Modelling the angular momentum evolution of low-mass stars with core-envelope decoupling

We present a model of angular momentum evolution for stars in the mass range 0.5 – 1.1 M , during their early stages of evolution. The model is based upon the following hypothesis: a constant surface rotational period during star-disk interaction, angular momentum loss through magnetic wind, and differential rotation parameterized with a constant coupling time. We investigate the effect of the different parameters, the initial velocity at the T Tauri age, the disk lifetime, the magnetic braking law, and we discuss the effect of introducing a core-envelope decoupling. The angular momentum transfer is parameterized by the use of a coupling time scale τc , which controls the exchanges of angular momentum between the – fast-rotating – radiative core and the convective envelope, both supposed to rotate as solid bodies. We present evolutionary tracks of a single star through the pre-main sequence and the main-sequence, for different masses and different coupling time-scales. We conclude that rapid rotators require solid-body rotation, and ZAMS slow rotator require a strong differential decoupling with a characteristic coupling time about 100 Myr.