The Minimum Mass of Rotating Main Sequence Stars and its Impact on the
  Nature of Extended Main Sequence Turnoffs in Intermediate-Age Star Clusters
  in the Magellanic Clouds
  Extended main sequence turn-offs (eMSTOs) are a common feature in
color-magnitude diagrams (CMDs) of young and intermediate-age star clusters in
the Magellanic Clouds. The nature of eMSTOs is still debated. The most popular
scenarios are extended star formation and ranges of stellar rotation rates.
Here we study implications of a kink feature in the main sequence (MS) of young
star clusters in the Large Magellanic Cloud (LMC). This kink shows up very
clearly in new \emph{Hubble Space Telescope} observations of the 700-Myr-old
cluster NGC 1831, and is located below the region in the CMD where multiple or
wide MSes, which are known to occur in young clusters and thought to be due to
varying rotation rates, merge together into a single MS. The kink occurs at an
initial stellar mass of $1.45 \pm 0.02\;M_{\odot}$; we posit that it represents
a lower limit to the mass below which the effects of rotation on the energy
output of stars are rendered negligible at the metallicity of these clusters.
Evaluating the positions of stars with this initial mass in CMDs of massive LMC
star clusters with ages of $\sim\,$1.7 Gyr that feature wide eMSTOs, we find
that such stars are located in a region where the eMSTO is already
significantly wider than the MS below it. This strongly suggests that stellar
rotation \emph{cannot} fully explain the wide extent of eMSTOs in massive
intermediate-age clusters in the Magellanic Clouds. A distribution of stellar
ages still seems necessary to explain the eMSTO phenomenon.
