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

TL;DR

This paper extends the empirical [C/N]-age calibration to older and metal-poor stars using globular clusters, enabling age estimation for a broader range of evolved stars in the Milky Way.

Contribution

The study broadens the [C/N]-age relationship to include globular clusters, allowing chemical clock application to older and metal-poor stellar populations.

Findings

Extended [C/N]-age calibration to older, metal-poor stars.

Validated the calibration with asteroseismic ages.

Applied the relationship to APOGEE DR17 data.

Abstract

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 APOGEE DR17 red giant stars, that have experienced the first dredge up but have not yet undergone any extra-mixing, in clusters usable for ages between $8.62 \leq \log(Age[{\rm yr}]) \leq 10.13$ and for metallicites of $-1.2\leq[Fe/H]\leq+0.3$. 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 asterosiesmic results of APOKASC and APO-K2.