On the rotational evolution of young low-mass stars

Observations of young clusters indicate that a significant fraction of solar-type stars are rotating very slowly, with equatorial velocities less than 10 km s(-1). So far, models have failed to reproduce a sufficiently large proportion of these stars on the zero-age main sequence. On the basis of the idea that the mixing length in convection theories could depend on the size of the convective zone (Canuto & Mazzitelli), we examine the influence of a varying mixing-length parameter alpha on the rotational evolution of solar-type stars. A decreasing alpha (owing to evolution) in the mixing-length theory (MLT) leads to a slower contraction rate and to a larger stellar moment of inertia. The stellar spin-up is consequently reduced, and this helps to increase the number of very slow rotators present in young clusters. We also investigate the possibility that alpha could depend on the rotation rate, and show the consequences of this parameterization for the lithium surface abundance.