Chemical Compositions of Field and Globular Cluster RR Lyrae Stars. II. ω Centauri

We present a detailed spectroscopic analysis of RR Lyrae (RRL) variables in the globular cluster NGC 5139 (omega Cen). We collected optical (4580-5330 angstrom), high-resolution (R similar to 34,000), high signal-to-noise ratio (similar to 200) spectra for 113 RRLs with the multifiber spectrograph M2FS at the Magellan/Clay Telescope at Las Campanas Observatory. We also analyzed high-resolution (R similar to 26,000) spectra for 122 RRLs collected with FLAMES/GIRAFFE at the Very Large Telescope, available in the ESO archive. The current sample doubles the literature abundances of cluster and field RRLs in the Milky Way based on high-resolution spectra. Equivalent-width measurements were used to estimate atmospheric parameters, iron, and abundance ratios for alpha (Mg, Ca, Ti), iron peak (Sc, Cr, Ni, Zn), and s-process (Y) elements. We confirm that omega Cen is a complex cluster, characterized by a large spread in the iron content: -2.58 <= [Fe/H] <= -0.85. We estimated the average cluster abundance as <[Fe/H]> = -1.80 +/- 0.03, with sigma = 0.33 dex. Our findings also suggest that two different RRL populations coexist in the cluster. The former is more metal-poor ([Fe/H] less than or similar to - 1.5), with almost solar abundance of Y. The latter is less numerous, more metal-rich, and yttrium enhanced ([Y/Fe] greater than or similar to 0.4). This peculiar bimodal enrichment only shows up in the s-process element, and it is not observed among lighter elements, whose [X/Fe] ratios are typical for Galactic globular clusters.