ROTATION OF LOW-MASS STARS - A NEW PROBE OF STELLAR EVOLUTION

We evolved rotating and nonrotating models for low-mass (0.4-1.3 M⊙) stars from the early pre-main sequence to an age of 1.7 × 109 yr. The models include angular momentum loss. We treat the internal transport of angular momentum, and the associated material mixing, with coupled diffusion equations using a phenomenological description of turbulence. The surface properties of these models, including lithium and beryllium abundances, are presented in tabular form. We then compare the results with surface rotation velocities and light-element abundances of low-mass cluster stars to obtain constraints on the basic properties of rotating stars. Different initial angular momenta produce different surface rotation velocities in models of stars younger than 108 yr and a range in surface lithium and beryllium abundance in otherwise identical models of older stars; both phenomena are seen in open clusters of the appropriate age. The mean lithium abundance as a function of temperature in clusters such as the Hyades, and the dispersion about the mean, therefore provide information about the distribution of initial angular momentum as a function of mass. The lithium gap in the Hyades F stars can be attributed to a transition from low average initial angular momentum for G stars to high average initial angular momentum in F stars. The inferred initial angular momenta as a function of mass are consistent with T Tauri rotation velocities. A dispersion in lithium abundance in clusters younger than 108 yr can be produced by a range in age within the cluster or by star-to-star differences in the lifetime of accretion disks. We propose observational tests for these phenomena, and discuss the meaning of correlations between rotation velocity and lithium abundance for clusters of different ages. The rotation velocities and periods as a function of time determine the angular momentum loss law; the small range in rotation period at fixed effective temperature in the Hyades implies that the angular momentum loss law is nearly unique by an early age. The surface rotation velocity is therefore an excellent age indicator for field stars. Shortcomings of lithium as an age indicator are also discussed. The spin-down of stars of different masses places strong constraints on the time scale of angular momentum transport from the interiors to the surfaces of stars. We find that the two extreme cases - the case of rigid rotation enforced on a short time scale, and that of no coupling between the surface convection zone and the interior - can be ruled out by the observations. Observational tests which can determine the amount of differential rotation with depth tolerated in low-mass stars are proposed, and constraints from the inferred soLar rotation curve and the properties of evolved stars are discussed. We also look at ways of distinguishing rotational mixing by hydrodynamic mechanisms from magnetic angular momentum transport and diffusive processes not directly related to rotation.