Tidal-locking-induced stellar rotation dichotomy in the open cluster NGC 2287?

Weijia Sun, Chengyuan Li, Licai Deng, Richard De Grijs

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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 (OCs) 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. Here, we show through analysis of the projected stellar rotational velocities (v sin i, where i represents the star's inclination angle) that the well-separated double MS in the young, ∼200 Myr old Milky Way OC NGC 2287 is tightly correlated with a dichotomous distribution of stellar rotation rates. 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.

Original languageEnglish
Article number182
Pages (from-to)1-9
Number of pages9
JournalAstrophysical Journal
Issue number2
Publication statusPublished - 1 Oct 2019


Bibliographical note

Copyright 2019 the American Astronomical Society. First published in Astrophysical Journal, volume 883, issue 2, article 182. The original publication is available at http://doi.org/10.3847/1538-4357/ab3cd0, published by IOP Publishing. Version archived for private and non-commercial use with the permission of the author/s and according to publisher conditions. For further rights please contact the publisher.

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