The exclusion of a significant range of ages in a massive star cluster

TL;DR

This study shows that the extended main-sequence turn-off regions in certain star clusters can be explained by stellar rotation effects rather than age spreads, challenging previous interpretations of cluster evolution.

Contribution

It demonstrates that extended turn-offs in intermediate-age clusters may result from stellar rotation, not from multiple stellar populations with different ages.

Findings

Extended turn-offs can be explained by rapid stellar rotation.

Similar morphologies are observed in at least five other clusters.

Extended turn-offs do not necessarily indicate age spreads.

Abstract

Stars spend most of their lifetimes on the main sequence in the Hertzsprung--Russell diagram. The extended main-sequence turn-off regions -- containing stars leaving the main sequence after having spent all of the hydrogen in their cores -- found in massive (more than a few tens of thousands of solar masses), intermediate-age (about one to three billion years old) star clusters are usually interpreted as evidence of cluster-internal age spreads of more than 300 million years, although young clusters are thought to quickly lose any remaining star-forming fuel following a period of rapid gas expulsion on timescales of order $10^7$ years. Here we report that the stars beyond the main sequence in the two billion-year-old cluster NGC 1651, characterized by a mass of $\sim 1.7 \times 10^5$ solar masses, can be explained only by a single-age stellar population, even though the cluster has clearly extended main-sequence turn-off region. The most plausible explanation for the extended main-sequence turn-offs invokes the presence of a population of rapidly rotating stars, although the secondary effects of the prolonged stellar lifetimes associated with such a stellar-population mixture are as yet poorly understood. From preliminary analysis of previously obtained data, we find that similar morphologies are apparent in the Hertzsprung--Russell diagrams of at least five additional intermediate-age star clusters, suggesting that an extended main-sequence turn-off does not necessarily imply the presence of a significant cinternal age dispersion.