Abstract

Classical T Tauri stars are the prototypes of contracting solar-mass proto-stars that are accreting material from their surrounding circumstellar disc. As a consequence of both accretion and contraction, they should undergo a spin-up during their accreting phase. But this is not what observations tell us. Instead, they seem to retain a rather constant angular speed as long as the disc remains present. This observational evidence has lead to the so called « disk-locking paradigm » and to one of the most challenging issues of star formation theory.

It will be argued that the star-disc interaction cannot, per se, lead to the required braking torque. Instead, the angular momentum excess of the star-inner disc system must be expelled away, the most natural way being probably in the form of outflows. Such a situation has been considered extensively in the past, in particular within the X-wind model (Shu et al 1994). But, after a critical exam of the necessary conditions prevailing in the X-wind launching zone, we will be lead to the conclusion that such a configuration may never be reached. An alternative process should then be searched for. It seems that one possibility of efficiently braking down protostars could lie in time dependent accretion-ejection events, as evidenced in some MHD numerical experiments.