New Metallicities of Rr Lyrae Stars in Ω Centauri: Evidence for a Non He-enhanced Metal-intermediate Population

We present new spectroscopic metal abundances for 74 RR Lyrae stars in ω Cen obtained with FLAMES. The well-known metallicity spread is visible among the RR Lyrae variables. The metal-intermediate (MInt) RR Lyrae stars ([Fe/H] ∼ −1.2) are fainter than the bulk of the dominant metal-poor population ([Fe/H] ∼ −1.7), in good agreement with the corresponding zero-age horizontal branch models with cosmological helium abundance Y = 0.246. This result conflicts with the hypothesis that the progenitors of the MInt RR Lyrae stars correspond to the anomalous blue main-sequence stars, which share a similar metallicity but whose properties are currently explained by assuming for them a large helium enhancement. Therefore, in this scenario, the coexistence within the cluster of two different populations with similar metallicities ([Fe/H] ∼ −1.2) and different helium abundances has to be considered. Subject headings: techniques: spectroscopic — stars: abundances — stars: Population II — stars: variables: RR Lyrae — globular clusters: ω Cen