We present a detailed spectroscopic analysis of RR Lyrae ( RRL ) variables in the globular cluster NGC 5139 ( \omega Cen ) .
We collected optical ( 4580–5330 Å ) , high resolution ( R \sim 34,000 ) , high signal-to-noise ratio ( \sim 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 \sim 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 \leq [ Fe/H ] \leq - 0.85 .
We estimated the average cluster abundance as \langle [ Fe/H ] \rangle = - 1.80 \pm 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 ] \lesssim - 1.5 ) , with almost solar abundance of Y .
The latter is less numerous , more metal-rich , and yttrium enhanced ( [ Y/Fe ] \gtrsim 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 .