The globular cluster Omega Centauri - The origin of the mysterious correlation between the RR Lyrae luminosity and Fe/H

The observed correlation between the RR Lyrae luminosity and Fe/H for all Omega Cen RR Lyrae stars for which Fe/H has been determined by the delta-S method is considered, and emphasis is placed on the lack of luminosity variation among the variables in Omega Cen. Model calculations are shown to suggest that the observed 'flat' correlation between the RR Lyrae luminosity and Fe/H for stars having Fe/H forbidden line less than -1.4 can be reproduced if the HB stars in Omega 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 considered to be a natural consequence of recent HB evolutionary models suggesting that the RR Lyrae mean luminosity depends on HB morphology as well as metallicity.