Homogeneous analysis of globular clusters from the APOGEE survey with the BACCHUS code - III. ω Cen

We study the multiple populations of omega Cen by using the abundances of Fe, C, N, O, Mg, Al, Si, K, Ca, and Ce from the high-resolution, high signal-to-noise (S/N > 70) spectra of 982 red giant stars observed by the SDSS-IV/APOGEE-2 survey. We find that the shape of the Al-Mg and N-C anticorrelations changes as a function of metallicity, continuous for the metal-poor groups, but bimodal (or unimodal) at high metallicities. There are four Fe populations, similarly to previous literature findings, but we find seven populations based on Fe, Al, and Mg abundances. The evolution of Al in omega Cen is compared to its evolution in the Milky Way and in five representative globular clusters. We find that the distribution of Al in metal-rich stars of omega Cen closely follows what is observed in the Galaxy. Other alpha-elements and C, N, O, and Ce are also compared to the Milky Way, and significantly elevated abundances are observed over what is found in the thick disc for almost all elements. However, we also find some stars with high metallicity and low [Al/Fe], suggesting that omega Cen could be the remnant core of a dwarf galaxy, but the existence of these peculiar stars needs an independent confirmation. We also confirm the increase in the sum of CNO as a function of metallicity previously reported in the literature and find that the [C/N] ratio appears to show opposite correlations between Al-poor and Al-rich stars as a function of metallicity.

Automated summary

The study of omega Centauri reveals multiple populations with different effects of metallicity on the Al-Mg and N-C anticorrelations, and the Fe, Al, and Mg abundances can be divided into seven populations. In comparison to the Milky Way and other globular clusters, some element abundances in omega Centauri are significantly higher, and the presence of peculiar stars might indicate it as the remnant core of a dwarf galaxy. Moreover, the increase in the sum of CNO with metallicity and the opposite correlations of the [C/N] ratio in Al-poor and Al-rich stars are observed.