Abstract

The physics of angular momentum evolution in low-mass stars is one of the oldest and most fundamental issues in stellar evolution, determined by initial conditions during star formation, stellar structure evolution, and the behavior of stellar magnetic fields. Previous theoretical work attained reasonable agreement with rotation data for solar-type stars, but failed drastically when applied to very low-mass fully convective stars. 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. Wr formulate a corrected braking law based on this, for the full range of low-mass stars (solar-tpe to fully convective). 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 across this range in stellar masses, without requiring baroque changes in stellar dynamos or field topologies. Further improvements and generalisations to the field geometry and wind velocity are also discussed.