Not all nitrogen-rich field stars originate from globular clusters

Abstract

Globular clusters (GCs) are important tracers of the early Galactic assembly process, with some 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 these 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 estimates from asteroseismology. Of the 20 enriched stars we identified, 13 have precise asteroseismic ages, of which 3 at most 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 indicates alternative enrichment and evolutionary scenarios, such as mass transfer or coalescence, rather than a globular-cluster origin for most field nitrogen-rich stars.