Logo
ScienceStack
Table of Contents
Fetching ...
Sign In

Not all nitrogen-rich field stars originate from globular clusters

Ellen I. Leitinger, Andrea Miglio, Josefina Montalbán, Davide Massari, Angela Bragaglia, Walter E. van Rossem, Karsten Brogaard, Alessandro Mazzi, Jeppe Sinkbæk Thomsen, Emma Willett

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.

Not all nitrogen-rich field stars originate from globular clusters

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 ( 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.
Paper Structure (17 sections, 4 figures, 1 table)

This paper contains 17 sections, 4 figures, 1 table.

Table of Contents

  1. Introduction
  2. Data sample selection and analysis
  3. APOGEE DR17
  4. Gaia
  5. Kepler seismology
  6. Classification of primordial and enriched stars
  7. Discussion
  8. Conclusions
  9. Population classification using the new abundance catalogue from 2025Kane
  10. Population classification using APOGEE DR17
  11. Possible origins of individual enriched stars
  12. KIC 5184073
  13. KIC 10796857
  14. KIC 8350894
  15. KIC 8110538
  16. ...and 2 more sections

Figures (4)

  • Figure 1: The primordial (blue) and enriched (red) field stars in the Kepler sample, with the sample of GC stars identified by 2024Schiavon (green), both using APOGEE DR17 abundances. Stars in the Kepler sample with ages determined through asteroseismology are shown with black borders. Upper left: [N/Fe] vs. [C/Fe] anticorrelation, lower left: [N/Fe] vs. [O/Fe] anticorrelation, upper middle: Toomre diagram, lower middle: integral of motion plot, upper right: histogram of ages estimated from asteroseismology against GC ages from 2010Forbes (grey histogram), lower right: histogram of masses derived from asteroseismology.
  • Figure 2: Primordial (blue) and enriched (red) stars identified in Kepler sample using abundances from the Gaia BP/RP catalogue 2025Kane with [Fe/H] $<-0.5$ dex (black points). Stars in the Kepler sample with ages (three primordial stars) determined through asteroseismology are shown with black borders.
  • Figure 3: Nitrogen abundance versus metallicity for our primordial (blue stars) and enriched (red stars) samples, as well as the bulge stars (black points) and nitrogen-rich stars (green squares) presented in 2017Schiavon (S17 - updated from APOGEE DR12 to DR17 abundances). The vertical dashed line represents the metallicity limit of our sample ([Fe/H] $< -0.5$).
  • Figure 4: Abundances of two enriched (shades of red) and five primordial (shades of blue) stars in our sample with abundances obtained by 2021Matsuno.