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The origin of extended main sequence turnoffs (eMSTOs) in intermediate-age(1-3 Gyr) clusters is one of the most intriguing questions in current starcluster research. Unlike the split main sequences found in some globularclusters. which are caused by bimodal populations in age and/or chemicalabundances, eMSTOs are believed to be owing to stellar rotation. We present aspectroscopic survey of MSTO stars in a nearby, intermediate-age(\unit[0.9]{Gyr}), low mass ($\sim 1.7\times10^3 M_\odot$) Galactic opencluster, NGC 5822. We derive a clean sample of member stars based on\textit{Gaia} proper motions and parallaxes and confirm the existence of aneMSTO. Using medium-resolution ($R\sim 4000$) Southern African Large Telescope(SALT) spectra, we derive the rotational velocities of 24 member stars(representing 20\% completeness around the eMSTO region) and find that the lociof the main sequence stars in the eMSTO region show a clear correlation withthe projected rotational velocities in the sense that fast rotators are locatedon the red side of the eMSTO and slow rotators are found on the blue side. Bycomparison with a synthetic cluster model, we show that the stellar rotationalvelocities and the eMSTO of NGC 5822 can be well reproduced and we concludethat stellar rotation is the main cause of the eMSTO in NGC 5822.

Stellar rotation and the extended main sequence turnoff in the open cluster NGC 5822