Not all nitrogen-rich field stars originate from globular clusters

TL;DR

This study investigates nitrogen-rich stars in the Galactic field, using asteroseismology and chemical analysis to assess their origins, suggesting most are unlikely to have come from globular clusters due to their young inferred ages.

Contribution

It demonstrates the use of combined chemical, kinematic, and asteroseismic data to evaluate the origins of nitrogen-rich stars beyond globular clusters.

Findings

Most nitrogen-rich field stars are too young to originate from GCs.

Asteroseismic ages suggest alternative evolutionary scenarios like binary interactions.

Only a few nitrogen-rich stars could plausibly come from GCs, based on age estimates.

Abstract

Globular clusters (GCs) are important tracers of the early Galactic assembly process, with part of their stars showing distinct chemical abundance patterns. When such stars are found in the Galactic field rather than within GCs, they are assumed to have originated from clusters. We expand the search for such chemically enriched stars in the Kepler field, targeting stars located in the halo, thin and thick disc, to show the potential in using asteroseismology to link the inferred masses and hence, ages, with chemical abundances and kinematics. Using data from APOGEE DR17, Gaia DR3, and the Kepler mission, we identify primordial stars as those with chemical signatures typical of field stars, and enriched stars as those exhibiting strong nitrogen enrichment, with corresponding carbon and oxygen depletion. We present our sample of 133 red giant branch and core-He-burning stars, 92 of which have measured masses and inferred age estimations from asteroseismology. Of the 20 enriched stars identified, 13 have precise asteroseismic ages, of which a maximum of 3 are old enough ($> 8$ Gyr) to plausibly originate from globular clusters. The inferred asteroseismic ages indicate that most enriched stars found in the field appear too young to have originated from GCs; however, these apparently young ages are likely the result of assuming single-star evolution, rather than accounting for binary interactions or mergers. This points to alternative enrichment and evolutionary scenarios, such as mass transfer or coalescence, rather than a globular-cluster origin for most field nitrogen-rich stars.