[C/N] Ages for Red Giants and their Implications for Galactic Archaeology

TL;DR

This study calibrates a [C/N]-age relation for red giants by anchoring to high-quality APOKASC-3 asteroseismic ages and APOGEE DR17 abundances, enabling age determinations for RGB and RC stars across 1–10 Gyr with ~1.64 Gyr precision. It carefully handles evolutionary-state effects, mass loss, and metallicity dependencies by splitting samples into calibration, validation, and mapping sets, and by aligning RC ages with LRGB via a mass-loss correction in $\Delta\mathrm{log(Age)}_{RC}$. The calibrated relation is then applied to a large APOGEE DR17 mapping sample to trace Galactic chemical evolution and to identify outward migrators, revealing only weak chemical evolution (~0.01–0.02 dex Gyr$^{-1}$) and fewer migrators than some radial-migration models predict. The results demonstrate the value of [C/N] as a robust chronometer for large spectroscopic surveys, particularly for luminous giants where asteroseismic masses are challenging, and provide new constraints on the Milky Way’s formation history and migration patterns.

Abstract

Red giants undergo the first dredge-up, a mixing event that creates a connection between their surface [C/N] and their mass and age. We derive a [C/N]-Age relationship for red giants calibrated on APOGEE DR17 abundances and APOKASC-3 asteroseismic ages. We find that we can use [C/N] to reliably recover asteroseismic ages between 1 and 10 Gyr with average uncertainties of 1.64 Gyr. We find that [C/N] yields concordant ages, with modest offsets, for stars in different evolutionary states. We also find that the [C/N]-birth mass relationship is robust for luminous giants, and argue that this is an advantage over direct asteroseismology for these stars. We use our ages to infer Galactic birth abundance trends in [Fe/H] and [Mg/H] as a function of position in the Galactic disk. We filter out stars with kinematic or chemical properties consistent with migrators and found the number of migrators to be much lower than expected by standard radial migration prescriptions. The remaining population shows weak chemical evolution trends, on the order of 0.01 dex/Gyr, over the last 10 Gyr across a wide range of radii.

Figures (18)

• Figure 1: Kiel diagram of the APOKASC-3 stars used in our calibration sample. The black points in the background are the full APOGEE DR17 sample, provided for context. The blue stars are LRGB stars, and the red stars are RC stars, determined asteroseismically.
• Figure 2: Age-[Fe/H] for the 21 clusters used in this paper.
• Figure 3: Kiel diagram of the APO-K2 stars used in this paper. The conventions and labeling are identical to Figure \ref{['fig:kascHR']}.
• Figure 4: Kiel diagram of the APOGEE DR17 stars used in this paper. All conventions remain the same as Figure \ref{['fig:kascHR']}. The stars on the left (hot) side of the giant branch are excluded due to our extra-mixing cut, which removes the metal-poor stars.
• Figure 5: Galactocentric X-Z map of the APOGEE DR17 giants used in this paper, separated by evolutionary state. The yellow star marks the Sun, and the red circle marks the galactic center.