The angular momentum evolution of very low mass stars

We present theoretical models of the angular momentum evolution of very low mass stars (0.1-0.5 M-.). We also present models of solar analogs (0.6-1.1 M-.) for comparison with previous work. We investigate the effect of rotat ion on the effective temperature and luminosity of these stars. Rotation lo wers the effective temperature and luminosity of the models relative to sta ndard models of the same mass and composition. We find that the decrease in T-eff and L can be significant at the higher end of our mass range but bec omes small below 0.4 M,. The effects of different assumptions about interna l angular momentum transport are discussed. Formulae for relating T-eff to mass and nu(rot) are presented. We demonstrate that the kinetic energy of r otation is not a significant contribution to the luminosity of low-mass sta rs. Previous studies of the angular momentum evolution of low-mass stars co ncentrated on solar analogs and were complicated by uncertainties related t o the internal transport of angular momentum. In this paper we extend our t heoretical models for the angular momentum evolution of stars down to 0.1 M -.. We compare our models to rotational data from young open clusters of di fferent ages to infer the rotational history of low-mass stars and the depe ndence of initial conditions and rotational evolution on mass. We find that the qualitative conclusions for stars below 0.6 M-. do not depend on the a ssumptions about internal angular momentum transport with the exception of a zero-point shift in the angular momentum loss saturation threshold. We ar gue that this makes these low-mass stars ideal candidates for the study of the angular momentum loss law and distribution of initial conditions. For s tars with masses between 0.6 and 1.1 M-., scaling the saturation threshold by the Rossby number can reproduce the observed mass dependence of the stel lar angular momentum evolution. We find that neither models with solid-body rotation nor differentially rotating models can simultaneously reproduce t he observed stellar spin-down in the 0.6-1.1 M-. range and for stars betwee n 0.1 and 0.6 M-.. We argue that the most Likely explanation is that the sa turation threshold drops more steeply at low masses than would be predicted with a simple Rossby scaling. In young clusters there is a systematic incr ease in the mean rotation rate with decreased temperature below 3500 K (0.4 M-.). This suggests either inefficient angular momentum loss or mass-depen dent initial conditions for stars near the fully convective boundary.