Stellar Rotation and the Extended Main-sequence Turnoff in the Open Cluster NGC 5822

Authors: Sun, Weijia; de Grijs, Richard; Deng, Licai; Albrow, Michael D. Bibcode: 2019ApJ...876..113S arXiv: 1904.03547

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 due to stellar rotation.

CMD of NGC 5822 with the member stars color-coded by their rotational velocities

We present a spectroscopic survey of MSTO stars in a nearby, intermediate-age (0.9 Gyr), low-mass Galactic open cluster, NGC 5822. The existence of an eMSTO is confirmed. Using medium-resolution (R ~ 4000) Southern African Large Telescope 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.

Pseudo-color vs. rotational velocity