The Mg-K Anticorrelation in ω Centauri

We present [K/Fe] abundance ratios for a sample of 450 stars in ω Centauri, using high-resolution spectra acquired with the multiobject spectrograph FLAMES@VLT. Abundances for Fe, Na, and Mg were also derived. We detected intrinsic K variations in the analyzed stars. Moreover, [K/Fe] shows a significant correlation with [Na/Fe] and an anticorrelation with [Mg/Fe]. The presence of a clear-cut Mg-K anticorrelation makes ω Centauri the third stellar system, after NGC 2419 and NGC 2808, hosting a subpopulation of stars with [Mg/Fe] < 0.0 dex, K-enriched in the case of ω Centauri by 1/40.3 dex with respect to Mg-rich stars ([Mg/Fe] > 0.0 dex). The correlation/anticorrelation between K and other light elements involved in chemical anomalies supports the idea that the spread in [K/Fe] can be associated with 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 a manifestation of an extreme self-enrichment process. Theoretical models face problems explaining K production in globular clusters. Indeed, models where asymptotic giant branch stars are responsible for the Mg-K anticorrelation 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.