Star clusters with dual red clumps

TL;DR

This paper investigates star clusters with dual red clumps in the Magellanic Clouds, revealing their age and stellar evolution constraints, and compares them with Galactic clusters to calibrate stellar population ages.

Contribution

It provides the first detailed analysis of clusters with dual red clumps, demonstrating their use in constraining stellar evolution parameters and age calibration.

Findings

Dual red clumps are real and indicate different stellar evolutionary paths.

Clusters with dual red clumps help constrain convective-core overshooting.

Comparison with Galactic clusters enhances understanding of stellar population ages.

Abstract

A few star clusters in the Magellanic Clouds exhibit composite structures in the red-clump region of their colour-magnitude diagrams. The most striking case is NGC419 in the SMC, where the red clump is composed of a main blob as well as a distinct secondary feature. This structure is demonstrated to be real and corresponds to the simultaneous presence of stars which passed through electron degeneracy after central-hydrogen exhaustion and those that did not. This rare occurrence in a single cluster allows us to set stringent constraints on its age and on the efficiency of convective-core overshooting during main-sequence evolution. We present a more detailed analysis of NGC419, together with a first look at other populous LMC clusters which are apparently in the same phase: NGC1751, NGC1783, NGC1806, NGC1846, NGC1852 and NGC1917. We also compare these Magellanic Cloud cases with their Galactic counterparts, NGC752 and NGC7789. We emphasise the extraordinary potential of these clusters as absolute calibration marks on the age scale of stellar populations.