# Chemical Compositions of Field and Globular Cluster RR~Lyrae Stars: II. omega Centauri

## TL;DR

This study provides a detailed spectroscopic analysis of RR Lyrae stars in omega Centauri, revealing a complex metallicity distribution and bimodal s-process element enrichment, enhancing understanding of the cluster's chemical evolution.

## Contribution

It offers the largest high-resolution spectroscopic dataset of RR Lyrae stars in omega Centauri, identifying two distinct populations with different metallicities and s-process element enhancements.

## Key findings

- Omega Centauri has a wide [Fe/H] range from -2.58 to -0.85.
- Two RR Lyrae populations coexist with distinct metallicities.
- A bimodal s-process element enrichment pattern is observed.

## Abstract

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.