The Variable-Star Population in Phoenix: Coexistence of Anomalous and Short-Period Classical Cepheids and Detection of RR Lyrae Variables

We present the results of a search for variable stars in the Local Groupdwarf galaxy Phoenix. Nineteen Cepheids, six candidate long-period variables,one candidate eclipsing binary and a large number of candidate RR Lyrae starshave been identified. Periods and light curves have been obtained for all theCepheid variables. Their distribution in the period-luminosity diagram revealsthat both Anomalous Cepheids (AC) and short-period Classical Cepheids s-pCC arefound in our sample. This is the first time that both types of variable starare identified in the same system even though they likely coexist, but havegone unnoticed so far, in other low-metallicity galaxies like Leo A and SextansA. We argue that the conditions for the existence of both types of variablestar in the same galaxy are a low metallicity at all ages, and the presence ofboth young and intermediate-age (or old, depending on the nature of AC) stars.The RR Lyrae candidates trace, together with the well developed horizontalbranch, the existence of an important old population in Phoenix. The differentspatial distributions of s-pCC, AC and RR Lyrae variables in the Phoenix fieldare consistent with the stellar population gradients found in Phoenix, in thesense that the younger population is concentrated in the central part of thegalaxy. The gradients in the distribution of the young population within thecentral part of Phoenix, which seem to indicate a propagation of the recentstar formation, are also reflected in the spatial distribution of the s-pCC.Comment: 44 pages, 11 figures, 5 tables. Astronomical Journal, in press, March 2004 issu