Extended Main Sequence Turnoffs in Open Clusters as Seen by Gaia: I. NGC 2818 and the Role of Stellar Rotation

TL;DR

This study analyzes the open cluster NGC 2818 using Gaia data and spectroscopy, revealing that stellar rotation likely causes the extended main sequence turn-off phenomenon observed in the cluster.

Contribution

It provides the first detailed analysis of an open cluster’s eMSTO, linking stellar rotation to the phenomenon in a low-mass, intermediate-age cluster.

Findings

Stars on the red side of eMSTO have high Vsini (>160 km/s).

Stars on the blue side of eMSTO have low Vsini (<160 km/s).

The extent of eMSTO correlates with cluster age.

Abstract

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.