Catastrophic rotational braking among Sun-like stars

A model of the Sun’s rotation evolution

European Space Agency, ESTEC − Postbus 299, 2200 AG Noordwijk, The Netherlands
e-mail: pgondoin@cosmos.esa.int

Received: 20 September 2016
Accepted: 16 December 2016

Abstract

Context. Observations of young open clusters show a bimodal distribution of stellar rotation. In those clusters, Sun-like stars group into two main populations of fast and slow rotators. Beyond an age of approximately 600 Myr, the two populations converge towards a single sequence of slow rotators.

Aims. The present study addresses the origin of this bimodal distribution and the cause of its observed evolution.

Methods. New prescriptions of mass-loss rate and Alfven radius dependences on Rossby number suggested by observations are implemented in a phenomenological model of angular-momentum loss and redistribution. The obtained model is used to calculate the time evolution of a rotation-period distribution of solar-mass stars similar to that observed in the 5 Myr-old NGC 2362 open cluster. The simulated distributions at subsequent ages are compared with those of h Per, the Pleiades, M 50, M 35, and M 37.

Results. The model is able to reproduce the appearance and disappearance of a bimodal rotation-period distribution in open clusters providing that a brief episode of large-angular-momentum loss is included in the early evolution of Sun-like stars.

Conclusions. I argue that a transitory episode of large-angular-momentum loss occurs on Sun-like stars with Rossby numbers between 0.13 and 0.3. This phenomenon of enhanced magnetic braking by stellar wind would be mainly driven by a rapid increase of mass loss at a critical rotation rate. This scenario accounts for the bimodal distribution of stellar rotation in open clusters with ages between 20−30 Myr and approximately 600 Myr. The mass-loss rate increase could account for a significant fraction of the X-ray luminosity decay of Sun-like stars in the 0.13−0.3 Rossby number range where a transition from the saturated to the non-saturated regime of X-ray emission is observed. Observed correlations between Li abundance and rotation sequences in the Pleiades and M 34 clusters support this scenario.