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