The influence of the C+N+O abundances on the determination of the relative ages of Globular Clusters: the case of NGC 1851 and NGC 6121 (M4)

TL;DR

This study investigates how variations in C+N+O abundances affect the perceived relative ages of globular clusters, showing that chemical composition differences can mimic age differences in color-magnitude diagrams.

Contribution

It demonstrates that differences in total CNO content can lead to misinterpretation of age differences between similar globular clusters.

Findings

CNO abundance variations influence SGB and RGB features

Similar clusters can appear different in age due to chemical differences

CNO variations can mimic age differences in CMD analysis

Abstract

The color magnitude diagram (CMD) of NGC 1851 presents two subgiant branches (SGB), probably due the presence of two populations differing in total CNO content. We test the idea that a difference in total CNO may simulate an age difference when comparing the CMD of clusters to derive relative ages. We compare NGC 1851 with NGC 6121 (M4), a cluster of very similar [Fe/H]. We find that, with a suitable shift of the CMDs that brings the two red horizontal branches at the same magnitude level, the unevolved main sequence and red giant branch match, but the SGB of NGC 6121 and its red giant branch "bump" are fainter than in NGC 1851. In particular, the SGB of NGC 6121 is even slightly fainter than the the faint SGB in NGC 1851. Both these features can be explained if the total CNO in NGC 6121 is larger than that in NGC 1851, even if the two clusters are coeval. We conclude by warning that different initial C+N+O abundances between two clusters, otherwise similar in metallicity and age, may lead to differences in the turnoff morphology that can be easily attributed to an age difference.