CNO abundances in the globular clusters NGC 1851 and NGC 6752

TL;DR

This study measures C+N+O abundances in red giant stars of two globular clusters, revealing a constant sum in NGC 6752 and a significant spread in NGC 1851, which impacts understanding of their stellar populations.

Contribution

It provides the first detailed comparison of C+N+O sums in these two clusters, highlighting differences in chemical complexity and stellar evolution.

Findings

NGC 6752 has a constant C+N+O sum with minimal dispersion.

NGC 1851 shows a large C+N+O spread, especially in the anomalous RGB.

The C+N+O variation supports the idea of different subpopulations with distinct chemical histories.

Abstract

We measure the C+N+O abundance sum in red giant stars in two Galactic globular clusters, NGC 1851 and NGC 6752. NGC 1851 has a split subgiant branch which could be due to different ages or C+N+O content while NGC 6752 is representative of the least complex globular clusters. For NGC 1851 and NGC 6752, we obtain average values of A(C+N+O) = 8.16 +/- 0.10 (\sigma = 0.34) and 7.62 +/- 0.02 (\sigma = 0.06), respectively. When taking into account the measurement errors, we find a constant C+N+O abundance sum in NGC 6752. The C+N+O abundance dispersion is only 0.06 dex, and such a result requires that the source of the light element abundance variations does not increase the C+N+O sum in this cluster. For NGC 1851, we confirm a large spread in C+N+O. In this cluster, the anomalous RGB has a higher C+N+O content than the canonical RGB by a factor of four (~0.6 dex). This result lends further support to the idea that the two subgiant branches in NGC 1851 are roughly coeval, but with different CNO abundances.