Journal Article

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

S.-C. Yang and A. Sarajedini

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 419, issue 2, pages 1362-1375
Published in print January 2012 | ISSN: 0035-8711
Published online December 2011 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2011.19792.x
HST/WFPC2 imaging of the dwarf satellites And XI and And XIII: horizontal branch morphology and RR Lyraes

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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 ( d), 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 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 (mM)0, V= 24.33 ± 0.05 for And XI and (mM)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.

Keywords: stars: variables: RR Lyrae; Local Group

Journal Article.  9837 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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