  Extended main-sequence turnoffs apparent in most young and intermediate-age
clusters (younger than ~2 Gyr) are known features caused by fast rotating
early-type (earlier than F-type) stars. Late-type stars are not fast rotators
because their initial angular momenta have been quickly dispersed due to
magnetic braking. However, the mass limit below which stars have been
magnetically braked has not been well constrained by observation. In this
paper, we present an analysis of the eMSTO of NGC 6819, an open cluster of an
intermediate-age (~2.5 Gyr), believed to be comparable to the lifetime of stars
near the mass limit for magnetic braking. By comparing the observation with
synthetic CMDs, we find that NGC 6819 does not harbor an obvious eMSTO. The
morphology of its TO region can be readily explained by a simple stellar
population considering the observational uncertainties as well as the
differential reddening. In addition, the MSTO stars in NGC 6819 have very small
values of average rotational velocity and dispersion, indicating that they have
undergone significant magnetic braking. Combining with results in the
literature for clusters of younger ages, our current work suggests that the
critical age for the disappearance of eMSTO in star clusters must be shorter
but very close to the age of NGC 6819, and this, in turn, implies a critical
stellar mass for magnetic braking at solar metallicity above but close to 1.54
$M_{\odot}$ based on the PARSEC model. We emphasize that the phenomenon of
eMSTO could provide a unique way to constrain the onset mass of magnetic
braking.
