The tight subgiant branch of the intermediate-age star cluster NGC 411 implies a single-aged stellar population

TL;DR

This study shows that the star cluster NGC 411's subgiant branch is consistent with a single-aged stellar population, challenging the idea that extended star-formation histories cause the observed features in intermediate-age clusters.

Contribution

The paper provides evidence that the morphology of NGC 411's subgiant branch supports a single-aged population, contradicting previous interpretations of extended star-formation histories in such clusters.

Findings

The width of NGC 411's subgiant branch favors a single-aged population.

The red clump morphology also supports a near single-age star formation history.

The extended main-sequence turn-off is inconsistent with a simple single-age population.

Abstract

The presence of extended main-sequence turn-off (eMSTO) regions in intermediate-age star clusters in the Large and Small Magellanic Clouds is often interpreted as resulting from extended star-formation histories (SFHs), lasting $\geq$ 300 Myr. This strongly conflicts with the traditional view of the dominant star-formation mode in stellar clusters, which are thought of as single-aged stellar populations. Here we present a test of this interpretation by exploring the morphology of the subgiant branch (SGB) of NGC 411, which hosts possibly the most extended eMSTO among all known intermediate-age star clusters. We show that the width of the NGC 411 SGB favours the single-aged stellar population interpretation and rules out an extended SFH. In addition, when considering the red clump (RC) morphology and adopting the unproven premise that the widths of all features in the colour--magnitude diagram are determined by an underlying range in ages, we find that the SFH implied is still very close to that resulting from a single-aged stellar population, with a minor fraction of stars scattering to younger ages compared with the bulk of the population. The SFHs derived from the SGB and RC are both inconsistent with the SFH derived from the eMSTO region. NGC 411 has a very low escape velocity and it has unlikely undergone significant mass loss at an early stage, thus indicating that it may lack the capacity to capture most of its initial, expelled gas from stellar evolutionary processes, a condition often required for extended SFHs to take root.