Extending the [C/N]–Age Calibration: Using Globular Clusters to Explore Older and Metal-poor Populations

In the coming years, detailed chemical abundances from large-scale high-resolution spectroscopic surveys will become available for vast numbers of stars across the Milky Way. Previous work has suggested that abundance ratios from these spectra can allow us to estimate ages from a large number of stars. These data will be leveraged to calibrate chemical clocks to age-date field stars, as reliable stellar ages remain elusive. In this work, we extended our empirical relationship between stellar age and their carbon-to-nitrogen ([C/N]) abundance ratio for evolved stars to older and more metal-poor stars by combining the original open cluster calibration sample and four globular clusters: 47 Tuc, M 71, M 4, and M 5. With this extension, [C/N] can be used as a chemical clock for evolved field stars to investigate not only regions within the metal-rich disk but also more metal-poor regions of our Galaxy. We have established the [C/N]–age relationship for Apache Point Observatory Galactic Evolution Experiment (APOGEE) DR17 red giant stars, which have experienced the first dredge-up but have not yet undergone any extramixing, in clusters usable for ages between 8.62 ≤ log ( Age [ yr ] ) ≤ 10.13 and for metallicities of −1.2 ≤ [Fe/H] ≤ +0.3. This relationship can be uniformly applied to these stars within the APOGEE DR17 sample. This measured [C/N]–age APOGEE DR17 relationship is also shown to be consistent with stellar ages derived from asteroseismic results of APOKASC and APO-K2.