Chemical tagging with APOGEE: Discovery of a large population of N-rich stars in the inner Galaxy

TL;DR

This study uses APOGEE infrared data to identify a large population of nitrogen-rich field stars in the inner Galaxy, suggesting possible origins from dissolved globular clusters or early Galactic formation processes.

Contribution

It reports the discovery of N-rich stars in the inner Galaxy with chemical signatures similar to globular cluster stars, providing new insights into Galactic formation and evolution.

Findings

N-rich stars are homogeneously distributed and kinematically similar to other field stars.

Their metallicity peaks at [Fe/H]~-1, differing from current globular cluster metallicity distributions.

The mass of dissolved GCs could be significantly larger than the surviving GC system.

Abstract

Formation of globular clusters (GCs), the Galactic bulge, or galaxy bulges in general, are important unsolved problems in Galactic astronomy. Homogeneous infrared observations of large samples of stars belonging to GCs and the Galactic bulge field are one of the best ways to study these problems. We report the discovery by APOGEE of a population of field stars in the inner Galaxy with abundances of N, C, and Al that are typically found in GC stars. The newly discovered stars have high [N/Fe], which is correlated with [Al/Fe] and anti-correlated with [C/Fe]. They are homogeneously distributed across, and kinematically indistinguishable from, other field stars in the same volume. Their metallicity distribution is seemingly unimodal, peaking at [Fe/H]~-1, thus being in disagreement with that of the Galactic GC system. Our results can be understood in terms of different scenarios. N-rich stars could be former members of dissolved GCs, in which case the mass in destroyed GCs exceeds that of the surviving GC system by a factor of ~8. In that scenario, the total mass contained in so-called "first-generation" stars cannot be larger than that in "second-generation" stars by more than a factor of ~9 and was certainly smaller. Conversely, our results may imply the absence of a mandatory genetic link between "second generation" stars and GCs. Last, but not least, N-rich stars could be the oldest stars in the Galaxy, the by-products of chemical enrichment by the first stellar generations formed in the heart of the Galaxy.