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Stars caught in the braking stage in young Magellanic Cloud 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 years1,2,3. A strongly debated question is whether such an extended turnoff is instead due to populations with different stellar rotations3,4,5,6. 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 models7,8,9,10. However, a complete theoretical characterization of the observed colour–magnitude diagram also seemed to require an age spread9. 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.

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Figure 1: Colour–magnitude diagram for four young LMC clusters at different ages.
Figure 2: Illustration of the role of braking at the age of NGC 1755 and NGC 1866.

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Acknowledgements

We thank C. Georgy and S. Ekström for creating and maintaining the interactive Web page for the Geneva stellar models at https://obswww.unige.ch/Recherche/evoldb/index/. A.M. acknowledges support by the Australian Research Council through Discovery Early Career Researcher Award DE150101816.

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Authors

Contributions

F.D. and A.M. jointly designed and coordinated this study. F.D. proposed and designed the rotational evolution model. F.D., E.V., A.M. and P.V. worked on the theoretical interpretation and implications of the observations. M.T. and M.D.C. carried out the simulations and the analysis. All authors read, commented on and approved submission of this article.

Corresponding author

Correspondence to Francesca D’Antona.

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The authors declare no competing financial interests.

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Supplementary Information

Supplementary Figures 1–7 and Supplementary Tables 1–2. (PDF 634 kb)

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D’Antona, F., Milone, A., Tailo, M. et al. Stars caught in the braking stage in young Magellanic Cloud clusters. Nat Astron 1, 0186 (2017). https://doi.org/10.1038/s41550-017-0186

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