Chemical Compositions of Field and Globular Cluster RR~Lyrae Stars: II.
  omega 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 A),
high resolution (R = 34,000), high signal-to-noise ratio (200) spectra for 113
RRLs with the multi-fiber spectrograph M2FS at the Magellan/Clay Telescope at
Las Campanas Observatory. We also analysed high resolution (R = 26,000) spectra
for 122 RRLs collected with FLAMES/GIRAFFE at the VLT, 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, characterised 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] < -1.5), with almost solar
abundance of Y. The latter is less numerous, more metal-rich, and yttrium
enhanced ([Y/Fe] > 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.
