Radius Dependent Angular Momentum Evolution in Low-Mass Stars. I

TL;DR

This paper proposes a new model for angular momentum evolution in low-mass stars that relates rotation to magnetic field strength and accounts for radius-dependent effects, explaining observed stellar rotation trends.

Contribution

It introduces a radius-dependent braking law based on magnetic field strength, providing a unified explanation for stellar rotation evolution across different star types.

Findings

The model explains the steep rotation transition at the convection boundary.

It aligns with the empirical Skumanich law for solar-type stars.

Predicted spin-down ages match observed magnetic activity lifetimes.

Abstract

Angular momentum evolution in low-mass stars is determined by initial conditions during star formation, stellar structure evolution, and the behaviour of stellar magnetic fields. Here we show that the empirical picture of angular momentum evolution arises naturally if rotation is related to magnetic field strength instead of to magnetic flux, and formulate a corrected braking law based on this. Angular momentum evolution then becomes a strong function of stellar radius, explaining the main trends observed in open clusters and field stars at a few Gyr: the steep transition in rotation at the boundary to full convection arises primarily from the large change in radius across this boundary, and does not require changes in dynamo mode or field topology. Additionally, the data suggest transient core-envelope decoupling among solar-type stars, and field saturation at longer periods in very low-mass stars. For solar-type stars, our model is also in good agreement with the empirical Skumanich law. Finally, in further support of the theory, we show that the predicted age at which low-mass stars spin down from the saturated to unsaturated field regimes in our model corresponds remarkably well to the observed lifetime of magnetic activity in these stars.