The ACS LCID Project: RR Lyrae Stars as Tracers of Old Population Gradients in the Isolated Dwarf Spheroidal Galaxy Tucana

Abstract

We present a study of the radial distribution of RR Lyrae variables,which present a range of photometric and pulsational properties, in thedwarf spheroidal galaxy Tucana. We find that the fainter RR Lyrae stars,having a shorter period, are more centrally concentrated than the moreluminous, longer period RR Lyrae variables. Through comparison with thepredictions of theoretical models of stellar evolution and stellarpulsation, we interpret the fainter RR Lyrae stars as a more metal-richsubsample. In addition, we show that they must be older than about 10Gyr. Therefore, the metallicity gradient must have appeared very earlyon in the history of this galaxy.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS5-26555. These observations are associated with program10505.