Stellar mid-life crisis: subcritical magnetic dynamos of solar-like stars and the breakdown of gyrochronology
B. M. Tripathi, Dibyendu Nandy, Soumitro Banerjee
Abstract
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.