Lithium depletion is a strong test of core-envelope recoupling

Rotational mixing is a prime candidate for explaining the gradual depletion of light elements from the photospheres of cool stars during the main sequence. However, previous mixing calculations have relied primarily on incomplete treatments of angular momentum (AM) transport in stellar interiors, which do not predict the rotational evolution of open clusters. In order to produce new mixing calculations, we modify our rotating stellar evolution code to include an additional source of AM transport, a necessary ingredient for explaining the open cluster rotation pattern. With this machinery, we show for the first time that the main sequence evolution of surface rotation in open clusters and the evolving abundance of lithium in cool stars can be simultaneously predicted. Our mixing-derived core-envelope coupling strengths agree well with previous work, confirming the reliability of our mixing calculations. Using Li abundances, we argue that the timescale for core-envelope re-coupling during the main sequence is a strong function of mass, thus placing an important constraint on models seeking to identify the mechanism responsible for core-envelope coupling. We discuss implications of this finding for stellar physics, including the viability of gravity waves and magnetic fields as agents of AM transport.