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 ] _ { 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 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 .