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

Abstract

We study the multiple populations of ω 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 ω 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 ω Cen closely follows what is observed in the Galaxy. Other α-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 ω 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.

Chemical Cartography with APOGEE: Mapping Disk Populations with a 2-process Model and Residual Abundances

“…Stars observed by APOGEE in the LMC that overlap our disk star metallicity, g log , and T eff range show a similar abundance pattern to the GSE stars, and a 0.2 dex enhancement of Ce. The 14 ω Cen members that fall in our disk sample show extreme (∼1 dex) enhancements of C+N and Ce and large (∼0.4 dex) enhancements of Na and Al, similar to the unusual abundance ratios found in more comprehensive studies of this cluster (Smith et al 2000;Johnson & Pilachowski 2010;Mészáros et al 2021). Stars in the outer disk (R = 15-17 kpc), either near the midplane (|Z| 2 kpc) or well above it (Z = 2-6 kpc), have abundances entirely consistent with those of our R = 3-13 kpc sample, with the slight exception of a 0.05 dex median depression of Ce in the high-Z population.…”

“…This star is a member of ω Cen, a globular cluster that is often hypothesized to be the stripped core of a dwarf galaxy because of its large internal [Fe/H] spread (e.g., Smith et al 2000). The well-established and distinctive pattern of enhanced N, Na, Al, and s-process elements is thought to be a signature of self-enrichment by the cluster's evolved AGB stars (Smith et al 2000;Johnson & Pilachowski 2010;Mészáros et al 2020Mészáros et al , 2021. A significant number of the most extreme χ 2 stars in our sample are members of ω Cen, and we discuss the abundance pattern of these stars further in Section 7.…”

“…Like the other ω Cen members, these three stars all have extremely elevated (0.7-1.1 dex) C+N, Na, Al, and Ce; positive Ca residuals (0.1-0.2 dex); and negative Ni residuals (0.05-0.25 dex). Mészáros et al (2021) present an APOGEE analysis of a much larger sample (982 stars) of ω Cen members, identifying multiple subpopulations in the (Fe, Al, Mg) distribution and examining abundance ratio trends in detail (see also Johnson & Pilachowski 2010).…”

Chemical Cartography with APOGEE: Mapping Disk Populations with a Two-Process Model and Residual Abundances

“…This star is a member of ω Cen, a globular cluster that is often hypothesized to be the stripped core of a dwarf galaxy because of its large internal [Fe/H] spread (e.g., Smith et al 2000). The well established and distinctive pattern of enhanced N, Na, Al, and s-process elements is thought to be a signature of self-enrichment by the cluster's evolved AGB stars (Smith et al 2000;Johnson & Pilachowski 2010;Mészáros et al 2020Mészáros et al , 2021. A significant number of the most extreme χ 2 stars in our sample are members of ω Cen, and we discuss the abundance pattern of these stars further in §7.…”

“…Like the other ω Cen members, these three stars all have extremely elevated (0.7-1.1 dex) C+N, Na, Al, and Ce, positive Ca residuals (0.1-0.2 dex) and negative Ni residuals (0.05-0.25 dex). Mészáros et al (2021) present an APOGEE analysis of a much larger sample (982 stars) of ω Cen members, identifying multiple sub-populations in the (Fe,Al,Mg) distribution and examining abundance ratio trends in detail (see also Johnson & Pilachowski 2010).…”