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 Sculpt or dwarf spheroidal galaxy reveal that the red horizontal branch (RHB) star s 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 dist ribution within Sculptor neither age nor metallicity can individually accou nt for the internal "second-parameter" problem observed within the galaxy. We have also identified an unusual "spur" of stars that extends from the ma in-sequence turnoff region and is located between the canonical blue stragg ler region and the subgiant branch. These stars are also more centrally con centrated than the oldest stars in Sculptor, although not as extremely as t he RHB stars. Unable to convincingly interpret the spur either as an unusua l young population or as a foreground population of stars, we conclude that binary stars offer the most reasonable explanation for the origin of the s pur 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 Sc ulptor may be binaries. We speculate that the possible radial gradient in t he binary-star population may be related to the variation of the HB morphol ogy in Sculptor.