The Mg-K anti-correlation in Omega Centauri
  We present [K/Fe] abundance ratios for a sample of 450 stars in Omega
Centauri, using high resolution spectra acquired with the multi-object
spectrograph FLAMES@VLT. Abundances for Fe, Na and Mg were also derived. We
detected intrinsic K variations in the analysed stars. Moreover, [K/Fe] shows a
significant correlation with [Na/Fe] and anti-correlation with [Mg/Fe]. The
presence of a clear-cut Mg-K anti-correlation makes Omega Centauri the third
stellar system, after NGC 2419 and NGC 2808, hosting a sub-population of stars
with [Mg/Fe]<0.0 dex, K-enriched in the case of Omega Centauri by ~0.3 dex with
respect to the Mg-rich stars ([Mg/Fe]>0.0 dex). The
correlation/anti-correlation between K and other light elements involved in
chemical anomalies supports the idea that the spread in [K/Fe] can be
associated to the same self-enrichment process typical of globular clusters. We
suggest that significant variations in K abundances perhaps can be found in the
most massive and/or metal-poor globular clusters as manifestation of an extreme
self-enrichment process. Theoretical models face problems to explain the K
production in globular clusters. Indeed, models where asymptotic giant branch
stars are responsible for the Mg-K anti-correlation only qualitatively agree
with the observations. Finally, we discovered a peculiar star with an
extraordinary K overabundance ([K/Fe]=+1.60 dex) with respect to the other
stars with similar [Mg/Fe]. We suggest that this K-rich star could be formed
from the pure ejecta of AGB stars before dilution with pristine material.
