The color-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 which may differ even by hundreds million years (Mackey et al. 2008,
Milone et al. 2009, Girardi et al. 2011). A strongly debated question is
whether such an extended turnoff is instead due to populations with different
stellar rotations (Girardi et al. 2011, Goudfrooij et al. 2011, Rubele et al.
2013, Li et al. 2014). The recent discovery of a `split' main sequence in some
younger clusters (about 80--400Myr) added another piece to this puzzle. The
blue (red) side of the main sequence is consistent with slowly (rapidly)
rotating stellar models (D'Antona et al. 2015, Milone et al. 2016, Correnti et
al. 2017, Milone et al 2016), but a complete theoretical characterization of
the observed color-magnitude diagram appeared to require also an age spread
(Correnti et al. 2017). We show here that, in three clusters so far analyzed,
if the blue main sequence stars are interpreted with models that have been
always slowly rotating, they must be about 30% younger than the rest of the
cluster. If they are instead interpreted as stars initially rapidly rotating,
but that have later slowed down, the age difference disappears, and "braking"
also helps to explain the apparent age differences of the extended turnoff. The
age spreads in Magellanic Cloud clusters are a manifestation of rotational
stellar evolution. Observational tests are suggested.Comment: Accepted for publication and in state of Advance Online Publication
(from 24 July 2017) on Nature Astronom
