Tidal-locking-induced Stellar Rotation Dichotomy in the Open Cluster NGC 2287?

Authors: Sun, Weijia; Li, Chengyuan; Deng, Licai; de Grijs, Richard Bibcode: 2019ApJ...883..182S arXiv: 1908.06530

Stars spend most of their lifetimes on the main sequence (MS) in the Hertzsprung--Russell diagram. The obvious double MSs seen in the equivalent color--magnitude diagrams characteristic of Milky Way open clusters pose a fundamental challenge to our traditional understanding of star clusters as "single stellar populations". The clear MS bifurcation of early-type stars with masses greater than 1.6 M is thought to result from a range in the stellar rotation rates. However, direct evidence connecting double MSs to stellar rotation properties has yet to emerge.

CMD of the OC NGC 2287 around its eMSTO and split-MS regions.

Here, we show through analysis of the projected stellar rotational velocities that the well-separated double MS in the young, ~200 Myr-old Milky Way open cluster NGC 2287 is tightly correlated with a dichotomous distribution of stellar rotation rates.

CMD of NGC 2287 with stars color-coded according to their projected rotational velocities.

We discuss whether our observations may reflect the effects of tidal locking affecting a fraction of the cluster's member stars in stellar binary systems. We show that the slow rotators could potentially be initially rapidly rotating stars that have been slowed down by tidal locking by a low mass-ratio companion in a cluster containing a large fraction of short-period, low-mass-ratio binaries. This demonstrates that stellar rotation drives the split MSs in young, < 300 Myr-old star clusters. However, special conditions, e.g., as regards the mass-ratio distribution, might be required for this scenario to hold.

Example of best-fitting rotation velocity distributions.