The Second-Parameter Effect in Metal-rich Globular Clusters

Recent Hubble Space Telescope observations have found that the horizontalbranches (HB's) in the metal-rich globular clusters NGC 6388 and NGC 6441 slopeupward with decreasing B-V. Such a slope is not predicted by canonical HBmodels and cannot be produced by either a greater cluster age or enhanced massloss along the red-giant branch (RGB). The peculiar HB morphology in theseclusters may provide an important clue for understanding the second-parametereffect. We have carried out extensive evolutionary calculations and numericalsimulations in order to explore three non-canonical scenarios for explainingthe sloped HB's in NGC 6388 and NGC 6441: i) A high cluster helium abundancescenario, where the HB evolution is characterized by long blue loops; ii) Arotation scenario, where internal rotation during the RGB phase increases theHB core mass; iii) A helium-mixing scenario, where deep mixing on the RGBenhances the envelope helium abundance. All three of these scenarios predictsloped HB's with anomalously bright RR Lyrae variables. We compare thisprediction with the properties of the two known RR Lyrae variables in NGC 6388.Other possible observational tests are suggested.Comment: 12 pages, including three embedded figures. ApJ (Letters), in pres