Two sequences in the age-metallicity relation as seen from [C/N] abundances in APOGEE

TL;DR

This study investigates the age-metallicity relation in the Milky Way disk using APOGEE data, revealing two distinct sequences in the solar neighborhood linked to different galactic regions, with [C/N] ratios serving as age proxies.

Contribution

It provides new evidence for two separate age-metallicity sequences in the solar neighborhood using APOGEE red clump giants and explores their properties across different galactic radii.

Findings

Two distinct sequences are only present in the solar neighborhood.

[C/N] ratios effectively serve as age proxies for red giants.

The sequences correlate with different galactic radii and kinematic properties.

Abstract

The age-metallicity relation is fundamental to study the formation and evolution of the disk. Observations have shown that this relation has a large scatter which can not be explained by observational errors only. That scatter is hence attributed to the effects of radial migration in which stars tracing different chemical evolution histories in the disk get mixed. However, the recent study of Nissen et al. 2020, using high precision observational data of solar type stars, found two relatively tight age-metallicity relations. One sequence of older and metal-richer stars probably traces the chemical enrichment history of the inner disk while the other sequence of younger and metal-poorer stars the chemical enrichment history of the outer disk. If uncertainties in age measurements increase, these sequences mix explaining the scatter of the one relation observed in other studies. This work follows up on these results, by analysing an independent sample of red clump giants observed by APOGEE. Since ages for red giants are significantly more uncertain, the [C/N] ratios are considered as a proxy for age. This larger dataset is used to investigate these relations at different Galactic radii, finding that these distinct sequences exist only in the solar neighbourhood. The APOGEE dataset is further used to explore different abundance and kinematical planes to shed light on the nature of these populations.