On the Production of Bright RR Lyrae Variables in Metal-Rich Globular Clusters

Recent HST observations of the metal-rich bulge clusters NGC 6388 and NGC 6441 by Rich et al. (1997) have found that the horizontal branches (HBs) in these clusters slope upward with decreasing B-V. Such an upward slope in the HB morphology is not predicted by canonical HB models. Moreover, it cannot be produced by either a greater cluster age or enhanced mass loss along the red-giant branch (RGB). The peculiar HB morphology in these clusters may provide an important clue for understanding the second-parameter effect. We have carried out extensive evolutionary calculations and numerical simulations in order to understand the cause of the sloped HBs in NGC 6388 and NGC 6441. Three scenarios have been investigated: i) A high cluster helium abundance scenario, where the HB morphology is determined by long blue loops; ii) A rotation scenario, where the core mass in the HB models is increased by internal rotation during the HB phase; iii) A helium-mixing scenario, where deep mixing on the RGB enhances the envelope helium abundance. All three of these scenarios predict sloped HBs with anomalously bright RR Lyrae variables. We compare this prediction with the properties of the two known RR Lyrae variables in NGC 6388 as well as with the properties of the metal-rich field RR Lyrae variables and V9 in 47 Tuc.