Breaking Gyrochronology through the Collapse of Coronal Winds

The breakdown of gyrochronology in old solar-type stars, revealed by asteroseismology (e.g., van Saders et al. (2016)), is commonly attributed to an abrupt transition in stellar dynamo efficiency. We investigate an alternative mechanism in which reduced coronal heating modifies the properties of the magnetized wind, reducing the loss of angular momentum.

 

We develop a coupled framework that includes a polytropic magnetohydrodynamic wind (Weber and Davis (1967)), internal angular-momentum redistribution(MacGregor and Brenner (1991)), and a dynamo prescription $B\propto\Omega$ linking magnetic field strength to rotation.

 

In the low coronal temperature limit ($T \lesssim 1.5$ MK), magnetocentrifugal effects remain non-negligible and exclude simple power-law parameterizations of $\dot{M}$ and $\dot{J}$ as a function of coronal temperature $T$. We find that $\dot{M}$ scales linearly with volumetric heating, establishing a physically motivated link between thermodynamics and wind properties. Imposing $T \propto \Omega^\sigma$, we recover reduced magnetic braking for $\sigma \gtrsim 1.2$, but reproducing the observed gyrochronology break requires $\sigma \gtrsim 1.5$, corresponding to a drop of several orders of magnitude in energy input but leading to unrealistic coronal temperature (Levesque et Charbonneau (2025)).

 

This appears inconsistent with observed chromospheric and coronal emission, suggesting that wind thermodynamics alone cannot account for the weakened spin-down, and reinforcing the need for dynamo-related contribution.