CCD photometry of the globular cluster ω centauri. I. Metallicity of RR Lyrae stars from Caby photometry

We present new measurements of the metallicity of 131 RR Lyrae stars in the globular cluster ω Centauri, using the hk index of the Caby photometric system. The hk method has distinct advantages over ΔS and other techniques in determining the metallicity of RR Lyrae stars and has allowed us to obtain the most complete and homogeneous metallicity data to date for the RR Lyrae stars in this cluster. For RR Lyrae stars in common with the ΔS observations of Butler, Dickens, & Epps and Gratton, Tornambe, & Ortolani, we have found that our metallicities, [Fe/H]_hk, deviate systematically from their AS metallicity, while our [Fe/H]_hk for well-observed, field RRab stars are consistent with previous spectroscopic measurements. We conclude that this is because of the larger errors associated with the previous ΔS observations for this cluster. The M_V(RR)-[Fe/H] and period shift-[Fe/H] relations obtained from our new data are consistent with the evolutionary models predicted by Y.-W. Lee, confirming that the luminosity of RR Lyrae stars depends on evolutionary status as well as metallicity. Using the period-amplitude diagram, we have also identified highly evolved RRab stars in the range -1.9 ≤ [Fe/H] < - 1.5, as predicted from the synthetic horizontal-branch models.