The main-sequence rotation-colour relation in the Coma Berenices open cluster

TL;DR

This study measures stellar rotation periods in the Coma Berenices cluster, revealing a tight period-colour relation that supports using rotation as an age indicator for F, G, and K stars around 600 million years old.

Contribution

It provides the first precise rotation period measurements for main-sequence stars in Coma Berenices and demonstrates the tight period-colour relation, supporting stellar rotation as an age diagnostic.

Findings

Rotation period correlates strongly with J-K colour with only 2% scatter.

Stars in Coma Berenices rotate as solid bodies, indicating rapid internal angular momentum coupling.

A modified magnetic braking law accurately estimates stellar ages within 9% precision.

Abstract

We present the results of a photometric survey of rotation rates in the Coma Berenices (Melotte 111) open cluster, using data obtained as part of the SuperWASP exoplanetary transit-search programme. The goal of the Coma survey was to measure precise rotation periods for main-sequence F, G and K dwarfs in this intermediate-age (~600 Myr) cluster, and to determine the extent to which magnetic braking has caused the stellar spin periods to converge. We find a tight, almost linear relationship between rotation period and J-K colour with a root-mean square scatter of only 2 percent. The relation is similar to that seen among F, G and K stars in the Hyades. Such strong convergence can only be explained if angular momentum is not at present being transferred from a reservoir in the deep stellar interiors to the surface layers. We conclude that the coupling timescale for angular momentum transport from a rapidly-spinning radiative core to the outer convective zone must be substantially shorter than the cluster age, and that from the age of Coma onward, stars rotate effectively as solid bodies. The existence of a tight relationship between stellar mass and rotation period at a given age supports the use of stellar rotation period as an age indicator in F, G and K stars of Hyades age and older. We demonstrate that individual stellar ages can be determined within the Coma population with an internal precision of order 9 percent (RMS), using a standard magnetic braking law in which rotation period increases with the square root of stellar age. We find that a slight modification to the magnetic-braking power law, P proportional to t^0.56, yields rotational and asteroseismological ages in good agreement for the Sun and other stars of solar age for which p-mode studies and photometric rotation periods have been published.