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

## TL;DR

This paper proposes that subcritical magnetic dynamos in aging solar-like stars explain the breakdown of gyrochronology, the bimodal sunspot distribution, and the transition to magnetic inactivity, challenging traditional stellar spin-down models.

## Contribution

It introduces the concept of subcritical magnetic dynamos as a unifying physical mechanism for various observed stellar phenomena and their evolution with age.

## Key findings

- Reproduces bimodal sunspot distribution with subcritical dynamos
- Suggests aging stars access weak magnetic field regimes
- Proposes disrupted magnetic braking causes slower spin-down

## Abstract

Recent observations reveal the surprising breakdown of stellar gyrochronology relations at about the Sun's age hinting that 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 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 {\emph{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.