An Internal Second-Parameter Problem in the Sculptor Dwarf Spheroidal Galaxy

We present BV photometry of the Sculptor dwarf galaxy to V=22. These datagive evidence for a bimodality in Sculptor's metallicity distribution based ona discontinuity in the luminosities of horizontal branch (HB) stars and by thepresence of two distinct red giant branch (RGB) bumps. A consistent picture ofthe evolved stars in Sculptor is given by the presence of (1) a metal-poorpopulation of [Fe/H] ~ -2.3 with an exclusively blue HB and that corresponds tothe blueward side of the Sculptor RGB and the more luminous RGB bump, and (2) aless metal-poor population of [Fe/H] ~ -1.5 required to explain the lessluminous red HB, the red side of the RGB, and a second, less luminous RGB bump.Best fits to the HB populations are obtained with enhanced oxygen abundances,[O/Fe] ~ +0.5. Variations in the global HB and RGB morphology of Sculptor canbe explained by differences in the radial distribution of these twopopulations. The presence of these two populations shows that the Sculptordwarf galaxy has an internal second parameter problem.Comment: 15 pages, 4 figures, LaTeX, uses aastex. Accepted by ApJ Letter