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Extended main-sequence turnoffs apparent in most young and intermediate-ageclusters (younger than ~2 Gyr) are known features caused by fast rotatingearly-type (earlier than F-type) stars. Late-type stars are not fast rotatorsbecause their initial angular momenta have been quickly dispersed due tomagnetic braking. However, the mass limit below which stars have beenmagnetically braked has not been well constrained by observation. In thispaper, we present an analysis of the eMSTO of NGC 6819, an open cluster of anintermediate-age (~2.5 Gyr), believed to be comparable to the lifetime of starsnear the mass limit for magnetic braking. By comparing the observation withsynthetic CMDs, we find that NGC 6819 does not harbor an obvious eMSTO. Themorphology of its TO region can be readily explained by a simple stellarpopulation considering the observational uncertainties as well as thedifferential reddening. In addition, the MSTO stars in NGC 6819 have very smallvalues of average rotational velocity and dispersion, indicating that they haveundergone significant magnetic braking. Combining with results in theliterature for clusters of younger ages, our current work suggests that thecritical age for the disappearance of eMSTO in star clusters must be shorterbut very close to the age of NGC 6819, and this, in turn, implies a criticalstellar 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 ofeMSTO could provide a unique way to constrain the onset mass of magneticbraking.

At what mass are stars braked? The implication from the turnoff morphology of NGC 6819