The Angular Momentum Evolution of Very Low Mass Stars

We present theoretical models of the angular momentum evolution of very lowmass stars (0.1 - 0.5 M_sun) and solar analogues (0.6 - 1.1 M_sun). Weinvestigate the effect of rotation on the effective temperature and luminosityof these stars. We find that the decrease in T_eff and L can be significant atthe higher end of our mass range, but becomes negligible below 0.4 M_sun.Formulae for relating T_eff to mass and v_rot are presented. We compare our models to rotational data from young open clusters ofdifferent ages to infer the rotational history of low mass stars, and thedependence of initial conditions and rotational evolution on mass. We find thatthe qualitative conclusions for stars below 0.6 M_sun do not depend on theassumptions about internal angular momentum transport, which makes these lowmass stars ideal candidates for the study of the angular momentum loss law anddistribution of initial conditions. We find that neither models with solid bodynor differential rotation can simultaneously reproduce the observed stellarspin down in the 0.6 to 1.1 M_sun mass range and for stars between 0.1 and 0.6M_sun. The most likely explanation is that the saturation threshold drops moresteeply at low masses than would be predicted with a simple Rossby scaling. Inyoung clusters there is a systematic increase in the mean rotation rate withdecreased temperature below 3500 K (0.4 M_sun). This suggests eitherinefficient angular momentum loss or mass-dependent initial conditions forstars near the fully convective boundary. (abridged)Comment: To appear in the May 10, 2000 Ap