Extended Main Sequence Turnoffs in Open Clusters as Seen by Gaia: I. NGC
  2818 and the Role of Stellar Rotation
  We present an analysis of the relatively low mass ($\sim2400$~M$_{\odot}$),
$\sim800$~Myr, Galactic open cluster, NGC~2818, using Gaia DR2 results combined
with VLT/FLAMES spectroscopy. Using Gaia DR2 proper motions and parallax
measurements we are able to select a clean sample of cluster members. This
cluster displays a clear extended main sequence turn-off (eMSTO), a phenomenon
previously studied mainly in young and intermediate age massive clusters in the
Magellanic clouds. The main sequence of NGC~2818 is extremely narrow, with a
width of $\sim0.01$ magnitudes (G$_{\rm BP} - $ G$_{\rm RP}$), suggesting very
low levels of differential extinction. Using VLT/FLAMES spectroscopy of 60
cluster members to measure the rotational velocity of the stars (Vsini) we find
that stars on the red side of the eMSTO have high Vsini ($>160$~km/s) while
stars on the blue side have low Vsini ($<160$~km/s), in agreement with model
predictions. The cluster also follows the previously discovered trend between
the age of the cluster and the extent of the eMSTO. We conclude that stellar
rotation is the likely cause of the eMSTO phenomenon.
