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

Abstract

Formation of globular clusters (GCs), the Galactic bulge, or galaxy bulges in general,
\nare important unsolved problems in Galactic astronomy. Homogeneous infrared
\nobservations of large samples of stars belonging to GCs and the Galactic bulge field are
\none of the best ways to study these problems. We report the discovery by APOGEE
\nof a population of field stars in the inner Galaxy with abundances of N, C, and Al
\nthat are typically found in GC stars. The newly discovered stars have high [N/Fe],
\nwhich is correlated with [Al/Fe] and anti-correlated with [C/Fe]. They are homogeneously
\ndistributed across, and kinematically indistinguishable from, other field stars
\nwithin the same volume. Their metallicity distribution is seemingly unimodal, peaking
\nat [Fe/H]�–1, thus being in disagreement with that of the Galactic GC system. Our
\nresults can be understood in terms of different scenarios. N-rich stars could be former
\nmembers of dissolved GCs, in which case the mass in destroyed GCs exceeds that of
\nthe surviving GC system by a factor of �8. In that scenario, the total mass contained
\nin so-called “first-generation” stars cannot be larger than that in “second-generation”
\nstars by more than a factor of �9 and was certainly smaller. Conversely, our results
\nmay imply the absence of a mandatory genetic link between “second generation” stars
\nand GCs. Last, but not least, N-rich stars could be the oldest stars in the Galaxy,
\nthe by-products of chemical enrichment by the first stellar generations formed in the
\nheart of the Galaxy.