Deep Hubble Space Telescope Imaging of IC 1613. I. Variable Stars and Distance

Abstract

We present WFPC2 V I photometry of a Ðeld in the halo of IC 1613, Ðnding 13 RR Lyrae stars and 11 Cepheids. Our photometry of the red giant branch tip and red clump is used to derive distances to IC 1613, which are consistent with each other and with distances based on the variable stars. We compare these values with similarly measured distances for the Magellanic Clouds, and are able to measure metallicity dependencies of the RR Lyrae and Cepheid distances by requiring consistent relative distance measurements from the four techniques. For metallicities of [Fe/H] \ [1.3 (RR Lyrae stars) and [1.0 (Cepheids), we Ðnd a relatively steep slope of 0.34 ^ 0.20 mag per dex for the RR Lyrae stars and a shallow slope of [0.07 ^ 0.16 mag per dex for the Cepheids, both values within the range of theoretical and empirical results in the literature. We Ðnd that a dependence of the red clump absolute magnitude on age, in addition to metallicity, is required to produce self-consistent relative distances between IC 1613 and the Magellanic Clouds. Adopting such a red clump calibration and self-consistent calibrations for the other three distance indicators, we Ðnd that the distances to all three objects are in excellent agreement. Our best distance modulus to IC 1613 is corresponding to a distance of k 0 \ 24.31 ^ 0.06, 730 ^ 20 kpc. This distance produces an RR Lyrae absolute magnitude of 0.61 ^ 0.08. Subject headings : Cepheids È galaxies : distances and redshifts È galaxies : individual (IC 1613) È Local Group È stars : variables : other 1. INTRODUCTION Studies of the stellar populations in nearby galaxies provide a powerful tool for determining the key physical parameters of galaxy evolution, such as the age (star forma-tion history), the chemical composition and enrichment history, the stellar initial mass function, environmental e ects, and the dynamical history of the system. Using the Hubble Space T elescope (HST), it is possible to photometer individual stars down to very faint magnitudes, and to interpret the observable parameters such as the morphol-ogy of the color-magnitude diagram (CMD). This approach is a logical stepping stone to understanding galaxy evolu-tion and provides a physical basis for understanding obser-vations of high-redshift galaxies and their implications for cosmological models. Detailed analysis of the intermediate