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B.J. Pritzl, H.A. Smith (Michigan State University), M. Catelan, A.V. Sweigart (NASA/GSFC)
The horizontal branches (HB's) in the metal-rich globular clusters NGC 6388 and NGC 6441 show prominent blue extensions not seen in other metal-rich globular clusters such as 47 Tuc (Rich et al. 1997, ApJ, 484, L25). Most surprisingly, the HB's in these clusters slope upward with decreasing B-V from the red clump to the top of the blue tail. Sweigart & Catelan (1998, ApJ, 501, L63) demonstrated that the upward slope of the HB's cannot be explained using the standard canonical models for HB morphology or by either greater cluster age or enhanced mass loss along the red giant branch. Therefore, the unusual morphology of these HB's may provide important clues in understanding the origin of the second parameter effect. To explain the HB morphology in NGC 6388 and NGC 6441, Sweigart & Catelan have proposed several noncanonical scenarios. To test these scenarios, we have obtained new photometry of the RR Lyrae stars in these two clusters to determine whether, as predicted, the HB's of these clusters are unusually bright. We present the results of these observations, including new periods and light curves, and discuss their implications for the noncanonical models.
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