Discovery of a large population of Nitrogen-Enhanced stars in the Magellanic Clouds

TL;DR

This paper reports the discovery of a large population of nitrogen-enhanced giant stars in the Magellanic Clouds, resembling globular cluster populations, suggesting ongoing formation of multiple stellar populations at lower redshifts.

Contribution

It presents the first identification of nitrogen-enhanced stars in the Magellanic Clouds, linking them to disrupted globular clusters and expanding understanding of stellar population evolution.

Findings

44 nitrogen-enhanced stars identified in the Magellanic Clouds

Presence of stars with [N/Fe] > +2.45 indicating extreme nitrogen enrichment

Evidence supporting the tidal disruption of globular clusters as a source of these stars

Abstract

We report the APOGEE-2S$+$ discovery of a unique collection of nitrogen-enhanced mildly metal-poor giant stars, peaking at [Fe/H]$\sim -0.89$ with no carbon enrichment, toward the Small and Large Magellanic Clouds (MCs), with abundances of light- (C, N), odd-Z (Al, K) and $\alpha-$elements (O, Mg, Si) that are typically found in Galactic globular clusters (GCs). Here we present 44 stars in the MCs that exhibit significantly enhanced [N/Fe] abundance ratios, well above ([N/Fe]$>+0.6$) typical Galactic levels at similar metallicity, and a star that is very nitrogen-enhanced ([N/Fe]$> +2.45$). Our sample consists of luminous evolved stars on the asymptotic giant branch (AGB), eight of which are classified as bonafide semi-regular (SR) variables, as well as low-luminosity stars similar to that of stars on the tip of the red giant branch of stellar clusters in the MCs. It seems likely that whatever nucleosynthetic process is responsible for these anomalous MC stars it is similar to that which caused the common stellar populations in GCs. We interpret these distinctive C-N patterns as the observational evidence of the result of tidally shredded GCs in the MCs. These findings might explain some previous conflicting results over bulge N-rich stars, and broadly help to understand GC formation and evolution. Furthermore, the discovery of such a large population of N-rich AGB stars in the MCs suggests that multiple stellar populations might not only be exotic events from the past but can also form at lower redshift.