L'évolution du moment cinétique des étoiles pré-séquence principale de faible masse
This thesis presents a study of the rotation of low-mass stars (from 0.5 to 1.2 MG) during their pre-main sequence - from T Tauri ages, at a few million years old - and main sequence - to a few billion years old. We used both observations, to mesure new rotational velocities of these objects, and modelisation, to understand the physical processes involved in rotational evolution. Observations were essentiaIly aimed at measuring rotational periods and velocities in young clusters: IC 4665, & Perseus and the Pleiades. At the age of these clusters, solartype stars just arrive on the main sequence, and a large fraction have velocities lower than 10 km.s-1. But, due to instrumental limitations, up to now, only upper limits hab been derived. We observed aIl the very slow rotators in & Perseus and the Pleiades with the instruments CORAVEL and ELODIE and resolved aIl the vsini for masses between 0.6 and 1.1 M. . Velocity distributions are computed for different masses in both clusters, and are compared to the models. A model of angular momentum evolution has been developped in order to take into account pre-main sequence evolution, changes in internaI structure, the effect of an accretion disk, angular momentum loss at the surface and angular momentum transfer between the core and the envelope. The new young clusters data provide strong constraints to the transfert of angular momentum in steIlar interiors. It is found that for rapid rotators, a very efficient transfer leads to a nearly solid-body rotation during aIl the evolution, from T Tauri to the Sun ages. On the other hand, the existence of a large fraction of slow rotators requires a strong decoupling between the core and the envelope, with a characteristic time-scale of 100 million years. The evolution of these slow rotators velocities during the first ages of pre-main sequence is also in good agreement with a long coupling time between the radiative core and the convective envelope.