A New Culprit in the Second-Parameter Problem in the Sculptor Dwarf Spheroidal Galaxy?

Color-magnitude diagrams from deep, wide-field CCD photometry of the Sculptor dwarf spheroidal galaxy reveal that the red horizontal branch (RHB) stars are strongly concentrated toward the center of the galaxy relative to the dominant old population in Sculptor, confirming an earlier claim of such a gradient. Since we find no radial gradients of the age or metallicity distribution within Sculptor, neither age nor metallicity can individually account for the internal "second-parameter" problem observed within the galaxy. We have also identified an unusual "spur" of stars that extends from the main-sequence turnoff region and is located between the canonical blue straggler region and the subgiant branch. These stars are also more centrally concentrated than the oldest stars in Sculptor, although not as extremely as the RHB stars. Unable to convincingly interpret the spur either as an unusual young population or as a foreground population of stars, we conclude that binary stars offer the most reasonable explanation for the origin of the spur that is also consistent with other features in the CMD of Sculptor. We infer that 30%-60% of all apparently single stars in the inner region of Sculptor may be binaries. We speculate that the possible radial gradient in the binary-star population may be related to the variation of the HB morphology in Sculptor.