The globular cluster ω centauri: The origin of the mysterious correlation between the RR lyrae luminosity and [Fe/H]

The lack of an observed correlation between the absolute magnitudes of the RR Lyrae stars and their metallicities in the unusual globular cluster ω Cen has been investigated. It is confirmed that the post-zero-age horizontal branch (ZAHB) evolution plays an important role in understanding this problem as speculated by Gratton, Tornambè, & Ortolani. The detailed model calculations suggest, however, that the observed "flat" correlation between the RR Lyrae luminosity and [Fe/H] for stars having [Fe/H] < - 1.4 can be reproduced only if the HB stars in ω Cen obey the nonmonotonic behavior of the HB morphology with decreasing [Fe/H] similar to that observed in the Galactic globular cluster system. This is a direct consequence of the recent HB evolutionary models by Lee, which suggest that the mean luminosity of the RR Lyrae stars within the instability strip depends sensitively on HB morphology as well as metallicity, especially when the HB type is extremely blue. This underscores once more that, when the distances to the Population II objects are to be estimated using the RR Lyrae stars, the HB type of the stellar population, as well as metallicity, must be known.