Discovery of two distinct red clumps in NGC419: a rare snapshot of a cluster at the onset of degeneracy

TL;DR

This study identifies two distinct red clumps in the star cluster NGC419, providing a rare observational snapshot of a cluster transitioning from classical to degenerate cores, and estimates its age and overshooting efficiency.

Contribution

The paper presents the first clear detection of two red clumps in NGC419, linking them to stellar evolution stages and using this to measure cluster age and overshooting parameters.

Findings

Detection of two red clumps in NGC419's CMD.

Confirmation that the secondary clump belongs to the cluster.

Estimation of cluster age at approximately 1.35 Gyr and overshooting parameter Lambda_c=0.47.

Abstract

Colour-magnitude diagrams (CMD) of the SMC star cluster NGC419, derived from HST/ACS data, reveal a well-delineated secondary clump located below the classical compact red clump typical of intermediate-age populations. We demonstrate that this feature belongs to the cluster itself, rather than to the underlying SMC field. Then, we use synthetic CMDs to show that it corresponds very well to the secondary clump predicted to appear as a result of He-ignition in stars just massive enough to avoid electron-degeneracy settling in their H-exhausted cores. The main red clump instead is made of the slightly less massive stars which passed through electron-degeneracy and ignited He at the tip of the RGB. In other words, NGC419 is the rare snapshot of a cluster while undergoing the fast transition from classical to degenerate H-exhausted cores. At this particular moment of a cluster's life, the colour distance between the main sequence turn-off and the red clump(s) depends sensitively on the amount of convective core overshooting, Lambda_c. By coupling measurements of this colour separation with fits to the red clump morphology, we are able to estimate simultaneously the cluster mean age (1.35(-0.04,+0.11) Gyr) and overshooting efficiency (Lambda_c=0.47(-0.04,+0.14)). Therefore, clusters like NGC419 may constitute important marks in the age scale of intermediate-age populations. After eye inspection of other CMDs derived from HST/ACS data, we suggest that the same secondary clump may also be present in the LMC clusters NGC1751, 1783, 1806, 1846, 1852, and 1917.