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.
