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

The origin of extended main sequence turnoffs (eMSTOs) in intermediate-age (1-3 Gyr) clusters is one of the most intriguing questions in current star cluster research. Unlike the split main sequences found in some globular clusters. which are caused by bimodal populations in age and/or chemical abundances, eMSTOs are believed to be owing to stellar rotation. We present a spectroscopic survey of MSTO stars in a nearby, intermediate-age (\unit[0.9]{Gyr}), low mass ($\sim 1.7\times10^3 M_\odot$) Galactic open cluster, NGC 5822. We derive a clean sample of member stars based on \textit{Gaia} proper motions and parallaxes and confirm the existence of an eMSTO. 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 loci of the main sequence stars in the eMSTO region show a clear correlation with the projected rotational velocities in the sense that fast rotators are located on the red side of the eMSTO and slow rotators are found on the blue side. By comparison with a synthetic cluster model, we show that the stellar rotational velocities and the eMSTO of NGC 5822 can be well reproduced and we conclude that stellar rotation is the main cause of the eMSTO in NGC 5822.