The ACS LCID project: Variable stars as tracers of population gradients

Abstract

We present a few highlights concerning the search for short-period variable stars in four galaxies, namely IC 1613, LGS3, Cetus and Tucana, based on very deep, multi-epoch HST/ACS photometry. These are discussed in the context of the star formation histories obtained from our very deep color-magnitude diagrams. In particular, we show how the pulsational properties of the RR Lyrae stars, which represent the vast majority of the observed variables, can trace subtle differences in the age and metallicity of the old population. For example, in the dwarf spheroidal galaxy Tucana we find that the fainter RR Lyrae stars, having a shorter period, are more centrally concentrated than the more luminous, longer period RR Lyrae variables. Through comparison with the predictions of theoretical models of stellar evolution and stellar pulsation, we interpret the fainter RR Lyrae stars as a more metal-rich subsample. In addition, we show that they must be older than about 10 Gyr, indicating that the metallicity gradient must have appeared very early on in the history of this galaxy. We also compare the populations of Cepheids in the galaxies of our sample based on their period-Wesenheit diagram. We tentatively classify them as classical short-period Cepheids in the two gas-rich galaxies (IC 1613 & LGS3), and as anomalous Cepheids in the dwarf spheroidals.