2016MNRAS.455.2163O

We have probed the pulsating variable star content of the isolated Local Group dwarf galaxy, DDO210 (Aquarius), using archival Advanced Camera for Surveys/Hubble Space Telescope imaging in the F475W and F814W passbands. We find a total of 32 RR Lyrae stars (24 ab-type; 8 c-type) and 75 Cepheid variables. The mean periods of the ab-type and c-type RR Lyrae stars are calculated to be <Pab> = 0.609±0.011 and <Pc> = 0.359±0.025 d, respectively. The light-curve properties of the fundamental mode RR Lyrae stars yield a mean metallicity of <[Fe/H]> = -1.63±0.11dex for this ancient population, consistent with a recent synthetic colour-magnitude diagram analysis. We find this galaxy to be Oosterhoff-intermediate and lacking in high-amplitude, short-period ab-type RR Lyrae, consistent with behaviour recently observed for many dwarf spheroidals and ultrafaint dwarfs in the Local Group. We find a distance modulus of µ = 25.07±0.12 as determined by the RR Lyrae stars, slightly larger but agreeing with recent distance estimates from the red giant branch tip. We also find a sizable population of Cepheid variables in this galaxy. We provide evidence in favour of most if not all of these stars being short-period classical Cepheids. Assuming all of these stars to be classical Cepheids, we find that most of these Cepheids are ∼ 300 Myr old, with the youngest Cepheids being offset from the older Cepheids and the centre of the galaxy. We conclude that this may have resulted from a migration of star formation in DDO210.