The ISLAnds Project III: Variable Stars in Six Andromeda Dwarf Spheroidal Galaxies

TL;DR

This study catalogs variable stars in six Andromeda dwarf spheroidal galaxies using Hubble data, deriving distances and comparing pulsational properties with Milky Way satellites, revealing similar ancient stellar populations.

Contribution

It provides the first homogeneous census of variable stars in these galaxies and compares their properties with Milky Way satellites, highlighting similarities in early stellar evolution.

Findings

870 RR Lyrae stars detected across six galaxies.

Distances derived with systematic uncertainties of 0.08 mag.

Old RR Lyrae populations show no significant differences between Andromeda and Milky Way satellites.

Abstract

We present a census of variable stars in six M31 dwarf spheroidal satellites observed with the Hubble Space Telescope. We detect 870 RR Lyrae (RRL) stars in the fields of And I (296), II (251), III (111), XV (117), XVI (8), XXVIII (87). We also detect a total of 15 Anomalous Cepheids, three Eclipsing Binaries, and seven field RRL stars compatible with being members of the M31 halo or the Giant Stellar Stream. We derive robust and homogeneous distances to the six galaxies using different methods based on the properties of the RRL stars. Working with the up-to-date set of Period-Wesenheit ($I$, $B$ - $I$) relations published by Marconi et al., we obtain distance moduli of $\mu_0$ = [24.49, 24.16, 24.36, 24.42, 23.70, 24.43] mag (respectively), with systematic uncertainties of 0.08 mag and statistical uncertainties $<$ 0.11 mag. We have considered an enlarged sample of sixteen M31 satellites with published variability studies, and compared their pulsational observables (e.g., periods, amplitudes), with those of fifteen Milky Way satellites for which similar data are available. The properties of the (strictly old) RRL in both satellite systems do not show any significant difference. In particular, we found a strikingly similar correlation between the mean period distribution of the fundamental RRL pulsators (RRab) and the mean metallicities of the galaxies. This indicates that the old RRL progenitors were similar at the early stage in the two environments, suggesting very similar characteristics for the earliest stages of evolution of both satellite systems.