_{ab}$ $\sim$ 0.5 days), yet their period -- V
band amplitude (P-Amp(V)) relations track the relation defined by the M31 field
halo RRL populations at $\sim$ 11 kpc from the center 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
RGB slope indicating that our measurements are not significantly affected by
RRL evolutionary away from the zero age horizontal branch. The distance to each
galaxy, based on the absolute V magnitudes of the RRab stars, is
$(m-M)_{0,V}$=24.33 $\pm$ 0.05 for And XI and $(m-M)_{0,V}$=24.62 $\pm$ 0.05
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.
