Stellar mid-life crisis: subcritical magnetic dynamos of solar-like stars and the breakdown of gyrochronology

Abstract

Recent observations have revealed the surprising breakdown of stellar gyrochronology relations at about the age of the Sun hinting the middle-aged, solar-like stars transition to a magnetically inactive future. We provide a theoretical basis for these intriguing observations inspired by simulations with a mathematical-dynamo model that can explore long-term solar cycle fluctuations. We reproduce the observed bimodal distribution of sunspot numbers, but only for subcritical dynamos. Based on a bifurcation analysis, we argue that the ageing of solar-like stars makes the magnetically weak dynamo regime readily accessible. Weak magnetic field production in this regime compromises wind-driven angular momentum losses, thus disrupting the hegemony of magnetic braking on stellar rotational spin-down. This hypothesis of subcritical magnetic dynamos of solar-like stars provides a self-consistent, unifying physical basis for a diversity of solar-stellar phenomena such as why stars beyond their mid-life do not spin-down as fast as in their youth, the break-down of stellar gyrochronology relations, the observed bimodal distribution of long-term sunspot observations, and recent findings suggesting that the Sun may be transitioning to a magnetically inactive future.