First Observational Signature of Rotational Deceleration in a Massive, Intermediate-age Star Cluster in the Magellanic Clouds

TL;DR

This study presents observational evidence of rotational deceleration in a massive star cluster, challenging traditional age spread explanations for extended main-sequence turn-offs and offering a new rotational evolution perspective.

Contribution

It provides the first observational signature supporting rotational deceleration as an explanation for features in star cluster color-magnitude diagrams.

Findings

Confirmed the reality of the color-dependent SGB width trend.

Showed that rotational deceleration explains the observed SGB features.

Challenged the age spread hypothesis for extended star-formation histories.

Abstract

While the extended main-sequence turn-offs (eMSTOs) found in almost all 1--2 Gyr-old star clusters in the Magellanic Clouds are often explained by postulating extended star-formation histories, the tight subgiant branches (SGBs) seen in some clusters challenge this popular scenario. Puzzlingly, the SGB of the eMSTO cluster NGC 419 is significantly broader at bluer than at redder colors. We carefully assess and confirm the reality of this observational trend. If we would assume that the widths of the features in color--magnitude space were entirely owing to a range in stellar ages, the star-formation histories of the eMSTO stars and the blue SGB region would be significantly more prolonged than that of the red part of the SGB. This cannot be explained by assuming an internal age spread. We show that rotational deceleration of a population of rapidly rotating stars, a currently hotly debated alternative scenario, naturally explains the observed trend along the SGB. Our analysis shows that a `converging' SGB could be produced if the cluster is mostly composed of rapidly rotating stars that slow down over time owing to the conservation of angular momentum during their evolutionary expansion from main-sequence turn-off stars to red giants.