Main-sequence stars of 10 and 30 M-circle dot: approaching the steady-state rotation

The evolution of the angular velocity profile in 10 and 30 M-. main-sequenc e (MS) stars has been calculated from the initial uniform rotation to the a symptotic steady-state rotation. Following Zahn (1992) we assume that both transport of angular momentum and mixing of chemical elements are produced by rotationally induced meridional circulation and turbulent diffusion. It is shown that for a sufficiently large surface rotational velocity, whose v alue can be estimated a priori, the relaxation time for star's achieving th e steady-state rotation is much shorter than the star's MS life-time. In th is case the assumption that a star is in a state of stationary rotation fro m the very beginning of its MS evolution is quite reasonable. On the other hand, for a star rotating slowly one has to solve the nonstationary angular momentum transport equation simultaneously with the stellar evolution calc ulations. Despite the fact that the rate of mixing of chemical elements by meridional circulation is strongly reduced by horizontal erosion, diffusion -like abundance profiles of C and N are built up in the radiative envelope by the end of the star's MS life. The surface N abundance begins to increas e after some delay time required for the diffusion wave to reach the atmosp here. If mixing penetrates the convective core the abundance of He is expec ted to behave like that of N. Internal gravity waves generated near the con vective core border are shown to probably play an important role as another angular momentum transport mechanism, especially in the inner part of the envelope.