Stars caught in the braking stage in young Magellanic Clouds clusters

Abstract

The colour–magnitude diagrams of many Magellanic Cloud clusters (with ages up to 2 billion years) display extended turnoff regions where the stars leave the main sequence, suggesting the presence of multiple stellar populations with ages that may differ even by hundreds of millions of years. A strongly debated question is whether such an extended turnoff is instead due to populations with different stellar rotations. The recent discovery of a ‘split’ main sequence in some younger clusters (~80–400 Myr) added another piece to this puzzle. The blue side of the main sequence is consistent with slowly rotating stellar models, and the red side consistent with rapidly rotating models. However, a complete the- oretical characterization of the observed colour–magnitude diagram also seemed to require an age spread. We show here that, in the three clusters so far analysed, if the blue main- sequence stars are interpreted with models in which the stars have always been slowly rotating, they must be ~30% younger than the rest of the cluster. If they are instead interpreted as stars that were initially rapidly rotating but have later slowed down, the age difference disappears, and this ‘braking’ also helps to explain the apparent age differences of the extended turnoff. The age spreads in Magellanic Cloud clusters are thus a manifestation of rotational stellar evolution. Observational tests are suggested.