[C/N] Ages and Extra-Mixing for [Fe/H] <- 0.5: Insights from the LMC and SMC

TL;DR

This study investigates how extra mixing affects the [C/N]-age relation in metal-poor stars, using Magellanic Clouds data to improve age estimations of stars in the Milky Way and external galaxies.

Contribution

It provides empirical and theoretical insights into the mass and metallicity dependence of extra mixing, enabling more accurate [C/N]-based stellar age determinations at low metallicities.

Findings

Empirical calibration works well for stars below 1.25 M_sun.

Theoretical models match observed mass dependence down to 1.25 M_sun.

Extra mixing becomes ineffective above 1.8 M_sun at [Fe/H] ~ -0.7.

Abstract

The [C/N]-age relation has become a powerful tool for reconstructing the formation history of the Milky Way (MW), providing the largest age sample for field giant stars. However, at metallicities below [Fe/H] $< -0.5$, stellar surfaces are altered by a poorly understood process known as extra mixing, which modifies [C/N] in a mass- and metallicity-dependent manner. This effect complicates the application of the traditional [C/N]-age relation in metal-poor regimes. Within the MW, constraining the mass dependence of extra mixing is particularly challenging because stars at [Fe/H] $< -0.5$ are predominantly old and therefore low-mass, leading to strong degeneracies between mass and metallicity. In this work, we explore the potential of the Magellanic Clouds (MCs) to disentangle these effects and constrain extra mixing as a function of age and metallicity. By comparing empirical corrections calibrated in the MW with predictions from thermohaline mixing models, we isolate the mass dependence of extra mixing in the MCs down to [Fe/H] $\sim-0.7$. We find that the empirical calibration performs well for lower-mass stars ($< 1.25$ $M_{\odot}$), while theoretical models successfully reproduce the observed mass dependence down to $\sim$ 1.25 $M_{\odot}$. We further present the first observational evidence that extra mixing becomes ineffective above $\sim$ 1.8 $M_{\odot}$ at [Fe/H] $\sim -0.7$. Our results demonstrate the feasibility of deriving [C/N]-based ages for individual stars in external galaxies. Future observations targeting higher-$\log g$ or fainter stars in the MCs will provide stronger constraints on extra-mixing processes and enable the calibration of [C/N]-age relation that can be applied to low-metallicity individual stars in the MW or external galaxies.