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 inves- tigate 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 c , which controls the exchanges of angu- lar momentum between the - fast-rotating - radiative core and the convective envelope, both supposed to rotate as solid bod- ies. 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 rota- tors require solid-body rotation, and ZAMS slow rotator require a strong differential decoupling with a characteristic coupling time about 100 Myr.