ANGULAR-MOMENTUM TRANSFER IN PRE-MAIN-SEQUENCE STARS OF INTERMEDIATE-MASS

Pre-main-sequence stars between 2 and 5M(.) (Herbig Ae/Be stars) posse ss strong stellar winds and extended chromospheres. The non-radiative heating necessary to fuel such chromospheres is considerable. Unlike s olar type stars, this heating can not be related to the existence of a subphotosopheric convection zone, as their envelope are in radiative equilibrium. Another possibility advanced for T Tauri stars is to use the gravitational energy contained in an accretion disk; however, the presence of optically thick disks around Herbig Ae/Be stars has been s eriously questioned recently. The kinetic energy of stellar rotation i s potentially sufficient to support this chromospheric heating during the evolution towards the main-sequence. The problem is to find an eff icient mechanism to extract rotational energy, to transfer and dissipa te it in the outer layers of the star. We investigate the effect of th e angular momentum losses driven by a strong stellar wind on the distr ibution of the angular momentum inside the star. We propose that the b raking torque exerted by the wind forces turbulent motions below the s tellar surface. Guided by an analogy with geophysical and experimental fluids, a simplified model shows that the wind-induced angular moment um losses are efficiently transferred through the stellar interior by these turbulent motions. This transfer occurs in a turbulent layer whi ch deepens towards the stellar interior in a time-scale of 10(6) years , comparable with the Kelvin timescale of Herbig Ae/Be stars. It resul ts that, during their pre-main-sequence evolution, Herbig Ae/Be stars convert part of their rotational energy into turbulent motions. This p rovides appropriate physical conditions to produce a magnetic field wh ich could transfer and dissipate this turbulent kinetic energy in the outer layers of the star.